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How Two-Stroke Engines Work

written by: Haresh Khemani • edited by: Swagatam • updated: 9/9/2008

In two-stroke engines, the cycle of operations of the engine are completed by two strokes of the piston inside the cylinder. Two-stroke petrol engines and diesel engines work in the same way. The cycle of operation of two-stroke engines will be further described.

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    The two-stroke internal combustion (IC) engine was invented by Dugald Clerk in the year 1878. In two-stroke engines, the cycle of operations of a petrol or diesel engine is completed in two-strokes of the piston inside the cylinder or one revolution of the crankshaft. During two-strokes of the cycle, one complete stroke of power is produced. This means that for every revolution of the crankshaft, power is produced once within the engine. In comparison, four-stroke engines have one power stroke for every two revolutions of the crankshaft. Ideally the power produced by the two-stroke engine is double of that produced by the four-stroke engine.

    Here are the various strokes of the two-stroke engine:

    1) Suction and compression stroke: As the piston moves from the bottom most position (or bottom dead center) to the top dead center position, air in the case of diesel engines, or air-fuel mixture in the case of the petrol engine, is inducted into the crankcase. There is an inlet valve in the crankcase that opens due to the pressure difference between the outside and inside of the crankcase.

    As the piston moves upwards, it also compresses the previously inducted air, or air-fuel in the upper portion of the cylinder. The air-fuel mixture is compressed in the clearance volume of the cylinder. When the piston reaches topmost position the inlet valve of the crankcase closes, the suction and compression stroke completes. The spark is instantly generated in the spark ignition (SI) engine which burns the fuel, while in compression ignition (CI) or diesel engines, the fuel burns due to high compression pressures.

    2) Expansion and exhaust stroke: Due to combustion of fuel, large amounts of thermal or heat energy is produced which creates very high pressures inside the cylinder. This causes the piston to move towards the bottom. With the expansion of gases and the movement of the piston, the air-fuel mixture, called charge, gets compressed in the crankcase. During this expansion stroke the power is produced by the engine, which is transmitted to the crankshaft via connecting rod.

    Due to combustion of fuel, residual gases are produced inside the cylinder, which should be removed to enable the fresh charge to enter the upper portion of the cylinder. Towards the end of the expansion stroke, the piston uncovers the exhaust port permitting the residual or exhaust gases to leave the cylinder.

    As the piston moves further down the cylinder the transfer port is uncovered, allowing slightly compressed fresh air-fuel mixture located in the crankcase to move towards the upper portion of the cylinder via the transfer port.

    The top portion of the piston usually has a projection that guides the fresh air-fuel mixture towards the top of the cylinder and prevents it from going toward the exhaust ports, thus the flow of fresh charge escaping through the exhaust port is prevented. Sometimes the deflector is located in the transfer port to prevent the fresh charge from escaping through the exhaust port.

    By the time piston reaches bottom dead center all the fresh charge is inducted above the cylinder and all the exhaust gases escape through the exhaust port. Now the piston starts moving in upward direction and the inlet valve opens. The piston starts compressing the air-fuel mixture and the fresh charge enters into the crankcase through the inlet valve. The cycle keeps on repeating continuously until the vehicle is running.