OM660

Daimler's own archive labels it plainly: the 0.8-liter three-cylinder OM 660 of 1998 was the world's smallest diesel engine fitted in a passenger car. Everything about it was miniature except the engineering — 799 cc from 65.5 × 79 mm cylinders, aluminum block and head, a single overhead camshaft with two valves per cylinder, 18.5:1 compression, a turbocharger and common-rail direct injection, all of it packaged behind the rear axle line of a car barely two and a half meters long. At launch it made 41 PS, which was never the point.

Economy was the point. Reworked in 2007 with a new common-rail system and more boost, the fortwo cdi was rated at 3.3 l/100 km and 88 g/km of CO₂ and sold as the most economical production car in the world: roughly 1,000 km on a 33-liter tank, or fifteen fills a year at average mileage. Output rose to 45 PS and then 54 PS by 2009, and assembly passed from DaimlerChrysler to MDC Power at Kölleda. The engine ran until the middle of the 2010s, and nothing smaller has worn a Mercedes-Benz diesel designation since.