How does the combustion engine work?

The four-stroke internal combustion engine seen in today's cars and trucks is an elegant study in fluid mechanics.
Its centerpiece: a piston in a cylinder.
Each stroke of the piston accomplishes a different task.
First, a downstroke creates a partial vacuum that sucks in vaporized, fuel mixed with air.
Nest, an upward stroke compresses this mixture and, at the top, sets off the spark plug, which ignites the gases.
In what's referred to as the power stroke, the hot gases expand, pushing the piston down and transferring power to the crankshaft.
An exhaust valve opens, and the piston rises again, expelling the combusted air-and-fuel mixture.
A third stroke draws the fuel mixture into the cylinder, and a fourth compresses it to start the cycle again.
This process is sometimes called the Otto cycle, after Nicolaus Otto, the German inventor who built the first working four-stroke internal combustion engine in 1876.