Hey I have a random question.. Does anyone know why an engine runs hotter when you run it lean? I had a Thermo teacher tell me in class that when an engine is run rich it runs hotter, and when it runs lean it runs cooler. I know this is not true, but didn't have a good reason why. If anyone knows that would be awesome.. Thanks!

I think a part of this is due to the cooling effect of vaporizing additional fuel. The transition from liquid to gas requires alot of energy.

I have a buddy in a fuel science course who asked about the same thing. Our conclusion was that stoich runs the hottest, and the temp drops on both sides. I think a lot of confusion comes from thermo profs considering lean as AFRs >14.7, on the lean side of stoich. When we talk about lean we still mean AFRs <14.7

What is common is the confustion between combustion temperature and exhaust temperature.

Combustion temperature peaks at the stoichiometric ratio of air and fuel. This is peak energy release.

Relative to a stoichiometric ratio of 14.7:1, a lean mixture will have a cooler combustion temperature and hotter exhuast temperature. A rich mixture will have a cooler combustion temperature and cooler exhaust temperature.

At some point, I believe it is ~19:1 AFR, exhaust temperature drops again due to engine misfire (unburnt fuel) present in the exhaust.

i think peak cylinder temperature (and temperature) occur when you have the fastest burn rate and best cylinder pressure which is slightly richer then stoich. Something like 14.0-14.2.

so your thermo teacher is technically correct. egt's, like others have said, don't necessarily follow the same trend.

I think the actual cooling effect of the fuel being sprayed into the cylinder has a significant impact (temperature difference-wise). That and going from liquid to gas (fuel evap) absorbs heat. Any confirmation on this as I don't have my engine book in front of me.

The extra fuel does absorb heat during combustion lowering peak cylinder temps and pressures, and many FI engines take advantage of that, but they are much richer then that for best power because they can offset the power loss with more boost.

I was thinking about it and your thermo teacher is probably thinking about adiabatic flame temperature which is highest at stoich, but a running engine is different. It has to burn all of its fuel in the shortest amount of time to get the highest cylinder temperatures/pressures which happens when the mixture is richer then stoich.

The Thermo Teacher was talking about diesel engines.
The only ones I have calibrated were Toyota turbo/Diesel 4WD engines. If it runs richer than 20.5:1 at full load, thermal load on the pistons will cause failure. 15:1 = big power increase, then big blowup! 23:1 is less power, much lower combustion temp, very long engine life.
p.s. Here in my world Petrol in near $1 per litre but Diesel is still $1.50
Cheers

Very true for diesels, although stoich for diesel is around 15:1, so even at that point you are not running rich yet. I always thought the limit is egts causing failure of exhaust valves or the turbo before the piston would go.