The four strokes of a four cycle engine.

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Intake Stroke - This is the first stroke in a four cycle engine.  The piston travels downward, causing a vacuum, around the same time the intake valve opens.  The vacuum pulls air and fuel into the combustion chamber and cylinder.

Compression Stroke - The intake valve closes, and then the piston begins to travel up the cylinder, compressing the air-fuel mixture.  Compressing the air-fuel mixture makes this mixture more volatile.

Power Stroke - The compressed air-fuel mixture is ignited.  The burning air-fuel mixture expands, forcing the piston down.

Exhaust Stroke - The exhaust valve opens.  The piston travels up the cyclinder, forcing the hot exhaust gasses out of the cylinder.  Toward the end of this stroke, on most engines, the intake valve opens.  This is known as valve overlap, and used to aid the intake of fresh air and fuel.  The cycle restarts after this stroke.

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