Theoretical Valve Timing-
Theoretically, the inlet valve opens exactly at the beginning of the suction stroke and closes at the end of the stroke. Both the valves remain closed during compression and power strokes. The exhaust valve opens exactly at the beginning of the exhaust stroke and closes at the end of the stroke. The theoretical valve timing diagram for a four-stroke Otto cycle engine is shown in Fig. 3.3. The opening and closing of valve reference to the position of the piston and crankshaft during the four strokes are described as follows:
Suction Stroke-
The inlet valve opens. The piston starts to move down from the top dead (T.D.C.) position and reaches the bottom dead center (B.D.C.) position. A fresh charge of air fuel enters the cylinder. The exhaust valve remains closed. The crankshaft rotates 180 degrees.
Compression stroke-
Both the valves remain closed. The piston starts to move upward thus compressing the charge till it reaches the T.D.C. This time the pressure and the temperature are increased in the cylinder. The piston compresses the fresh charge. This time crankshaft rotates 180 degrees.
Power stroke-
Both valves remain closed in this stroke. Sparking takes place from the spark plug which is the compressed charge. The piston moves downward from T.D.C. and reaches B.D.C. A sudden thrust is placed on the top of the piston and the adiabatic process starts. The pressure is much higher in this stroke compared to the atmospheric pressure. This time also crankshaft rotates 180 degrees.
Exhaust stroke-
In actual practice, the above cycle is slightly modified. The exact moments of opening and closing valves with reference to the piston and crankshaft are shown graphically in the figure. This diagram is an actual valve timing diagram. The opening and closing of the inlet and exhaust valves are described as follows:
Actual Valve Timing Diagram-
Inlet valve-
The inlet valve starts opening 10 degrees to 30 degrees before T.D.C. as measured in degrees of crankshaft rotation. It remains open during 180″ of the normal suction stroke and, in addition, 30° to 40° or even 60° after B.D.C., at the beginning of the compression stroke. The reason for opening the inlet valve before the start of the suction 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 open sufficiently during the suction stroke. The valves are made to open and close slowly to provide silent operation under high-speed conditions.
The column of charge in the inlet pipe requires to be accelerated before the suction stroke starts, so that sufficient charge may enter the cylinder during the suction stroke.
The reason for remaining open the inlet valve after the suction stroke is also important. As the piston moves down and during the suction stroke, the pressure decreases inside the cylinder which causes the gases to rush in and fill up the space above the piston. The piston reaches the end of the stroke before a complete charge has time to enter through the small inlet valve opening. Therefore, pressure in the combustion space will still be below atmospheric, and the gases will be moving in the direction of the motion of the piston with high velocity. If the inlet valve is closed at this point so that no more charge enters, less charge will remain in the cylinder. Thus, the inlet valve is made to remain open until the piston reaches a point in its next stroke at which the pressure in the cylinder equals the pressure outside. Also. the actual closing point of the valve coincides with the point when the motion of the rushing charge would reverse the direction.
Exhaust valve-
1. Gases under compression exceed the pressure in the manifold and will continue to flow out because of this difference in pressure.
When the crankarms are in a position as at A, for a certain number of degrees, say 15° movement of the crankshaft, the piston will move upward for a considerable distance. When the crankarms are as at B, for the same 15° turning of the crankshaft, the distance moved by the piston will be less. When the crankarms are as at C, for the same 15° turning, there is a very little upward movement of the piston. It can be seen that between certain points there is practically no motion
of the piston. Travel in this region is called the rock of the piston. Within this region usually, the exhaust valve is closed after the top dead center.
Timing Data of Valves-
Inlet valve-
This shows the timing data of the inlet valve for a popular engine. In this engine, the inlet valve starts to open 5° before the top dead center. This pre-admission allows the valve to be open during 5° of the exhaust stroke. It remains open during the 180° of the normal suction stroke, and in addition, during 44° of the beginning of the compression stroke. This gives a total inlet valve opening of 229° of crankshaft rotation.
Exhaust valve-
This shows the timing data of the exhaust valve for a popular engine. In this engine, the exhaust valve opens 47° before the bottom bead center. This pre-release causes the valve to be open during the last 47° of the power stroke. It remains open during the 180° of the normal exhaust stroke, and in addition, during 12° of the beginning of the suction stroke. This gives a total exhaust valve opening of 239° of crankshaft rotation.