"In a four-stroke engine cycle, the crankshaft makes two revolutions for every complete combustion cycle. First is the intake stroke. The intake valves open and the piston moves down, creating a vacuum that draws the air and fuel mixture into the cylinder. There may be some overlap with the exhaust valves being open, but let's stick to the basics of your question. Next is the compression stroke. The intake valves close, and the piston moves up, compressing and heating the air/fuel mixture. Somewhere around top dead center of the piston's travel, the spark will occur. It might be a little bit before or a little bit after, depending on the operating conditions. The resulting 'explosion' in the cylinder will force the piston down, making the crankshaft rotate and producing useful power. Finally, on the exhaust stroke, the exhaust valves open, and the piston moves up, forcing the spent fuel and air out and down the tail pipe."