Many cylinders, one smooth ride

A one-cylinder engine delivers a push once every 720°, and then nothing for two full turns. It is simple, but it shakes: the torque on the crankshaft goes from a big peak to zero and even to negative, and the piston's up-and-down inertia shakes the whole engine. Big flywheels help, but there is a better cure: more cylinders, firing in turn.

With n cylinders firing evenly, a power stroke begins every 720° / n:

• 1 cylinder: every 720°
• 2 cylinders: every 360°
• 4 cylinders: every 180°
• 6 cylinders: every 120°
• 8 cylinders: every 90°

In an inline-4, one power stroke ends just as the next begins, so the crankshaft always gets a push. The torque ripples much less than in a single cylinder.

In an inline-4 the crank throws are set in pairs: pistons 1 and 4 move together, and pistons 2 and 3 move together, half a turn out of step. When the pair is at the top, one of the two is finishing compression (about to fire) while its partner is finishing exhaust (about to take in fresh air), so the two are always half a cycle apart. Two firing orders are common: 1-3-4-2 (the most usual) and 1-2-4-3. Both fire a cylinder every 180°, and a cylinder never fires right after its next-door neighbour, which reduces the strain on the crankshaft and evens out the gas flowing in the intake pipe.

Even firing is not the whole story. A basic inline-4 still has a small leftover shake at twice the engine speed because the pistons do not move symmetrically up and down; larger inline-4 engines often add balance shafts to cancel it. An inline-6 firing every 120° is naturally very smooth, which is why it has long been prized, while a V8 fires every 90° with its throws arranged to balance.

Does more cylinders mean more power? Not by itself. Power comes from how much air and fuel the engine burns each second, which depends on its displacement, its speed, and how hard each cycle works. For a four-stroke engine, P = p × V_d × N / 2, where p is the mean effective pressure (the steady pressure that would do the same work as the real one), V_d the total displacement and N the rotations per second. For a 2.0-litre engine with p ≈ 1.0 MPa at 100 rev/s (6 000 rpm): P = 1.0×10⁶ × 0.002 × 100 / 2 = 100 kW. Spread the same 2.0 litres over six or eight cylinders and the power is about the same; you get smoothness and a different sound, not extra power.

A turbocharger forces more air in, raising p and so the power from the same displacement, which is why modern small engines can be strong.