[soutthpaw]

Cody, great explanation.
a simple version is that the throttle in a gasser controls how much air gets into the engine. In a diesel the throttle controls how much fuel gets into the engine.

as for why lean gas engines run hot. the gasoline vapor like water absorbs the heat in the cylinder from the previous combustion cycle much faster than air does think of a water mist spray in front of a fan on a hot day. So a lean engine puts less fuel in the engine and thus absorbs less heat from the cylinders and so the engine runs hotter...