piston

When the fuel/air mixture] lights, it explodes, expanding and pushing the piston down. The piston is connected to the crankshaft, so the shaft spins. All you have to do is hook the shaft to a transmission, and voila, you have useful movement.

The face of the piston gets coated with carbon (a by-product of combustion) very quickly, which isn't a problem unless it gets really thick, which won't happen unless the fuel-air mixture is really wrong, or oil seeps into the combustion chamber.

On the side of the piston, there are two or three rings, which seal the top end from the bottom end. See, there's lots of oil flying around to lubricate the various moving parts, and it's important (in the case of a four-stroke engine) that it not leak into the combustion chamber. The rings seal oil against this. Similarly, it would be bad for fuel to leak into the crankcase, and the rings also guard against this.

Pis"ton (?), n. [F. piston; cf. It. pistone piston, also pestone a large pestle; all fr. L. pinsere, pistum, to pound, to stamp. See Pestle, Pistil.] Mach.

A sliding piece which either is moved by, or moves against, fluid pressure. It usually consists of a short cylinder fitting within a cylindrical vessel along which it moves, back and forth. It is used in steam engines to receive motion from the steam, and in pumps to transmit motion to a fluid; also for other purposes.

Piston head Steam Eng., that part of a piston which is made fast to the piston rod. -- Piston rod, a rod by which a piston is moved, or by which it communicates motion. -- Piston valve Steam Eng., a slide valve, consisting of a piston, or connected pistons, working in a cylindrical case which is provided with ports that are traversed by the valve.

© Webster 1913.