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The four-stroke engine operates on four basic principles: intake, compression, combustion, and exhaust. Each stroke represents a stage in the engine’s operation.
1. Intake Stroke
The intake stroke begins when the piston is at the top of the cylinder. The intake valve opens, and the piston moves downward, creating a vacuum that draws air and fuel into the cylinder.
2. Compression Stroke
As the piston moves back upward, it compresses the air-fuel mixture, thereby increasing its pressure. The compression stroke heats up the mixture and makes the ensuing combustion more powerful. Both the intake and exhaust valves are closed during this stage.
3. Combustion Stroke
At the top of the compression stroke, the spark plug ignites the compressed air-fuel mixture, causing an explosion. This explosion forces the piston back down the cylinder, generating the power that turns the crankshaft. This is often referred to as the power stroke.
4. Exhaust Stroke
Finally, the piston moves back upward to expel the exhaust gases produced during combustion through the open exhaust valve. Once the exhaust gases are cleared, the exhaust valve closes, the intake valve opens, and the cycle begins again.
History and Development
The four-stroke engine was first patented by Nikolaus Otto in 1876, hence it’s often referred to as the “Otto cycle.” Otto’s groundbreaking design offered significant improvements over the existing steam-powered and two-stroke engines, primarily in terms of fuel efficiency and power output.
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