OM639

It is an OM640 with a cylinder removed. Same 83 × 92 mm bore and stroke as the A-Class four, same chain-driven twin camshafts, same common-rail injection, same 18:1 compression — but 1,493 cc in three pots, an aluminum block and head, a turbocharger, and a dual-mass flywheel to keep what a three-cylinder does to a driveline out of the cabin. Developed with Mitsubishi and built for two jointly engineered cars, the smart forfour and the Mitsubishi Colt, assembled together at NedCar in the Netherlands.

There were two tunes, 50 kW with 160 N·m and 70 kW with 210 N·m, and in the stronger one it briefly held the title of the world's most powerful three-cylinder passenger-car diesel — a record BMW eventually took with the B37. It had nowhere to go. The forfour was canceled in 2006 after two years, Mitsubishi dropped the diesel Colt at the 2008 facelift citing lack of demand, and the engine finished in 2009, a genuine technical high-water mark stranded by the sales of the only two cars that used it.