If you can find a pull-start gas engine lawn mower, you can learn lots from it. These things generally have a single piston. The pull string that you use to start it gets it moving in the right direction, and once it starts running on its own, it's momentum that brings it back around for another go.

Also keep in mind that this single piston engine is two-stroke and not four as in the article.

Back to four stroke engines: Momentum and the firing of other pistons keeps the whole thing cycling back around. The more pistons you have, the more there are firing between each revolution of a given piston (and thus overcoming resistance and supplementing that momentum) and thus the more power available to drive the vehicle.