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What is a four-stroke engine and how does it work?

A four-stroke engine refers to the number of strokes a piston makes in one cycle, or in two rotations of the crankshaft. The four strokes are intake, compression, power, and exhaust. During the intake stroke, the piston travels down the cylinder to let fresh air/fuel in through the intake valve (Mechanical Booster). The air/fuel mixture is sucked into the cylinder because "the piston is an airtight seal inside the cylinder... " which "...creates a partial vacuum" (EpicPhysics.com). The compression stroke starts as the piston travels up the cylinder and compresses the fuel/air mixture (Jenkins). Then a "spark plug ignites the..." fuel/air mixture, forcing the piston down the cylinder during the power stroke (Jenkins). It is important to note that the power stroke "... is the only point at which the engine creates energy" (EpicPhysics.com). Moreover, "as the piston moves up and down in the cylinder it rotates the crankshaft" (AutoEducation). Lastly, once the piston completes the power stroke, "the exhaust valve opens" to expel the exhaust gasses (AutoEducation).