The fuel system doesn't provide restriction on the air going through the engine. A diesel engine that's not dumping in fuel (because your foot isn't on the ped

The fuel system doesn't provide restriction on the air going through the engine. A diesel engine that's not dumping in fuel (because your foot isn't on the pedal) has about as much engine braking as a gas engine that's run out of fuel but the operator has floored the pedal. A gas engine has a throttle that can restrict airflow. A diesel can either be equipped with an exhaust brake or compression brake. The latter is tons more effective but louder.