Valve Timing Diagram
What is the Valve timing Diagram?
- Valve timing is the regulation of the points in the cycle at which the valves are set to open and close.
- A valve timing diagram is a graphical representation of the exact moments, in the sequence of operations, at which the two valves (i.e. inlet and exhaust valves) open and close as well as the firing of the fuel. It is, generally, expressed in terms of angular positions of the crankshaft. Here we shall discuss theoretical valve timing diagrams for four-stroke and two-stroke cycle engines.
- In the ideal cycle inlet and exhaust valves open and close at dead centers, but in actual cycles they open or close before or after dead centers as explained below.
Theoretical Vs Actual Valve Timing Diagram :
There are two factors, one mechanical and other dynamics, for the actual valve timing to be different from the theoretical valve timing.
(a) Mechanical factor.
The poppet valves of the reciprocating engines are opened and closed by cam mechanisms. The clearance between cam, tappet, and valve must be slowly taken up and the valve slowly lifted, at first, if noise and wear is to be avoided. For the same reasons the valve cannot be closed abruptly, else it will 'bounce' on its seat. (Also the cam contours should be so designed as to produce gradual and smooth changes in directional acceleration). Thus the valve opening and closing periods are spread over a considerable number of crankshaft degrees. As a result, the opening of the valve must commence ahead of the time at which it is fully opened (i.e., before dead centers).
b) Dynamic factor;
Besides the mechanical factor of opening and closing of valves, the actual valve timing is set taking into consideration the dynamic effects of gas flow.
Theoretical valve timing diagram :
1. Theoretical valve timing diagram for four-stroke cycle engine
- The theoretical valve timing diagram for a four-stroke cycle engine is shown In this diagram, the inlet valve opens at A and the suction takes place from A to B.
- The crankshaft revolves through 180º and the piston moves from T.D.C. to B.D.C. At B, the inlet valve closes and the compression takes place from B to C.
- The crankshaft revolves through 180º and the piston moves from B.D. C. to T.D. C. At C, the fuel is fired and the expansion takes place from C to D.
- The crankshaft revolves through 180º and the piston again moves from T.D.C. to B.D.C. At D, the exhaust valve opens and the exhaust takes place from D to E. The crankshaft again revolves through 180º and the piston moves back to T.D.C.
2. Theoretical valve timing diagram for the two-stroke cycle engine.
- The theoretical valve timing diagram for a two-stroke cycle engine is shown. In this diagram, the fuel is fired at A and the expansion of gases takes place from A to B.
- The crankshaft revolves through approximately 120º and the piston moves from T.D.C. towards B.D.C. At B, the valves open, and suction, as well as exhaust, take place from B to C.
- The crankshaft revolves through approximately 120º and the piston moves first to B.D.C and then little upwards. At C. both the valves close and compression takes place from C to A. The crankshaft revolves through approximately 120º and the piston moves to T.D.C
Valve Timing Of Four-Stroke Petrol /Spark Ignition Engine. ( SI Engine)
Or we can represent valve timing as shown in below,
- The valve timing diagram, as shown we see that the inlet valve opens before the piston reaches TDC or in other words, while the piston is still moving up before the beginning of the suction stroke.
- Now the piston reaches the TDC and the suction stroke starts. The piston reaches the BDC and then starts moving up. The inlet valve closes, when the crank has moved a little beyond the BDC This is done as the incoming charge continues to flow into the cylinder although the piston is moving upwards from BDC Now the charge is compressed (with both valves closed) and then and temperature) push the piston downwards with full force and the expansion or working stroke takes place.
- Now the exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder. Now the piston reaches BDC and then starts moving up, thus performing the exhaust stroke.
- The inlet valve opens before the piston reaches TDC to start suction stroke. This is done as the fresh incoming charge helps in pushing out the burnt gases.
- Now the piston again reaches TDC, and the suction stroke starts. The exit valve closes after the crank has moved a little beyond the TDC. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards. It may be noted that for a small fraction of a crank revolution, both the inlet and outlet valves are open. This is known as valve overlap.
Valve Timing Diagram For A Four Stroke Cycle Diesel Engine :
- In the valve timing diagram as shown we see that the inlet valve opens before the piston reaches TDC; or in other words while the piston is still moving up before the beginning of the suction stroke.
- Now the piston reaches the TDC and the suction stroke starts. The piston reaches the BDC and then starts moving up. The inlet valve closes, when the crank has moved a little beyond the BDC.
- This is done as the incoming air continues to flow into the cylinder although the piston is moving upwards from BDC. Now the air is compressed with both valves closed. The fuel valve opens a little before the piston reaches the TDC. Now the fuel is injected in the form of very fine spray, into the engine cylinder, which gets ignited due to the high temperature of the compressed air. The fuel valve closes after the piston has come down a little from the TDC. This is done as the required quantity of fuel is injected into the engine cylinder. The burnt gases (under high pressure and temperature) push the piston downwards, and the expansion or working stroke takes place.
- Now the exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder.
- Now the piston reaches BDC and then starts moving up thus performing the exhaust stroke. The inlet valve opens before the piston reaches TDC to start suction stroke. This is done as the fresh air helps in pushing out the burnt gases.
- Now the piston again reaches TDC, and the suction starts. The exhaust valve closes when the crank has moved a little beyond the TDC. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards.
Valve timing Diagram For Two Stroke Petrol / SI Engine : ( Port Timing Diagram For SI Engine )
- In the valve timing diagram, as shown we see that the expansion of the charge (after ignition) starts as the piston moves from TDC towards BDC.
- First of all, the exhaust port opens a fraction of the crank revolution, the transfer port also opens and the fresh fuel-air mixture enters into the engine cylinder. This is done as the fresh incoming charge helps in pushing out the burnt gases.
- Now the piston reaches BDC and then starts moving upwards. As the crank moves a little beyond BDC, first the transfer port closes and then the exhaust port also closes. This is done to suck fresh charge through the transfer port and to exhaust the burnt gases through the exhaust port simultaneously.
- Now the charge is compressed with both ports closed and then ignited with the help of a spark plug before the end of the compression stroke. This is done as the charge requires some time to ignite. By the time the piston reaches TDC, the burnt gases (under high pressure and temperature) push the piston downwards with full force, and the expansion of the burnt gases takes place.
- It may be noted that the exhaust and transfer ports open and close at equal angles on either side of the BDC position.
Valve Timing Diagram For A Two-Stroke Diesel Engine : ( Port Timing Diagram For CI Engine )
- In the valve timing diagram, as shown, we see that the expansion of the charge (after ignition) starts as the piston moves from TDC towards BDC. First of all, the exhaust port opens before the piston reaches BDC and the burnt gases start leaving the cylinder. After a small fraction of the crank revolution, the transfer port also opens and the fresh air enters into the engine cylinder. This is done as the fresh incoming air helps in pushing out the burnt gases.
- Now the piston reaches BDC and then starts moving upwards. As the crank moves a little beyond BDC, first the transfer port closes and then the exhaust port also closes. This is done to suck fresh air through the transfer port and to exhaust the burnt gases through the exhaust port simultaneously.
- Now the charge is compressed with both the ports closed. The fuel valve opens a little before the piston reaches the TDC.
- Now the fuel is injected in the form of very fine spray into the engine cylinder, which gets ignited due to the high temperature of the compressed air. The fuel valve closes after the piston has come down a little from the TDC. 'This is done as the required quantity of fuel is injected into the engine cylinder.
- Now the burnt gases (under high pressure and temperature) push the piston downwards with full force and expansion of the gases takes place. It may be noted that in a two-stroke cycle diesel engine, like a two-stroke petrol engine, the exhaust and transfer ports open and close at equal angles on either side of the BDC position.
Difference Between Actual Valve Diagram and Theoretical Valve Timing
| Sr. No. | Actual Valve Diagram | Theoretical Valve Timing |
|---|---|---|
| 1. | The inlet valve starts opening 10 degrees to 30 degrees before the beginning of suction stroke (TDC) and closes after 30 degrees to 40 degrees at the end of the stroke (BDC) | The inlet valve opens exactly at the beginning of suction stroke (TDC) and closes at the end of the stroke (BDC) |
| 2. | The exhaust valve starts opening 30 degrees to 60 degrees before the beginning of exhaust stroke (BDC) and closes after 8 degrees to 10 degrees at the end of the stroke (TDC) | The exhaust valve opens exactly at the beginning of exhaust stroke (BDC) and closes at the end of the stroke (TDC) |
| 3. | The inertia of the valve operating mechanism is considered | Inertia of the valve operating mechanism is not considered |
| 4. | Time for the charge to fill completely into the cylinder is considered | Time for the charge to fill completely into the cylinder is not considered |
| 5. | Time for the exhaust gases to escape out of the cylinder is considered | Time for the exhaust gases to escape out of the cylinder is not considered |
| 6. | The inlet valve is closed when the piston reaches a point in its next stroke at which the pressure in the cylinder equals the pressure outside. | The inlet valve is closed when the piston reaches TDC |
| 7. | The valves are opened or closed slowly. | The valves are closed or opened instantaneously |
| 8. | There is valve overlap. | There is no valve overlap. |
Other Posts Related to Valve Timing :
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Theoretical and Actual valve timing diagram For Four Stroke SI Engine
IC Engine Articles , Notes , Question and Answer
Mechanical Subjectwise Basic Concept Notes ,Articles
Sachin Thorat
Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.
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