[skywal_l]

In a 4-stroke engine, throttle or not, intake valves are shut down when in compression so cylinders are sealed off, compression happens anyway, diesel or gas. Indeed, in 2-cycles engines there are not intake valve so LinuxBender's point is valid.

[ska]

I think you misread; I should have been clearer. This is how I understand/remember it although to be fair it's been a while since I've worked on either so might mess it up a bit.

Anyway it has nothing to do with compression or the intake valve in either case. Compression happens in both cases, and doesn't affect anything.

In diesel, Jake type breaks steal energy by opening the exhaust valve right after TDC, e.g. what would be the power stroke. The energy stored in compressed air escapes out the exhaust valve rather than being (mostly) reclaimed by the crank on expansion - this slows down the crank and hence (if not in neutral) the vehicle slows. NB this is not when the exhaust valve would normally open, but rather a cycle earlier.

In gas, on the intake stroke the intake is blocked (not by the valve, further up by throttle) so the intake motion creates vaccuum - this takes energy, which slows down the crank, and hence etc. etc. The exhaust valve doesn't change timing.

The latter approach only works if you have something blocking the intake "above" the intake valve. In a diesel engine the airflow is kept the same and the fuel adjusted (unlike gas) so there is no natural mechanism to do this with the throttle.

[sokoloff]

Most of the energy stored in the cylinder charge during the compression stroke is returned (as if an air spring) on the (what would be the) power stroke. The difference between a gas and diesel engine shows up in the higher pumping losses on the intake stroke (if you're pulling air past a closed throttle plate or not).