Valve Timing Diagram of Petrol Engine

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


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.
Note:-
(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.

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