Last-modified: 18 Jan 1995
Version: 1.00
FAQ: Automotive Gasoline
Bruce Hamilton
B.Hamilton@irl.cri.nz
---------------------------------------------------------------------------
Subject: 1. Introduction and Intent
The intent of this FAQ is to provide some basic information on gasolines
and other fuels for spark ignition engines used in automobiles. The
toxicity and environmental reasons for recent and planned future changes to
gasoline are discussed, along with recent and proposed changes in
composition of gasoline. This FAQ intended to help readers choose the most
appropriate fuel for vehicles, assist with the diagnosis of fuel-related
problems, and to understand the significance of most gasoline properties
listed in fuel specifications. I make no apologies for the fairly heavy
emphasis on chemistry, it is the only sensible way to describe the
oxidation of hydrocarbon fuels to produce energy, water, and carbon
dioxide.
---------------------------------------------------------------------------
Subject: 2. Table of Contents
1. Introduction and Intent
2. Table of Contents
3. What Advantage will I gain from reading this FAQ?
4. What is Gasoline?
4.1 Where does crude oil come from?.
4.2 When will we run out of crude oil?.
4.3 What is the history of gasoline?
4.4 What are the hydrocarbons in gasoline?
4.5 What are oxygenates?
4.6 Why were alkyl lead compounds added?
4.7 Why not use other organometallic compounds?
4.8 What do the refining processes do?
4.9 What energy is released when gasoline is burned?
4.10 What are the gasoline specifications?
4.11 What are the effects of the specified fuel properties?
4.12 Are brands different?
4.13 What is a typical composition?
4.14 Is gasoline toxic or carcinogenic?
4.15 Is unleaded gasoline more toxic than leaded?
5. Why is Gasoline Composition Changing?
5.1 Why pick on cars and gasoline?
5.2 Why are there seasonal changes?
5.3 Why were alkyl lead compounds removed?
5.4 Why are evaporative emissions a problem?
5.5 Why control tailpipe emissions?
5.6 Why do exhaust catalysts influence fuel composition?
5.7 Why are "cold start" emissions so important?
5.8 When will the emissions be "clean enough"?
5.9 Why are only some gasoline compounds restricted?
5.10 What does "renewable" fuel/oxygenate mean?
5.11 Will oxygenated gasoline damage my vehicle?
5.12 What does "reactivity" of emissions mean?
5.13 What are "carbonyl" compounds?
5.14 What are "gross polluters"?
6. What do Fuel Octane ratings really indicate?
6.1 Who invented Octane Ratings?
6.2 Why do we need Octane Ratings?
6.3 What fuel property does the Octane Rating measure?
6.4 Why are two ratings used to obtain the pump rating?
6.5 What does the Motor Octane rating measure?
6.6 What does the Research Octane rating measure?
6.7 Why is the difference called "sensitivity"?
6.8 What sort of engine is used to rate fuels?
6.9 How is the Octane rating determined?
6.10 What is the Octane Distribution of the fuel?
6.11 What is a "delta Research Octane number"?
6.12 How do other fuel properties affect octane?
6.13 Can higher octane fuels give me more power?
6.14 Does low octane fuel increase engine wear?
6.15 Can I mix different octane fuel grades?
6.16 What happens if I use the wrong octane fuel?
6.17 Can I tune the engine to use another octane fuel?
6.18 How can I increase the fuel octane?
6.19 Are aviation gasoline octane numbers comparable?
7. What parameters determine octane requirement?
7.1 What is the effect of Compression ratio?
7.2 What is the effect of changing the air/fuel ratio?
7.3 What is the effect of changing the ignition timing
7.4 What is the effect of engine management systems?
7.5 What is the effect of temperature and Load?
7.6 What is the effect of engine speed?
7.7 What is the effect of engine deposits?
7.8 What is the Road octane requirement of an vehicle?
7.9 What is the effect of air temperature?.
7.10 What is the effect of altitude?.
7.11 What is the effect of humidity?.
7.12 What does water injection achieve?.
8. How can I identify and cure other fuel-related problems?
8.1 What causes an empty fuel tank?
8.2 Is knock the only abnormal combustion problem?
8.3 Can I prevent carburetter icing?
8.4 Should I store fuel to avoid the oxygenate season?
8.5 Can I improve fuel economy by using quality gasolines?
8.6 What is "stale" fuel, and should I use it?
8.7 How can I remove water in the fuel tank?
8.8 Can I use unleaded on older vehicles?
9. Alternative Fuels and Additives
9.1 Do fuel additives work?
9.2 Can a quality fuel help a sick engine?
9.3 What are the advantages of alcohols and ethers?
9.4 Why are CNG and LPG considered "cleaner" fuels.
9.5 Why are hydrogen-powered cars not available?
9.6 What are "fuel cells" ?
9.7 What is a "hybrid" vehicle?
9.8 What about other alternative fuels?
9.9 What about alternative oxidants?
10. Historical Legends
10.1 The myth of Triptane
10.2 From Honda Civic to Formula 1 winner.
11. References
11.1 Books and Research Papers
11.2 Suggested Further Reading
---------------------------------------------------------------------------
Subject: 3. What Advantage will I gain from reading this FAQ?
This FAQ is intended to provide a fairly technical description of what
gasoline contains, how it is specified, and how the properties affect the
performance in your vehicle. The regulations governing gasoline have
changed, and are continuing to change. These changes have made much of the
traditional lore about gasoline obsolete. Motorists may wish to understand
a little more about gasoline to ensure they obtain the best value, and the
most appropriate fuel for their vehicle. There is no point in prematurely
destroying your second most expensive purchase by using unsuitable fuel,
just as there is no point in wasting hard-earned money on higher octane
fuel that your automobile can not utilize. Note that this FAQ does not
discuss the relative advantages of specific brands of gasolines, it is only
intended to discuss the generic properties of gasolines.
---------------------------------------------------------------------------
Subject: 4. What is Gasoline?
4.1 Where does crude oil come from?.
The generally-accepted origin of crude oil is from plant life up to 3
billion years ago, but predominantly from 100 to 600 million years ago [1].
"Dead vegetarian dino dinner" is more correct than "dead dinos". The
molecular structure of the hydrocarbons and other compounds present in
fossil fuels can be linked to the leaf waxes and other plant molecules of
marine and terrestrial plants believed to exist during that era. There are
various biogenic marker chemicals such as isoprenoids from terpenes,
porphyrins and aromatics from natural pigments, pristane and phytane from
the hydrolysis of chlorophyll, and normal alkanes from waxes, whose size
and shape can not be explained by known geological processes [2]. The
presence of optical activity and the carbon isotopic ratios also indicate a
biological origin [3]. There is another hypothesis that suggests crude oil
is derived from methane from the earth's interior. The current main
proponent of this abiotic theory is Thomas Gold, however abiotic and
extraterrestrial origins for fossil fuels were also considered at the turn
of the century, and were discarded then.
4.2 When will we run out of crude oil?
It has been estimated that the planet contains over 1.4 x 10^15 tonnes of
petroleum, however much of this is too dilute or inaccessible for current
technology to recover [4]. The petroleum industry uses a measure called the
Reserves/Production ratio (R/P) to monitor how production and exploration
are linked. This is based on the concept of "proved" reserves of crude oil,
which are generally taken to be those quantities which geological and
engineering information indicate with reasonable certainty can be recovered
in the future from known reservoirs under existing economic and operating
conditions. The Reserves/Production ratio is the above reserves divided by
the production in the last year, and the result is the length of time that
those remaining reserves would last if production were to continue at the
current level [5]. It is important to note those definitions, as the price
of oil increases, marginal fields become "proved reserves", thus we are
unlikely to "run out" of oil, as more fields will become economic as the
price rises. If the price exceeds $30/bbl then alternative fuels may become
competitive, and at $50-60/bbl coal-derived liquid fuels are economic, as
are many biomass-derived fuels and other energy sources [6]. One barrel of
oil equals 0.158987 m3. The current price for Brent Crude is approx.
$18/bbl. The R/P ratio has increased from 27 years (1979) to 43.1 years
(1993) [5]. Now, some numbers.
( billion = 1 x 10^9. trillion = 1 x 10^12 ).
Crude Oil Proved Reserves R/P Ratio
Middle East 89.6 billion tonnes 95.1 year
USA 4.0 9.9 years
Total World 136.7 43.1 years
Coal Proved Reserves R/P Ratio
USA 240.56 billion tonnes 267 years
Total World 1,039.182 236 years
Natural Gas Proved Reserves R/P Ratio
USA 4.7 trillion cubic metres 8.8 years
Total World 142.0 64.9 years.
4.3 What is the history of gasoline?
In the late 19th Century the most suitable fuels for the automobile were
coal tar distillates and the lighter fractions from the distillation of
crude oil. During the early 20th Century the oil companies were producing
gasoline as a simple distillate from petroleum, but the automotive engines
were rapidly being improved and required a more suitable fuel. During the
1910s, laws prohibited the storage of gasolines on residential properties,
so Charles F. Kettering ( yes - he of ignition system fame ) modified an IC
engine to run on kerosine. However the kerosine-fuelled engine would
"knock" and crack the cylinder head and pistons. He assigned Thomas Midgley
Jr. to confirm that the cause was from the kerosine droplets vaporising on
combustion as they presumed . Midgley demonstrated that the knock was
caused by a rapid rise in pressure after ignition, not during preignition
as believed [7]. This then lead to the long search for anti-knock agents,
culminating in tetra ethyl lead [8]. Typical mid-1920s gasolines were 40 -
60 Octane [9].
Because sulfur in gasoline inhibited the octane-enhancing effect of the
alkyl lead, the sulfur content of the thermally-cracked refinery streams
for gasolines was restricted. By the 1930s, the petroleum industry had
determined that the larger hydrocarbon molecules (kerosine) had major
adverse effects on the octane of gasoline, and were developing consistent
specifications for desired properties. By the 1940s catalytic cracking was
introduced, and gasoline compositions became fairly consistent between
brands during the various seasons.
The 1950s saw the start of the increase of the compression ratio, requiring
higher octane fuels. Lead levels were increased, and some new refining
processes ( such as hydrocracking ), specifically designed to provide
hydrocarbons components with good lead response and octane, were
introduced. Minor improvements were made to gasoline formulations to
improve yields and octane until the 1970s - when unleaded fuels were
introduced to protect the exhaust catalysts that were also being introduced
for environmental reasons. From 1970 until 1990 gasolines were slowly
changed as lead was phased out. In 1990 the Clean Air Act started forcing
major compositional changes on gasoline, and these changes will continue
into the 21st Century because gasoline is a major pollution source.
4.4 What are the hydrocarbons in gasoline?
Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and
carbon, both of which are fuel molecules that can be burnt ( oxidised ) to
form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is not
complete, carbon monoxide ( CO ) may be formed. As CO can be burnt to
produce CO2, it is also a fuel.
The way the hydrogen and carbons hold hands determines which hydrocarbon
family they belong to. If they only hold one hand they are called
"saturated hydrocarbons" because they can not absorb additional hydrogen.
If the carbons hold two hands they are called "unsaturated hydrocarbons"
because they can be converted into "saturated hydrocarbons" by the addition
of hydrogen to the double bond. Hydrogens are omitted from the following,
but if you remember C = 4 hands, H = 1 hand, and O = 2 hands, you can draw
the full structures of most HCs.
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. Detailed descriptions of structures
can be found in any chemical or petroleum text discussing gasolines [10].
4.4.1 Saturated hydrocarbons ( aka paraffins, alkanes )
* stable, the major component of gasolines
* tend to burn in air with a clean flame
alkanes
normal = continuous chain of carbons ( Cn H2n+2 )
normal heptane C-C-C-C-C-C-C C7H16
iso = branched chain of carbons ( Cn H2n+2 )
iso octane = C C
( aka 2,2,4-trimethylpentane ) | |
C-C-C-C-C C8H18
|
C
cyclic = circle of carbons ( Cn H2n )
( aka Naphthenes )
cyclohexane = C
/ \
C C
| | C6H12
C C
\ /
C
4.4.2 Unsaturated Hydrocarbons
* Unstable, are the remaining component of gasoline.
* Tend to burn in air with a smoky flame.
Alkenes ( aka olefins, have carbon=carbon double bonds )
These are unstable, and are usually limited to a few %.
C
| C5H10
2-methyl-2-butene C-C=C-C
Alkynes ( aka acetylenes, have carbon-carbon triple bonds )
These are even more unstable, are only present in trace amounts, and only
in some poorly-refined gasolines.
_
Acetylene C=C C2H2
Arenes ( aka aromatics )
Used to be up to 40%, gradually being reduced to <20%.
C C
// \ // \
C C C-C C
Benzene | || Toluene | ||
C C C C
\\ / \\ /
C C
C6H6 C7H8
Polynuclear Aromatics ( aka PNAs or PAHs )
These are high boiling, and are only present in small amounts in gasoline.
They contain benzene rings joined together, and the simplest is
Naphthalene. The multi-ringed PNAs are highly toxic, and are not present in
gasoline.
C C
// \ / \\
C C C
Naphthalene | || | C10H8
C C C
\\ / \ //
C C
4.5 What are oxygenates?
Oxygenates are just preused hydrocarbons :-). They contain oxygen, which
can not provide energy, but their structure provides a reasonable
anti-knock value, thus they are good substitutes for aromatics, and they
may also reduce the smog-forming tendencies of the exhaust gases [11].
Ethanol C-C-O-H C2H5OH
C
|
Methyl tertiary butyl ether C-C-O-C C4H90CH3
(aka tertiary butyl methyl ether ) |
C
They can be produced from fossil fuels eg methanol (MeOH), methyl tertiary
butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg
ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). Most oxygenates used in
gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain 1
to 6 carbons. MTBE is produced by reacting methanol ( from natural gas )
with isobutylene in the liquid phase over an acidic ion-exchange resin
catalyst at 100C. The isobutylene was initially from refinery catalytic
crackers or petrochemical olefin plants, but these days larger plants
produce it from butanes. Production has increased at the rate of 10 to 20%
per year, and the spot market price in June 1993 was around $270/tonne
[11]. The "ether" starting fluids for vehicles are usually diethyl ether (
liquid ) or dimethyl ether ( aerosol ). Note that " petroleum ether " is
actually a volatile hydrocarbon fraction, it is not a Cx-O-Cy compound.
Oxygenates are added to gasolines to reduce the reactivity of emissions,
but they are only effective if the hydrocarbon fractions are carefully
modified to utilise the octane and volatility properties of the oxygenates.
If the hydrocarbon fraction is not correctly modified, oxygenates can
increase the undesirable smog-forming and toxic emissions. The major
reduction in the reactivity of exhaust and evaporative emissions will occur
with reformulated gasolines, due to be introduced in January 1995, which
have oxygenates and major composition changes to the hydrocarbon component.
Oxygenates do not necessarily reduce all individual exhaust toxins, nor are
they intended to.
Oxygenates have significantly different physical properties to
hydrocarbons, and the levels that can be added to gasolines are controlled
by the EPA in the US, with waivers being granted for some combinations. The
change to reformulated gasoline requires oxygenates, but also that the
hydrocarbon composition must be significantly more modified than the
existing oxygenated gasolines to reduce unsaturates, volatility, benzene,
and the reactivity of emissions.
Oxygenates that are added to gasoline function in two ways. Firstly they
have high blending octane, and so can replace high octane aromatics in the
fuel. These aromatics are responsible for disproportionate amounts of CO
and HC exhaust emissions. This is called the "aromatic substitution
effect". Oxygenates also cause engines without sophisticated engine
management systems to move to the lean side of stoichiometry, thus reducing
emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can
reduce HC by 10%). However, on vehicles with engine management systems, the
fuel volume will be increased to bring the stoichiometry back to the
preferred optimum setting. Oxygen in the fuel can not contribute energy,
consequently the fuel has less energy content. For the same efficiency and
power output, more fuel has to be burnt, and the slight improvements in
efficiency that oxygenates provide on some engines usually do not
completely compensate for the oxygen [12].
There are huge number of chemical mechanisms involved in the pre-flame
reactions of gasoline combustion. Although both alkyl leads and oxygenates
are effective at suppressing knock, the chemical modes through which they
act are entirely different. MTBE works by retarding the progress of the low
temperature or cool-flame reactions, consuming radical species,
particularly OH radicals and producing isobutene. The isobutene in turn
consumes additional OH radicals and produces unreactive, resonantly
stabilised radicals such as allyl and methyl allyl, as well as stable
species such as allene, which resist further oxidation [13,14].
4.6 Why were alkyl lead compounds added?
The efficiency of a spark-ignited gasoline engine can be related to the
compression ratio up to at least compression ratio 17:1 [15]. However any
"knock" caused by the fuel will rapidly mechanically destroy an engine, and
General Motors was having major problems trying to improve engines without
inducing knock. The problem was to identify economic additives that could
be added to gasoline or kerosine to prevent knock, as it was apparent that
engine development was being hindered. The kerosine for home fuels soon
became a secondary issue, as the magnitude of the automotive knock problem
increased throughout the 1910s, and so more resources were poured into the
quest for an effective "anti-knock". A higher octane aviation gasoline was
required urgently once the US entered WWI, and almost every possible
chemical ( including melted butter ) was tested for anti-knock ability
[16].
Originally, iodine was the best anti-knock available, but was not a
practical gasoline additive, and was used as the benchmark. In 1919 aniline
was found to have superior antiknock ability to iodine, but also was not a
practical additive, however aniline became the benchmark anti-knock, and
various compounds were compared to it. The discovery of tetra ethyl lead,
and the scavengers required to remove it from the engine were made by teams
lead by Thomas Midgley Jr. in 1922 [7,8,16]. They tried selenium
oxychloride which was an excellent antiknock, however it reacted with iron
and "dissolved" the engine. Midgley was able to predict that other
organometallics would work, and slowly focused on organoleads. They then
had to remove the lead, which would otherwise accumulate and coat the
engine and exhaust system with lead. They discovered and developed the
halogenated lead scavengers that are still used in leaded fuels. The
scavengers, ( ethylene dibromide and ethylene dichloride ), function by
providing halogen atoms that react with the lead to form volatile lead
halide salts that can escape out the exhaust. The quantity of scavengers
added to the alkyl lead concentrate is calculated according to the amount
of lead present. If sufficient scavenger is added to theoretically react
with all the lead present, the amount is called one "theory". Typically,
1.0 to 1.5 theories are used, but aviation gasolines must only use one
theory. This ensures there is no excess bromine that could react with the
engine. The alkyl leads rapidly became the most cost-effective method of
enhancing octane.
The development of the alkyl leads ( tetra methyl lead, tetra ethyl lead )
and the toxic halogenated scavengers meant that petroleum refiners could
then configure refineries to produce hydrocarbon streams that would
increase octane with small quantities of alkyl lead. If you keep adding
alkyl lead compounds, the lead response of the gasoline decreases, and so
there are economic limits to how much lead should be added.
Up until the late 1960s, alkyl leads were added to gasolines in increasing
concentrations to obtain octane. The limit was 1.14g Pb/l, which is well
above the diminishing returns part of the lead response curve for most
refinery streams, thus it is unlikely that much fuel was ever made at that
level. I believe 1.05 was about the maximum, and articles suggest that 1970
100 RON premiums were about 0.7-0.8 g Pb/l and 94 RON regulars 0.6-0.7 g
Pb/l, which matches published lead response data [17] eg.
For Catalytic Reformate Straight Run Naphtha.
Lead g/l Research Octane Number
0 96 72
0.1 98 79
0.2 99 83
0.3 100 85
0.4 101 87
0.5 101.5 88
0.6 102 89
0.7 102.5 89.5
0.8 102.75 90
The alkyl lead anti-knocks work in a different stage of the pre-combustion
reaction to oxygenates. In contrast to oxygenates, the alkyl lead
interferes with hydrocarbon chain branching in the intermediate temperature
range where HO2 is the most important radical species. Lead oxide, either
as solid particles, or in the gas phase, reacts with HO2 and removes it
from the available radical pool, thereby deactivating the major chain
branching reaction sequence that results in undesirable,
easily-autoignitable hydrocarbons [13,14].
4.7 Why not use other organometallic compounds?
As the toxicity of the alkyl lead and the halogenated scavengers became of
concern, alternatives were considered. The most famous of these is
methylcyclopentadienyl manganese tricarbonyl (MMT), which was used in the
USA until banned by the EPA from 27 Oct 1978 [18], but is approved for use
in Canada and Australia. It is more expensive than alkyl leads and has been
reported to increase unburned hydrocarbon emissions and block exhaust
catalysts [19]. Other compounds that enhance octane have been suggested,
but usually have significant problems such as toxicity, cost, increased
engine wear etc.. Examples include dicyclopentadienyl iron and nickel
carbonyl.
4.8 What do the refining processes do?
Crude oil contains a wide range of hydrocarbons, organometallics and other
compounds containing sulfur, nitrogen etc. The HCs contain between 1 and 60
carbon atoms. Gasoline requires hydrocarbons with carbon atoms between 3
and 12, arranged in specific ways to provide the desirable properties.
Obviously, a refinery has to either sell the remainder as marketable
products, or convert the larger molecules into smaller gasoline molecules.
A refinery will distill crude oil into various fractions and, depending on
the desired final products, will further process and blend those fractions.
Typical final products could be:- gases for chemical synthesis and fuel
(CNG), liquified gases (LPG), butane, aviation and automotive gasolines,
aviation and lighting kerosines, diesels, distillate and residual fuel
oils, lubricating oil base grades, paraffin oils and waxes. Many of the
common processes are intended to increase the yield of blending feedstocks
for gasolines.
Typical modern refinery processes for gasoline components include
* Catalytic cracking - breaks larger, higher-boiling, hydrocarbons into
gasoline range product that contains 30% aromatics and 20-30% olefins.
* Hydrocracking - cracks and adds hydrogen to molecules, producing a
more saturated, stable, gasoline fraction.
* Isomerisation - raises gasoline fraction octane by converting straight
chain hydrocarbons into branched isomers.
* Reforming - converts saturated, low octane, hydrocarbons into higher
octane product containing about 60% aromatics.
* Alkylation - reacts gaseous olefin streams with isobutane to produce
liquid high octane iso-alkanes.
The changes that the Clean Air Act and other legislation ensures that the
refineries will continue to modify their processes to produce a less
volatile gasoline with fewer toxins and toxic emissions. Options include:-
* Reducing the "severity" of reforming to reduce aromatic production.
* Distilling the C5/C6 fraction from reformer feeds and treating that
stream to produce non-aromatic high octane components.
* Distilling the higher boiling fraction ( which contains 80-100% of
aromatics that can be hydrocracked ) from catalytic cracker product
[20].
* Convert butane to isobutane or isobutylene for alkylation or MTBE
feed.
4.9 What energy is released when gasoline is burned?
It is important to note that the theoretical energy content of gasoline
when burned in air is only related to the hydrogen and carbon contents.
Octane rating is not fundamentally related to the energy content, and the
actual hydrocarbon and oxygenate components used in the gasoline will
determine both the energy release and the anti-knock rating.
Two important reactions are:-
1. C + O2 = CO2
2. H + O2 = H2O
The mass or volume of air required to provide sufficient oxygen to achieve
this complete combustion is the "stoichiometric" mass or volume of air.
Insufficient air = "rich", and excess air = "lean", and the stoichiometric
mass of air is related to the carbon:hydrogen ratio of the fuel. The
procedures for calculation of stoichiometric air/fuel ratios are fully
documented in an SAE standard [21].
Atomic masses used are:- Hydrogen = 1.00794, Carbon = 12.011, Oxygen =
15.994, Nitrogen = 14.0067, and Sulfur = 32.066.
The composition of sea level air ( 1976 data, hence low CO2 value ) is
Gas Fractional Molecular Weight Relative
Species Volume kg/mole Mass
N2 0.78084 28.0134 21.873983
O2 0.209476 31.9988 6.702981
Ar 0.00934 39.948 0.373114
CO2 0.000314 44.0098 0.013919
Ne 0.00001818 20.179 0.000365
He 0.00000524 4.002602 0.000021
Kr 0.00000114 83.80 0.000092
Xe 0.000000087 131.29 0.000011
CH4 0.000002 16.04276 0.000032
H2 0.0000005 2.01588 0.000001
---------
Air 28.964419
For normal heptane C7H16 with a molecular weight = 100.204
C7H16 + 11O2 = 7CO2 + 8H2O
thus 1.000 kg of C7H16 required 3.513 kg of O2 = 15.179 kg air.
The chemical stoichiometric combustion of hydrocarbons with oxygen can be
written as:-
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
Often, for simplicity, the remainder of air is assumed to be nitrogen,
which can be added to the equation when exhaust compositions are required.
As a general rule, maximum power is achieved at slightly rich, whereas
maximum fuel economy is achieved at slightly lean.
The energy content of the gasoline is obtained by burning all the fuel
inside a bomb calorimeter and measuring the temperature increase. The
energy available depends on what happens to the water produced from the
combustion of the hydrogen. If the water remains as a gas, then it cannot
release the heat of vaporisation, thus producing the Nett Calorific Value.
If the water were condensed back to the original fuel temperature, then
Gross Calorific Value of the fuel, which will be larger, is obtained.
The calorific values are fairly constant for families of HCs, which is not
surprising, given their fairly consistent carbon/hydrogen ratios. For
liquid ( l ) or gaseous ( g ) fuel converted to gaseous products - except
for the 2-methylbutene-2, where only gaseous is reported. * = Blending
Octane Number
Typical Heats of Combustion are [22]:-
Fuel State Heat of Combustion Research Motor
MJ/kg Octane Octane
n-heptane l 44.592 0 0
g 44.955
i-octane l 44.374 100 100
g 44.682
toluene l 40.554 124* 112*
g 40.967
2-methylbutene-2 44.720 176* 141*
Because all the data is available, the calorific value of fuels can be
estimated quite accurately from hydrocarbon fuel properties such as the
density, sulfur content, and aniline point ( which indicates the aromatics
content ).
It should be noted that because oxygenates contain oxygen that can not
provide energy, they will have significantly lower energy contents. They
are added to provide octane, not energy. For an engine that can be
optimised for oxygenates, more fuel is required to obtain the same power,
but they can burn slightly more efficiently, thus the power ratio is not
identical to the energy content ratio. They also require more energy to
vaporise.
Energy Content Heat of Vaporisation Oxygen Content
Nett MJ/kg MJ/kg wt%
Methanol 19.95 1.154 49.9
Ethanol 26.68 0.913 34.7
MTBE 35.18 0.322 18.2
ETBE 36.29 0.310 15.7
TAME 36.28 0.323 15.7
Gasoline 42 - 44 0.297 0.0
Typical values for commercial fuels in megajoules/kilogram are [23]:-
Gross Nett
Hydrogen 141.9 120.0
Carbon to Carbon monoxide 10.2 -
Carbon to Carbon dioxide 32.8 -
Sulfur to sulfur dioxide 9.16 -
Natural Gas 53.1 48.0
Liquified petroleum gas 49.8 46.1
Aviation gasoline 46.0 44.0
Automotive gasoline 45.8 43.8
Kerosine 46.3 43.3
Diesel 45.3 42.5
Obviously, for automobiles, the nett calorific value is appropriate. The
calorific value is the maximum energy that can be obtained from the fuel,
but the reality of modern SI engines is that efficiencies of 20-40% may be
obtained, this limit being due to engineering and material constraints that
prevent optimum combustion conditions being used. The CI engine can achieve
higher efficiencies, usually over a wider operating range as well.
4.10 What are the gasoline specifications?
Gasolines are usually defined by government regulation, where properties
and test methods are clearly defined. In the US, several government and
state bodies can specify gasoline properties. The US gasoline
specifications and test methods are listed in several readily available
publications, including the Society of Automotive Engineers (SAE) [24], and
the American Society for Testing Materials (ASTM) [25]. The 1994 ASTM
edition has:-
D4814-93a Specification for Automotive Spark-Ignition Engine Fuel.
This specification lists various properties that all fuels have to comply
with, and may be updated throughout the year. Typical properties are:-
4.10.1 Vapour Pressure and Distillation Classes.
6 different classes according to location and/or season.
As gasoline is distilled, the temperatures at which various fractions are
evaporated are calculated. Specifications define the temperatures at which
various percentages of the fuel are evaporated. Distillation limits include
maximum temperatures that 10% is evaporated (50-70C), 50% is evaporated
(110-121C), 90% is evaporated (185-190C), and the final boiling point
(225C). A minimum temperature for 50% evaporated (77C), and a maximum
amount of Residue (2%) after distillation. Vapour pressure limits for each
class ( 54, 62, 69, 79, 93, 103 kPa ) are also specified. Note that the EPA
has issued a waiver that does not require gasoline/ethanol blends to meet
the required specifications.
4.10.2 Vapour Lock Protection Classes
5 classes for vapour lock protection, according to location and/or season.
The limit is a maximum Vapour/Liquid ratio of 20 at test temperatures of
41, 47, 51, 56, 60C.
4.10.3 Antiknock Index ( aka (RON+MON)/2, "Pump Octane" )
The ( Research Octane Number + Motor Octane Number ) divided by two. Limits
are not specified, but changes in engine requirements according season and
location are discussed. Fuels with an Antiknock index of 87, 89, 91 (
Unleaded), and 88 ( Leaded ) are listed as typical for the US.
4.10.4 Lead Content
Leaded = 1.1 g Pb / L maximum, and Unleaded = 0.013 g Pb / L maximum.
4.10.5 Copper strip corrosion
Ability to tarnish clean copper, indicating the presence of any corrosive
sulfur compounds
4.10.6 Maximum Sulfur content
Sulfur adversely affects exhaust catalysts and fuel hydrocarbon lead
response, and also may be emitted as polluting sulfur oxides.
Leaded = 0.15 %mass maximum, and Unleaded = 0.10 %mass maximum.
Typical US gasoline levels are 0.03 %mass.
4.10.7 Maximum Existent Gum
Limits the amount of gums present in fuel at the time of testing to 5
mg/100mls. The results do not correlate well with actual engine deposits
caused by fuel vaporisation [26].
4.10.8 Minimum Oxidation Stability
This ensures the fuel remains chemically stable, and does not form
additional gums during periods in distribution systems, which can be up to
3-6 months. The sample is heated with oxygen inside a pressure vessel, and
the delay until significant oxygen uptake is measured.
4.10.9 Water Tolerance
Highest temperature that causes phase separation of oxygenated fuels. The
limits vary according to location and month. For Alaska - North of 62
latitude, it changes from -41C in Dec/Jan to 9C in July, but remains 10C
all year in Hawaii.
As well as the above, there are various restrictions introduced by the
Clean Air Act and state bodies such as California's Air Resources Board
(CARB) that often have more stringent limits for the above properties, as
well as additional limits. The Clean Air Act also specifies some regions
that exceed air quality standards have to use reformulated gasolines (RFGs)
all year, starting January 1995. Other regions are required to use
oxygenated gasolines for four winter months, beginning November 1992. The
RFGs also contain oxygenates. Metropolitan regions with severe ozone air
quality problems must use reformulated gasolines in 1995 that;- contain at
least 2.0 wt% oxygen, reduce 1990 volatile organic carbon compounds by 15%,
and reduce specified toxic emissions by 15% (1995) and 25% (2000).
Metropolitan regions that exceeded carbon monoxide limits were required to
use gasolines with 2.7 wt% oxygen during winter months, starting in 1992.
Because phosphorus adversely affects exhaust catalysts, the EPA limits
phosphorus in all gasolines to 0.0013 gP/L.
The 1990 Clean Air Act (CAA) amendments and CARB phase 2 (1996)
specifications for reformulated gasoline establish the following limits,
compared with typical 1990 gasoline. Because of a lack of data, the EPA
were unable to define the CAA required parameters , so they instituted a
two-stage system. The first stage, the "Simple Model" is an interim stage
that run from 1/Jan/1995 to 1/May/1997. The second stage, the "Complex
Model" would be developed, with the following parameters likely to be
controlled - reid vapour pressure, benzene, oxygen, sulfur, olefins
distillation ( 90% Evaporated ), and aromatics. Each refiner must have
their RFG recertified using the Complex model by 1/May/1997 [27].
1990 Clean Air Act CARB
benzene 2 % 1 % maximum 1.0 vol% maximum
oxygen 0.2 % 2 % minimum 1.8-2.0 mass%
sulfur 150 ppm no increase 40 ppm
aromatics 32.0 % 25 % maximum 25 vol% maximum
olefins 9.9 % 5 % maximum 6 vol% maximum
reid vapour pressure 60 kPa 56 kPa (north) 48 kPa
50 kPa (south)
90% evaporated 170 C - 149 C
These regulations also specify emissions criteria. eg CAA specifies no
increase in nitric oxides (NOx) emissions, reductions in VOC by 15% during
the ozone season, and specified toxins by 15% all year. These criteria
indirectly establish vapour pressure and composition limits that refiners
have to meet. Note that the EPA also can issue CAA Section 211 waivers that
allow refiners to choose which oxygenates they use. In 1981, the EPA also
decided that fuels with up to 2% alcohols and ethers (except methanol) were
"substantially similar" to 1974 unleaded gasoline, and thus were not "new"
gasoline additives. That level was increased to 2.7 wt% in 1991. Some other
oxygenates have also been granted waivers, eg ethanol to 3.5 wt% in
1979/1982, and tert-butyl alcohol to 3.5 wt% in 1981.
4.11 What are the effects of the specified fuel properties?
Volatility
This affects evaporative emissions and driveability, it is the property
that must change with location and season. Fuel for mid-summer Arizona
would be difficult to use in mid-winter Alaska. The US is divided into
zones, according to altitude and seasonal temperatures, and the fuel
volatility is adjusted accordingly. Incorrect fuel may result in difficult
starting in cold weather, carburetter icing, vapour lock in hot weather,
and crankcase oil dilution. Volatility is controlled by distillation and
vapour pressure specifications. The higher boiling fractions of the
gasoline have significant effects on the emission levels of undesirable
hydrocarbons and aldehydes, and a reduction of 40C in the final boiling
point will reduce the levels of benzene, butadiene, formaldehyde and
acetaldehyde by 25%, and will reduce HC emissions by 20% [28].
Combustion Characteristics
As gasolines contain mainly hydrocarbons, the only significant variable
between different grades is the octane rating of the fuel, as most other
properties are similar. Octane is discussed in detail in Section 6. There
are only slight differences in combustion temperatures ( most are around
2000C in isobaric adiabatic combustion [29]). Note that the actual
temperature in the combustion chamber is also determined by other factors,
such as load and engine design. The addition of oxygenates changes the
pre-flame reaction pathways, and also reduces the energy content of the
fuel. The levels of oxygen in the fuel is regulated according to regional
air quality standards.
Stability
Motor gasolines may be stored up to six months, consequently they must not
form gums which may precipitate. Gums are usually the result of
copper-catalysed reactions of the unsaturated HCs, so antioxidants and
metal deactivators are added. Existent Gum is used to measure the gum in
the fuel at the time tested, whereas the Oxidation Stability measures the
time it takes for the gasoline to break down at 100C with 100psi of oxygen.
A 240 minutes test period has been found to be sufficient for most storage
and distribution systems.
Corrosiveness
Sulfur in the fuel creates corrosion, and when combusted will form
corrosive gases that attack the engine, exhaust and environment. Sulfur
also adversely affects the alkyl lead octane response and may poison
exhaust catalysts. The copper strip corrosion test and the sulfur
specification are used to ensure fuel quality. The copper strip test
measures active sulfur, whereas the sulfur content reports the total sulfur
present.
4.12 Are brands different?
Yes. The above specifications are intended to ensure minimal quality
standards are maintained, however as well as the fuel hydrocarbons, the
manufacturers add their own special ingredients to provide additional
benefits. A quality gasoline additive package would include:-
* octane-enhancing additives ( improve octane ratings )
* anti-oxidants ( inhibit gum formation, improve stability )
* metal deactivators ( inhibit gum formation, improve stability )
* deposit modifiers ( reduce deposits, spark-plug fouling and
preignition )
* surfactants ( prevent icing, improve vaporisation, inhibit deposits,
reduce NOx emissions )
* freezing point depressants ( prevent icing )
* corrosion inhibitors ( prevent gasoline corroding storage tanks )
* dyes ( product colour for safety or regulatory purposes ).
During the 1980s significant problems with deposits accumulating on intake
valve surfaces occurred as new fuel injections systems were introduced.
These intake valve deposits (IVD) were different to the injector deposits,
in part because the valve can reach 300C. Engine design changes that
prevent deposits usually consist of ensuring the valve is flushed with
liquid gasoline, and provision of adequate valve rotation. Gasoline factors
that cause deposits are the presence of alcohols or olefins. Gasoline
manufacturers now routinely use additives that prevent IVD and also
maintain the cleanliness of injectors. These usually include a surfactant
and light oil to maintain the wetting of important surfaces. A more
detailed description of additives is provided in Section 9.1.
Texaco demonstrated that a well-formulated package could improve fuel
economy, reduce NOx emissions, and restore engine performance because, as
well as the traditional liquid-phase deposit removal, some additives can
work in the vapour phase to remove existing engine deposits without
adversely affecting performance ( as happens when water is poured into a
running engine to remove carbon deposits:-) )[30]. Most suppliers of
quality gasolines will formulate similar additives into their products, and
cheaper lines are less like to have such additives added. As different
brands use different additives and oxygenates, it is probable that
important parameters, such as octane distribution, are different, even
though the pump octane ratings are the same.
So, if you know your car is well-tuned, and in good condition, but the
driveability is pathetic on the correct octane, try another brand. Remember
that the composition will change with the season, so if you lose
driveability, try yet another brand. As various Clean Air Act changes are
introduced over the next few years, gasoline will continue to change.
4.13 What is a typical composition?
There seems to be a perception that all gasolines of one octane grade are
chemically similar, and thus general rules can be promulgated about "energy
content ", "flame speed", "combustion temperature" etc. etc.. Nothing is
further from the truth. The behaviour of manufactured gasolines in octane
rating engines can be predicted, using previous octane ratings of special
blends intended to determine how a particular refinery stream responds to
an octane-enhancing additive. Refiners can design and reconfigure
refineries to efficiently produce a wide range of gasolines feedstocks,
depending on market and regulatory requirements.
The last 10 years of various compositional changes to gasolines for
environmental and health reasons have resulted in fuels that do not follow
historical rules, and the regulations mapped out for the next decade also
ensure the composition will remain in a state of flux. The reformulated
gasoline specifications, especially the 1/May/1997 Complex model, will
probably introduce major reductions in the distillation range, as well as
the various limits on composition and emissions.
I'm not going to list all 500+ HCs in gasolines, but the following are
representative of the various classes typically present in a gasoline. The
numbers after each chemical are:- Research Blending Octane : Motor Blending
Octane : Boiling Point (C): Density (g/ml @ 15C) : Minimum Autoignition
Temperature (C). It is important to realise that the Blending Octanes are
derived from a 20% mix of the HC with a 60:40 iC8:nC7 base, and the
extrapolation of this 20% to 100%. This is different from rating the pure
fuel, which often requires adjustment of the test engine conditions outside
the acceptable limits of the rating methods. Generally the actual octanes
of the pure fuel are similar for the alkanes, but are up to 30 octane
numbers lower than the blending octanes for the aromatics and olefins [31].
A traditional composition I have dreamed up would be like the following,
whereas newer oxygenated fuels reduce the aromatics and olefins, narrow the
boiling range, and add oxygenates up to about 12-15% to provide the octane.
15% n-paraffins RON MON BP d AIT
n-butane 113 : 114 : -0.5: gas : 370
n-pentane 62 : 66 : 35 : 0.626 : 260
n-hexane 19 : 22 : 69 : 0.659 : 225
n-heptane (0:0 by definition) 0 : 0 : 98 : 0.684 : 225
n-octane -18 : -16 : 126 : 0.703 : 220
( you would not want to have the following alkanes in gasoline,
so you would never blend kerosine with gasoline )
n-decane -41 : -38 : 174 : 0.730 : 210
n-dodecane -88 : -90 : 216 : 0.750 : 204
n-tetradecane -90 : -99 : 253 : 0.763 : 200
30% iso-paraffins
2-methylpropane 122 : 120 : -12 : gas : 460
2-methylbutane 100 : 104 : 28 : 0.620 : 420
2-methylpentane 82 : 78 : 62 : 0.653 : 306
3-methylpentane 86 : 80 : 64 : 0.664 : -
2-methylhexane 40 : 42 : 90 : 0.679 :
3-methylhexane 56 : 57 : 91 : 0.687 :
2,2-dimethylpentane 89 : 93 : 79 : 0.674 :
2,2,3-trimethylbutane 112 : 112 : 81 : 0.690 : 420
2,2,4-trimethylpentane 100 : 100 : 98 : 0.692 : 415
( 100:100 by definition )
12% cycloparaffins
cyclopentane 141 : 141 : 50 : 0.751 : 380
methylcyclopentane 107 : 99 : 72 : 0.749 :
cyclohexane 110 : 97 : 81 : 0.779 : 245
methylcyclohexane 104 : 84 : 101 : 0.770 : 250
35% aromatics
benzene 98 : 91 : 80 : 0.874 : 560
toluene 124 : 112 : 111 : 0.867 : 480
ethyl benzene 124 : 107 : 136 : 0.867 : 430
meta-xylene 162 : 124 : 138 : 0.868 : 463
para-xylene 155 : 126 : 138 : 0.866 : 530
ortho-xylene 126 : 102 : 144 : 0.870 : 530
3-ethyltoluene 162 : 138 : 158 : 0.865 :
1,3,5-trimethylbenzene 170 : 136 : 163 : 0.864 :
1,2,4-trimethylbenzene 148 : 124 : 168 : 0.889 :
8% olefins
2-pentene 154 : 138 : 37 : 0.649 :
2-methylbutene-2 176 : 140 : 36 : 0.662 :
2-methylpentene-2 159 : 148 : 67 : 0.690 :
cyclopentene 171 : 126 : 44 : 0.774 :
( the following olefins are not present in significant amounts
in gasoline, but have some of the highest blending octanes )
1-methylcyclopentene 184 : 146 : 75 : 0.780 :
1,3 cyclopentadiene 218 : 149 : 42 : 0.805 :
dicyclopentadiene 229 : 167 : 170 : 1.071 :
Oxygenates
Published octane values vary a lot because the rating conditions are
significantly different to standard conditions, for example the API Project
45 numbers used above for the hydrocarbons, reported in 1957, gave MTBE
blending RON as 148 and MON as 146, however that was based on the lead
response, whereas today we use MTBE in place of lead.
methanol 133 : 105 : 65 : 0.796 : 385
ethanol 129 : 102 : 78 : 0.794 : 365
iso propyl alcohol 118 : 98 : 82 : 0.790 : 399
methyl tertiary butyl ether 116 : 103 : 55 : 0.745 :
ethyl tertiary butyl ether 118 : 102 : 72 : 0.745 :
tertiary amyl methyl ether 111 : 98 : 86 : 0.776 :
There are some other properties of oxygenates that have to be considered
when they are going to be used as fuels, particularly their ability to form
very volatile azeotropes that cause the fuel's vapour pressure to increase,
the chemical nature of the emissions, and their tendency to separate into a
separate water/oxygenate phase when water is present. The reformulated
gasolines address these problems more successfully than the original
oxygenated gasolines.
Before you rush out to make a highly aromatic or olefinic gasoline to
produce a high octane fuel, remember they have other adverse properties, eg
the aromatics attack elastomers and generate smoke, and the olefins are
unstable ( besides smelling foul ) and form gums. The art of correctly
formulating a gasoline that does not cause engines to knock apart, does not
cause vapour lock in summer - but is easy to start in winter, does not form
gums and deposits, burns cleanly without soot/residues, and does not
dissolve or poison the car catalyst or owner, is based on knowledge of the
gasoline composition.
4.14 Is gasoline toxic or carcinogenic?
There are several known toxins in gasoline, some of which are confirmed
human carcinogens. The most famous of these toxins are lead and benzene,
and both are regulated. The other aromatics and some toxic olefins are also
controlled. Lead alkyls also require ethylene dibromide and/or ethylene
dichloride scavengers to be added to the gasoline, both of which are
suspected human carcinogens. In 1993 an International Symposium on the
Health Effects of Gasoline was held [32]. Major review papers on the
carcinogenic, neurotoxic, reproductive and developmental toxicity of
gasoline, additives, and oxygenates were presented. The oxygenates are also
being evaluated for carcinogenicity, and even ethanol and ETBE may be
carcinogens. It should be noted that the oxygenated gasolines were not
expected to reduce the toxicity of the emissions, however the reformulated
gasolines will produce different emissions, and specific toxins must be
reduced by 15% all year.
There is little doubt that gasoline is full of toxic chemicals, and should
therefore be treated with respect. However the biggest danger remains the
flammability, and the relative hazards should always be kept in
perspective. The major toxic risk from gasolines comes from breathing the
tailpipe, evaporative, and refuelling emissions, rather than occasional
skin contact from spills. Breathing vapours and skin contact should always
be minimised.
4.15 Is unleaded gasoline more toxic than leaded?
The short answer is no. However that answer is not global, as some
countries have replaced the lead compound octane-improvers with aromatic or
olefin octane-improvers without introducing exhaust catalysts. Some
aromatics are more toxic that paraffins. Unfortunately, the manufacturers
of alkyl lead compounds have embarked on a worldwide misinformation
campaign in countries considering emulating the lead-free US. The use of
lead precludes the use of exhaust catalysts, thus the emissions of
aromatics are only slightly diminished, and other pollutants can not
reduced by exhaust catalysts.
The use of unleaded on modern vehicles with engine management systems and
catalysts can reduce aromatic emissions to 10% of the level of vehicles
without catalysts [33]. Alkyl lead additives can only substitute for some
of the aromatics in gasoline, consequently they do not eliminate aromatics,
which will produce benzene emissions [34]. Alkyl lead additives also
require toxic organohalogen scavengers, which also react in the engine to
form and emit other organohalogens, including highly toxic dioxin [35].
Leaded fuels emit lead, organohalogens, and much higher levels of regulated
toxins because they preclude the use of exhaust catalysts. In the USA the
gasoline composition is being changed to reduce fuel toxins ( olefins,
aromatics ) as well as emissions of specific toxins.
---------------------------------------------------------------------------
Subject: 5. Why is Gasoline Composition Changing?
5.1 Why pick on cars and gasoline?
Cars emit several pollutants as combustion products out the tailpipe,
(tailpipe emissions), and as losses due to evaporation (evaporative
emissions, refuelling emissions). The volatile organic carbon (VOC)
emissions from these sources, along with nitrogen oxides (NOx) emissions
from the tailpipe, will react in the presence of ultraviolet light
(wavelengths of less than 430nm) to form ground-level (tropospheric) ozone,
which is one of the major components of photochemical smog [36]. Smog has
been a major pollution problem ever since coal-fired power stations were
developed in urban areas, but their emissions are being cleaned up. Now
it's the turn of the automobile.
Cars currently use gasoline that is derived from fossil fuels, thus when
gasoline is burned to completion, it produces additional CO2 that is added
to the atmospheric burden. The effect of the additional CO2 on the global
environment is not known, but the quantity of man-made emissions of fossil
fuels must cause the system to move to a new equilibrium. Even if current
research doubles the efficiency of the IC engine/gasoline combination, and
reduces HC, CO, NOx, SOx, VOCs, particulates, and carbonyls, the amount of
carbon dioxide from the use of fossil fuels may still cause global warming.
More and more scientific evidence is accumulating that warming is occurring
[37]. The issue is whether it is natural, or induced by human activities.
There are international agreements to limit CO2 emissions to 1990 levels, a
target that will require more efficient, lighter, or appropriately-sized
vehicles, - if we are to maintain the current usage. One option is to use
"renewable" fuels in place of fossil fuels. Consider the amount of
energy-related CO2 emissions for selected countries in 1990 [38].
CO2 Emissions
( tonnes/year/person )
USA 20.0
Canada 16.4
Australia 15.9
Germany 10.4
United Kingdom 8.6
Japan 7.7
New Zealand 7.6
The number of new vehicles provides an indication of the magnitude of the
problem. Although vehicle engines are becoming more efficient, the distance
travelled is increasing, resulting in a gradual increase of gasoline
consumption. The world production of vehicles (in thousands) over the last
few years was [39];-
Cars
Region 1990 1991 1992 1993
Africa 222 213 194 201
Asia-Pacific 12,064 12,112 11,869 11,467
Central & South America 800 888 1,158 1,524
Eastern Europe 2,466 984 1,726 1,783
Middle East 35 24 300 377
North America 7,762 7,230 7,470 8,172
Western Europe 13,688 13,286 13,097 11,124
Total World 37,039 34,739 35,815 34,649
Trucks ( including heavy trucks and buses )
Region 1990 1991 1992 1993
Africa 133 123 108 109
Asia-Pacific 5,101 5,074 5,117 5,054
Central & South America 312 327 351 417
Eastern Europe 980 776 710 708
Middle East 36 28 100 110
North America 4,851 4,554 5,371 6,037
Western Europe 1,924 1,818 1,869 1,345
Total World 13,336 12,701 13,627 13,779
To fuel all operating vehicles, considerable quantities of gasoline and
diesel have to be consumed. Major consumption in 1993 of gasoline and
middle distillates ( which may include some heating fuels, but not fuel
oils ) in million tonnes.
Gasoline Middle Distillates
USA 335.6 233.9
Canada 25.0 24.4
Western Europe 166.0 264.0
Japan 56.4 89.6
Total World 802.0 989.0
The USA consumption of gasoline increased from 294.4 (1982) to 335.6 (1989)
then dipped to 324.2 (1991), and has continued to rise since then to reach
335.6 million tonnes in 1993. In 1993 the total world production of crude
oil was 3164.8 million tonnes, of which the USA consumed 787.5 million
tonnes [40]. Transport is a very significant user of crude oil products,
thus improving the efficiency of utilisation, and minimising pollution from
vehicles, can produce immediate reductions in emissions of CO2, HCs, VOCs,
CO, NOx, carbonyls, and other chemicals.
5.2 Why are there seasonal changes?
Only gaseous hydrocarbons burn, consequently if the air is cold, then the
fuel has to be very volatile. But when summer comes, a volatile fuel can
boil and cause vapour lock, as well as producing high levels of evaporative
emissions. The solution was to adjust the volatility of the fuel according
to altitude and ambient temperature. This volatility change has been
automatically performed for decades by the oil companies without informing
the public of the changes. It is one reason why storage of gasoline through
seasons is not a good idea. Gasoline volatility is being reduced as modern
engines, with their fuel injection and management systems, can
automatically compensate for some of the changes in ambient conditions -
such as altitude and air temperature, resulting in acceptable driveability
using less volatile fuel.
5.3 Why were alkyl lead compounds removed?
" With the exception of one premium gasoline marketed on the east coast and
southern areas of the US, all automotive gasolines from the mid-1920s until
1970 contained lead antiknock compounds to increase antiknock quality.
Because lead antiknock compounds were found to be detrimental to the
performance of catalytic emission control system then under development,
U.S. passenger car manufacturers in 1971 began to build engines designed to
operate satisfactorily on gasolines of nominal 91 Research Octane Number.
Some of these engines were designed to operate on unleaded fuel while
others required leaded fuel or the occasional use of leaded fuel. The 91
RON was chosen in the belief that unleaded gasoline at this level could be
made available in quantities required using then current refinery
processing equipment. Accordingly, unleaded and low-lead gasolines were
introduced during 1970 to supplement the conventional gasolines already
available.
Beginning with the 1975 model year, most new car models were equipped with
catalytic exhaust treatment devices as one means of compliance with the
1975 legal restrictions in the U.S. on automobile emissions. The need for
gasolines that would not adversely affect such catalytic devices has led to
the large scale availability and growing use of unleaded gasolines, with
all late-model cars requiring unleaded gasoline."[41].
There was a further reason why alkyl lead compounds were subsequently
reduced, and that was the growing recognition of the highly toxic nature of
the emissions from a leaded-gasoline fuelled engine. Not only were toxic
lead emissions produced, but the added toxic lead scavengers ( ethylene
dibromide and ethylene dichloride ) could react with hydrocarbons to
produce highly toxic organohalogen emissions such as dioxin. Even if
catalysts were removed, or lead-tolerant catalysts discovered, alkyl lead
compounds would remain banned because of their toxicity and toxic emissions
[42].
5.4 Why are evaporative emissions a problem?
As tailpipe emissions are reduced due to improved exhaust emission control
systems, the hydrocarbons produced by evaporation of the gasoline during
distribution, vehicle refuelling, and from the vehicle, become more and
more significant. A recent European study found that 40% of man-made
volatile organic compounds came from vehicles [43]. Many of the problem
hydrocarbons are the aromatics and olefins that have relatively high octane
values. Any sensible strategy to reduce smog and toxic emissions will
attack evaporative and tailpipe emissions.
The health risks to service station workers, who are continuously exposed
to refuelling emissions remain a concern [44]. Vehicles will soon be
required to trap the refuelling emissions in larger carbon canisters, as
well as the normal evaporative emissions that they already capture. This
recent decision went in favour of the oil companies, who were opposed by
the auto companies. The automobile manufacturers felt the service station
should trap the emissions. The activated carbon canisters adsorb organic
vapours, and these are subsequently desorbed from the canister and burnt in
the engine during normal operation, once certain vehicle speeds and coolant
temperatures are reached. A few activated carbons used in older vehicles do
not function efficiently with oxygenates.
5.5 Why control tailpipe emissions?
Tailpipe emissions were responsible for the majority of pollutants in the
late 1960s after the crankcase emissions had been controlled. Ozone levels
in the Los Angeles basin reached 450-500ppb in the early 1970s, well above
the typical background of 30-50ppb [45].
Tuning a carburetted engine can only have a marginal effect on pollutant
levels, and there still had to be some frequent, but long-term, assessment
of the state of tuning. Exhaust catalysts offered a post-engine solution
that could ensure pollutants were converted to more benign compounds. As
engine management systems and fuel injection systems have developed, the
volatility properties of the gasoline have been tuned to minimise
evaporative emissions, and yet maintain low exhaust emissions.
The design of the engine can have very significant effects on the type and
quantity of pollutants, eg unburned hydrocarbons in the exhaust originate
mainly from combustion chamber crevices, such as the gap between the piston
and cylinder wall, where the combustion flame can not completely use the
HCs. The type and amount of unburned hydrocarbons are related to the fuel
composition (volatility, olefins, aromatics, final boiling point), as well
as state of tune, engine condition, and age/condition of the engine
lubricating oil [46]. Particulate emissions, especially the size fraction
smaller than ten micrometres, are a serious health concern. The current
major source is from compression ignition ( CI = diesel ) engines, and the
modern SI engine system has no problem meeting regulatory requirements.
The ability of reformulated gasolines to actually reduce smog has not yet
been confirmed. The composition changes will reduce some compounds, and
increase others, making predictions of environmental consequences extremely
difficult. Planned future changes, such as the CAA 1997 Complex model
specifications, that are based on several major ongoing government/industry
gasoline and emission research programmes, are more likely to provide
unambiguous environmental improvements. The rules for tailpipe emissions
will continue to become more stringent as countries try to minimise local
problems ( smog, toxins etc.) and global problems ( CO2 ). Reformulation
does not always lower all emissions, as evidenced by the following
aldehydes from an engine with an adaptive learning management system [33].
FTP-weighted emission rates (mg/mi)
Gasoline Reformulated
Formaldehyde 4.87 8.43
Acetaldehyde 3.07 4.71
The type of exhaust catalyst and management system can have significant
effects on the emissions [33].
FTP-weighted emission rates. (mg/mi)
Total Aromatics Total Carbonyls
Gasoline Reformulated Gasoline Reformulated
Noncatalyst 1292.45 1141.82 174.50 198.73
Oxidation Catalyst 168.60 150.79 67.08 76.94
3-way Catalyst 132.70 93.37 23.93 23.07
Adaptive Learning 111.69 105.96 17.31 22.35
If we take the five compounds listed as toxics under the Clean Air Act,
then the beneficial effects of catalysts are obvious [33].
FTP-weighted emission rates. (mg/mi)
Benzene Formaldehyde Acrolein
Gas Reform Gas Reform Gas Reform
Noncatalyst 156.18 138.48 73.25 85.24 11.62 13.20
Oxidation Cat. 27.57 25.01 28.50 35.83 3.74 3.75
3-way Catalyst 19.39 15.69 7.27 7.61 1.11 0.74
Adaptive Learn. 19.77 20.39 4.87 8.43 0.81 1.16
Acetaldehyde 1,3 Butadiene
Gas Reform Gas Reform
Noncatalyst 19.74 21.72 2.96 1.81
Oxidation Cat. 11.15 11.76 0.02 0.33
3-way Catalyst 4.43 3.64 0.07 0.05
Adaptive Learn. 3.07 4.71 0.00 0.14
The author reports analytical problems with the 1,3 Butadiene, and only
Noncatalyst values are considered reliable.
Emission Standards
There are several bodies responsible for establishing standards, and they
promulgate test cycles, analysis procedures, and the % of new vehicles that
must comply each year. The test cycles and procedures do change ( usually
indicated by an anomalous increase in the numbers in the table ), and I
have not listed the percentages of the vehicle fleet that are required to
comply. This table is only intended to convey where we have been, and where
we are going. It does not cover any regulation in detail - readers are
advised to refer to the relevant regulations. Additional limits for other
pollutants, such as formaldehyde and particulates, are omitted. The 1994
tests signal the transition from 50,000 to 75,000 mile compliance testing,
and I have not listed the subsequent 50,000 mile limits [47,48].
Year Federal California
HCs CO NOx Evap HCs CO NOx Evap
g/mi g/mi g/mi g/test g/mi g/mi g/mi g/test
Before regs 10.6 84.0 4.1 47 10.6 84.0 4.1 47
add crankcase +4.1 +4.1
1966 6.3 51.0 6.0
1968 6.3 51.0 6.0
1970 4.1 34.0 4.1 34.0 6
1971 4.1 34.0 4.1 34.0 4.0 6
1972 3.0 28.0 2.9 34.0 3.0 2
1973 3.0 28.0 3.0 2.9 34.0 3.0 2
1974 3.0 28.0 3.0 2.9 34.0 2.0 2
1975 1.5 15.0 3.1 2 0.90 9.0 2.0 2
1977 1.5 15.0 2.0 2 0.41 9.0 1.5 2
1980 0.41 7.0 2.0 6 0.41 9.0 1.0 2
1981 0.41 3.4 1.0 2 0.39 7.0 0.7 2
1993 0.41 3.4 1.0 2 0.25 3.4 0.4 2
1994 50,000 0.26 3.4 0.3 ? TLEV 0.13 3.4 0.4
1994 75,000 0.31 4.2 0.6 ?
1997 LEV 0.08 3.4 0.2
1997 ULEV 0.04 1.7 0.2
1998 ZEV 0.0 0.0 0.0
2004 0.13 1.8 0.16 ?
It's also worth noting that exhaust catalysts also emit platinum, and the
soluble platinum salts are some of the most potent sensitizers known. Early
research [49] reported the presence of 10% water-soluble platinum in the
emissions, however later work on monolithic catalysts has determined the
quantities of water soluble platinum emissions are negligible [50]. The
particle size of the emissions has also been determined, and the emissions
have been correlated with increasing vehicle speed. Increasing speed also
increases the exhaust gas temperature and velocity, indicating the
emissions are probably a consequence of physical attrition.
Estimated Fuel Median Aerodynamic
Speed Consumption Emissions Particle Diameter
km/h l/100km ng/m-3 um
60 7 3.3 5.1
100 8 11.9 4.2
140 10 39.0 5.6
US Cycle-75 6.4 8.5
Using the estimated fuel consumption, and about 10m3 of exhaust gas per
litre of gasoline, the emissions are 2-40ng/km. These are 2-3 orders of
magnitude lower than earlier reported work on pelletised catalysts. These
emissions may be controlled directly in the future. They are currently
indirectly controlled by the cost of platinum, and the new requirement for
the catalyst to have an operational life of at least 100,000 miles.
5.6 Why do exhaust catalysts influence fuel composition?
Modern adaptive learning engine management systems control the combustion
stoichiometry by monitoring various ambient and engine parameters,
including exhaust gas recirculation rates, the air flow sensor, and exhaust
oxygen sensor outputs, This closed loop system using the oxygen sensor can
compensate for changes in fuel content and air density. The oxygen sensor
is also known as the lambda sensor, because the stoichiometric mass
Air/Fuel ratio is known as lambda. Typical stoichiometric air/fuel ratios
are [51]:-
6.4 methanol
9.0 ethanol
11.7 MTBE
12.1 ETBE, TAME
14.6 gasoline without oxygenates
The engine management system rapidly switches the stoichiometry between
slightly rich and slightly lean, except under wide open throttle conditions
- when the system runs open loop. The response of the oxygen sensor to
composition changes is about 3 ms, and closed loop switching is typically
1-3 times a second, going between 50mV ( lambda = 1.05 (Lean)) to 900mV
(lambda = 0.99 ( Rich)). The catalyst oxidises about 80% of the H2, CO, and
HCs, and reduces the NOx [47].
Typical reactions that occur in a modern 3-way catalyst are:-
2H2 + O2 -> 2H2O
2CO + O2 -> 2CO2
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
2CO + 2NO -> N2 + 2CO2
CxHy + 2(x + (y/4))NO -> (x + (y/4))N2 + (y/2)H2O + xCO2
2H2 + 2NO -> N2 + 2H2O
CO + H20 -> CO2 + H2
CxHy + xH2O -> xCO + (x + (y/2))H2
The use of exhaust catalysts have resulted in reaction pathways that can
accidentally be responsible for increased pollution. An example is the
CARB-mandated reduction of fuel sulfur. A change from 450ppm to 50ppm,
which will reduce HC & CO emissions by 20%, may increase formaldehyde by
45% [19].
The requirement that the exhaust catalysts must now endure for 10 years or
100,000 miles will also encourage automakers to push for lower levels of
known catalyst "poisons" such as sulfur and phosphorus in both the gasoline
and lubricant. Modern catalysts are unable to reduce the relatively high
levels of NOx that are produced during lean operation down to approved
levels, thus preventing the application of lean-burn engine technology.
Recently Mazda has announced they have developed a "lean burn" catalyst,
which may enable automakers to move the fuel combustion towards the lean
side, and different gasoline properties may be required to optimise the
combustion and reduce pollution. Mazda claim that fuel efficiency is
improved by 5-8% while meeting all emission regulations [52] .
Catalysts also inhibit the selection of gasoline octane-improving and
cleanliness additives ( such as MMT and phosphorus-containing additives )
that may result in refractory compounds known to physically coat the
catalyst and increase pollution.
5.7 Why are "cold start" emissions so important?
The catalyst requires heat to reach the temperature ( >300-350C ) where it
functions most efficiently, and the delay until it reaches operating
temperature can produce more hydrocarbons than would be produced during the
remainder of many typical urban short trips. It has been estimated that
70-80% of the non-methane HCs that escape conversion by the catalysts are
emitted during the first two minutes after a cold start. As exhaust
emissions have been reduced, the significance of the evaporative emissions
increases. Several engineering techniques are being developed, including
the Ford Exhaust Gas Igniter ( uses a flame to heat the catalyst - lots of
potential problems ), zeolite hydrocarbon traps, and relocation of the
catalyst closer to the engine [47].
Reduced gasoline volatility and composition changes, along with cleanliness
additives and engine management systems, can help minimise cold start
emissions, but currently the most effective technique appears to be rapid,
deliberate heating of the catalyst, and the new generation of low thermal
inertia "fast light-up" catalysts reduce the problem, but further research
is necessary [53].
As the evaporative emissions are also starting to be reduced, the emphasis
has shifted to the refuelling emissions. These will be mainly controlled on
the vehicle, and larger canisters may be used to trap the vapours emitted
during refuelling.
5.8 When will the emissions be "clean enough"?
The California ZEV regulations effectively preclude IC vehicles, because
they stipulate zero emissions. However, the concept of regulatory forcing
of alternative vehicle propulsion technology may have to be modified to
include hybrid or fuel-cell vehicles, as the major failing of EVs remains
the lack of a cheap, light, safe, and easily-rechargeable electrical
storage device [54,55]. There are several major projects intending to
further reduce emissions from automobiles, mainly focusing on vehicle mass
and engine fuel efficiency, but gasoline specifications and alternative
fuels are also being investigated. It may be that changes to IC engines and
gasolines will enable the IC engine to continue well into the 21st century
as the prime motive force for personal transportation.
5.9 Why are only some gasoline compounds restricted?
The less volatile hydrocarbons in gasoline are not released in significant
quantities during normal use, and the more volatile alkanes are
considerably less toxic than many other chemicals encountered daily. The
newer gasoline additives also have potentially undesirable properties
before they are even combusted. Most hydrocarbons are very insoluble in
water, with the lower aromatics being the most soluble, however the
addition of oxygen to hydrocarbons significantly increases the mutual
solubility with water.
Compound in Water Water in Compound
% mass/mass @ C % mass/mass @ C
normal decane 0.0000052 25 0.0072 25
iso-octane 0.00024 25 0.0055 20
normal hexane 0.00125 25 0.0111 20
cyclohexane 0.0055 25 0.010 20
1-hexene 0.00697 25 0.0477 30
toluene 0.0515 25 0.0334 25
benzene 0.1791 25 0.0635 25
methanol complete 25 complete 25
ethanol complete 25 complete 25
MTBE 4.8 20 1.4 20
TAME - 0.6 20
The concentrations and ratios of benzene, toluene, ethyl benzene, and
xylenes ( BTEX ) in water are often used to monitor groundwater
contamination from gasoline storage tanks or pipelines. The oxygenates and
other new additives may increase the extent of water and soil pollution by
acting as co-solvents for HCs.
Various government bodies ( EPA, OSHA, NIOSH ) are charged with ensuring
people are not exposed to unacceptable chemical hazards, and maintain
ongoing research into the toxicity of liquid gasoline contact, water and
soil pollution, evaporative emissions, and tailpipe emissions [56]. As
toxicity is found, the quantities in gasoline of the specific chemical (
benzene ), or family of chemicals ( alkyl leads, aromatics, olefins ) are
regulated.
The recent dramatic changes caused by the need to reduce alkyl leads,
halogens, olefins, aromatics has resulted in whole new families of
compounds ( ethers, alcohols ) being introduced into fuels without prior
detailed toxicity studies being completed. If adverse results appear, these
compounds are also likely to be regulated to protect people and the
environment.
Also, as the chemistry of emissions is unravelled, the chemical precursors
to toxic tailpipe emissions ( such as higher aromatics that produce benzene
emissions ) are also controlled, even if they are not toxic.
5.10 What does "renewable" fuel/oxygenate mean?
The general definition of "renewable" is that the carbon originates from
recent biomass, and thus does not contribute to the increased CO2
emissions. A truly "long-term" view could claim that fossil fuels are
"renewable" on a 100 million year timescale :-). There is currently a major
battle between the ethanol/ETBE lobby ( agricultural, corn growing ), and
the methanol/MTBE lobby ( oil company, petrochemical ) over an EPA mandate
demanding that a specific percentage of the oxygenates in gasoline are
produced from "renewable" sources [57].
Unfortunately, "renewable" ethanol is not cost competitive when crude oil
is $18/bbl, so a federal subsidy ( $0.54/US Gallon ) and additional state
subsidies ( 11 states - from $0.08(Michigan) to $0.66(Tenn.)/US Gal.) are
provided. A judgement on the use of "renewable" oxygenates is expected in
early 1995.
5.11 Will oxygenated gasoline damage my vehicle?
The following comments assume that your vehicle was designed to operate on
unleaded, if not, then damage like valve seat recession may also occur.
Damage should not occur if the gasoline is correctly formulated, and you
select the appropriate octane, but oxygenated gasoline will hurt your
pocket. In the first year of mandated oxygenates, it appears some refiners
did not carefully formulate their oxygenated gasoline, and driveability and
emissions problems occurred. Most reputable brands are now carefully
formulated. Some older activated carbon canisters may not function
efficiently with oxygenated gasolines, but this is a function of the type
of carbon used. How your vehicle responds to oxygenated gasoline depends on
the engine management system and state of tune. A modern system will
automatically compensate for all of the currently-permitted oxygenate
levels, thus your fuel consumption will increase. Older, poorly-maintained,
engines may require a tune up to maintain acceptable driveability.
Be prepared to try several different brands of reformulated gasolines to
identify the most suitable brand for your vehicle, and be prepared to
change again with the seasons. This is because the refiners can choose the
oxygenate they use to meet the regulations, and may choose to set some fuel
properties, such as volatility, differently to their competitors.
Most stories of corrosion etc, are derived from anhydrous methanol
corrosion of light metals (aluminum, magnesium), however the addition of
either 0.5% water to pure methanol, or corrosion inhibitors to
methanol/gasoline blends will prevent this. If you observe corrosion, talk
to your gasoline supplier. Oxygenated fuels may either swell or shrink some
elastomers on older cars, depending on the aromatic and olefin content of
the fuels. Cars later than 1990 should not experience compatibility
problems, and cars later than 1994 should not experience driveability
problems, but they will experience increased fuel consumption, depending on
the state of tune and engine management system.
5.12 What does "reactivity" of emissions mean?
The traditional method of exhaust regulations was to specify the actual HC,
CO, NOx, and particulate contents. With the introduction of oxygenates and
reformulated gasolines, the volatile organic carbon (VOC) species in the
exhaust also changed. The "reactivity" refers to the ozone-forming
potential of the VOC emissions when they react with NOx, and is being
introduced as a regulatory means of ensuring that automobile emissions do
actually reduce smog formation. The ozone-forming potential of chemicals is
defined as the number of molecules of ozone formed per VOC carbon atom, and
this is called the Incremental Reactivity. Typical values ( big is bad :-)
) are [45]:
Maximum Incremental Reactivities as mg Ozone / mg VOC
carbon monoxide 0.054
alkanes methane 0.0148
ethane 0.25
propane 0.48
n-butane 1.02
olefins ethylene 7.29
propylene 9.40
1,3 butadiene 10.89
aromatics benzene 0.42
toluene 2.73
meta-xylene 8.15
1,3,5-trimethyl benzene 10.12
oxygenates methanol 0.56
ethanol 1.34
MTBE 0.62
ETBE 1.98
5.13 What are "carbonyl" compounds?
Carbonyls are produced in large amounts under lean operating conditions,
especially when oxygenated fuels are used. Most carbonyls are toxic, and
the carboxylic acids can corrode metals. The emission of carbonyls can be
controlled by combustion stoichiometry and exhaust catalysts.
Typical carbonyls are:-
* aldehydes ( containing -CHO ),
o formaldehyde (HCHO) - which is formed in large amounts during
lean combustion of methanol [58].
o acetaldehyde (CH2CHO) - which is formed during ethanol
combustion.
o acrolein (CH2=CHCHO) - a very potent irritant.
* ketones ( containing C=0 ),
* acetone (CH3COCH3)
* carboxylic acids ( containing -COOH ),
o formic acid (HCOOH) - formed during lean methanol combustion.
o acetic acid (CH3COOH).
5.14 What are "gross polluters"?
It has always been known that the EPA emissions tests do not reflect real
world conditions. There have been several attempts to identify vehicles on
the road that do not comply with emissions standards. Recent remote sensing
surveys have demonstrated that the highest 10% of CO emitters produce over
50% of the pollution, and the same ratio applies for the HC emitters -
which may not be the same vehicles [59,60,61]. 20% of the CO emitters are
responsible for 80% of the CO emissions, consequently modifying gasoline
composition is only one aspect of pollution reduction. The new additives
can help maintain engine condition, but they can not compensate for
out-of-tune, worn, or tampered-with engines.
The most famous of these remote sensing systems is the FEAT ( Fuel
Efficiency Automobile Test ) team from the University of Denver [62]. This
team is probably the world leader in remote sensing of auto emissions to
identify grossly polluting vehicles. The system measures CO/CO2 ratio, and
the HC/CO2 ratio in the exhaust of vehicles passing through an infra-red
light beam crossing the road 25cm above the surface. The system also
includes a video system that records the licence plate, date, time,
calculated exhaust CO, CO2, and HC. The system is effective for traffic
lanes up to 18 metres wide, however rain, snow, and water spray can cause
scattering of the beam. Reference signals monitor such effects and, if
possible, compensate. The system has been comprehensively validated,
including using vehicles with on-board emissions monitoring instruments.
They can monitor up to 1000 vehicles an hour and, as an example,they were
invited to Provo, Utah to monitor vehicles, and gross polluters would be
offered free repairs [63]. They monitored over 10,000 vehicles and mailed
114 letters to owners of vehicles newer than 1965 that had demonstrated
high CO levels. They received 52 responses and repairs started in Dec 1991,
and continued to Mar 1992. They offered to purchase two vehicles at blue
book price. They were declined, and so attempted to modify those vehicles,
even though their condition did not justify the expense.
The entire monitored fleet at Provo (Utah) during Winter 1991/1992
Model year Grams CO/gallon Number of
(Median value) (mean value) Vehicles
92 40 80 247
91 55 1222
90 75 1467
89 80 1512
88 85 1651
87 90 1439
86 100 300 1563
85 120 1575
84 125 1206
83 145 719
82 170 639
81 230 612
80 220 500 551
79 350 667
78 420 584
77 430 430
76 770 317
75 760 950 163
Pre 75 920 1060 878
As observed elsewhere, over half the CO was emitted by about 10% of the
vehicles. If the 47 worst polluting vehicles were removed, that achieves
more than removing the 2,500 lowest emitting vehicles from the total tested
fleet.
Surveys of vehicle populations have demonstrated that emissions systems had
been tampered with on over 40% of the gross polluters, and an additional
20% had defective emission control equipment [64]. No matter what changes
are made to gasoline, if owners "tune" their engines for power, then the
majority of such "tuned" vehicle will become gross polluters. Professional
repairs to gross polluters usually improves fuel consumption, resulting in
a low cost to owners ( $32/pa/Ton CO year ). The removal of CO in the Provo
example above was costed at $200/Ton CO, compared to Inspection and
Maintenance programs ($780/Ton CO ), and oxygenates ( $1034-$1264/Ton CO in
Colorado 1991-2 ), and UNOCALs vehicle scrapping programme ( $1025/Ton of
all pollutants ).
Thus, identifying and repairing or removing gross polluters can be far more
cost-effective than playing around with reformulated gasolines and
oxygenates.
---------------------------------------------------------------------------
Subject: 6. What do Fuel Octane ratings really indicate?
6.1 Who invented Octane Ratings?
Since 1912 the spark ignition internal combustion engine's compression
ratio had been constrained by the unwanted "knock" that could rapidly
destroy engines. "Knocking" is a very good description of the sound heard
from an engine using fuel of too low octane. The engineers had blamed the
"knock" on the battery ignition system that was added to cars along with
the electric self-starter. The engine developers knew that they could
improve power and efficiency if knock could be overcome.
Kettering assigned Thomas Midgley, Jr. to the task of finding the exact
cause of knock [16]. They used a Dobbie-McInnes manograph to demonstrate
that the knock did not arise from preignition, as was commonly supposed,
but arose from a violent pressure rise _after_ ignition. The manograph was
not suitable for further research, so Midgley and Boyd developed a
high-speed camera to see what was happening. They also developed a
"bouncing pin" indicator that measured the amount of knock [7]. Ricardo had
developed an alternative concept of HUCF ( Highest Useful Compression Ratio
) using a variable-compression engine. His numbers were not absolute, as
there were many variables, such as ignition timing, cleanliness, spark plug
position, engine temperature. etc.
In 1926 Graham Edgar suggested using two hydrocarbons that could be
produced in sufficient purity and quantity [9]. These were "normal
heptane", that was already obtainable in sufficient purity from the
distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl
pentane " that he first synthesized. Today we call it " iso-octane " or
2,2,4-trimethyl pentane. The octane had a high anti-knock value, and he
suggested using the ratio of the two as a reference fuel number. He
demonstrated that all the commercially- available gasolines could be
bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane.
The reason for using normal heptane and iso-octane was because they both
have similar volatility properties, specifically boiling point, thus the
varying ratios 0:100 to 100:0 should not exhibit large differences in
volatility that could affect the rating test.
Heat of
Melting Point Boiling Point Density Vaporisation
C C g/ml MJ/kg
normal heptane -90.7 98.4 0.684 0.365 @ 25C
iso octane -107.45 99.3 0.6919 0.308 @ 25C
Having decided on standard reference fuels, a whole range of engines and
test conditions appeared, but today the most common are the Research Octane
Number ( RON ), and the Motor Octane Number ( MON ).
6.2 Why do we need Octane Ratings?
To obtain the maximum energy from the gasoline, the compressed fuel/air
mixture inside the combustion chamber needs to burn evenly, propagating out
from the spark plug until all the fuel is consumed. This would deliver an
optimum power stroke. In real life, a series of pre-flame reactions will
occur in the unburnt "end gases" in the combustion chamber before the flame
front arrives. If these reactions form molecules or species that can
autoignite before the flame front arrives, knock will occur [13,14].
Simply put, the octane rating of the fuel reflects the ability of the
unburnt end gases to resist spontaneous autoignition under the engine test
conditions used. If autoignition occurs, it results in an extremely rapid
pressure rise, as both the desired spark-initiated flame front, and the
undesired autoignited end gas flames are expanding. The combined pressure
peak arrives slightly ahead of the normal operating pressure peak, leading
to a loss of power and eventual overheating. The end gas pressure waves are
superimposed on the main pressure wave, leading to a sawtooth pattern of
pressure oscillations that create the "knocking" sound.
The combination of intense pressure waves and overheating can induce piston
failure in a few minutes. Knock and preignition are both favoured by high
temperatures, so one may lead to the other. Under high-speed conditions
knock can lead to preignition, which then accelerates engine destruction
[17].
6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite
before the flame-front arrives. Different reaction mechanisms, occurring at
various stages of the pre-flame compression stroke, are responsible for the
undesirable, easily-autoignitable, end gases.
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed
one at a time from the molecule by reactions with small radical species
(such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen
bonds depends on what the carbon is connected to. Straight chain HCs such
as normal heptane have secondary C-H bonds that are significantly weaker
than the primary C-H bonds present in branched chain HCs like iso-octane
[13,14].
The octane rating of hydrocarbons is determined by the structure of the
molecule, with long, straight hydrocarbon chains producing large amounts of
easily-autoignitable pre-flame decomposition species, while branched and
aromatic hydrocarbons are more resistant. This also explains why the octane
ratings of paraffins consistently decrease with carbon number. In real
life, the unburnt "end gases" ahead of the flame front encounter
temperatures up to about 700C due to piston motion and radiant and
conductive heating, and commence a series of pre-flame reactions. These
reactions occur at different thermal stages, with the initial stage ( below
400C ) commencing with the addition of molecular oxygen to alkyl radicals,
followed by the internal transfer of hydrogen atoms within the new radical
to form an unsaturated, oxygen-containing species. These new species are
susceptible to chain branching involving the HO2 radical during the
intermediate temperature stage (400-600C), mainly through the production of
OH radicals. Above 600C, the most important reaction that produces chain
branching is the reaction of one hydrogen atom radical with molecular
oxygen to form O and OH radicals.
The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Anti-knock additives
work by interfering at different points in the pre-flame reactions, with
the oxygenates retarding undesirable low temperature reactions, and the
alkyl lead compounds react in the intermediate temperature region to
deactivate the major undesirable chain branching sequence [13,14].
The antiknock ability is related to the "autoignition temperature" of the
hydrocarbons. Antiknock ability is _not_ substantially related to:-
1. The energy content of fuel, this should be obvious, as oxygenates have
lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this should be
evident from the similarities of the two reference hydrocarbons.
Although flame speed does play a minor part, there are many other
factors that are far more important. ( such as compression ratio,
stoichiometry, combustion chamber shape, chemical structure of the
fuel, presence of antiknock additives, number and position of spark
plugs, turbulence etc.) Flame speed does not correlate with octane.
6.4 Why are two ratings used to obtain the pump rating?
The correct name for the (RON+MON)/2 formula is the "antiknock index", and
it remains the most important quality criteria for motorists [25].
The initial octane method developed in the 1920s was the Motor Octane
method and, over several decades, a large number of octane test methods
appeared. These were variations to either the engine design, or the
specified operating conditions [65]. During the 1950-1960s attempts were
made to internationally standardise and reduce the number of Octane Rating
test procedures.
During the late 1930s - mid 1960s, the Research method became the important
rating because it more closely represented the octane requirements of the
motorist using the fuels/vehicles/roads then available. In the late 1960s
German automakers discovered their engines were destroying themselves on
long Autobahn runs, even though the Research Octane was within
specification. They discovered that either the MON or the Sensitivity ( the
numerical difference between the RON and MON numbers ) also had to be
specified. Today it is accepted that no one octane rating covers all use.
In fact, during 1994, there have been increasing concerns in Europe about
the high Sensitivity of some commercially-available unleaded fuels.
The design of the engine and car significantly affect the fuel octane
requirement for both RON and MON. In the 1930s, most vehicles would run on
the specified Research Octane fuel, almost regardless of the Motor Octane,
whereas most 1990s engines have a 'severity" of one, which means the engine
is unlikely to knock if a changes of one RON is matched by an equal and
opposite change of MON [19].
6.5 What does the Motor Octane rating measure?
The conditions of the Motor method represent severe, sustained high speed,
high load driving. For most hydrocarbon fuels, including those with either
lead or oxygenates, the motor octane number (MON) will be lower than the
research octane number (RON).
Test Engine conditions Motor Octane
Test Method ASTM D2700-92 [66]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm
6.6 What does the Research Octane rating measure?
The Research method settings represent typical mild driving, without
consistent heavy loads on the engine.
Test Engine conditions Research Octane
Test Method ASTM D2699-92 [67]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm )
6.7 Why is the difference called "sensitivity"?
RON - MON = Sensitivity. Because the two test methods use different test
conditions, especially the intake mixture temperatures and engine speeds,
then a fuel that is sensitive to changes in operating conditions will have
a larger difference between the two rating methods. Modern fuels typically
have sensitivities around 10. The US 87 (RON+MON/2) unleaded gasoline is
required to have a 82+ MON, thus preventing very high sensitivity fuels
[25].
6.8 What sort of engine is used to rate fuels?
Automotive octane ratings are determined in a special single-cylinder
engine with a variable compression ratio ( CR 4:1 to 18:1 ) known as a
Cooperative Fuels Research ( CFR ) engine. The cylinder bore is 82.5mm, the
stroke is 114.3mm, giving a displacement of 612 cm3. The piston has four
compression rings, and one oil control ring. The intake valve is shrouded.
The head and cylinder are one piece, and can be moved up and down to obtain
the desired compression ratio. The engines have a special four-bowl
carburettor that can adjust individual bowl air/fuel ratios. This
facilitates rapid switching between reference fuels and samples. A
magnetorestrictive detonation sensor in the combustion chamber measures the
rapid changes in combustion chamber pressure caused by knock, and the
amplified signal is measured on a "knockmeter" with a 0-100 scale [66,67].
A complete Octane Rating engine system costs about $200,000 with all the
services installed. Only one company manufactures these engines, the
Waukesha Engine Division of Dresser Industries, Waukesha. WI 53186.
6.9 How is the Octane rating determined?
To rate a fuel, the engine is set to an appropriate compression ratio that
will produce a knock of about 50 on the knockmeter for the sample when the
air/fuel ratio is adjusted on the carburettor bowl to obtain maximum knock.
Normal heptane and iso-octane are known as primary reference fuels. Two
blends of these are made, one that is one octane number above the expected
rating, and another that is one octane number below the expected rating.
These are placed in different bowls, and are also rated with each air/fuel
ratio being adjusted for maximum knock. The higher octane reference fuel
should produce a reading around 30-40, and the lower reference fuel should
produce a reading of 60-70. The sample is again tested, and if it does not
fit between the reference fuels, further reference fuels are prepared, and
the engine readjusted to obtain the required knock. The actual fuel rating
is interpolated from the knockmeter readings [66,67].
6.10 What is the Octane Distribution of the fuel?
The combination of vehicle and engine can result in specific requirements
for octane that depend on the fuel. If the octane is distributed
differently throughout the boiling range of a fuel, then engines can knock
on one brand of 87 (RON+MON/2), but not on another brand. This "octane
distribution" is especially important when sudden changes in load occur,
such as high load, full throttle, acceleration. The fuel can segregate in
the manifold, with the very volatile fraction reaching the combustion
chamber first and, if that fraction is deficient in octane, then knock will
occur until the less volatile, higher octane fractions arrive [17].
Some fuel specifications include delta RONs, to ensure octane distribution
throughout the fuel boiling range was consistent. Octane distribution was
seldom a problem with the alkyl lead compounds, as the tetra methyl lead
and tetra ethyl lead octane volatility profiles were well characterised,
but it can be a major problem for the new, reformulated, low aromatic
gasolines, as MTBE boils at 55C, whereas ethanol boils at 78C. Drivers have
discovered that an 87 (RON+MON/2) from one brand has to be substituted with
an 89 (RON+MON/2) of another, and that is because of the combination of
their driving style, engine design, vehicle mass, fuel octane distribution,
fuel volatility, and the octane-enhancers used.
6.11 What is a "delta Research Octane number"?
To obtain an indication of behaviour of a gasoline during any manifold
segregation, an octane rating procedure called the Distribution Octane
Number was used. The rating engine had a special manifold that allowed the
heavier fractions to be separated before they reached the combustion
chamber [17]. That method has been replaced by the "delta" RON procedure.
The fuel is carefully distilled to obtain a distillate fraction that boils
to the specified temperature, which is usually 100C. Both the parent fuel
and the distillate fraction are rated on the octane engine using the
Research Octane method [68]. The difference between these is the delta
RON(100C), usually just called the delta RON.
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant determinant of
octane is the chemical structure of the hydrocarbons and their response to
the addition of octane enhancing additives. Other factors include:-
* Front End Volatility - Paraffins are the major component in gasoline,
and the octane number decreases with increasing chain length or ring
size, but increases with chain branching. Overall, the effect is a
significant reduction in octane if front end volatility is lost, as
can happen with improper or long term storage. Fuel economy on short
trips can be improved by using a more volatile fuel, at the risk of
carburettor icing and increased evaporative emissions.
* Final Boiling Point.- Decreases in the final boiling point increase
fuel octane. Aviation gasolines have much lower final boiling points
than automotive gasolines. Note that final boiling points are being
reduced because the higher boiling fractions are responsible for
disproportionate quantities of pollutants and toxins.
* Preignition tendency - both knock and preignition can induce each
other.
6.13 Can higher octane fuels give me more power?
Not if you are already using the proper octane fuel. The engine will be
already operating at optimum settings, and a higher octane should have no
effect on the management system. Your driveability and fuel economy will
remain the same. The higher octane fuel costs more, so you are just
throwing money away. If you are already using a fuel with an octane rating
slightly below the optimum, then using a higher octane fuel will cause the
engine management system to move to the optimum settings, possibly
resulting in both increased power and improved fuel economy. You may be
able to change octanes between seasons ( reduce octane in winter ) to
obtain the most cost-effective fuel without loss of driveability.
Once you have identified the fuel that keeps the engine at optimum
The manufacturer's recommendation is conservative, so you may be able to
carefully reduce the fuel octane. The penalty for getting it badly wrong,
and not realising that you have, could be expensive engine damage.
6.14 Does low octane fuel increase engine wear?
Not if you are meeting the octane requirement of the engine. If you are not
meeting the octane requirement, the engine will rapidly suffer major damage
due to knock. You must not use fuels that produce sustained audible knock,
engine damage will occur. If the octane is just sufficient, the engine
management system will move settings to a less optimal position, and the
only major penalty will be increased costs due to poor fuel economy.
Whenever possible, engines should be operated at the optimum position for
long-term reliability. Engine wear is mainly related to design,
manufacturing, maintenance and lubrication factors. Once the octane and
run-on requirements of the engine are satisfied, increased octane will have
no beneficial effect on the engine. The quality of gasoline, and the
additive package used, would be more likely to affect the rate of engine
wear, rather than the octane rating.
6.15 Can I mix different octane fuel grades?
Yes, however attempts to blend in your fuel tank should be carefully
planned. You should not allow the tank to become empty, and then add 50% of
lower octane, followed by 50% of higher octane. The fuels may not
completely mix immediately, especially if there is a density difference.
You may get a slug of low octane that causes severe knock. You should
refill when your tank is half full. In general the octane response will be
linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will
give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain higher
octane are useless. The lead response of the unleaded fuel does not
overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will
give 94. Some blends of oxygenated fuels with ordinary gasoline can result
in undesirable increases in volatility due to volatile azeotropes, and that
some oxygenates can have negative lead responses. Also note that the octane
requirement of some engines is determined by the need to avoid run-on, not
to avoid knock.
6.16 What happens if I use the wrong octane fuel?
If you use a fuel with an octane rating below the requirement of the
engine, the management system may move the engine settings into an area of
less efficient combustion, resulting in reduced power and reduced fuel
economy. You will be losing both money and driveability. If you use a fuel
with an octane rating higher than what the engine can use, you are just
wasting money by paying for octane that you can not utilise. Forget the
stories about higher octanes having superior additive packages - they do
not. If your vehicle does not have a knock sensor, then using an octane
significantly below the requirement means that the little men with hammers
will gleefully pummel your engine to pieces.
You should initially be guided by the vehicle manufacturer's
recommendations, however you can experiment, as the variations in vehicle
tolerances can mean that Octane Number Requirement for a given vehicle
model can range over 6 Octane Numbers. Caution should be used, and remember
to compensate if the conditions change, such as carrying more people or
driving in different ambient conditions. You can often reduce the octane of
the fuel you use in winter because the temperature decrease and possible
humidity changes may significantly reduce the octane requirement of the
engine.
Use the octane that provides cost-effective driveability and performance,
using anything more is waste of money, and anything less could result in an
unscheduled, expensive visit to your mechanic.
6.17 Can I tune the engine to use another octane fuel?
In general, modern engine management systems will compensate for fuel
octane, and once you have satisfied the optimum octane requirement, you are
at the optimum overall performance area of the engine map. Tuning changes
to obtain more power will probably adversely affect both fuel economy and
emissions. Unless you have access to good diagnostic equipment that can
ensure regulatory limits are complied with, it is likely that adjustments
may be regarded as illegal tampering by your local regulation enforcers. If
you are skilled, you will be able to legally wring slightly more
performance from your engine by using a dynamometer in conjunction with
engine and exhaust gas analyzers and a well-designed, retrofitted,
performance engine management chip.
6.18 How can I increase the fuel octane?
Not simply, you can purchase additives, however these are not
cost-effective and a survey in 1989 showed the cost of increasing the
octane rating of one US gallon by one unit ranged from 10 cents ( methanol
), 50 cents (MMT), $1.00 ( TEL ), to $3.25 ( xylenes ) [69]. It is
preferable to purchase a higher octane fuel such as racing fuel, aviation
gasolines, or methanol. Sadly, the price of chemical grade methanol has
almost doubled during 1994. If you plan to use alcohol blends, ensure your
fuel handling system is compatible, and that you only use dry gasoline by
filling up early in the morning when the storage tanks are cool. Also
ensure that the service station storage tank has not been refilled
recently. Retailers are supposed to wait several hours before bringing a
refilled tank online, to allow suspended undissolved water to settle out,
but they do not always wait the full period.
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two numbers,
with common grades being 80/87, 100/130, and 115/145 [70]. The first number
is the Aviation rating ( aka Lean Mixture rating ), and the second number
is the Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new
grade, 100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was
introduced to replace the 80/87 and 100/130. Soon after the introduction,
there was a spate of plug fouling, and high cylinder head temperatures
resulting in cracked cylinder heads [71]. The old 80/87 grade was
reintroduced on a limited scale. The Aviation rating is determined using
the automotive Motor Octane test procedure, and then corrected to an
Aviation number using a table in the method - it's usually only 1 - 2
Octane units different to the Motor value up to 100, but varies significant
above that eg 110MON = 128AN.
The second Avgas number is the Rich Mixture method Performance Number ( PN
- they are not commonly called octane numbers when they are above 100 ),
and is determined on a supercharged version of the CFR engine which has a
fixed compression ratio. The method determines the dependence of the
highest permissible power ( in terms of indicated mean effective pressure )
on mixture strength and boost for a specific light knocking setting. The
Performance Number indicates the maximum knock-free power obtainable from a
fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an
engine designed to utilise the fuel can obtain 150% of the knock-limited
power of iso-octane at the same mixture ratio. This is an arbitrary scale
based on iso-octane + varying amounts of TEL, derived from a survey of
engines performed decades ago. Aviation gasoline PNs are rated using
variations of mixture strength to obtain the maximum knock-limited power in
a supercharged engine. This can be extended to provide mixture response
curves which define the maximum boost ( rich - about 11:1 stoichiometry )
and minimum boost ( weak about 16:1 stoichiometry ) before knock [71].
The 115/145 grade is being phased out, but even the 100LL has more octane
than any automotive gasoline.
---------------------------------------------------------------------------
Subject: 7. What parameters determine octane requirement? 7.1 What is the
effect of Compression ratio?
Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency of an
engine according to the standard equation
Efficiency = 1 - (1/compression ratio)^gamma-1
where gamma = ratio of specific heats at constant pressure and constant
volume of the working fluid ( for most purposes air is the working fluid,
and is treated as an ideal gas ). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1 [15].
The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of
the ideal combustion power to the flywheel. For a typical carburetted
engine, without engine management [17,24]:-
Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.
7.2 What is the effect of changing the air/fuel ratio?
Traditionally, the greatest tendency to knock was near 13.5:1 air/fuel
ratio, but was very engine specific. Modern engines, with engine management
systems, now have their maximum octane requirement near to 14.5:1. For a
given engine using gasoline, the relationship between thermal efficiency,
air/fuel ratio, and power is complex. Stoichiometric combustion ( Air/Fuel
Ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum
power - which occurs around A/F 12-13:1 (Rich), nor maximum thermal
efficiency - which occurs around A/F 16-18:1 (Lean). The air-fuel ratio is
controlled at part throttle by a closed loop system using the oxygen sensor
in the exhaust. Conventionally, enrichment for maximum power air/fuel ratio
is used during full throttle operation to reduce knocking while providing
better driveability [24]. If the mixture is weakened, the flame speed is
reduced, consequently less heat is converted to mechanical energy, leaving
heat in the cylinder walls and head, potentially inducing knock. It is
possible to weaken the mixture sufficiently that the flame is still present
when the inlet valve opens again, resulting in backfiring.
7.3 What is the effect of changing the ignition timing
The tendency to knock increases as spark advance is increased, eg 2 degrees
BTDC requires 91 octane, whereas 14 degrees BTDC requires 96 octane. If you
advance the spark, the flame front starts earlier, and the end gases start
forming earlier in the cycle, providing more time for the autoigniting
species to form before the piston reaches the optimum position for power
delivery, as determined by the normal flame front propagation. It becomes a
race between the flame front and decomposition of the increasingly-squashed
end gases. High octane fuels produce end gases that take longer to
autoignite, so the good flame front reaches and consumes them properly.
The ignition advance map is partly determined by the fuel the engine is
intended to use. The timing of the spark is advanced sufficiently to ensure
that the fuel/air mixture burns in such a way that maximum pressure of the
burning charge is about 15-20 degree after TDC. Knock will occur before
this point, usually in the late compression/early power stroke period. The
engine management system uses ignition timing as one of the major variables
that is adjusted if knock is detected. If very low octane fuels are used (
several octane numbers below the vehicle's requirement at optimal settings
), both performance and fuel economy will decrease.
The actual Octane Number Requirement depends on the engine design, but for
some 1978 vehicles using standard fuels, the following (R+M)/2 Octane
Requirements were measured. "Standard" is the recommended ignition timing
for the engine, probably a few degrees before Top Dead Centre [24].
Basic Ignition Timing
Vehicle Retarded 5 degrees Standard Advanced 5 degrees
A 88 91 93
B 86 90.5 94.5
C 85.5 88 90
D 84 87.5 91
E 82.5 87 90
The actual ignition timing to achieve the maximum pressure from normal
combustion of gasoline will depend mainly on the speed of the engine and
the flame propagation rates in the engine. Knock increases the rate of the
pressure rise, thus superimposing additional pressure on the normal
combustion pressure rise. The knock actually rapidly resonates around the
chamber, creating a series of abnormal sharp spikes on the pressure
diagram. The normal flame speed is fairly consistent for most gasoline HCs,
regardless of octane rating, but the flame speed is affected by
stoichiometry. Note that the flame speeds in this FAQ are not the actual
engine flame speeds. A 12:1 CR gasoline engine at 1500 rpm would have a
flame speed of about 16.5 m/s, and a similar hydrogen engine yields 48.3
m/s, but such engine flame speeds are also very dependent on stoichiometry.
7.4 What is the effect of engine management systems?
Engine management systems are now an important part of the strategy to
reduce automotive pollution. The good news for the consumer is their
ability to maintain the efficiency of gasoline combustion, thus improving
fuel economy. The bad news is their tendency to hinder tuning for power. A
very basic modern engine system could monitor and control:- mass air flow,
fuel flow, ignition timing, exhaust oxygen ( lambda oxygen sensor ), knock
( vibration sensor ), EGR, exhaust gas temperature, coolant temperature,
and intake air temperature. The knock sensor can be either a nonresonant
type installed in the engine block and capable of measuring a wide range of
knock vibrations ( 5-15 kHz ) with minimal change in frequency, or a
resonant type that has excellent signal-to-noise ratio between 1000 and
5000 rpm [72].
A modern engine management system can compensate for altitude, ambient air
temperature, and fuel octane. The management system will also control cold
start settings, and other operational parameters. There is a new
requirement that the engine management system also contain an on-board
diagnostic function that warns of malfunctions such as engine misfire,
exhaust catalyst failure, and evaporative emissions failure. The use of
fuels with alcohols such as methanol can confuse the engine management
system as they generate more hydrogen which can fool the oxygen sensor [47]
.
The use of fuel of too low octane can actually result in both a loss of
fuel economy and power, as the management system may have to move the
engine settings to a less efficient part of the performance map. The system
retards the ignition timing until only trace knock is detected, as engine
damage from knock is of more consequence than power and fuel economy.
7.5 What is the effect of temperature and load?
Increasing the engine temperature, particularly the air/fuel charge
temperature, increases the tendency to knock. The Sensitivity of a fuel can
indicate how it is affected by charge temperature variations. Increasing
load increases both the engine temperature, and the end-gas pressure, thus
the likelihood of knock increases as load increases.
7.6 What is the effect of engine speed?.
Faster engine speed means there is less time for the pre-flame reactions in
the end gases to occur, thus reducing the tendency to knock. On engines
with management systems, the ignition timing may be advanced with engine
speed and load, to obtain optimum efficiency at incipient knock. In such
cases, both high and low engines speeds may be critical.
7.7 What is the effect of engine deposits?
A new engine may only require a fuel of 6-9 octane numbers lower than the
same engine after 25,000 km. This Octane Requirement Increase (ORI) is due
to the formation of a mixture of organic and inorganic deposits resulting
from both the fuel and the lubricant. They reach an equilibrium amount
because of flaking, however dramatic changes in driving styles can also
result in dramatic changes of the equilibrium position. When the engine
starts to burn more oil, the octane requirement can increase again. ORIs up
to 12 are not uncommon, depending on driving style [17,19]. The deposits
produce the ORI by several mechanisms:-
- they reduce the combustion chamber volume, effectively increasing the
compression ratio. - they also reduce thermal conductivity, thus increasing
the combustion chamber temperatures. - they catalyse undesirable pre-flame
reactions that produce end gases with low autoignition temperatures.
7.8 What is the Road Octane requirement of an vehicle?
The actual octane requirements of a vehicle is called the Octane Number
Requirement ( ONR ), and is determined by using standard octane fuels that
can be blends of iso-octane and normal heptane, or commercial gasolines.
The vehicle is tested under a wide range of conditions and loads, using
different octane fuels until trace knock is detected. The conditions that
require maximum octane are not consistent, but often are full-throttle
acceleration from low starting speeds using the highest gear available.
They can even be at constant speed conditions [17]. Engine management
systems that adjust the octane requirement may also reduce the power output
on low octane fuel, resulting in increased fuel consumption. The maximum
ONR is of most interest, as that usually defines the recommended fuel.
The octane rating engines do not reflect actual conditions in a vehicle,
consequently there are standard procedures for evaluating the performance
of the gasoline in an engine. The most common are:- 1. The Modified
Uniontown Procedure. Full throttle accelerations are made from low speed
using primary reference fuels. The ignition timing is adjusted until trace
knock is detected at some stage. Several reference fuels are used, and a
Road Octane Number v Basic Ignition timing graph is obtained. The fuel
sample is tested, and the ignition timing setting is read from the graph to
provide the Road Octane Number. This is a rapid procedure but provides
minimal information. 2. The Modified Borderline Knock Procedure. The
automatic spark advance is disabled, and a manual adjustment facility
added. Accelerations are performed as in the Modified Uniontown Procedure,
however trace knock is maintained throughout the run. A map of ignition
advance v engine speed is made for several reference fuels and the sample
fuels. This procedure can show the variation of road octane with engine
speed.
7.9 What is the effect of air temperature?
An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [17,24]. When the combined
effects of air temperature and humidity are considered, it is often
possible to use one octane grade in summer, and use a lower octane rating
in winter. The Motor octane rating has a higher charge temperature, and
increasing charge temperature increases the tendency to knock, so fuels
with low Sensitivity ( the difference between RON and MON numbers ) are
less affected by air temperature.
7.10 What is the effect of altitude?
The effect of increasing altitude may be nonlinear, with one study
reporting a decrease of the octane requirement of 1.4 RON/300m from sea
level to 1800m and 2.5 RON/300m from 1800m to 3600m [17]. Other studies
report the octane number requirement decreased by 1.0 - 1.9 RON/300m
without specifying altitude [24]. Modern engine management systems can
accommodate this adjustment, and in some recent studies, the octane number
requirement was 0.2 - 0.5 Antiknock Index/300m. The reduction on older
engines was due to:-
- reduced air density provides lower combustion temperature and pressure. -
fuel is metered according to air volume, consequently as density decreases
the stoichiometry moves to rich, with a lower octane number requirement. -
manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance.
7.11 What is the effect of humidity?.
An increase of absolute humidity of 1.0 g water/ kg of dry air lowers the
octane requirement of an engine by 0.25 - 0.32 MON [17,24].
7.12 What does water injection achieve?.
Water injection was used in WWII aviation engine to provide a large
increase in available power for very short periods. The injection of water
does decrease the dew point of the exhaust gases. This has potential
corrosion problems. The very high specific heat and heat of vaporisation of
water means that the combustion temperature will decrease. It has been
shown that a 10% water addition to methanol reduces the power and
efficiency by about 3%, and doubles the unburnt fuel emissions, but does
reduce NOx by 25% [73]. A decrease in combustion temperature will reduce
the theoretical maximum possible efficiency of an otto cycle engine that is
operating correctly, but may improve efficiency in engines that are
experiencing abnormal combustion on existing fuels.
Some aviation SI engines still use boost fluids. The water/methanol
mixtures are used to provide increased power for short periods, up to 40%
more - assuming adequate mechanical strength of the engine. The 40/60 or
45/55 water/methanol mixtures are used as boost fluids for aviation engines
because water would freeze. Methanol is just "preburnt" methane,
consequently it only has about half the energy content of gasoline, but it
does have a higher heat of vaporisation, which has a significant cooling
effect on the charge. Water/methanol blends are more cost-effective than
gasoline for combustion cooling. The high Sensitivity of alcohol fuels has
to be considered in the engine design and settings.
Boost fluids are used because they are far more economical than using the
fuel. When a supercharged engine has to be operated at high boost, the
mixture has to be enriched to keep the engine operating without knock. The
extra fuel cools the cylinder walls and the charge, thus delaying the onset
of knock which would otherwise occur at the associated higher temperatures.
The overall effect of boost fluid injection is to permit a considerable
increase in knock-free engine power for the same combustion chamber
temperature. The power increase is obtained from the higher allowable
boost. In practice, the fuel mixture is usually weakened when using boost
fluid injection, and the ratio of the two fuel fluids is approximately 100
parts of avgas to 25 parts of boost fluid. With that ratio, the resulting
performance corresponds to an effective uprating of the fuel of about 25%,
irrespective of its original value. Trying to increase power boosting above
40% is difficult, as the engine can drown because of excessive liquid [71].
Note that for water injection to provide useful power gains, the engine
management and fuel systems must be able to monitor the knock and adjust
both stoichiometry and ignition to obtain significant benefits. Aviation
engines are designed to accommodate water injection, most automobile
engines are not. Returns on investment are usually harder to achieve on
engines that do not normal extend their performance envelope into those
regions. Water injection has been used by some engine manufacturers -
usually as an expedient way to maintain acceptable power after regulatory
emissions baggage was added to the engine, but usually the manufacturer
quickly produces a modified engine that does not require water injection.
---------------------------------------------------------------------------
Subject: 8. How can I identify and cure other fuel-related problems? 8.1
What causes an empty fuel tank?
* You forgot to refill it.
* Your friendly neighbourhood thief "borrowed" the gasoline - the
unfriendly one took the vehicle.
* The fuel tank leaked.
* Your darling child/wife/husband/partner/mother/father used the car.
* The most likely reason is that your local garage switched to an
oxygenated gasoline, and the engine management system compensated for
the oxygen content, causing the fuel consumption to increase
significantly.
8.2 Is knock the only abnormal combustion problem?
No. Many of the abnormal combustion problems are induced by the same
conditions, and so one can lead to another.
Preignition occurs when the air/fuel mixture is ignited prematurely by
glowing deposits or hot surfaces - such as exhaust valves and spark plugs.
If it continues, it can increase in severity and become Run-away Surface
Ignition (RSI) which prevents the combustion heat being converted into
mechanical energy, thus rapidly melting pistons. The Ricardo method uses an
electrically-heated wire in the engine to measure preignition tendency. The
scale uses iso-octane as 100 and cyclohexane as 0.
Some common fuel components:-
paraffins 50-100
benzene 26
toluene 93
xylene >100
cyclopentane 70
di-isobutylene 64
hexene-2 -26
There is no direct correlation between anti-knock ability and preignition
tendency, however high combustion chamber temperatures favour both, and so
one may lead to the other. An engine knocking during high-speed operation
will increase in temperature and that can induce preignition, and
conversely any preignition will result in higher temperatures than may
induce knock.
Misfire is commonly caused by either a failure in the ignition system, or
fouling of the spark plug by deposits. The most common cause of deposits
was the alkyl lead additives in gasoline, and the yellow glaze of various
lead salts was used by mechanics to assess engine tune. From the upper
recess to the tip, the composition changed, but typical compounds ( going
from cold to hot ) were PbClBr; 2PbO.PbClBr; PbO.PbSO4; 3Pb3(PO4)2.PbClBr.
Run-on is the tendency of an engine to continue running after the ignition
has been switched off. It is usually caused by the spontaneous ignition of
the fuel/air mixture, rather than by surface ignition from hotspots or
deposits, as commonly believed. The narrow range of conditions for
spontaneous ignition of the fuel/air mixture ( engine speed, charge
temperature, cylinder pressure ) may be created when the engine is switched
off. The engine may refire, thus taking the conditions out of the critical
range for a couple of cycles, and then refire again, until overall cooling
of the engine drops it out of the critical region. The octane rating of the
fuel is the appropriate parameter, and it is not rare for an engine to
require a higher Octane fuel to prevent run-on than to avoid knock [17].
8.3 Can I prevent carburetter icing?
Yes, carburettor icing is caused by the combination of highly volatile
fuel, high humidity and low ambient temperature. The extent of cooling,
caused by the latent heat of the vaporised gasoline in the carburettor, can
be as much as 20C, perhaps dropping below the dew point of the charge. If
this happens, water will condense on the cooler carburettor surfaces, and
will freeze if the temperature is low enough. The fuel volatility can not
always be reduced to eliminate icing, so anti-icing additives are used.
Two types of additive are added to gasoline to inhibit icing:-
- surfactants that form a monomolecular layer over the metal parts that
inhibits ice crystal formation. These are usually added at concentrations
of 30-150 ppm. - cryoscopic additives that depress the freezing point of
the condensed water so that it does not turn to ice. Alcohols ( methanol,
ethanol, iso-propanol, etc. ) and glycols ( hexylene glycol, dipropylene
glycol ) are used at concentrations of 0.03% - 1%. If you have icing
problems, the addition of 100-200mls of methanol to a full tank of dry
gasoline will prevent icing under most conditions. If you believe there is
a small amount of water in the fuel tank, add 500mls of isopropanol as the
first treatment. Oxygenated gasolines using alcohols can also be used.
8.4 Should I store fuel to avoid the oxygenate season?
No. The fuel will be from a different season, and will have significantly
different volatility properties that may induce driveability problems. You
can tune your engine to perform on oxygenated gasoline as well as it did on
traditional gasoline, however you will have increased fuel consumption due
to the useless oxygen in the oxygenates. Some engines may not initially
perform well on some oxygenated fuels, usually because of the slightly
different volatility and combustion characteristics. A good mechanic should
be able to recover any lost performance or driveability, providing the
engine is in reasonable condition.
8.5 Can I improve fuel economy by using quality gasolines?
Yes, several manufacturers have demonstrated that their new gasoline
additive packages are more effective than traditional gasoline
formulations. Texaco claim their new vapour phase fuel additive can reduce
existing deposits by up to 30%, improve fuel economy, and reduce NOx
tailpipe emissions by 15%, when compared to other advanced liquid phase
additives. These claims appear to have been verified in independent tests
[30]. Other reputable gasoline brands will have similar additive packages
in their quality products. Quality gasolines, of whatever octane ratings,
will include a full range of gasoline additives designed to provide
consistent fuel quality.
Note that oxygenated gasolines must decrease fuel economy for the same
power. If your engine is initially well-tuned on hydrocarbon gasolines, the
stoichiometry will move to lean, and maximum power is slightly rich, so
either the management system ( if you have one ) or your mechanic has to
increase the fuel flow. The minor improvements in combustion efficiency
that oxygenates may provide, can not compensate for 2+% of oxygen in the
fuel that will not provide energy.
8.6 What is "stale" fuel, and should I use it?
"Stale" fuel is caused by improper storage, and usually smells sour. The
gasoline has been allowed to get warm, thus catalysing olefin decomposition
reactions, and perhaps also losing volatile material in unsealed
containers. Such fuel will tend to rapidly form gums, and will usually have
a significant reduction in octane rating. The fuel can be used by blending
with twice the volume of new gasoline. Some stale fuels can drop several
octane numbers, so be generous with the dilution.
8.7 How can I remove water in the fuel tank?
If you only have a small quantity of water, then the addition of 500mls of
dry isopropanol (IPA) to a near-full 30-40 litre tank will absorb the
water, and will not significantly affect combustion. Once you have mopped
up the water with IPA. Small, regular doses of any anhydrous alcohol will
help keep the tank dry. This technique will not work if you have very large
amounts of water, and the addition of greater amounts of IPA may result in
poor driveability.
Water in fuel tanks can be minimised by keeping the fuel tank near full,
and filling in the morning from a service station that allows storage tanks
to stand for several hours after refilling before using the fuel. Note that
oxygenated gasolines have greater water solubility, and should cope with
small quantities of water.
8.8 Can I used unleaded on older vehicles?
Yes, providing the octane is appropriate. There are some older engines that
cut the valve seats directly into the cylinder head ( eg BMC minis ). The
absence of lead, which lubricated the valve seat, causes the very hard
oxidation products of the valve to wear down the seat. This valve seat
recession is usually corrected by installing seat inserts. Most other
problems arise because the fuels have different volatility, or the
reduction of combustion chamber deposits. These can usually be cured by
reference to the vehicle manufacturer, who will probably have a publication
with the changes. Some vehicles will perform as well on unleaded with a
slightly lower octane than recommended leaded fuel, due to the significant
reduction in deposits from modern unleaded gasolines.
---------------------------------------------------------------------------
Section: 9. Alternative Fuels and Additives 9.1 Do fuel additives work?
Most aftermarket fuel additives are not cost-effective. These include the
octane-enhancer solutions discussed in section 6.18. There are various
other pills, tablets, magnets, filters, etc. that all claim to improve
either fuel economy or performance. Some of these have perfectly sound
scientific mechanisms, unfortunately they are not cost-effective. Some do
not even have sound scientific mechanisms. Because the same model
production vehicles can vary significantly, it's expensive to unambiguously
demonstrate these additives are not cost-effective. If you wish to try
them, remember the biggest gain is likely to be caused by the lower mass of
your wallet/purse.
There is one aftermarket additive that may be cost-effective, the lubricity
additive used with unleaded gasolines to combat valve seat recession on
engines that do not have seat inserts. The long-term solution is to install
inserts at the next top overhaul.
Some other fuel additives work, especially those that are carefully
formulated into the gasoline by the manufacturer at the refinery.
A typical gasoline may contain [17,19,24]:-
* Oil-soluble Dye, initially added to leaded gasoline at about 10 ppm to
prevent its misuse as an industrial solvent * Antioxidants, typically
phenylene diamines or hindered phenols, are added to prevent oxidation of
unsaturated hydrocarbons. * Metal Deactivators, typically about 10ppm of
chelating agent such as N,N'-disalicylidene-1,2-propanediamine is added to
inhibit copper, which can rapidly catalyze oxidation of unsaturated
hydrocarbons. * Corrosion Inhibitors, about 5ppm of oil-soluble surfactants
are added to prevent corrosion caused either by water condensing from
cooling, water-saturated gasoline, or from condensation from air onto the
walls of almost-empty gasoline tanks that drop below the dew point. If your
gasoline travels along a pipeline, it's possible the pipeline owner will
add additional corrosion inhibitor to the fuel. * Anti-icing Additives,
used mainly with carburetted cars, and usually either a surfactant, alcohol
or glycol. * Anti-wear Additives, these are used to control wear in the
upper cylinder and piston ring area that the gasoline contacts, and are
usually very light hydrocarbon oils. Phosphorus additives can also be used
on engines without exhaust catalyst systems. * Deposit-modifying Additives,
usually surfactants. 1. Carburettor Deposits, additives to prevent these
were required when crankcase blow-by (PCV) and exhaust gas recirculation
(EGR) controls were introduced. Some fuel components reacted with these gas
streams to form deposits on the throat and throttle plate of carburettors.
2. Fuel Injector tips operate about 100C, and deposits form in the annulus
during hot soak, mainly from the oxidation and polymerisation of the larger
unsaturated hydrocarbons. The additives that prevent and unclog these tips
are usually polybutene succinimides or polyether amines. 3. Intake Valve
Deposits caused major problems in the mid-1980s when some engines had
reduced driveability when fully warmed, even though the amount of deposit
was below previously acceptable limits. It is believed that the new fuels
and engine designs were producing a more absorbent deposit that grabbed
some passing fuel vapour, causing lean hesitation. Intake valves operate
about 300C, and if the valve is is kept wet deposits tend not to form, thus
intermittent injectors tend to promote deposits. Oil leaking through the
valve guides can be either harmful or beneficial, depending on the type and
quantity. Gasoline factors implicated in these deposits include unsaturates
and alcohols. Additives to prevent these deposits contain a detergent
and/or dispersant in a higher molecular weight solvent or light oil whose
low volatility keeps the valve surface wetted. 4. Combustion Chamber
Deposits have been targeted in the 1990s, as they are responsible for
significant increases in emissions. Recent detergent-dispersant additives
have the ability to function in both the liquid and vapour phases to remove
existing carbon and prevent deposit formation. * Octane Enhancers, these
are usually formulated blends of alkyl lead or MMT compounds in a solvent
such as toluene, and added at the 100-1000 ppm levels. They have been
replaced by hydrocarbons with higher octanes such as aromatics and olefins.
These hydrocarbons are now being replaced by a mixture of saturated
hydrocarbons and and oxygenates. If you wish to play with different fuels
and additives, be aware that some parts of your engine management systems,
such as the oxygen sensor, can be confused by different exhaust gas
compositions. An example is increased quantities of hydrogen from methanol
combustion.
9.2 Can a quality fuel help a sick engine?
It depends on the ailment. Nothing can compensate for poor tuning and wear.
If the problem is caused by deposits or combustion quality, then modern
premium quality gasolines have been shown to improve engine performance
significantly. The new generation of additive packages for gasolines
include components that will dissolve existing carbon deposits, and have
been shown to improve fuel economy, NOx emissions, and driveability.
9.3 What are the advantages of alcohols and ethers?
This section discusses only the use of high ( >80% ) alcohol or ether
fuels. Alcohol fuels can be made from sources other than imported crude
oil, and the nations that have researched/used alcohol fuels have mainly
based their choice on import substitution. Alcohol fuels can burn more
efficiently, and can reduce photochemically-active emissions. Most vehicle
manufacturers favoured the use of liquid fuels over compressed or liquified
gases. The alcohol fuels have high research octane ratings, but also high
sensitivity and high latent heats [6,17,51,74].
Methanol Ethanol Unleaded Gasoline
RON 106 107 92 - 98
MON 92 89 80 - 90
Heat of Vaporisation (MJ/kg) 1.154 0.913 0.3044
Nett Heating Value (MJ/kg) 19.95 26.68 42 - 44
Vapour Pressure @ 38C (kPa) 31.9 16.0 48 - 108
Flame Temperature ( C ) 1870 1920 2030
Stoich. Flame Speed. ( m/s ) 0.43 - 0.34
Minimum Ignition Energy ( mJ ) 0.14 - 0.29
Lower Flammable Limit ( vol% ) 6.7 3.3 1.3
Upper Flammable Limit ( vol% ) 36.0 19.0 7.1
Autoignition Temperature ( C ) 460 360 260 - 460
Flash Point ( C ) 11 13 -43 - -39
The major advantages are gained when pure fuels ( M100, and E100 ) are
used, as the addition of hydrocarbons to overcome the cold start problems
also significantly reduces, if not totally eliminates, any emission
benefits. Methanol will produce significant amounts of formaldehyde, a
suspected human carcinogen, until the exhaust catalyst reaches operating
temperature. Ethanol produces acetaldehyde. The cold-start problems have
been addressed, and alcohol fuels are technically viable, however with
crude oil at <$30/bbl they are not economically viable, especially as the
demand for then as precursors for gasoline oxygenates has elevated the
world prices. Methanol almost doubled in price during 1994. There have also
been trials of pure MTBE as a fuel, however there are no unique or
significant advantages that would outweigh the poor economic viability
[11].
9.4 Why are CNG and LPG considered "cleaner" fuels.
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20%
ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to
butane. The fuel has a high octane and usually only trace quantities of
unsaturates. The emissions from CNG have lower concentrations of the
hydrocarbons responsible for photochemical smog, reduced CO, SOx, and NOx,
and the lean misfire limit is extended [75]. There are no technical
disadvantages, providing the installation is performed correctly. The major
disadvantage of compressed gas is the reduced range. Vehicles may have
between one to three cylinders ( 25 MPa, 90-120 litre capacity), and they
usually represent about 50% of the gasoline range. As natural gas pipelines
do not go everywhere, most conversions are dual-fuel with gasoline. The
ignition timing and stoichiometry are significantly different, but good
conversions will provide about 85% of the gasoline power over the full
operating range, with easy switching between the two fuels [76].
CNG has been extensively used in Italy and New Zealand ( NZ had 130,000
dual-fuelled vehicles with 380 refuelling stations in 1987 ). The
conversion costs are usually around US$1000, so the economics are very
dependent on the natural gas price. The typical 15% power loss means that
driveability of retrofitted CNG-fuelled vehicles is easily impaired,
consequently it is not recommended for vehicles of less than 1.5l engine
capacity, or retrofitted onto engine/vehicle combinations that have
marginal driveability on gasoline. The low price of crude oil, along with
installation and ongoing CNG tank-testing costs, have reduced the number of
CNG vehicles in NZ. The US CNG fleet continues to increase in size ( 60,000
in 1994 ).
LPG ( Liquified Petroleum Gas ) is predominantly propane with iso-butane
and n-butane. It has one major advantage over CNG, the tanks do not have to
be high pressure, and the fuel is stored as a liquid. The fuel offers most
of the environmental benefits of CNG, including high octane. Approximately
20-25% more fuel is required, unless the engine is optimised ( CR 12:1 )
for LPG, in which case there is no decrease in power or increase in fuel
consumption [17,76].
methane propane iso-octane
RON 120 112 100
MON 120 97 100
Heat of Vaporisation (MJ/kg) 0.5094 0.4253 0.2712
Net Heating Value (MJ/kg) 50.0 46.2 44.2
Vapour Pressure @ 38C ( kPa ) - - 11.8
Flame Temperature ( C ) 1950 1925 1980
Stoich. Flame Speed. ( m/s ) 0.45 0.45 0.31
Minimum Ignition Energy ( mJ ) 0.30 0.26 -
Lower Flammable Limit ( vol% ) 5.0 2.1 0.95
Upper Flammable Limit ( vol% ) 15.0 9.5 6.0
Autoignition Temperature ( C ) 540 - 630 450 415
9.5 Why are hydrogen-powered cars not available?
The Hindenburg. The technology to operate IC engines on hydrogen has been
investigated in depth since before the turn of the century. One attraction
was to use the hydrogen in airships to fuel the engines instead of venting
it. Hydrogen has a very high flame speed ( 3.24 - 4.40 m/s ), wide
flammability limits ( 4.0 - 75 vol% ), low ignition energy ( 0.017 mJ ),
high autoignition temperature ( 520C ), and flame temperature of 2050 C.
Hydrogen has a very high specific energy ( 120.0 MJ/kg ), making it very
desirable as a transportation fuel. The problem has been to develop a
storage system that will pass all safety concerns, and yet still be light
enough for automotive use. Although hydrogen can be mixed with oxygen and
combusted more efficiently, most proposals use air [73,77].
Unfortunately the flame temperature is sufficiently high to dissociate
atmospheric nitrogen and form undesirable NOx emissions. The high flame
speeds mean that ignition timing is at TDC, except when running lean, when
the ignition timing is advanced 10 degrees. The high flame speed, coupled
with a very small quenching distance mean that the flame can sneak past
inlet narrow inlet valve openings and cause backfiring. The advantage of a
wide range of mixture strengths and high thermal efficiencies are matched
by the disadvantages of pre-ignition and knock unless weak mixtures, clean
engines, and cool operation are used.
Interested readers are referred to the group sci.energy.hydrogen for
details about this fuel.
9.6 What are "fuel cells" ?
Fuel cells are electrochemical cells that directly oxidise the fuel at
electrodes producing electrical and thermal energy. The oxidant is usually
oxygen from the air and the fuel is usually gaseous, with hydrogen
preferred. There has, so far, been little success using low temperature
fuel cells ( < 200C ) to perform the direct oxidation of hydrocarbon-based
liquids or gases. Methanol can be used as a source for the hydrogen by
adding an on-board reformer. The main advantage of fuel cells is their high
fuel-to- electricity efficiency of about 40-60% of the nett calorific value
of the fuel. As fuel cells also produce heat that can be used for vehicle
climate control. Fuel Cells are the most likely candidate to replace the IC
engine as a primary energy source. Fuel cells are quiet and produce
virtually no toxic emissions, but they do require a clean fuel ( no
halogens, CO, S, or ammonia ) to avoid poisoning. They currently are
expensive to produce, and have a short operational lifetime, when compared
to an IC engine [78,79].
9.7 What is a "hybrid" vehicle?
A hybrid vehicle has three major systems [80].
1. A primary power source, either an IC engine driven generator where the
IC engine only operates in the most efficient part of it's performance
map, or alternatives such as fuel cells and turbines.
2. A power storage unit, which can be a flywheel, battery, or
ultracapacitor.
3. A drive unit, almost always now an electric motor that can used as a
generator during braking. Regenerative braking may increase the
operational range about 8-13%.
Battery technology has not yet advanced sufficiently to economically
substitute for an IC engine, while retaining the carrying capacity, range,
performance, and driveability of the vehicle. Hybrid vehicles may enable
this problem to be at least partially overcome, but they remain expensive,
and the current ZEV proposals exclude fuel cells and hybrids systems, but
this is being re-evaluated.
9.8 What about other alternative fuels?
9.8.1 Ammonia
Anhydrous ammonia has been researched because it does not contain any
carbon, and so would not release any CO2. The high heat of vaporisation
requires a pre-vaporisation step, preferably also with high jacket
temperatures ( 180C ) to assist decomposition. Power outputs of about 70%
of that of gasoline under the same conditions have been achieved [73].
9.8.2 Water
Mr Gunnerman has been promoting his patents that claim mixing one part of
gasoline with 2 parts water can provide as much power from an IC engine as
the same flow rate of gasoline. He claims the increased efficiency is from
catalysed dissociation of water to H2 and 02, as the combustion chamber of
the test engine contained a catalyst. It takes the same amount of energy to
dissociate water, as you reclaim when you burn the H2 with 02. So he has to
use heat energy that is normally lost. He appears to have modified his
claims a little with his new A55 fuel. A recent article claims a 29%
increase in fuel economy for a test bus in Reno, but also claims that his
fuel combusts so efficiently that it can pass an emissions test without
requiring a catalytic converter [81]. Caterpillar are working with
Gunnerman to evaluate his claims and develop the product.
9.9 What about alternative oxidants?
9.9.1 Nitrous Oxide
Nitrous oxide ( N2O ) contains 33 vol% of oxygen, consequently the
combustion chamber is filled with less useless nitrogen. It is also metered
in as a liquid, with can cool the incoming charge further, thus effectively
increasing the charge density. With all that oxygen, a lot more fuel can be
squashed into the combustion chamber. The advantage of nitrous oxide is
that it has a flame speed, when burned with hydrocarbon and alcohol fuels,
that can be handled by current IC engines, consequently the power is
delivered in an orderly fashion, but rapidly. The same is not true for pure
oxygen combustion with hydrocarbons, so leave that oxygen cylinder on the
gas axe alone :-). The following are for common premixed flames [82].
Temperature Flame Speed
Fuel Oxidant ( C ) ( m/s )
Acetylene Air 2400 1.60 - 2.70
" Nitrous Oxide 2800 2.60
" Oxygen 3140 8.00 - 24.80
Hydrogen Air 2050 3.24 - 4.40
" Nitrous Oxide 2690 3.90
" Oxygen 2660 9.00 - 36.80
Propane Air 1925 0.45
Natural Gas Air 1950 0.39
Nitrous oxide is not yet routinely used on standard vehicles, but the
technology is well understood.
9.9.2 Membrane Enrichment of Air
Over the last two decades, extensive research has been performed on the use
of membranes to enrich the oxygen content of air. Increasing the oxygen
content can make combustion more efficient due to the higher flame
temperature and less nitrogen. The optimum oxygen concentration for
existing automotive engine materials is around 30 - 40%. There are several
commercial membranes that can provide that level of enrichment. The problem
is that the surface area required to produce the necessary amount of
enriched air for an SI engine is very large. The membranes have to be laid
close together, or wound in a spiral, and significant amounts of power are
required to force the air along the membrane surface for sufficient
enriched air to run a slightly modified engine. Most research to date has
centred on CI engines, with their higher efficiencies. Several systems have
been tried on research engines and vehicles, however the higher NOx
emissions remain a problem [83,84].
---------------------------------------------------------------------------
Subject: 10. Historical Legends 10.1 The myth of Triptane
This post is an edited version of some posted after JdA posted
some claims from a hot-rod enthusiast reporting that triptane +
4cc TEL had a rich power octane rating of 270. This was followed
by another that claimed the unleaded octane was 150.
In WWII there was a major effort to increase the power of the aviation
engines continuously, rather than just for short periods using boost
fluids. Increasing the octane of the fuel had dramatic effects on engines
that could be adjusted to utilise the fuel ( by changing boost pressure ).
There was a 12% increase in cruising speed, 40% increase in rate of climb,
20% increase in ceiling, and 40% increase in payload for a DC-3, if the
fuel went from 87 to 100 Octane, and further increases if the engine could
handle 100+ PN fuel [85]. A 12 cylinder allison aircraft engine was
operated on a 60% blend of triptane ( 2,2,3-trimethylbutane ) in 100 octane
leaded gasoline to produce 2500hp when the rated take-off horsepower with
100 octane leaded was 1500hp [10].
Triptane was first shown to have high octane in 1926 as part of the General
Motors Research Laboratories investigations [86]. As further interest
developed, gallon quantities were made in 1938, and a full size production
plant was completed in late 1943. The fuel was tested, and the high lead
sensitivity resulted in power outputs up to 4 times that of iso-octane, and
as much as 25% improvement in fuel economy over iso-octane [10].
All of this sounds incredibly good, but then, as now, the cost of octane
enhancement has to be considered, and the plant producing triptane was not
really viable. the fuel was fully evaluated in the aviation test engines,
and it was under the aviation test conditions - where mixture strength is
varied, that the high power levels were observed over a narrow range of
engine adjustment. If turbine engines had not appeared, then maybe triptane
would have been used as an octane agent in leaded aviation gasolines.
Significant design changes would have been required for engines to utilise
the high anti-knock rating.
As an unleaded additive, it was not that much different to other
isoalkanes, consequently the modern manufacturing processes for aviation
gasolines are alkylation of unsaturated C4 HCs with isobutane, to produce a
highly iso-paraffinic product, and/or aromatization of naphthenic fractions
to produce aromatic hydrocarbons possessing excellent rich-mixture
antiknock properties.
So, the myth that triptane was the wonder anti-knock agent that would
provide heaps of power arose. In reality, it was one of the best of the
iso-alkanes ( remember we are comparing it to iso-octane which just
happened to be worse than most other iso-alkanes), but it was not _that_
different from other members. It was targeted, and produced, for
supercharged aviation engines that could adjust their mixture strength,
used highly leaded fuel, and wanted short period of high power for takeoff,
regardless of economy.
The blending octane number, which is what we are discussing, of triptane is
designated by the American Petroleum Institute Research Project 45 survey
as 112 Motor and 112 Research [31]. Triptane does not have a significantly
different blending number for MON or RON, when compared to iso-octane. When
TEL is added, the lead response of a large number of paraffins is well
above that of iso-octane ( about +45 for 3ml TEL/US Gal ), and this can
lead to Performance Numbers that can not be used in conventional automotive
engines [10].
10.2 From Honda Civic to Formula 1 winner.
The following is edited from a post in a debate over the
advantages of water injection. I tried to demonstrate what
modifications would be required to convert my own 1500cc Honda
Civic into something worthwhile :-).
There are many variables that will determine the power output of an engine.
High on the list will be the ability of the fuel to burn evenly without
knock. No matter how clever the engine, the engine power output limit is
determined by the fuel it is designed to use, not the amount of oxygen
stuffed into the cylinder and compressed. Modern engines designs and
gasolines are intended to reduce the emission of undesirable exhaust
pollutants, consequently engine performance is mainly constrained by the
fuel available.
My Honda Civic uses 91 RON fuel, but the Honda Formula 1 turbocharged 1.5
litre engine was only permitted to operate on 102 Research Octane fuel, and
had limits placed on the amount of fuel it could use during a race, the
maximum boost of the turbochargers was specified, as was an additional 40kg
penalty weight. Standard 102 RON gasoline would be about 96 R+M/2 if sold
as a pump gasoline. The normally-aspirated 3.0 litre engines could use
unlimited amounts of 102RON fuel. The F1 race duration is 305 km or 2
hours, and it's perhaps worth remembering that Indy cars run at 7.3 psi
boost.
Engine Standard Formula One
Year 1986 1987 1989
Size 1.5 litre 1.5 litre 1.5 litre
Cylinders 4 12 12
Aspiration normal turbo turbo
Maximum Boost - 58 psi 36.3 psi
Maximum Fuel - 200 litres 150 litres
Fuel 91 RON 102 RON 102 RON
Horsepower @ rpm 92 @ 6000 994 @ 12000 610 @ 12500
Torque (lb-ft @ rpm) 89 @ 4500 490 @ 9750 280 @ 10000
Lets consider the transition from Standard to Formula 1, without
considering materials etc.
1. Replace the exhaust system. HP and torque climb to 100.
2. Double the rpm while improving breathing, you now have 200hp but still
only about 100 torque.
3. Boost it to 58psi which equals 4 such engines, so 1000hp and 500
torque.
Simple?, not with 102RON fuel, the engine would detonate to pieces.
so..
4. Lower the compression ratio to 7.4:1, and the higher rpm is a big
advantage - there is much less time for the end gases to ignite and
cause detonation.
5. Optimise engine design. 80 degree bank angles V for aerodynamic
reasons and go to six cylinders = V-6
6. Cool the air. The compression of 70F air at 14.7psi to 72.7psi raises
its temperature to 377F. The turbos churn the air and although they
are about 75% efficient the air is now at 479F. The huge intercoolers
could reduce the air to 97F, but that was too low to properly vaporise
the fuel.
7. Bypass the intercoolers to maintain 104F.
8. Change the Air:Fuel ratio to 23% richer than stoichiometric to reduce
combustion temperature.
9. Change to 84:16 toluene/heptane fuel, harder to vaporise, but complies
with the 102 RON requirement
10. Add sophisticated electronic timing and engine manangement controls to
ensure reliable combustion with no detonation.
You now have a six-cylinder, 1.5 litre, 1000hp Honda Civic.
For subsequent years the restrictions were even more severe, 150 litres and
36.3 maximum boost, in a still vain attempt to give the 3 litre,
normally-aspirated engines a chance. Obviously Honda took advantage of the
reduced boost by increasing CR to 9.4:1, and only going to 15% rich
air/fuel ratio. They then developed an economy mode that involved heating
the liquid fuel to 180F to improve vaporisation, and increased the air temp
to 158F, and leaned out the air-fuel ratio to just 2% rich. The engine
output dropped to 610hp @ 12,500 ( from 685hp @ 12,500 and about 312 lbs-ft
of torque @ 10,000 rpm ), but 32% of the energy in the fuel was converted
to mechanical work. The engine still had crisp throttle response, and still
beat the normally aspirated engines that did not have the fuel limitation.
So turbos were banned. No other F1 racing engine has ever come close to
converting 32% of the fuel energy into work [87].
---------------------------------------------------------------------------
Subject: 11. References
11.1 Books and Research Papers
1. Modern Petroleum Technology - 5th edition.
Editor, G.D.Hobson.
Wiley. ISBN 0 471 262498 (1984).
- Chapter 1. G.D.Hobson.
2. Hydrocarbons from Fossil Fuels and their Relationship with Living
Organisms.
I.R.Hills, G.W.Smith, and E.V.Whitehead.
J.Inst.Petrol., v.56 p.127-137 (May 1970).
3. Reference 1.
- Chapter 9. R.E.Banks and P.J.King.
4. Ullmann's Encyclopedia of Industrial Chemistry - 5th edition.
Editor, B.Elvers.
VCH. ISBN 3-527-20123-8 (1993).
- Volume A23. Resources of Oil and Gas.
5. BP Statistical Review of World Energy - June 1994.
- Proved Reserves at end 1993. p.2.
6. Kirk-Othmer Encyclopedia of Chemical Technology - 4th edition.
Editor M.Howe-Grant.
Wiley. ISBN 0-471-52681-9 (1993)
- Volume 1. Alcohol Fuels.
7. Midgley: Saint or Serpent?.
G.B.Kauffman.
Chemtech, December 1989. p.717-725.
8. ?
T.Midgley Jr., T.A.Boyd.
Ind. Eng. Chem., v.14 p.589,849,894 (1922).
9. Measurement of the Knock Characteristics of Gasoline in terms of a
Standard Fuel.
G. Edgar.
Ind. Eng. Chem., v.19 p.145-146 (1927).
10. The Effect of the Molecular Structure of Fuels on the Power and
Efficiency of Internal Combustion Engines.
C.F.Kettering.
Ind. Eng. Chem., v.36 p.1079-1085 (1944).
11. Experiments with MTBE-100 as an Automobile Fuel.
K.Springer, L.Smith.
Tenth International Symposium on Alcohol Fuels.
- Proceedings, v.1 p.53 (1993).
12. Oxygenates for Reformulated Gasolines.
W.J.Piel, R.X.Thomas.
Hydrocarbon Processing, July 1990. p.68-73.
13. The Chemical Kinetics of Engine Knock.
C.K.Westbrook, W.J. Pitz.
Energy and Technology Review, Feb/Mar 1991. p.1-13.
14. The Chemistry Behind Engine Knock.
C.K.Westbrook.
Chemistry & Industry (UK), 3 August 1992. p.562-566.
15. A New Look at High Compression Engines.
D.F.Caris and E.E.Nelson.
SAE Paper 812A. (1958)
16. Problem + Research + Capital = Progress
T.Midgley,Jr.
Ind. Eng. Chem., v.31 p.504-506 (1939).
17. Reference 1.
- Chapter 20. K.Owen.
18. Automotive Gasolines - Recommended Practice
SAE J312 Jan93.
- Section 3.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
19. Reference 6.
- Volume 12. Gasoline and Other Motor Fuels
20. Refiners have options to deal with reformulated gasoline.
G.Yepsin and T.Witoshkin.
Oil & Gas Journal, 8 April 1991. p.68-71.
21. Stoichiometric Air/Fuel Ratios of Automotive Fuels - Recommended
Practice.
SAE J1829 May92.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
22. Chemical Engineers' Handbook - 5th edition
R.H.Perry and C.H.Chilton.
McGraw-Hill. ISBN 07-049478-9 (1973)
- Chapter 3.
23. Alternative Fuels
E.M.Goodger.
MacMillan. ISBN 0-333-25813-4 (1980)
- Appendix 4.
24. Automotive Gasolines - Recommended Practice.
SAE J312 Jan93.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
25. Standard Specification for Automotive Spark-Ignition Engine Fuel.
ASTM D 4814-93a.
Annual Book of ASTM Standards v.05.03 (1994).
26. Criteria for Quality of Petroleum Products.
Editor, J.P. Allinson.
Applied Science. ISBN 0 85334 469 8
- Chapter 5. K.A.Boldt and S.T.Griffiths.
27. Meeting the challenge of reformulated gasoline.
R.J. Schmidt, P.L.Bogdan, and N.L.Gilsdorf.
Chemtech, February 1993. p.41-42.
28. The Relationship between Gasoline Composition and Vehicle Hydrocarbon
Emissions: A Review of Current Studies and Future Research Needs.
D. Schuetzle, W.O.Siegl, T.E.Jensen, M.A.Dearth, E.W.Kaiser, R.Gorse,
W.Kreucher, and E.Kulik.
Environmental Health Perspectives Supplements v.102 s.4 p.3-12. (1994)
29. Reference 23.
- Chapter 5.
30. Texaco to introduce clean burning gasoline.
Oil & Gas Journal, 28 February 1994. p.22-23.
31. Knocking Characteristics of Pure Hydrocarbons.
ASTM STP 225. (1958)
32. Health Effects of Gasoline.
Environmental Health Perspectives Supplements v.101. s.6 (1993)
33. Speciated Measurements and Calculated Reactivities of Vehicle Exhaust
Emissions from Conventional and Reformulated Gasolines.
S.K.Hoekman.
Environ. Sci. Technol., v.26 p.1206-1216 (1992).
34. Effect of Fuel Structure on Emissions from a Spark-Ignited Engine.
2. Naphthene and Aromatic Fuels.
E.W.Kaiser, W.O.Siegl, D.F.Cotton, R.W.Anderson.
Environ. Sci. Technol., v.26 p.1581-1586 (1992).
35. Determination of PCDDs and PCDFs in Car Exhaust.
A.G.Bingham, C.J.Edmunds, B.W.L.Graham, and M.T.Jones.
Chemosphere, v.19 p.669-673 (1989).
36. Volatile Organic Compounds: Ozone Formation, Alternative Fuels and
Toxics.
B.J.Finlayson-Pitts and J.N.Pitts Jr..
Chemistry and Industry (UK), 18 October 1993. p.796-800.
37. The rise and rise of global warming.
R.Matthews.
New Scientist, 26 November 1994. p.6.
38. Energy-related Carbon Dixode Emissions per Capita for OECD Countries
during 1990.
International Energy Agency. (1993)
39. Market Data Book - 1991, 1992, 1993 and 1994 editions.
Automobile News
- various tables
40. BP Statistical Review of World Energy - June 1994.
- Crude oil consumption p.7.
41. Automotive Gasolines - Recommended Practice
SAE J312 Jan93.
- Section 4
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
42. The Rise and Fall of Lead in Petrol.
IDG Berwick
Phys. Technol., v.18 p.158-164 (1987)
43. E.C. seeks gasoline emission control.
Hydrocarbon Processing, September 1990. p.43.
44. Health Effects of Gasoline Exposure. I. Exposure assessment for U.S.
Distribution Workers.
T.J.Smith, S.K.Hammond, and O.Wong.
Environmental Health Perspectives Supplements. v.101 s.6 p.13 (1993)
45. Atmospheric Chemistry of Tropospheric Ozone Formation: Scientific and
Regulatory Implications.
B.J.Finlayson-Pitts and J.N.Pitts, Jr.
Air & Waste, v.43 p.1091-1100 (1993).
46. Trends in Auto Emissions and Gasoline Composition.
R.F.Sawyer
Environmental Health Perspectives Supplements. v.101 s.6 p.5 (1993)
47. Reference 6.
- Volume 9. Exhaust Control, Automotive.
48. Achieving Acceptable Air Quality: Some Reflections on Controlling
Vehicle Emissions.
J.G.Calvert, J.B.Heywood, R.F.Sawyer, J.H.Seinfeld
Science v261 p37-45 (1993).
49. Radiometric Determination of Platinum and Palladium attrition from
Automotive Catalysts.
R.F.Hill and W.J.Mayer.
IEEE Trans. Nucl. Sci., NS-24, p.2549-2554 (1977).
50. Determination of Platinum Emissions from a three-way
catalyst-equipped Gasoline Engine.
H.P.Konig, R.F.Hertel, W.Koch and G.Rosner.
Atmospheric Environment, v.26A p.741-745 (1992).
51. Alternative Automotive Fuels - SAE Information Report.
SAE J1297 Mar93.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
52. Lean-burn Catalyst offers market boom.
New Scientist, 17 July 1993. p.20.
53. Catalysts in cars.
K.T.Taylor.
Chemtech, September 1990. p.551-555.
54. Advanced Batteries for electric vehicles.
G.L.Henriksen, W.H.DeLuca, D.R.Vissers.
Chemtech, November 1994. p.32-38.
55. The great battery barrier.
IEEE Spectrum, November 1992. p.97-101.
56. Exposure of the general Population to Gasoline.
G.G.Akland
Environmental Health Perspectives Supplements. v.101 s.6 p.27-32 (1993)
57. Court Ruling Spurs Continued Debate Over Gasoline Oxygenates.
G.Peaff.
Chemical & Engineering News, 26 September 1994. p.8-13.
58. The Application of Formaldehyde Emission Measurement to the
Calibration of Engines using Methanol as a Fuel.
P.Waring, D.C.Kappatos, M.Galvin, B.Hamilton, and A.Joe.
Sixth International Symposium on Alcohol Fuels.
- Proceedings, v.2 p.53-60 (1984).
59. Emissions from 200,000 vehicles: a remote sensing study.
P.L.Guenther, G.A.Bishop, J.E.Peterson, D.H.Stedman.
Sci. Total Environ., v.146/147 p.297-302 (1994)
60. Remote Sensing of Vehicle Exhaust Emissions.
S.H.Cadle and R.D.Stephens.
Environ. Sci. Technol., v.28 p.258A-264A. (1994)
61. Real-World Vehicle Emissions: A Summary of the Third Annual CRC-APRAC
On-Road Vehicle Emissions Workshop.
S.H.Cadle, R.A.Gorse, D.R.Lawson.
Air & Waste, v.43 p.1084-1090 (1993)
62. IR Long-Path Photometry: A Remote Sensing Tool for Automobile
Emissions.
G.A.Bishop, J.R.Starkey, A.Ihlenfeldt, W.J.Williams, and D.H.Stedman.
Analytical Chemistry, v.61 p.671A-677A (1989)
63. A Cost-Effectiveness Study of Carbon Monoxide Emissions Reduction
Utilising Remote Sensing.
G.A.Bishop, D.H.Stedman, J.E.Peterson, T.J.Hosick, and P.L.Guenther
Air & Waste, v.42 p.978-985 (1993)
64. A presentation to the California I/M Review Committee of results of
a 1991 pilot programme.
D.R.Lawson, J.A.Gunderson
29 January 1992.
65. Methods of Knock Rating. 15. Measurement of the Knocking
Characteristics of Automotive Fuels.
J.M.Campbell, T.A.Boyd.
The Science of Petroleum. Oxford Uni. Press. v.4 p.3057-3065 (1938).
66. Standard Test Method for Knock Characteristics of Motor and Aviation
Fuels by the Motor Method.
ASTM D 2700 - 92. IP236/83
Annual Book of ASTM Standards v.05.04 (1994).
67. Standard Test Method for Knock Characteristics of Motor Fuels by the
Research Method.
ASTM D 2699 - 92. IP237/69
Annual Book of ASTM Standards v.05.04 (1994).
68. Preparation of distillates for front end octane number ( RON 100C )
of motor gasoline
IP 325/82
Standard Methods for Analysis and Testing of Petroleum and Related
Products. Wiley. ISBN 0 471 94879 9 (1994).
69. Octane Enhancers.
D.Simanaitis and D.Kott.
Road & Track, April 1989. p.82,83,86-88.
70. Specification for Aviation Gasolines
ASTM D 910 - 93
Annual Book of ASTM Standards v.05.01 (1994).
71. Reference 1.
- Chapter 19. R.A.Vere
72. Automotive Sensors Improve Driving Performance.
L.M.Sheppard.
Ceramic Bulletin, v.71 p.905-913 (1992).
73. Reference 23.
- Chapter 7.
74. Investigation of Fire and Explosion Accidents in the Chemical, Mining
and Fuel-Related Industries - A Manual.
Joseph M. Kuchta.
US Dept. of the Interior. Bureau of Mines Bulletin 680 (1985).
75. Natural Gas as an Automobile Fuel, An Experimental study.
R.D.Fleming and J.R.Allsup.
US Dept. of the Interior. Bureau of Mines Report 7806 (1973).
76. Comparative Studies of Methane and Propane as Fuels for Spark Ignition
and Compression Ignition Engines.
G.A.Karim and I.Wierzba.
SAE Paper 831196. (198?).
77. The Outlook for Hydrogen.
N.S.Mayersohn.
Popular Science, October 1993. p.66-71,111.
78. Reference 6.
- Volume 11. Fuel Cells.
79. The Clean Machine.
R.H.Williams.
Technology Review, April 1994. p.21-30.
80. Hybrid car promises high performance and low emissions.
M. Valenti.
Mechanical Engineering, July 1994. p.46-49.
81. ?
Automotive Industries Magazine, December 1994.
82. Instrumental Methods of Analysis - 6th edition.
H.H.Willard, L.L.Merritt, J.A.Dean, F.A.Settle.
D.Van Nostrand. ISBN 0-442-24502-5 (1981).
83. Research into Asymmetric Membrane Hollow Filter Device for Oxygen-
Enriched Air Production.
A.Z.Gollan. M.H.Kleper.
Dept.of Energy Report DOE/ID/12429-1 (1985).
84. New Look at Oxygen Enrichment. I. The diesel engine.
H.C.Watson, E.E.Milkins, G.R.Rigby.
SAE Technical Paper 900344 (1990)
85. Thorpe's Dictionary of Applied Chemistry - 4th edition.
Longmans. 1949.
- Petroleum
86. Detonation Characteristics of Some Paraffin Hydrocarbons.
W.G.Lovell, J.M.Campbell, and T.A.Boyd.
Ind. Eng. Chem., v.23 p.26-29. (1931)
87. Secrets of Honda's horsepower heroics.
C. Csere.
Road & Track/Car & Driver?, May 1991. p.29.
11.2 Suggested Further Reading
1. Modern Petroleum Technology - any edition.
Editor, G.D.Hobson.
Wiley. ISBN 0 471 262498 (5th=1984).
2. Hydrocarbon Fuels.
E.M.Goodger.
MacMillan. (1975)
3. Alternative Fuels
E.M.Goodger.
MacMillan. ISBN 0-333-25813-4 (1980)
4. Kirk-Othmer Encyclopedia of Chemical Technology - 4th edition.
Editor, M.Howe-Grant.
Wiley. ISBN 0-471-52681-9 (1993)
- especially Alcohol Fuels, Gasoline and Other Motor Fuels, and
Fuel Cells chapters.
5. The Automotive Handbook. - any edition.
Bosch.
6. SAE Handbook, volume 1. - issued annually.
SAE. ISBN 1-56091-461-0 (1994).
- especially J312, and J1297.
7. Proceedings of the xxth International Symposium on Alcohol Fuels.
- Held every two years and most of the 10 conferences have lots of
good technical information, especially the earlier ones.
- various publishers.
8. Alternative Transportation Fuels - An Environmental and Energy
solution.
Editor, D.Sperling.
Quorum Books. ISBN 0-89930-407-9 (1989).
9. The Gasohol Handbook.
V. Daniel Hunt.
Industrial Press. ISBN 0-8311-1137-2 (1981).