Originally Posted By: Cujet
Originally Posted By: OVERKILL
Originally Posted By: Cujet
Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.
The only part of your post I have an issue with it this. An engine running lean has HIGHER EGT's, not lower. A lean engine will cause the headers to glow (as many a hot rodder can attest to) unless we are looking at relatively lean vs really lean here
This is incorrect. Peak EGT generally occurs at an AFR of around 14.7- 15.0 to 1 on gasoline. This is far too lean for maximum power and is dangerous under continuous WOT conditions. Many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry- about 15 to 1 for our purposes. If you go richer than 15 to 1, EGT will drop and if you go leaner than 15 to 1 EGT will ALSO drop. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will occur at an EGT somewhat colder than peak EGT.
In cruise conditions, such as a car on the highway, operation "lean of peak EGT" can result in as much as a 25% improvement in MPG, with some well configured engines. While resulting in LOWER exhaust valve temperatures.
Interestingly, engines configured with high compression, high swirl combustion chambers and staggered intake valve openings can very effectively run (at modest loads, such as highway driving) at leaner than 17 to 1 and often as lean as 20 to 1.
Remember, such a lean mixture burns slowly and won't detonate. Counter intuitive, I know. But true.
OK, my position relative to what constitutes "lean" here is off. I was thinking WOT conditions, most of the cars I'm thinking of are running 12:1-13.5:1, not stoich, as stoich is "too lean" at WOT and results in glowing headers and can lead to potential engine damage, LOL! But stoich isn't technically lean, which is where the disconnect was. Fuel is added at WOT to both increase power output, lower combustion temps and control detonation. Too much fuel of course will lower power output, but peak power always occurs below stoich, I'm sure we can agree there. Go above that (heading towards stoich) and you start losing power, driving up EGT's to dangerous levels, can burn valves....etc.
I didn't know that EGT's dropped off though when you went extremely lean however, so I learned something there, thanks for that