In a two-stroke engine, the cycle is completed in two strokes only or in one complete rotation of crankshaft. For completing the cycle in two strokes, suction and exhaust strokes are eliminated and these processes are combined with compression and exhaust strokes in operation.
Two-Stroke SI Engine:
A two-stroke SI engine consists of a cylinder with one end fitted with a cover for the placement of spark plug and the other fitted with a sealed crank case which will act as leak-proof pump during operation. The cylinder is provided with inlet port, transfer port, and exhaust port. These ports are opened and closed by the movement of the piston only. The exhaust port is located slightly above the transfer port and diametrically opposite. Let us study the operation sequence wise.
First Stroke:
In the beginning of the first stroke, the piston is at TDC and compressed charge, i.e., the mixture of petrol vapor and air, is available in combustion chamber. In this position, only the inlet port is kept opened and other two ports are closed. The mixture of air and petrol vapor is sucked into the crank case due to the vacuum created by the rotation of crank in crank case.
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This is due to the upward movement of the piston when compression is in progress. At this position, the spark plug ignites the compressed charge. The combustion releases hot gas which increases pressure in the cylinder and a force is given on the piston. The piston starts moving downward (Fig. 4.35).
Second Stroke:
During the downward travel of the piston, the inlet port is also closed due to which the mixture gets compressed in side crank case. The exhaust port gets opened at about 70-75° from BDC and burnt gases are exhausted to the atmosphere.
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At about 60-65° from BDC, the transfer port opens due to which the compressed mixture from crank case enters the cylinder. The piston is given a special shape. This is known as crowning of piston. This has the advantage that the unburnt charge is deviated from going out directly with burnt gases through the opening of exhaust port. During this process, there is every possibility that some unburnt charges may go with the burnt gases. This is a great loss in SI engine, i.e., the wastage of fuel.
Incoming fresh charge through transfer port also helps in driving out the exhaust gases. The driving out of exhaust gases by incoming fresh charge is known as scavenging. Thus, during the downward motion of the piston, the power stroke is completed and the crankshaft rotates by half revolution. The process is repeated again and again.
Two-Stroke CI Engine:
The operation of two-stroke CI engine is similar to two-stroke SI engine. In a two- stroke diesel engine, the cylinder is provided with inlet port, transfer port, and exhaust port. All the ports are closed and opened by the movement of the piston.
First Stroke:
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In the beginning of the first stroke, the piston occupies TDC position. At this position, only the inlet port is open and the other two ports are kept closed. The amount of air is drawn into the crank case from atmosphere due to the vacuum created due to upward movement of the piston.
At the end of the compression when the piston occupies TDC, the pressure and temperature of the air become high. The temperature achieved after compression becomes more than the ignition temperature of diesel due to high compression ratio (11-22). At that moment, diesel is injected in an atomized form into high-temperature compressed air.
The supply of diesel is made by the injector which is fitted in cylinder head. As soon as the diesel particles come in contact with the compressed air, they catch fire. The combustion of fuel starts and the force is exerted at the crown of the piston. The piston is pushed down.
Second Stroke:
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During the downward motion of the piston the inlet port is also closed due to which air is compressed inside the crank casing. The exhaust port gets opened at about 70-75° from BDC and burnt gases are exhausted in the atmosphere. The transfer port opens at about 60-65° from BDC and compressed air enters into the cylinder. Because of the special shape of the piston head, the air is deflected and thus prevented from going out directly with the burnt gas through the exhaust port. In addition, the deflected air helps in pushing the exhaust gases out.