VALVE TIMING DIAGRAM OF A FOUR-STROKE, OTTO CYCLE ENGINE

Hello friend's in this post we will talk about valve 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 valve opens during the exhaust stroke. The exact moment at winch each of the valves opens and closes with reference to the position of piston and crank can be shown 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 remain closed during compression and power strokes. The exhaust valve opens exactly at the beginning of exhaust stroke and closes at the end of stroke. The theoretical valve timing diagram for a four-stroke Otto cycle engine is shown in Fig. 3.3. The opening and closing of valves with reference to the position of piston and crank shaft during the four strokes are described as follows:

1. Suction stroke. The inlet valve opens. The piston starts to move downward from top dead centre (T.D.C.) position and reaches to bottom dead centre (B.D.C.) position. Fresh charge of air-fuel mixture 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 compressing the charge till it reaches the T.D.C. 

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

In actual practice, the above cycle is slightly modified. The exact moments of opening and closing the valves with reference to the piston and crankshaft are shown graphically in Fig. 3.4. This diagram is known as actual valve timing diagram. The opening and closing of inlet and exhaust valves are described as follows:

Inlet valve. The ialet valve starts opening 10" to 30 before T.D.C. as measured in degrees of crankshaft rolation. It remans open during 180 of the normal suction stroke and, in addition, 30 to 40° or even 60 afler B.D.C, in the begining of the compression stroke. The reason of opening the inlet valve before the start of 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 suficiently during the suetion stroke. The valves are made O open and elose slewly to provide silent oporation under high speed conditions. The column of charge in the inlet pipe requires to be accelerated before the suction stroke starts, so that sufticient charge may enter the eylinder during the suction stroke. 

The reason of remaining open the inlet valve after the suction stroke is also important. As the piston moves downward during the suction stroke, the pressure decreases inside the eylinder which causes the gases to nush in and fill up the space above the piston. The piston reaches at 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 eylinder 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. The exhaust valve starts opening 30° to 60° before B.D.C., remains open during 180° of the normal exhaust stroke, and in addition, 8° to 10° or even 25° 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 reducing the amount of work to the waste due to early release of gases. During the next outward stroke, the remaining gases are forced out through the open exhaust valve. This causes a slight compression of the gases ahead of the piston. When it reaches to T.D.C. position, there will be a certain amount of compressed exhaust gases in the clearance space. If the exhaust valve is closed at this point, this amount of the exhaust gases will remain in the eylinder. Thus, the exhaust valve is closed a little after, the end of the exhaust stroke. It may result in drawing the exhaust gases back into the cylinder. But this drawing back is prevented by two conditions: done by the piston on its return stroke. This reduction compensates.

1. Gases under compression exceed the pressure in the manifold and will continue to flow out because of this difference in pressure. 

2. The piston, while at the top of the stroke, moves but very little for 10° to 15° movement of the crankshaft. This does not materially increases combustion space as shown in Fig. 3.5.

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 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 top dead centre. 

Fig. 3.6 shows timing data of inlet valve for a popular engine. In this engine, the inlet valve starts to open 5° before top dead centre. 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 total inlet valve opening of 229° of crankshaft rotation. 

Fig. 3.7 shows timing data of exhaust valve for a popular engine. In this engine, the exhaust valve opens 47° before bottom bead centre. 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 total exhaust valve opening of 239° of crankshaft rotation.