Valve timing diagram for four-stroke petrol engine

In the valve timing diagram, the valves open and close at the dead center positions of the piston. But, in actual practice, they do not open and close instantaneously at dead centers. The valves operate to some degree before or after the dead centers. The ignition is also timed to occur a little before the top dead center. The timing of these sequences of events can be shown graphically in terms of crank angles from the dead center position. This diagram is known as the valve timing diagram.