rich/lean mixture vs. temperature questions

[242ATL]

I haven't posted in a long while, but I've had this question, or series of questions for some time. Lots of reading on this topic seems contradictory (not on this forum of course).

I understand that at a stoichiometric mixture (assume we're talking gasoline) of 14.7:1 (14.7 pounds of air with 1 pound fuel) we get complete combustion with no fuel left over.

Also, at 14.7, we get the most energy released from that combustion which means in theory, most heat transferred to the combustion chamber and hottest exhaust gas.

My question comes in as to what happens on either side of that point. The common lore is that lean of 14.7, combustion temperatures get hotter, whereas rich of 14.7, they get cooler. I get the rich-cooler... evaporating fuel absorbs some of the heat, but I do not get lean=hot.

Enter the plot below taken from the airplane people, also working with 4stroke gasoline engines.
http://www.n66ap.alexap.com/Mixture_article.htm



This plot clearly shows maximum EGT and CHT (cylinder head temp) at stoich (stoich is the position of max EGT), and then both falling on either side of stoich... both dropping... be it lean or rich, both temperatures falling.

So someone explain to me know how a lean mixture runs hotter, or gives higher cylinder head temps, or whatever.

Your thoughts.

Mike

[Bernard Fife]

Mike,

The lean mixture doesn't burn at a higher temperature. However, a lean mixture has an excess of oxygen (by definition), and this relatively hot oxygen is looking to combine with anything (i.e., burn). The leaner the mixture, the more oxygen there is looking to oxidize surrounding materials (up to the point where there is a misfire, and there's no heat to initiate the oxidation). Usually it is the aluminum piston that burns. The oxidation itself process does let off a small amount of heat, but this is quite small compared to the heat of combustion.

So lean combustion is not hotter, but it is more likely to burn pistons.

Lance.