[tomxor]

Interesting. I was only taught engine breaking from the practical perspective of down-shifting, but not the details of why it works. I understood the implicit effects of shifting down - maintaining the same high RPM with the same high resistance as a vehicle slows... but never gave much thought to what exactly those resistances were, I just assumed it was a combination of friction, compression, driving an alternator, other arbitrary mechanical losses etc.

Would there really be no significant braking effect without that "high manifold vacuum"? I suppose the engine does have a lot of mass so I could believe the effect could be too slow to be useful.

[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.