Fyrtakts

Fyrtakts, commonly called the four-stroke cycle, is an internal combustion engine cycle used in many piston engines. The cycle comprises four piston strokes—intake, compression, power, and exhaust—that occur over two revolutions of the crankshaft. During the intake stroke, the intake valve opens and the piston moves downward, drawing an air-fuel mixture (or air in diesel engines) into the cylinder. The compression stroke closes the valves; the piston rises, compressing the charge. In spark-ignition engines, the spark plug fires near the end of compression, causing a rapid rise in pressure that drives the piston downward in the power stroke. Finally, during the exhaust stroke, the exhaust valve opens and the piston pushes spent combustion gases out of the cylinder.

The concept was developed in the late 19th century, notably by Nikolaus Otto and Eugen Langen, and became the dominant cycle for automotive and many other piston engines. Four-stroke designs are generally more fuel-efficient and produce fewer emissions per unit power than two-stroke designs, at the cost of greater mechanical complexity and weight. They rely on a valvetrain, usually operated by a camshaft, to open and close intake and exhaust valves; modern engines may use overhead camshafts, variable valve timing, direct fuel injection, and turbocharging.

Four-stroke engines are used in cars, motorcycles, lawn equipment, power generators, and many aircraft engines, among other applications. They exist in numerous configurations, including petrol (gasoline) and diesel variants, and can be arranged in multiple cylinders and banks to suit power and packaging needs.