Bill, Longston is absolutely correct with his discription as to how a diesel engine works. Now as to why you don't need electric (alternator) to keep your diesel engine running, your engine has a mechanical gear driven fuel injection fuel pump. As long as engine is running, this pump is working. Inside this pump is a control rod which either allows fuel in the pump or shuts off the fuel. On some diesel engines this control rod itself is hooked up to a steel cable, electric shutoff solenoid, or a vacuum solenoid. On your MB, it is vacuum controlled. Diesel engines do not make vacuum and if they use vacuum controls, they must have a vacuum pump. With todays diesels, things are controlled by electric. Electric controls, injectors, and computers all in the name of EPA and progress.