The OM651 is a family of inline-four cylinder automobile diesel engines introduced by Mercedes-Benz in 2008.

One requirement of the design was the ability of the engine to be mounted both longitudinally and transversely.

Improved fuel efficiency and compliance with Euro 5 emission standards were also design objectives, by 2010 being updated to the Euro 6 standard; four piezo-electric injectors fed with very high pressure fuel from a common rail inject fuel directly into the combustion chambers to improve combustion compared to previous Mercedes Diesel engines and recirculated exhaust gas reduces the oxygen in the cylinders to "starve" any reactions that would produce NO(x).

The 163 PS (120 kW), 170 PS (125 kW) and 204 PS (150 kW) versions employ a 2-stage (bi-turbo) charging setup with a small, high pressure turbo providing quick boost at low RPM fed by a large, lower pressure turbo providing increased performance at higher RPM, then at highest rpm ranges and loads, using both simultaneously.

There are also 2 shorter stroke "square" variants of this engine with 1.8 litre displacement 109 PS (80 kW) (badged A/B180) and 136 PS (100 kW) (badged A/B200 and used only in smaller front wheel drive models like the A-Bclass).