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How does a two-stroke engine work?

- Jan 20, 2019 -

How does a two-stroke engine work?

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A two-stroke engine is a type of internal combustion engine which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaft revolution. Compared to four-stroke engines, two-stroke engines have a greatly reduced number of moving parts, and so can be more compact and significantly lighter.

2 Stroke Petrol Engine-

Its two strokes are described as follows:

  • Upward Stroke: During the upward stroke, the piston moves from bottom dead centre to top dead centre, compressing the air-petrol mixture in the cylinder. The cylinder is connected to a closed crank chamber. Due to upward movement of the piston, a partial vacuum is created in the crankcase, and a new charge is drawn into the crankcase through the uncovered inlet port. The exhaust port and transfer port are covered when the piston is at the top dead centre position. The compressed charge is ignited in the combustion chamber by a spark provided by the spark plug.

  • Downward Stroke: As soon as the charge is ignited, the hot gases force the piston to move downwards, rotating the crankshaft, thus doing the useful work. During this stroke the inlet port is covered by the piston and the new charge is compressed in the crank case. Further downward movement of the piston uncovers first the exhaust port and then the transfer port. The burnt gases escape through the exhaust port. As soon as the transfer port opens, the compressed charge from the crankcase flows into the cylinder. The charge is deflected upwards by the pump provided on the head of the piston and pushes out most of the exhaust gases. It may be noted that the incoming air-petrol mixture helps the removal of burnt gases from the engine cylinder, which is called as the scavenging process. If in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease. The cycle of events is then repeated.

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2 Stroke Diesel Engine-

In a two stroke cycle diesel engine, only air is compressed inside the cylinder and the diesel is injected by an injector. There is no spark plug in this engine. The remaining operations of the two stroke cycle diesel engine are exactly the same as those of the two stroke cycle petrol engine. Its two strokes are described as follows:

  • Upward Stroke: During the upward stroke, the piston moves from bottom dead centre to top dead centre, compressing the air in the cylinder. The cylinder is connected to a closed crank chamber. Due to upward movement of the piston, a partial vacuum is created in the crankcase, and a new charge is drawn into the crankcase through the uncovered inlet port. The exhaust port and transfer port are covered when the piston is at the top dead centre position. The compressed charge is mixed with the injected diesel and gets auto-ignited in the combustion chamber because of the high pressure and temperature present.

  • Downward Stroke: As soon as the fuel is ignited, the hot gases force the piston to move downwards, rotating the crankshaft, thus doing the useful work. During this stroke the inlet port is covered by the piston and the new charge is compressed in the crank case. Further downward movement of the piston uncovers first the exhaust port and then the transfer port. The burnt gases escape through the exhaust port. As soon as the transfer port opens, the compressed charge from the crankcase flows into the cylinder. The charge is deflected upwards by the pump provided on the head of the piston and pushes out most of the exhaust gases. It may be noted that the incoming air helps the removal of burnt gases from the engine cylinder, which is called as scavenging process. If in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease. The cycle of events is then repeated.

Quote from:www.quora.com

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