fourstroke

A four-stroke engine, or four-stroke cycle engine, is an internal combustion engine that completes a power cycle in four piston strokes: intake, compression, power, and exhaust. The cycle typically spans two revolutions of the crankshaft and is driven by one or more camshafts that open and close the intake and exhaust valves.

The four-stroke design was developed in the late 19th century, with Nikolaus Otto and his collaborators playing a defining role. It became the standard for modern gasoline engines due to its greater fuel efficiency and smoother operation compared with earlier or contemporary two-stroke designs, and it allowed cleaner combustion and more durable operation.

Operation: During the intake stroke, the intake valve opens and the piston moves downward, drawing air (and fuel in spark-ignition engines) into the cylinder. In the compression stroke, both valves close and the piston rises to compress the charge. The power stroke follows when the mixture is ignited (by a spark in SI engines or by compression in diesel engines), forcing the piston downward. Finally, the exhaust stroke opens the exhaust valve as the piston moves upward to expel spent gases.

Design and variants: Most four-stroke engines use a valve train driven by a camshaft, with configurations including overhead cam and pushrod layouts. They can be spark-ignition gasoline engines or compression-ignition diesel engines. They typically employ lubrication and cooling systems and may be configured as inline or V-type cylinders.

Applications: Four-stroke engines power most contemporary automobiles, motorcycles, boats, lawn equipment, and stationary generators, offering efficiency and durability at the cost of higher weight and complexity relative to two-stroke designs.