The four-stroke engine is also referred to as the Otto cycle engine after its inventor N.A. Otto. Most cars use the four-stroke engine. An individual cycle comprises four strokes: 1, intake stroke; 2, compression stroke; 3, power stroke and 4, exhaust stroke. These four strokes repeat to generate the crankshaft revolution.
1. Intake stroke: the intake stroke draws air and fuel into the combustion chamber. The piston descends in the cylinder bore to evacuate the combustion chamber. When the inlet valve opens, atmospheric pressure forces the air-fuel charge into the evacuated chamber. As a result, the combustible mixture of fuel and air fills the chamber.
2. Compression stroke: at the end of the intake stroke, both inlet and exhaust valves are closed. The inertial action of the crankshaft in turn lifts the piston which compresses the mixture. The ratio of the combustion
chamber volume before and after compression is called the compression ratio. Typically the value is approximately 9:1 in spark ignition engines and 15:1 in diesel engines.
3. Power stroke: when the piston ascends and reaches top dead center, an electric current ignites the spark plug and as the mixed gas burns, it expands and builds pressure in the combustion chamber. The resulting
pressure pushes the piston down with several tons of force. 4. Exhaust stroke: during the exhaust stroke, the inlet valve remains closed whilst the exhaust valve opens. The moving piston pushes the burned fumes through the now open exhaust port and another intake stroke starts again.
During one cycle, the piston makes two round trips and the crankshaft revolves twice. The inlet and exhaust valves open and close only once. The ignition plug also sparks only once. A petrol engine, whether four- or two-stroke, is called a spark ignition (SI) engine because it fires with an ignition plug. The four-stroke-cycle engine contains the lubricating oil in the crankcase. The oil both lubricates the crankshaft bearings and cools the hot piston.
The science and technology of materials in automotive engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
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