In the petrol engine, various strokes are performed to obtain the results from an engine. By denoting the corresponding position of the piston attached to the crankshaft at 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 in automobile engineering. The valve timing diagram is modified to set better charging and exhausting performance as there is always a difference between theory and practice.
The valve timing diagram is drawn for two complete revolutions of the crankshaft means for one complete cycle.
Piston starts moving from top dead center resulting opening of inlet valve. The fresh charge of air-fuel mixture enters in cylinder. Piston moves further to bottom dead center.
02) Compression Stroke
Inlet and exhaust valves are in close position and piston starts moving upward from bottom dead center to top dead center there by compression of charge took place.
03) Expansion Stroke
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 takes place which is followed by expansion of hot gases pushing the piston to its bottom dead center. In this stroke the useful work is obtained from the engine that is why, it is also called as working stroke.
04) Exhaust Stroke
Piston starts moving upward from bottom dead center, resulting the burnt gases are pushed out through the exhaust valve till it reaches to top dead center. 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 an inlet valve and delayed closing of an exhaust valve. The details are as below.(valve timing diagram)
Valve Timing Diagram
The inlet valve is opened 10 to 30 degree in advance to the top dead center of the piston to facilitate the inrush of fresh charge and out a rush of burnt gases.The piston moves down during suction stroke which is continued up to 30 to 40 degree or even 60 degrees after the bottom dead center. 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 center of the piston. The pressure rises up and attains a maximum value when the piston is about 10 degrees past to top dead center.
The exhaust valve is open about 30 to 60 degrees before the piston reaches to bottom dead center. The burnt exhaust gases pushed out of the 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 degrees past the top dead center.
The angle between the position of the crank at the inlet valve opening and that exhaust valve closing is known as valve overlap.
All these angular positions of the crank can be plotted by a circular line corresponding to one vertical line; where the top dead center can be taken at top of the line and bottom dead center at bottom of the vertical line.(As shown above)
I hope, this information about valve timing diagram will clear your all doubt.If still, you have any doubt about valve timing diagram, put your question in a comment.