lean fuel mix = higher engine T. Why??

[drwebb]

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.

 

[ivymike]

Peak temp is only slightly lean of stoich, and you've got the most important factors I can think of already stated.

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.

 

[automatic2]

I like that idea, but I'm sure the flame front increases in speed when slightly lean. The charge combusts quickly, with subsequent low energy in later part of cycle. Results are the same though, reduced effiency leads to higher heat generation.

 

[8quid]

L4ean Burn engines tend to have cooler combustion temps so I am not sure of the answer. Lean engines do however have a hotter exhaust temp as the combustion of the mixture is slower. This flame ceases later than a stioch engine and so more heat is on the exhaust valve and pipe

 

[automatic2]

Normal combustion is never complete; by the phenomenon of thermal equilibrium, there remains an unburned boundary layer of air-fuel mixture insulating the metal components of the combustion chamber from the propagating flame front, that fractal beast of burning air/fuel mixture originating at the spark plug. A dangerously lean air/fuel mixture burns with most efficiency, so much that the insulating boundary layer also gets consumed and the flame front touches the metal walls. At those locations, there is a dramatic rise in temperature, high enough to cause subsequent charges of air and fuel to spontaneously ignite resulting in multiple flame fronts. This is pre-ignition. Preceding each flame front is its sonic pressure wave whose collisions we hear as knocking and pinging. Allowed to persist, colliding sonic pressure waves will concentrate on the irregular shapes present (edges of pistons, valves, even the spark plug) to cause severe damage, just as you could with a hammer in your hand; this is detonation.

compliments of

http://www.channel1.com/users/graham/MyToyotaPrius/SideBars/LeanBurn.htm

 

[drwebb]

OK gang, thanks for the input. I have consulted some other sources as well. C.F.Taylor in "The IC Engine in Theory and Practice, Vol. 1" states, "The notion that engines overheat with lean mixtures is probably derived from the fact that the carburetors of many gasoline engines are set to give FsubR about 1.2. Figure 8-16 shows that if the carburetor is adjusted to give FsubR = 1.0 AND THE SAME POWER IS DEVELOPED (emphasis added) overheating may result." This sounds along the line of what strokersix is suggesting, and that excessively lean mixtures CAUSE inefficiency just like excessively rich mixtures; but in contrast to rich, lean does not have an additional evaporative cooling mechanism to counteract the excess heat.

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."