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Four-stroke engine rotation
Duty cycle of a four-stroke engine:
The four-stroke duty cycle is the basic principle behind the operation of internal combustion engines, whether gasoline (auto) or diesel engines. This cycle consists of four main stages that the piston completes during two complete revolutions of the crankshaft, while the camshaft rotates only once. Here is a detailed explanation of each stage:
1. Intake stroke:
The piston descends from top dead center (TDC) to bottom dead center (BDC) leaving a vacuum inside the cylinder.
The intake valve opens, allowing a mixture of air and gasoline (for gasoline engines) or just air (for diesel engines) to enter the cylinder.
The intake valve closes when the piston reaches bottom dead center.
2. Compression stroke:
Both the intake valve and the exhaust valve are closed.
The piston rises from bottom dead center (BDC) to top dead center (TDC) compressing the air-gasoline mixture (or just air in diesel engines).
3. Combustion stroke (or power):
Just before the piston reaches top dead center (TDC) in gasoline engines, the compressed air-gasoline mixture is ignited by an electric spark from the spark plug.
In diesel engines, the pressure and temperature inside the cylinder rise high enough for the air-fuel mixture to spontaneously ignite without the need for an external spark.
The combustion of the mixture forces the piston downward, creating energy that is used to turn the crankshaft.
4. Exhaust stroke:
The piston rises from bottom dead center (BDC) to top.
The exhaust valve opens, allowing combustion exhaust gases from the cylinder to escape the engine.
The exhaust valve closes when the piston reaches top dead center (TDC) in preparation for starting a new cycle.
Important Notes:
During a complete engine cycle, each piston completes two complete revolutions, i.e. four strokes.
The crankshaft rotates two complete revolutions for every revolution of the piston.
The camshaft rotates only once per revolution of the piston, controlling the timing of opening and closing of the intake and exhaust valves.
Diesel engines have higher fuel efficiency and lower emissions of pollutants compared to gasoline engines, due to the difference in the way fuel is burned in each type.
