Four-stroke cycle

The four-stroke cycle (or Otto cycle) of an internal combustion engine is the cycle most commonly used for automotive and industrial purposes today (cars and trucks, generators, etc). It was invented by German engineer Nikolaus Otto in 1876. The four-stroke cycle is more fuel-efficient and clean burning than the two-stroke cycle, but requires considerably more moving parts and manufacturing expertise and the resulting engine is larger and heavier than a two-stroke engine of comparable power output. The later-invented Wankel engine has four similar phases but is a rotary combustion engine rather than the much more usual, reciprocating engine of the four-stroke cycle.

Related Topics:
Internal combustion engine - Cars - Truck - Generator - Nikolaus Otto - 1876 - Fuel-efficient - Two-stroke cycle - Wankel engine - Rotary combustion engine - Reciprocating engine

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The Otto cycle is characterized by four strokes, or straight movements alternately, back and forth, of a piston inside a cylinder:

Related Topics:
Piston - Cylinder

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• intake (induction) stroke
• compression stroke
• power (ignition) stroke
• exhaust stroke

The cycle begins at top dead center, when the piston is at its topmost point. On the first downward stroke (intake) of the piston, a mixture of fuel and air is drawn into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s), and the following upward stroke (compression) compresses the fuel-air mixture.

Related Topics:
Top dead center - Fuel - Air - Valve

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Starting position, intake stroke, and compression stroke. View an animation.

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The air-fuel mixture is then ignited, usually by a spark plug for a gasoline or Otto cycle engine, or by the heat and pressure of compression for a Diesel cycle of compression ignition engine, at approximately the top of the compression stroke. The resulting expansion of burning gases then forces the piston downward for the third stroke (power), and the fourth and final upward stroke (exhaust) evacuates the spent exhaust gases from the cylinder past the then-open exhaust valve or valves, through the exhaust port.

Related Topics:
Spark plug - Gasoline - Otto cycle - Diesel cycle - Compression ignition engine

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Ignition of fuel, power stroke, and exhaust stroke. View an animation.

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Four-stroke cycle: Valve Timing ►