Four-stroke cycle : Four stroke cycle 

On the first (downward) stroke of the piston, fuel/air is drawn into the cylinder through the intake valve. The intake valve then closes and the following (upward) stroke compresses the fuel-air mixture, which is then ignited, usually by a spark plug, at approximately the top of the compression stroke. The resulting expansion of burning gases then forces the piston downward for the third stroke, and the fourth and final (upward) stroke evacuates the spent exhaust gases from the cylinder through the then-open exhaust valve.

An easy way to remember the four strokes and their functions is the series "suck, squeeze, pop, phooey."

The four-stroke cycle is more efficient than the two-stroke cycle, but requires considerably more moving parts and manufacturing expertise.

External link: Animated Engines: Four Stroke Engine (http://www.keveney.com/otto.html)

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