In petrol engine various strokes are performed to obtained the results from engine.By denoting the corresponding position of the piston attached to the crankshaftat which these strokes occurs,we can draw the exact moment in the sequence of events at which the valves are open and close.The graphical diagrams are known as valve timing diagram.The valve timing are modified to set better charging and exhausting performance as there is always a difference between theory and practical.
The diagram is drawn for two complete revolution of the crankshaft means for one complete cycle.
Following strokes are performed in four stroke petrol engine .
Piston starts moving from top dead centre resulting opening of inlet valve.The fresh charge of air-fuel mixture enters in cylinder.Piston moves further to bottom dead centre.
Inlet and exhaust valves are in close position and piston starts moving upward from bottom dead centre to top dead centre there by compression of charge took place.
The ignition is started by spark plug just before the end of compression stroke.Both inlet and exhaust valves are still closed.A rapid explosion take place which is followed by expansion of hot gases pushing the piston to its bottom dead centre.In this stroke the useful work is obtained from the engine that is why ,it is also called as working stroke.
Piston starts moving upward from bottom dead centre resulting the burnt gases are pushed out through the exhaust valve till it reaches to top dead centre.Up to this point inlet valve remain closed.when the burnt gases totally exhausted to atmosphere, piston starts moving down,inlet valve opens,fresh charge is sucked and the cycle is again repeated as earlier.
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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