Emissions Testing in the UK
by Bob Wallace
"I can find many sources quoting upper limits as far as the passing of various Govt. tests is concerned and indeed methods of attaining those levels but, as we all know, what's good for the Govt (and probably our own lungs!) is not necessarily automatically healthy for a VW engine."
First, for the benefit of those not here in the UK, here are the emissions tests which are preformed on all petrol vehicles (kit cars, amatuer built cars and Wankel rotory engines are slightly different) each year as a SMALL part of our annual test for the years covered by most aircooled VW busses....
All tests should be done with the engine warm.
All vehicles will be checked for emission of dense blue smoke. First rev the engine to 2500rpm (or approx half maximum revs) for 20 seconds then allow it to return to idle and check for dense smoke from the exhaust. The emission of such smoke is a reason for failure.
[Note: if the engine will not run a a reasonable idling speed, operating at too many rpm it will be failed. But if the throttle has to be applied to stop the engine from stalling (idle speed is too low) it won't result in a fail.]
Cars first used before 1 August 1975: Emissions not measured
Cars first used on or after 1 August 1975 but
before 1 August 1983:
Hydrocarbon content of the exhaust gasses must
not exceed 1200 parts per million (0.12%) for a continuous period
of 5 seconds
Carbon Monoxide content of the exhaust gasses must not exceed 6%
for a continuous period of 5 seconds.
No test for Nitrous Oxide
Cars first used on or after 1 August
1983:
Hydrocarbon content of the exhaust gasses must
not exceed 1200 parts per million (0.12%) for a continuous period
of 5 seconds
Carbon Monoxide content of the exhaust gasses must not exceed 4.5%
for a continuous period of 5 seconds.
No test for Nitrous Oxide
Now for the relationships between mixture, fuel ecconomy, and power output:
- The ideal air to fuel ratio for a petrol internal combustion engine is 14.7 air : 1 fuel.
- Maximum power is achieved with a rich mixture: 12.6 air : 1 fuel
- Minimum fuel consumption is achieved with a lean mixture - 15.4 air : 1 fuel
- A Rich mixture will increase HC and CO (but lower NOX) emissions
- A Lean mixture will reduce HC and CO emissions but NOX goes up (unless mixture is very lean)
- A rich mixture will burn cooler than a lean mixture.
- Emissions control began by detuning carb engines to give a leaner mixture.
Fuel Injected engines hold the mixture range between 14.5:1 to 14.7:1 giving below max power, a below max MPG, but a good compromise between CO, HC and NOX emissions. Aggressive emissions control passed to the catalytic converter which relies on this mixture; used outside of this range a catalytic converter can, in fact, result in worse NOX emissions than with no Cat!!!!
So how do you get good emissions, power, and economy with a carb engine when any mixture set during tune up will not be held by the carb over the full rev range, and any setting is a compromise? well you (or the government) choses which one is more important.
In the UK because of the emissions requirements, a post August '75 car will tend to be tuned by a garage to minimise emissions (epecially if the tune up follows a test failure). This should give good MPG too but poor power output and higher engine temps.
Because of the variability of mixture through the rev range (and with air pressure / altitude / temperature) carbs are traditionally tuned slighty rich giving good power, lower temps but below best MPG and bad emissions. This "traditional" state of tune will be achieved by following the procedures laid out in (for example) the Haynes manual.
My old '73 1600 TP had exhaust so clear it was invisible and easily passed the emissions for post 1.8.75 busses when it was tuned as per Haynes.
In my opinion this slightly rich mixture is better for your VW.
It is of course in contradition with the mixture I would like you to set for the benefit of the environment but that's exactly the horns of dilemma on which you are impailed.
