Valve timing diagram of a four stroke engine gives a clear idea about the actual position of the piston during the opening & closing of inlet & exhaust valves. In practice, the events of the four-stroke cycle do not start and finish exactly at the two ends of the strokes - to improve the breathing and exhausting, the inlet valve is arranged to open before TDC and to close after BDC and the exhaust valve opens before BDC and closes after TDC. These early and late opening and closing events can be shown on a valve timing diagram such as Fig. 1.1-4.
Valve lead This is where a valve opens so many degrees of crankshaft rotation before either TDC or BDC.
Valve lag This is where a valve closes so many degrees of crankshaft rotation after TDC or BDC.
Valve overlap This is the condition when both the inlet and the exhaust valves are open at the same time during so many degrees of crankshaft rotation.
