Timing Diagram Of 2 Stroke Diesel Engine

The Valve Timing Diagrams of a Marine Diesel Engine is a graphical representation of the exact moments, in the sequence of operations, at which the two valves (i.e. inlet & exhaust valves) open and close as well as the firing of fuel in a 4-stroke marine diesel engine. It is, generally, expressed in terms of angular positions of the crankshaft.

In the case of 2 stroke marine diesel engines, inlet valves are replaced by scavenge ports and exhaust ports are replaced by the centrally mounted exhaust valve (Uniflow Scavenging).

Valve Timing Diagrams of 4-Stroke Marine Diesel Engine

1. Theoretical

The theoretical valve timing diagram for a 4-stroke marine diesel 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.

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2. Actual

As we have seen in the theoretical valve timing diagram, the valves open and close at the dead centre position of the piston.

But, in actual practice, they do not open and close instantaneously at dead center.

The valve operates some degrees before or after the dead center.

The ignition also occurs a little before the top dead center.

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The valve timing diagram shown is the actual valve timing diagram, where we see that the inlet valve opens before the piston reaches T.D.C. or in other words while the piston is moving up before the beginning of the suction stroke.

Now the piston reaches the T.D.C. and the suction stroke starts.

The piston reaches the B.D.C. and then starts moving up.

The inlet valve closes, when the crank has moved a little beyond the B.D.C.

This is done as the incoming air continues to flow into the cylinder although the piston is moving upwards from B.D.C. Now the air is compressed with both valves closed.

The fuel valve opens a little before the piston reaches the T.D.C. Now the fuel is injected in the form of very fine spray, into the engine cylinder, which gets ignited due to 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 the fuel is injected into the engine cylinder.

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The burnt gases (under high pressure and temperature) push the piston downwards, and the expansion or working stroke takes place. Now the exhaust valves open before the piston again reaches B.D.C. and the burnt gases start leaving the engine cylinder.

Now the piston reaches B.D.C and then starts moving up thus performing the exhaust stroke. The inlet valve opens before the piston reaches T.D.C. to start a suction stroke. This is done as the fresh air helps in pushing out the burnt gases.

Now the piston again reaches T.D.C and the suction starts. The exhaust valve closes when the crank has moved a little beyond the T.D.C. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards.

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Valve Timing Diagrams of 2-Stroke Marine Diesel Engine

1. Theoretical

The theoretical valve timing diagram for a two-stroke cycle engine is shown in fig.

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 120 deg. and the piston moves from T.D.C. towards B.D.C.

At B, both the valves open and suction, as well as exhaust, take place from B to C.

The crankshaft revolves through approximately 120 deg. and the piston moves first to BDC and then little upwards.

At C, both the valves close and compression takes place from C to A. The crankshaft revolves through approximately 120 deg and the piston moves to B.D.C.

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2. Actual

As like as four-stroke cycle engines, the actual valve timing diagram of the two-stroke cycle engine is also different from its theoretical valve timing diagram.

The actual valve timing diagram of the two-stroke cycle engine is shown in fig.

As we see that the expansion of the charge (after ignition) starts as the piston moves T.D.C. towards B.D.C.

First of all, the exhaust port/exhaust valve opens before the piston reaches B.D.C. and the burnt gases start leaving the cylinder.

After a small fraction of the crank revolution, the transfer port/scavenge ports also open 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 B.D.C. and then starts moving upwards.

As the crank moves a little beyond B.D.C first the transfer port/scavenge ports close 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.

Fuel valve opens a little before the piston reaches the T.D.C.

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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 T.D.C.

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.

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