BMW W20K06 (i3 REx)

The 0.6 twin-cylinder range extender in the BMW i3 REx.

⚑ It is a motorcycle engine and it never drives the wheels. BMW took the twin from its own C650 scooter, mounted it beside the rear motor, and gave it one job: turning a generator when the battery runs low. Every complaint about acceleration, response or drivability on an i3 belongs to the electric drive system, and diagnosing it at this engine is looking in the wrong half of the car.

Its own problems come from how little it runs. A range extender in a car charged nightly may go months without starting, and the consequences are the ones any unused engine has: stale fuel, a flat-spotted starter battery, and seals that dry out. BMW's own maintenance mode runs it deliberately for that reason, and an owner who disables it is creating the fault.

The fuel tank is very small — a few litres — so a fuel-system complaint on one of these is usually about age rather than volume.

Everything about the high-voltage system applies, and the precautions a purely combustion car does not need are mandatory here.