Engine braking on diesels

[mat777]

The following is a snippet from the 80s Racing Trucks thread:

wst said:

mat777 said:

wst said:

mat777 said:

wst said:

Wait, how... how the hell does he drive 15 laps with no brakes without going around at a snail's pace? That sounds amazing.

engine braking?

Didn't think diesels had that much engine braking, unless the racing trucks are all fitted with jake brakes!

wait, what?????

Diesel engines have a massive amount of engine braking, many times more than petrol. Think of the compression ratio

No throttle. No vacuum. Nothing to inhibit the engine from turning. They have pretty much no engine braking...

Seriously, WTF? According to this, a diesel engined vehicles has no engine braking if you lift off the throttle. Hmmmmm.... every single bit of off roading I'ev ever seen, not to mention my own experiences learning to drive in a diesel, are wrong then? As is the physics of compressing a charge in the cylinder with the valves closed, even if there is very little fuel in it?

[mat777]

So surely that means that every time I or everyone else goes "feet off" in a Defender off road, it is defying what it should do by crawling down the hill with increased revs from idle, instead of shooting off the don the hill at idle?
No to mention every off road driving manual I've ever read being wrong in stating that "the engine braking effect in a diesel 4x4 will be greater than that in a petrol, in some cases requiring a higher gear to be selected for hill descents"?

Or am I missing something?

[mat777]

Ive been thinking somewhat and I now realise that this "vacuum" talk is misleading.

Correct me if I am wrong, but a diesel engine draws in the same amount of air in every induction stroke, the only thing controlling engine speed is how much fuel is injected each time. Ergo, the engine braking is the piston compressing all or mainly air, which doesnt combust so provides no power on the power stroke, at the massively high compression ratio.