Date: Sun, 8 Dec 1996 19:51:55 -0800 (PST)
From: Mike West <mwest@cdsnet.net>
Subject: Valves: "The Gates of Hell"

The "gates of hell" is not about a bar in Pacoima, that's the other place.

 This is about the valves in the combustion chamber of your car or van
engine.
 
 I read up on it because I wanted a better understanding and I hope 
some of you might feel the same way.

 This isn't a "fix your car thing". But it may explain why you need to fix
it sometimes.

  There are other kinds of engines that don't have valves per se, but yours
does.

 This is the world famous "VW aircooled flat four"!

 There are two types of valves in the head of the combustion chamber, an
Intake Valve and an Exhaust Valve.

The Intake lets the gasoline/air mix into the chamber and the Exhaust lets
the burnt gas fumes out of the chamber after it's exploded and pushed the
piston along it's path  toward the crankshaft.

 On the VW aircooled , 4 cylinder puppy, it does one of each every other stroke.
 This is  called a "2 stroke " engine. 

 Looking at the exhaust valve first will explain the most so that's our plan.

 At the beginning of the intake stroke, during the overlap period, (has to
do with the camshaft) it is subjected to a chilling blast of fuel-laden air,
then it is slammed closed with a force from 100 to 400 lbs, probably
bouncing a couple times on it's seat that may or may not have carbon
granules imbedded in it.
 
Then it is blasted with a hot flame, hot enough to melt aluminum, then it is
yanked off the seat while this is still going on and brought to a
temperature ranging from 1300-1800 deg.F., depending on where you measure
it, and then it gets hit with the cold blast of fuel/air again.

 This whole cycle happens from 850 to 5500 times a minute and three/fourths
of the time when it's not doing that, it's supposed to form a gas seal that
will contain pressures from 100 to as much as 1800 psi. Have you lapped your
valves lately?

 The intake valve is expected to do much the same but it doesn't get nearly
as much of the heat.

 What about the spark plug? 
 It is subjected to the heat and pressures but none of the beating and you
take it out and throw it away every tune-up.

 Now to add insult to the extreme injury:

  Neither valve recieves any direct cooling from the engine cooling system.

The heat crawls up the valve stem and is passed to the heads thru the valve
guides and then on to the engine oil but it's pathetic.

 You know those sloppy valve guides you left in there?
 Less contact with the stem means you have even less cooling.

 That oil you think is flowing down the valve stem to lubricate is 
carburizing and forming an insulation in the valve guide.

 Cooling of the Intake valve is pretty good from the continual flow of
air/fuel from the outside, but the Exhaust valve, except for a brief chill
in the "overlap" period recieves no cooling whatsoever unless you want to
call a 1300 deg. gas "cool". It's cherry red. 
  If it makes 1800F. you get "auto-ignition".

 A note here in that the higher your compression, the hotter that 
package of flame is.

 The duty imposed on the simplest valve head, when compared to any other
part of the engine is mind boggling.  

Bibliography: Petersons "Basic cams, valves 'n exhaust systems" and 
some other stuff I can't remember.

Smart-assed remarks are my own.

 west