Q: Power Stroke

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does the power stroke start when the piston begins moving down after the spark plug fires

Hello, As you know, there are 4 parts to the combustion cycle; the intake, combustion, compression and exhaust. The intake stroke takes place as the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The combustion cycle begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage. The compression stage is the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. During the exhaust stroke, the piston returns from B.D.C. to T.D.C. while the exhaust valve is open. This expels the spent air-fuel mixture through the exhaust valve.

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