Valve timing is characterized by the camshaft and valve mechanism. There are two timing processes. Both of these processes involve inlet flow and exhaust interaction, but the difference between the two is the time of interaction. In SI engine typical theoretical P-V and valve timing as shown in fig.

In ideal valve timing diagram valves open/close instantaneously at dead centers. In actual case Inlet valve and Exhaust Valve open/close before and after dead centers due to mechanical factor and dynamic factor of gas flow

Valves are opened and closed by cam mechanism. Valves will bounce on its seat if closed abruptly. Opening/closing of valves spread over a certain crank angle.Therefore actual valve timing diagram is differed and shown in figure

In actual SI engines valves operation:

i) IV opens 20o before TDC

ii) IV closes 35o after BDC to take the advantage of momentum of rapidly moving gases(Ram effect)

iii) Ignition occurs 35o before TDC; this is to allow the time delay between the spark and commencement of combustion

iv) EV opens at 35o before BDC; else pressure will rise enormously and the work required to expel the gas will increase.

v) EV closes at 10o after TDC; this is to increase the volumetric efficiency.