Theoretically above cycle is well perfect but in actual practice,it is slightly modified by the opening of inlet valve and delayed closing of exhaust valve.The details are as below.
The inlet valve is opened 10 to 30 degree in advance to the top dead centre of the piston to facilitate the inrush of fresh charge and out rush of burnt gases.
The piston moves down during suction stroke which is continued up to 30 to 40 degree or even 60 degree after the bottom dead centre.The inlet valve is then closed and compression stroke starts.
To give some extra time to fuel to burn,the spark is produced at 30 to 40 degree before the top dead centre of piston.The pressure rises up and attains a maximum value when the piston is about 10 degree past to top dead centre.
The exhaust valve is open about 30 to 60 degree before piston reaches to bottom dead centre.The burnt exhaust gases pushed out of cylinder as the piston starts moving upward.This exhaust stroke continuous till the exhaust valve closed when the piston is about 8 to 10 degree or even 25 degree past the top dead centre.
The angle between the position of the crank at the inlet valve opening and that exhaust valve closing is known as valve overlap.
All this angular positions of crank can be plotted by a circular line corresponding to one vertical line;where top dead center can be taken at top of the line and bottom dead center at bottom of the vertical line.
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