Combustion temp question

greasemonkee · Post by **greasemonkee** » Sat Dec 06, 2008 7:32 pm

It's been said that highest combustion temps/egt's will often occur at stoichiometry. If this is typical among all cylinder pressures, I don't know, (maybe someone can elaborate.) If this were true, then why do NoX emissions increase at such a rapid rate leaner than stoichiometry. This is in regard to pump gasoline.

xenocron · Post by **xenocron** » Mon Dec 08, 2008 1:56 pm

Have any links to where you read that highest combustion temps are at stoich?

greasemonkee · Post by **greasemonkee** » Mon Dec 08, 2008 8:51 pm

Nothing solid, seemed to be popular belief for a while. More accurate sources invalidate this fallacy. I was hoping someone could confirm that's all it was so I could put it out of my mind.

xenocron · Post by **xenocron** » Mon Dec 08, 2008 9:00 pm

I just think it depends on a lot of variables... Compression, timing...etc. And combustion temps and EGTs are two very different things as well obviously.

james25 · Post by **james25** » Mon Dec 08, 2008 10:20 pm

I thought high combustion temps are a result of a lean running engine. I mean if the engine is running lean it would create a lot of heat and combustion temps would sky rocket and form NoX. My 1990 Nissan D21 truck failed emissions 3 times for high NoX. My neighbor told me to dump some iso heet in the gas tank, he said the alcohol or methanol burns cooler. I put 2 bottles of it and gave it another shot and sure did, passed emissions and it lowered the NoX level.

JaredKaragen · Post by **JaredKaragen** » Tue Dec 09, 2008 1:25 am

heat range of plugs can directly affect combustion temps; for instance a truck I took to smog; it had one step hotter plugs and failed from an overheating catalytic converter. new plugs, passed like a dream.

blenz · Post by **blenz** » Sun Dec 14, 2008 5:06 pm

\"Chemical kinetics show that the formation of NO and other oxides of nitrogen increase very strongly with increasing flame temperature. This would imply that the highest concentration of NOx should be for slightly rich mixtures, those that have the highest flame temperature. However, oxygen is also needed for the formation of NO, so the maximum NO emissions occur just weak of stoichiometric. NOx formation will also be influenced by the flame speed. Lower flame speeds with lean mixtures provide a longer time for NOx to form. Similarly NOx emissions increase with reduced engine speed.\" -\"Introduction to Internal Combustion Engines\" - Stone Some other points made in Stone regarding NOx emissions: -Retarding ignition very effective in reducing NOx -Between 5 and 10% EGR is likely to half NOx emissions -NOx emissions are worst for an equivalence ratio of 0.95 (slightly lean) Some explanation of combustion temps from some class notes: \"Slightly rich mixtures are the most reactive but slightly lean mixtures have a higher \"k\" value (Cp/Cv) and lead to higher compression temperatures. Richening the mixture with excess fuel reduces the temperature and reduces knock.\" If the compression stroke is modelled as isentropic, then the temperature ratio T2/T1 = r^k where r is the compression ratio. Since lean mixtures have a higher k value, they will have a higher temperature before combustion even begins, just from compression. Hope this helped a bit

Civicman86 · Post by **Civicman86** » Thu Jan 15, 2009 1:07 pm

http://www.autoshop101.com/forms/h55.pdf http://www.autoshop101.com/forms/h56.pdf Take a look at those. They help show what is going on with NOx, HC, and CO in the combustion process. They deal more with AF ratios rather than engine temps though.

Orthello · Post by **Orthello** » Thu Jan 15, 2009 3:21 pm

what was is again? Leaner is hotter combustion More ignition retard is higher EGT

JaredKaragen · Post by **JaredKaragen** » Thu Jan 15, 2009 4:00 pm

retarded cam timing/ignition causes EGT temps to rise because more fuel is still burning when coming out of the exhaust ports than on an advanced setup... this is a major factor in that clause.