The valve timing diagram, as shown we see that the inlet valve opens before the piston reaches TDC or in other words, while the piston is still moving up before the beginning of the suction stroke.Or we can represent valve timing as shown in below, Valve timing diagram for 4 stroke SI Engine diagram ( SI Engine) Actual Valve Timing Diagram For Four Stroke petrol Engine Valve Timing Of Four-Stroke Petrol /Spark Ignition Engine. Theoretical valve timing diagram for two-stroke cycle engine The crankshaft revolves through approximately 120º and the piston moves to T.D.C both the valves close and compression takes place from C to A. The crankshaft revolves through approximately 120º and the piston moves first to B.D.C and then little upwards.At B, the valves open, and suction, as well as exhaust, take place from B to C. The crankshaft revolves through approximately 120º and the piston moves from T.D.C.In this diagram, the fuel is fired at A and the expansion of gases takes place from A to B. The theoretical valve timing diagram for a two-stroke cycle engine is shown.Theoretical valve timing diagram for the two-stroke cycle engine. Theoretical valve timing diagram for four-stroke cycle engineĢ. The crankshaft again revolves through 180º and the piston moves back to T.D.C. At D, the exhaust valve opens and the exhaust takes place from D to E. The crankshaft revolves through 180º and the piston again moves from T.D.C.At C, the fuel is fired and the expansion takes place from C to D. The crankshaft revolves through 180º and the piston moves from B.D.At B, the inlet valve closes and the compression takes place from B to C. The crankshaft revolves through 180º and the piston moves from T.D.C. ![]() The theoretical valve timing diagram for a four-stroke cycle engine is shown In this diagram, the inlet valve opens at A and the suction takes place from A to B.Theoretical valve timing diagram for four-stroke cycle engine As a result, the opening of the valve must commence ahead of the time at which it is fully opened (i.e., before dead centers).īesides the mechanical factor of opening and closing of valves, the actual valve timing is set taking into consideration the dynamic effects of gas flow. Thus the valve opening and closing periods are spread over a considerable number of crankshaft degrees. (Also the cam contours should be so designed as to produce gradual and smooth changes in directional acceleration). For the same reasons the valve cannot be closed abruptly, else it will ‘bounce’ on its seat. The clearance between cam, tappet, and valve must be slowly taken up and the valve slowly lifted, at first, if noise and wear is to be avoided. The poppet valves of the reciprocating engines are opened and closed by cam mechanisms. There are two factors, one mechanical and other dynamics, for the actual valve timing to be different from the theoretical valve timing. Theoretical Vs Actual Valve Timing Diagram : ![]()
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