A four-stroke engine is a type of internal combustion engine that completes four separate strokes while turning the crankshaft. The four strokes are:

1. Intake Stroke: The piston moves downward to the bottom, increasing the volume to allow a fuel-air mixture to enter the chamber.
2. Compression Stroke: The intake valve is closed, and the piston moves up the chamber to the top, compressing the fuel-air mixture. At the end of this stroke, a spark plug provides the compressed fuel with the activation energy required to begin combustion.
3. Power Stroke: As the fuel reaches the end of its combustion, the heat released from combusting hydrocarbons increases the pressure, causing the gas to push down on the piston and create the power output.
4. Exhaust Stroke: The piston moves up again, pushing the exhaust gases out of the chamber.

The four-stroke engine is the most common type of internal combustion engine design for motorized land transport, being used in automobiles, trucks, diesel trains, light aircraft, and motorcycles. The major alternative design is the two-stroke cycle.

Four-stroke engines provide a good balance of power, reliability, and efficiency. They mechanically separate each event, reducing unburned fuel emissions, and separate oil from fuel, which significantly reduces carbon monoxide emissions. The Briggs & Stratton 4-stroke engine, also referred to as a 4-cycle engine, powers an array of outdoor power equipment, including lawn mowers, generators, lawn tractors, and tillers.

In summary, a four-stroke engine is a type of internal combustion engine that completes four separate strokes while turning the crankshaft. It is the most common type of engine design for motorized land transport and provides a good balance of power, reliability, and efficiency.