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How Four Stroke Engine Works

Four Stroke Engine
The four-stroke engine was first demonstrated by Nikolaus Otto in 18761, hence it is also known as the Otto cycle. The technically correct term is actually a four-stroke cycle. The four-stroke engine is probably the most common engine type nowadays. It powers almost all cars and trucks.
The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston; Therefore, the complete cycle requires two revolutions of the crankshaft to complete.


Intake
During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel / air mixture. The illustrated engine features a poppet intake valve that is drawn by the vacuum produced by the intake stroke. Some early engines worked this way; However, most modern engines incorporate an extra cam / lifter arrangement as seen on the exhaust valve. The exhaust valve is closed by a spring (not illustrated here).

Compression

As the piston rises, the poppet valve is forced by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel / air mixture.









Power
At the top of the compression stroke, the spark plug fires, igniting the compressed fuel. As the fuel burns, it expands, driving the piston downward.






Exhaust
At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.

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