How a Petrol Engine Works

Sit at the lights with the engine ticking over and look at the rev counter. The needle rests somewhere under 1,000: even with the car standing still, the crankshaft is turning that many times a minute, and every two turns each cylinder breathes in, squeezes, burns and breathes out once. This lesson follows one cylinder through that cycle.

Inside each cylinder, a piston slides down and up, down and up. A connecting rod links it to the crankshaft, which turns that back-and-forth into rotation, the way your legs turn the pedals of a bike. Mechanics sum up the four strokes as suck, squeeze, bang, blow: intake, compression, power and exhaust.

One full cycle takes two turns of the crankshaft. At 3,000 rpm, an ordinary cruising speed for many engines, each cylinder runs the whole cycle 25 times a second. Only one stroke in four, the power stroke, actually pushes. The crankshaft’s momentum, a heavy flywheel on its end and the other cylinders carry the piston through the other three.