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 of 4 stroke engine
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
During the exhaust stroke, the gas pressure in the cylinder will fall from the exhaust-valve opening pressure (which may vary from 2 to 5 bar, depending on the engine speed and the throttle-opening position) to atmospheric pressure or even less as the piston nears the innermost position towards the cylinder head.
Cycle of events in a four-cylinder engine – induction, compression, power, and exhaust – is phased in a four-cylinder engine. The relationship between cylinder pressure and piston stroke position over the four strokes is clearly , it can be seen that a figures of eight is repeatedly being traced.