Why do engines have different cylinder layouts?
Because the number and arrangement of cylinders determines smoothness, size, weight, cost and where the engine can physically fit — and every layout is a compromise between those.
Why smoothness varies. A piston stops and reverses twice per revolution, producing forces that must be balanced. More cylinders means more, smaller power pulses, which overlap and produce less vibration. Some configurations cancel their own forces geometrically; others require balance shafts or accept the roughness.
The common layouts:
Inline-three. Compact, cheap, increasingly common in small cars. Inherently unbalanced in a way that needs a balance shaft, and has a distinctive thrum many people like.
Inline-four. The default for decades. Compact and inexpensive, but suffers a secondary imbalance that grows with capacity — which is why larger fours use balance shafts and why four-cylinder engines above about 2.5 litres feel coarse.
Inline-six. Perfectly balanced in both primary and secondary terms without any balance shafts — mechanically the smoothest common layout. Its drawback is length, which is why it fell out of favour when cars went transverse and front-wheel drive, and why it has returned in longitudinal premium cars.
V6. A six that fits where an inline-six will not, at the cost of extra complexity and, depending on the vee angle, some imbalance.
V8. Compact for its capacity, with a characteristic sound produced by the firing order of a cross-plane crankshaft.
Flat or boxer, where opposed pistons cancel each other. Very low centre of gravity, wide, and awkward to service.
V12 and flat-12, inherently smooth, expensive, and now rare.
Why layouts are changing. Downsizing and turbocharging replaced capacity with boost, so smaller-cylinder-count engines do work that once needed more. Electrification changes the calculation again — a hybrid can tolerate a rougher engine because the motor covers the worst of it, and an electric motor has no reciprocating parts to balance at all.