BYD DM-i (badge, not an engine)

DM-i is a badge, not an engine — BYD's name for its plug-in hybrid system, where DM is Dual Mode and the -i denotes the efficiency-focused version.

And what it names is a system in which the engine mostly does not drive the wheels. Like Honda's e:HEV, a DM-i car is electric at almost all speeds with the engine acting as a generator, and a clutch connects the engine directly only at steady high speed.

So the symptom that alarms new owners is normal: engine speed that does not track road speed at all, rising and holding under acceleration, and an engine that starts and stops with no relation to the accelerator. There is no transmission searching for a ratio — there is barely a transmission.

The combustion side is a naturally aspirated Atkinson-cycle petrol four designed for thermal efficiency rather than output, and its faults are ordinary. The interesting failures are electrical: the motor, the power electronics and their coolant loop, which is separate from the engine's.

DM-p is the performance version of the same idea with more motor and a different balance; the letter is the only place the difference is written.