This is my understanding from extensive work with Bosch fuel injection:
14.7:1 is about perfect stoichiometric mixture. At this mixture there is a very small amount of fuel unburned and a very small amount of oxygen that is unused. There is also a small amount of NOX or oxides of nitrogen from the heat of combustion. These levels are such that they can be combined evenly in a three way catylitic converter to form CO2 and H2O, leaving no oxygen, fuel or NOX at all. This is what we look for for a very clean running automotive engine.
Richer and there is less oxygen to combust the fuel, leaving much more unburned and giving you cooler combustion temperatures. But richer to a point gives you slightly more power, at the cost of efficiency and emissions, to around the 13:1 neighborhood, and then power starts to drop off precipitously.
Leaner provides enough oxygen to combust ALL of the fuel. This is why the combustion is hotter. The hotter combustion temperatures are enough to cause reactions that lead to the creation of excessive amounts of odd oxides of other elements in the air, mostly oxides of nitrogen. It's generally agreed that NOX is worse in the air than unburned hydrocarbons, so a lean mixture is actually worse for the air than a rich one. But for economy the fuel injection systems on modern automobile engines are programmed to run very lean in certain situations, like idle and part throttle cruise, for fuel efficiency. This is why you have EGR systems on cars. The small amount of "inert" exhaust gas run back into the intake stream under these conditions cools the very hot lean combustion reaction to inhibit formation of NOX. Not only does lean mixture cause NOX emissions, it causes preignition and detonation, which you hear as ping. Because there is plenty of oxygen to burn all of the fuel, the reaction comes closer to an explosion than a controlled burn and you get detonation. You can hear this as ping if your EGR system is closed off and the fuel injection is still running a leaner mixture. Leaner gives slightly less power.
Obviously if the mixture is grossly lean combustion temperatures are going to cool off, but in the area between 15:1-18:1 or so they are most definitely hotter.
Now, this is based on what Bosch factory manuals say, can you point me to material that would contradict this? I would be curious to read it, would like to learn more if my understanding is not correct.
I have found documentation stating that leaner mixtures burn cooler, but only with natural gas, not gasoline.
