When starting up, diesel engine needs approximately 5s before turning the key for ignition. Why is this neccessary and what can be damaged if you don't do that?
Since diesel engines rely on compression to run and not a spark like a gasoline engine, heat is directly involved in the combustion of the diesel in the combustion chamber. The way this occurs is that when the piston comes up it compresses the air which is in the cylinder. This creates heat. When the engine is cold, a lot of this heat gets transferred out of the combustion chamber and into the cylinder head, pistons, and cylinder walls. Since the engine needs this heat to run, it has to get it from somewhere else. When you first turn the ignition on, there are little parts called glow plugs. These heat the combustion chamber and gets them ready so combustion can occur. The five seconds is required to give the glow plugs time to do their thing. If this didn't occur, you could potentially flood the engine with diesel and it would be a really hard time starting it after that. No real damage would occur, but how long it would take to start (if it ever did start) would be forever (well, it would probably seem that way).