As you might've guessed, four stroke engines have four stages. Intake,
compression, power, and exhaust. In summary: "Intake"
references the intake of air and fuel into a cylinder of the engine.
"Compression" happens when that air and fuel mixture is
compressed into a smaller area. The "Power" stroke is
when the spark plug ignites the area where the air and fuel mixture is
present, causing combustion. The "Exhaust" stage is
when the exhaust valve opens to expell the spent air/fuel mixture.
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by Hayley Davis