I could never understand it. Lean AFR means less fuel gets in cylinders. How can less fuel produce more heat?

 

Yes lean means less fuel. Don't think of it that way. Think way more Oxygen. Get the picture now... more oxygen =hotter fire. Like blowing on coals in a campfire makes a big flare up. Less air just smolders.

 

What an eloquently simple explanation.

 

Actually, it's not a case of less fuel but more oxygen and under pressure...

One example is the blast furnace used in a smelting process...add more oxygen with the same amount of fuel and get much higher temps.

Another example, sadly, would be Apollo 1...the 100% oxygen atmosphere in the crew cabin allowed that fire to become deadly in just a couple of seconds.

 

One gram of gas (wood, coal, whatever) can produce exact amount of heat if burned completely. I understand that more oxygen makes it burn faster but it cannot make more calories.

 

http://www.youtube.com/watch?v=Eg9ReJAJzgw

That kinda shows the effect of an oxygen rich environment has on burn rates. The faster the burn rate the higher the temperature. You are correct in assuming the potential of a given amount of fuel is the same but the durration is what makes the difference.

 

The extra fuel absorbs heat and therefore makes the engine run cooler.

So its a matter of perspective. The ideal mix has just enough oxygen to burn the fuel completely, extracting the maximum calories/energy from the fuel used. The ideal mix develops the most heat and from most's perspective is mistakenly considered lean. It is leaner than you want to run on the street WOT because the engine cannot take those high temperatures for long. In this case extra fuel is squirted in to cool things off.

So starting from the perspective that the mix is normally set richer than ideal (all the fuel doesn't get burned and the extra fuel absorbs some of the heat energy), leaning the mix to the point of ideal will make a hotter flame. Going even more leaner from the ideal burn A/F mix, there is plenty of oxygen but not as much fuel so not as much energy gets developed and it actually burns with less heat.... to the point where it won't burn at all.... then there is no heat.

I know its not that simple because there are burn rates and one tank of fuel will have a different ideal mix than another, and different fuels and cylider shapes react differently, but thats the basics of the normally aspirated gas engine.