valve timing diagram for four strock S. I. Engine

     The exact moment at which the inlet and outlet valve opens and closes with reference to the position of piston and crank shown diagrammatically is known as valve timing Diagram . the timing  is expressed in terms of degrees of crank rotation.

SUCTION STROKE : 
     inlet valve is open . piston moves from the top dead center (TDS) to bottom dead center (BDC) . Air -fuel mix is sucked in by negative pressure in cylinder.

COMPRESSION STROKE : 
     Inlet and Outlet valves closed . Piston moves upwards from BDC to TDC. Air -fuel mix is compressed.

expansion / power stroke :
     inlet and outlet remains closed here also. piston moves from down from TDC to BDC. This happens as a result of ignition of the mixture inside the cylinder. Ignition is started by spark plug.