A four-stroke engine (also known as four-cycle) is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust—during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.
There are two common types of four-stroke engines. They are closely related to each other, but have major differences in design and behavior. The earliest of these to be developed is the Otto cycle engine developed in 1876 by Nikolaus August Otto in Cologne, Germany, after the operation principle described by Alphonse Beau de Rochas in 1861. This engine is most often referred to as a petrol engine or gasoline engine, after the fuel that powers it. The second type of four-stroke engine is the Diesel engine developed in 1893 by Rudolph Diesel, also of Germany. Diesel created his engine to maximize efficiency, which the Otto engine lacked. There are several major differences between the Otto cycle engine and the four-stroke diesel engine. The diesel engine is made in both a two-stroke and a four-stroke version. Otto's company, Deutz AG, now primarily produces diesel engines.
The Otto cycle is named after the 1876 engine of Nikolaus A. Otto, who built a successful four-stroke engine based on the work of Jean Joseph Etienne Lenoir. It was the third engine type that Otto developed. It used a sliding flame gateway for ignition of its fuel — a mixture of illuminating gas and air. After 1884, Otto also developed the magneto to create an electrical spark for ignition, which had been unreliable on the Lenoir engine.
Today, the internal combustion engine (ICE) is used in motorcycles, automobiles, boats, trucks, aircraft, ships, heavy duty machinery, and in its original intended use as stationary power both for kinetic and electrical power generation. Diesel engines are found in virtually all heavy duty applications such as trucks, ships, locomotives, power generation, and stationary power. Many of these diesel engines are two-stroke with power ratings up to 105,000 hp (78,000 kW).
The four strokes refer to intake, compression, combustion (power) and exhaust strokes that occur during two crankshaft rotations per power cycle. The cycle begins at Top Dead Centre (TDC), when the piston is farthest away from the axis of the crankshaft. A stroke refers to the full travel of the piston from Top Dead Centre (TDC) to Bottom Dead Centre (BDC). (See Dead centre.)
- INTAKE stroke: on the intake or induction stroke of the piston, the piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. A mixture of fuel and air, or just air in a diesel engine, is forced by atmospheric (or greater) pressure into the cylinder through the intake port. The intake valve(s) then closes. The volume of air/fuel mixture that is drawn into the cylinder, relative to the maximum volume of the cylinder, is called the volumetric efficiency of the engine.
- COMPRESSION stroke: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the combustion chamber of the cylinder head. During the compression stroke the temperature of the air or fuel-air mixture rises by several hundred degrees.
- POWER stroke: this is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed air–fuel mixture in a gasoline engine is ignited, usually by a spark plug, or fuel is injected into a diesel engine, which ignites due to the heat generated in the air during the compression stroke. The resulting pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead centre.
- EXHAUST stroke: during the exhaust stroke, the piston once again returns to top dead centre while the exhaust valve is open. This action expels the spent fuel-air mixture through the exhaust valve(s).
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