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 provi

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.