I'm a mechanic who is currently doing a study automotive engineering. Being someone who worked on both diesel and petrol cars I know what a higher or lower air fuel ratio (AFR) will do for both engines. However I don't fully understand how the following is possible. "Adding fuel in a diesel engine will result into more heat, however adding fuel in a petrol engine removes heat. Removing fuel from a diesel results into lower temperatures, however removing fuel from a petrol engine increases heat." It's like saying adding wood (fuel) to a fire will make a bigger fire with more heat, but then also saying adding more oxygen a fire increases heat. Both statements seem true, but how is that possible?! Here I will sum up some things so they won't have to be mentioned. For diesels- Adding fuel in a diesel engine rises EGT's (exhaust gas temperature) Diesel engines always run lean (even though their might be rich pockets) Diesel engines achieve combustion by pressure and heat Diesel engines use their injection system to time their combustion (so no pre-ignition is possible) Diesel engines don't have a real throttle valve For petrols- Going rich results into unburned fuel which turns into gas absorbing heat (temperature decreases) Leaning out in a petrol engine rises temperatures. Since leaning out increases temperatures pre-ignition and knocking might occur Petrol engines have to stay within a small range of AFR's Petrol engines have a throttle valve limiting air going into the engine I've also got a few questions. If we would run a diesel engine very rich, for example with a AFR of 13:1 will have the same cooling effects we see in petrol engines running rich mixture? (ignoring practical problems such as soot and NOx production) If we go lean in a petrol engine it will run hot. But what if we go beyond slightly lean, let's just say we run a AFR of 30:1. Would we see a drop in temperatures? What is the limiting factor in going lean? Can we simply keep going leaner till the engine starts struggling with combustion? Or is the limiting factor lubrication (from fuel)? Do diesel's never run rich enough to experience any cooling effects? And that petrol engines never run lean enough to experience temperature drops in combustion? Is it simply the case that diesel and petrol engines run the hottest between 14.7:1 and 18:1? And ratio's above or below that see lower temperatures because they need more fuel or oxygen to create heat?