Fiat TwinAir (two-cylinder)

Fiat's 875 cc two-cylinder — in the 500, Panda, Punto and the Alfa MiTo and Lancia Ypsilon.

A two-cylinder petrol engine in a modern car is rare enough to be the whole story, and everything about how it behaves follows from having two cylinders: it is inherently unbalanced, so what feels like a misfire or a mount problem on any other engine is often just the engine running.

Its distinctive technology is MultiAir, an electro-hydraulic system that opens the inlet valves through oil pressure rather than a cam lobe. That makes the oil part of the valve gear rather than merely a lubricant — and the MultiAir actuator unit is the engine's known expensive failure, giving a rough idle, a misfire and a loss of power that reads as ignition and is not.

⚑ The oil specification and interval are therefore not negotiable on this engine in the way they are on a conventional one, and a TwinAir that has been serviced late or with the wrong oil is the common version of a failed actuator.

Its real-world fuel consumption famously disappoints, and that is not a fault: a small engine worked hard uses what a larger one worked gently would.