[mcguire]

Gasoline engines have a throttle plate that, when you let off the throttle, prevents intake air from reaching the cylinders. The pistons try to draw air into the cylinders and create a pretty decent vacuum. (Respect to the throttle plate. :-))

Diesel engines don't; the throttle controls fuel flow into the cylinders. Let off the throttle and air flows through the intake, cylinders, and exhaust just without producing any power.

The effects of friction are roughly the same on both engines, and they are what engine designers and builders want to minimize to maximize fuel efficiency and power.