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VALUE TIMING DIAGRAM OF A FOUR STROKE, OTTO CYCLE ENGINE

 

VALUE TIMING DIAGRAM OF A FOUR STROKE, OTTO CYCLE ENGINE

It has already been explained in the operation of four stroke, Otto cycle engine that the inlet valve opens during the suction stroke and the exhaust stroke and the exhaust valve opens during the exhaust stroke. The exact moment at which each of the valves opens and closes with reference to the position of the piston and crank can be show graphically in a diagram. This diagram is known as valve timing diagram.

Theoretically, the inlet valve opens exactly at the beginning of suction stroke and closes at the end of the stroke. Both the valves close during power and compression stroke. The exhaust valve opens exactly at the beginning of exhaust stroke and closes at the end of the stroke. The theoretical valve timing diagram for a four stroke Otto cycle engine in shown in diagram. The opening and closing valves with reference to the position of piston and crank shaft during the four strokes are described as follow.


1.      Suction stroke. The inlet valve opens. The piston starts to move downward from the top dead centre (T.D.C) position and reaches to bottom dead centre (B.D.C) position. Fresh charge of air-fuel mix enters the cylinder. The exhaust valve remains closed.

 

2.      Compression stroke. Both the valves remain closed. The piston starts to move upward from B.D.C  thus comprising the charge till it reaches the T.D.C.


3.      Working stroke. Both the valves remain closed. Sparking takes place from spark plug which ignites the compressing charge. The piston moves downward from T.D.C and reaches the B.D.C.


4.      Exhaust Stroke. The exhaust valve opens. The piston moves from B.D.C and reaches to T.D.C. thus rushing out the burnt gases from the cylinder. The inlet valve remains close.

 

In actual practice the above cycle slightly modified. the exact moments of opening and closing the valves with reference to the piston and crankshaft are show in graphically. this diagram is known as actual valve timing diagram. opening and closing of inlet and exhaust valves are  described as follows.


Inlet valve. The inlet valve start opening 10 degree to 30 degree before T.D.C as measure in degrees of crankshaft rotation.

 It remains open during 180 degree of the normal suction stroke and in addition 30 degree to 40 degree or even 60 degree after B.D.C  in the beginning of the compression stroke. the reason of opening the inlet Valve before the start of suction of stroke is that the valve is made to open and close very slowly and this timing of opening the valve is necessary to permit it to be opens sufficiently during the suction stroke. The valves are made to open and close slowly to provide silent operation under high speed conditions.

 

 

Exhaust valve. The exhaust valve start opening 30 degree to 60 degree before B.D.C remains open during 180 degrees of the normal exhaust stroke and in addition 8 degree to 10 degree or even 25 after degree after T.D.C in the beginning of the suction stroke. The reason of opening the exhaust valve before the start of the exhaust stroke is that the gases have an outlet for expansion and begin to rush of their own pressure. This removes the greater part of the gases producing the amount of work to be done by the pistol on its return stroke. This reduction compensate the waste due to early release of gases.

 

 

 

 

 

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