Inside the cylinder

The last lesson treated the cylinder as a black box. This one opens it. Three parts do the mechanical heavy lifting: the piston that takes the push, the connecting rod that carries it, and the crankshaft that turns a straight shove into rotation. Together they are called the crank-slider mechanism, and it is the beating heart of every piston engine on Earth.

The piston's hard life

A piston looks like a simple metal cup, but consider its working day: slammed by combustion at up to 100 bar, scorched by 2000 °C flame on its crown, and — hardest of all — reversed direction thousands of times a minute. At 6,000 rpm a road piston stops dead and accelerates the other way 200 times a second, pulling accelerations of over 2,000 g. It survives because it is small, light (aluminium, a few hundred grams) and cooled from below by oil spray.

Around it, three thin springy rings seal the moving piston against the bore: the gap they close is where pressure would otherwise leak, and worn rings are why old engines lose punch and burn oil.

Straight push, round output

The crankshaft is a shaft with offset arms — crank throws — one per cylinder. The connecting rod links the piston to a throw, so as the piston pumps up and down, the throw is dragged around in a circle, exactly like your leg on a bicycle pedal. Push down on the pedal: the crank arm converts your straight leg-push into rotation of the chainring. An engine is a machine for pedalling four hundred times a second.

The geometry has a subtlety that matters everywhere in engine design: the piston does not move like a pure sine wave. Because the rod has finite length, the piston hurries near the top of its stroke and dawdles near the bottom — an asymmetry that feeds straight into vibration (next lesson) and valve timing.

Bore, stroke, character

Two numbers describe a cylinder: bore (its diameter) and stroke (how far the piston travels). Their ratio shapes an engine's personality. A long stroke gives leverage on the crank — lazy, torquey engines that pull from low revs. A big bore with a short stroke keeps piston speeds sane at stratospheric rpm and leaves room for big valves — the recipe for engines that scream. A Honda motorcycle engine and a tractor diesel are the same mechanism with opposite bore-stroke bets.