Here comes a small repetition in the four-stroke engine, the Otto engine as well as the Diesel engine, all to make it easier to understand Hydrogen as the fuel in an engine with these principles as a basis.     In a four-stroke engine, the entire working process takes place in four strokes, one stroke is the piston moving from one dead center to another, for example from top dead center to bottom dead center or vice versa. The term dead center means that when the piston reaches its bottom position in the cylinder, or top, it slows down so much that it is usually called standing still before it reverses direction and goes in the other direction, it reaches its dead center and in the different strokes, different things happen. Top dead center is sometimes abbreviated as TDC and bottom dead center as BDC. In one working cycle, the crankshaft rotates two revolutions, the camshaft rotates one. At least one inlet valve and one outlet valve are required for each cylinder. A spark plug is required to ignite the fuel-air mixture.