<font color="blue"> Isn't it hard on a DIESEL engine with a muffler? Why?" </font>
In my research on this issue, I chanced on an article by Banks Performance on their diesel brake, intended for motorhomes and such.
Basically, they said a diesel only compresses air. Air comes into the cylinder, is compressed until it's hot enough to ignite fuel, and the fuel is then injected and ignited by the hot, compressed air. When you close the throttle on a diesel, all you're shutting off is the fuel, not the air. The same amount of air enters the cylinder, is compressed and exhausted, regardless of the throttle setting. Because there is no resistance to the air, there is no engine braking.
Therefore, to provide engine braking, a diesel brake deliberately shuts off a portion of the exhuast when the throttle is closed, forcing some of the air to stay in the cylinder and provide pressure to (a) keep more air from entering; and (b) force the cylinder to slow down against the resistance of less exhaust. The limit is how much pressure the exhaust manifold and head gasket can stand - if the exhaust was closed fully, the engine would come to a stop, the tires would start screeching, and something would "pop". So, the diesel brake is calibrated to supply just less than harmful backpressure, yet enough to slow down the engine.
This doesn't hurt the diesel, because it's just air, not an air/fuel mixture, or burned combustion gases.
When the throttle is open, and there is a fuel mixture being burned, the compression is so high that all the exhaust is expelled efficiently. This isn't the case with a gas engine, especially one with more "overlap" in the cam timing.
Interesting stuff that I hadn't known before.