In a gasoline engine, engine speed is controlled by a throttle valve which meters the air entering the engine. A carburetor or fuel injection system then provides an appropriate amount of fuel for the incoming air. If you release the accelerator the throttle valve shuts and the engine effectively becomes a vacuum pump sucking air through small openings that only flow enough air to idle, which causes it to actively slow down.

A diesel engine does not have a throttle valve. Engine speed is controlled purely by the amount of fuel being injected. This is why diesel engines can "run away" if an uncontrolled fuel source such as oil leaking from a turbocharger enters the intake, and why older pre-computerization diesels can be smoky under hard acceleration (or any time if poorly tuned). When you release the accelerator the engine stops receiving fuel and thus stops producing power, but aside from friction there's nothing working to slow it down. Air is sucked in more or less unrestricted and some energy is spent compressing it as the pistons rise in the cylinders, but much of that energy comes back out as the pistons come back down so the only energy loss in the system is what's converted to heat. You would get the same effect in a gasoline engine if you shut off the fuel pump but kept the throttle wide open.

Am I sure? No, but I have been told this numerous times by actual diesel mechanics, and otherwise why was it necessary for Jacobs to invent the compression brake?

Your car also has rolling resistance and aerodynamic drag to contend with. Might work well at slowing a passenger car, not so much with a 50K lb. vehicle, hence the Jake brake.

> Your car also has rolling resistance and aerodynamic drag to contend with

I like to believe I'd recognize the difference, especially since it's a manual car and I can tell the difference between letting it roll in neutral and shifting down a gear which slows down the car :) Maybe it's not that it doesn't work on diesel cars but the effect is just less than with petrol?

Manual tranny, why didn't you say so? :-) Yeah, I guess you would notice the difference.

But I think you've hit on the difference, as I vastly simplified what is going on. Not that I expected you to read the link I gave, but it does give some explanation as to what's going on. And what's going is that the pistons are still going up and down because air is continually drawn into the engine. Air is compressed, and even though there is no fuel to make it go bang, that air still needs to uncompress and so returns a lot of the energy back to the crankshaft. Ergo, very little engine braking.

As a personal example, our diesel Sprinter van (automatic tranny, FWIW) had some degree of engine braking, but so little that I wouldn't rely on it for much more than coasting to a stoplight or other low-stakes slowing. If I'd like to stop sometime in the next day or two, I hit the brake pedal.