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Two Stroke Cycle Diesel Engine

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Four Stroke Cycle Compression Ignition (Diesel) Engine
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Two Stroke Cycle Diesel Engine
Comparison of Two and Four Stroke Cycle Diesel Engine
Comparison of S.I. and C.I. Engine
Piston Displacement or Swept Volume
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Compression Ratio

The pump scavenge two stroke diesel engine designed by Sir Dugald Clerk in 1879 was the first successful two-stroke engine; thus the two-stroke-cycle engine is sometimes called the Clerk engine. Uniflow scavenging took place - fresh charge entering the combustion chamber above the piston while the exhaust outflow occurred through ports uncovered by the piston at its outermost position.

Low- and medium-speed two-stroke marine diesels engines still use this system, but high-speed two-stroke diesel engines reverse the scavenging flow by blowing fresh charge through the bottom inlet ports, sweeping up through the cylinder and out of the exhaust ports in the cylinder head (Fig. 1.1-9(a)).

Internal Combustion Engine

With the two-stroke diesel engine, intake and exhaust phases take place during part of the compression and power stroke respectively, so that a cycle of operation is completed in one crankshaft revolution or two piston strokes. Since there are no separate intake and exhaust strokes, a blower is necessary to pump air into the cylinder for expelling the exhaust gases and to supply the cylinder with fresh air for combustion.

Scavenging (induction and exhaust) phase (Fig. 1.1-9(a)) The piston moves away from the cylinder head and, when it is about half-way down its stroke, the exhaust valves open. This allows the burnt gases to escape into the atmosphere. Near the end of the power stroke, a horizontal row of inlet air ports is uncovered by the piston lands (Fig. 1.1-9(a)). These ports admit pressurized air from the blower into the cylinder. The space above the piston is immediately filled with air, which now blows up the cylinder towards the exhaust valves in the cylinder head. The last remaining exhaust gases will thus be forced out of the cylinder into the exhaust system. This process of fresh air coming into the cylinder and pushing out unwanted burnt gas is known as scavenging.

Compression phase (Fig. 1.1-9(b)) Towards the end of the power stroke, the inlet ports will be uncovered. The piston then reaches its outermost position and reverses its direction of motion. The piston now moves upwards so that the piston seals and closes the inlet air ports, and just a little later the exhaust valves close. Any further upward movement will now compress the trapped air (Fig. 1.1-9(b)). This air charge is now reduced to about 1/15 to 1/18 of its original volume as the piston reaches the innermost position. This change in volume corresponds to a maximum cylinder pressure of about 30-40 bar. Power phase (Fig. 1.1-9(c)) Shortly before the piston reaches the innermost position to the cylinder head on its upward compression stroke, highly pressurized liquid fuel is sprayed into the dense intensely heated air charge (Fig. 1.1-9(c)). Within a very short period of time, the injected fuel droplets will vaporize and ignite, and rapid burning will be established by the time the piston is at the top of its stroke. The heat liberated from the charge will be converted mainly into gas-pressure energy which will expand the gas and so do useful work in driving the piston outwards.

An overall view of the various phases of operation in a two-stroke-cycle three-cylinder diesel engine is shown in Figs. 1.1-9(d), and Figs. 1.1-9(e) and (f) show the cycle of events in one crankshaft revolution expressed in terms of piston displacement and cylinder pressure.