lean fuel mix = higher engine T. Why??
lean fuel mix = higher engine T. Why??
(OP)
In as simple terms as possible, what is the mechanism by which an excessively lean fuel mix causes higher engine temperatures?
I have searched through the archives and Stone's "Intro to Internal Combustion Engines" and haven't found a direct explanation. Someone posed this question and I was at a bit of a loss. I understand the evaporative cooling effect of a rich mixture leading to cooler temperatures, but it seems that lean mixtures should also be cooler, unless there is a contribution from compression of the excess air. Thermodynamics anyone? What's up here- surely someone must know.
I have searched through the archives and Stone's "Intro to Internal Combustion Engines" and haven't found a direct explanation. Someone posed this question and I was at a bit of a loss. I understand the evaporative cooling effect of a rich mixture leading to cooler temperatures, but it seems that lean mixtures should also be cooler, unless there is a contribution from compression of the excess air. Thermodynamics anyone? What's up here- surely someone must know.





RE: lean fuel mix = higher engine T. Why??
With a richer than stoichiometric mixture, there is a significant amount of leftover fuel, and lots of heat is absorbed due to evaporation. You react (nearly) all of the air, which means that you'll get about as much energy release as you're going to get out of a charge of that density.
With a stoich. mixture, you have less heat loss due to evaporation, but there will still be some amount of fuel & air that doesn't get burned, and some amount of energy absorbtion due to evaporation of fuel that doesn't get burned.
With a slightly lean mixture, (nearly) all of the fuel reacts, and there is a slight amount of leftover air that absorbs some heat, but not much evaporation to speak of. You have less energy release than in the above cases, but you also soak up less heating the charge.
A little leaner than that you're releasing less energy and absorbing more (relative to the amount released), and the temp starts to fall off again.
RE: lean fuel mix = higher engine T. Why??
RE: lean fuel mix = higher engine T. Why??
RE: lean fuel mix = higher engine T. Why??
compliments of http://www.channel1.com/users/graham/MyToyotaPrius/SideBars/LeanBurn.htm
RE: lean fuel mix = higher engine T. Why??
F.A.F.Schmidt in "The IC Engine" 1965 p. 114 hints at a possible mechanism in stating "At high excess air ratios, due to lower combustion velocity and to the more marked effect of uneven mixture distribution, irregular running of the engine and after-burning during the expansion stroke occur giving rise to higher exhaust temperatures and poor consumption."