[lelandstanford]

I am kind of confused on this issue of how much fuel is injected into the combustion chamber.

If air/fuel ratio of a gasoline is too lean, pistons may get damaged from high temperature.

So, besides the issue of emission, why can't the cooling system remove the heat?

Since a diesel engine almost always get full dose of air in every intake cycle regardless of RPM. Why won't the pistons get damaged if idling (little fuel injected).