Valve Timing Diagram for a Four Stroke Cycle Petrol Engine – The petrol engines are also known as spark ignition engines. The valve timing diagram for a four stroke cycle petrol engine is shown in Figure below:

The following particulars are important for a four stroke cycle petrol engine regarding valve timing diagram :

(a) The inlet valve opens (IVO) at 10° — 20° before top dead center (TDC) and closes 30° — 40° after bottom dead center (BDC).

(b) The compression of charge starts at 30° — 40° after BDC and ends at 20° — 30° before TDC.

(c) The ignition (IGN) of charge takes place at 20°— 30° before TDC.

(d) The expansion starts at 20° — 30° before TDC and ends at 30° — 50° before BDC.

(e) The exhaust valve opens (EVO) at 30° — 50° before BDC and closes at 10° —15° after TDC.

Notes:

(i) The inlet valve of a four stroke I. C. engine remains open for 230°.

(ii) The charge is compressed when both the valves (i.e. inlet valve and exhaust valve) are closed.

(iii) The charge is ignited with the help of a spark plug.

(iv) The pressure inside the engine cylinder is above the atmospheric pressure during the exhaust stroke.