Four Stroke Engine
The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.
1. Intake. During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture.
2.Compression. As the piston rises the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.
3.Power. At the top of the compression stroke the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.
4.Exhaust. At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.
The link below has an excellent animation of a single cylinder 4- stroke engine. Cars have multiple cylinders working in alternating strokes of each other.(when one piston is going down , it is pushing it's opposing piston up and vice versa. This force is transferred through a crankshaft to which the pistons are connected to.)