Mercedes-Benz OM651
Overview
Manufacturer	Mercedes-Benz
Production	2008–present
Layout
Configuration	Inline 4[1]
Displacement	1.8 L (1,796 cc) 2.1 L (2,143 cc)
Cylinder bore	83 mm (3.27 in)
Piston stroke	83 mm (3.27 in) 99 mm (3.9 in)
Cylinder block material	Cast iron[2]
Cylinder head material	Aluminium alloy
Valvetrain	DOHC, chain-driven
Compression ratio	16.2:1
Combustion
Turbocharger	Single turbo / twin-turbo
Fuel system	Common rail
Fuel type	Diesel
Cooling system	Water cooled
Output
Power output	80–150 kW (107–201 hp)
Chronology
Predecessor	Mercedes-Benz OM646 engine
Successor	Mercedes-Benz OM654 engine

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

The main goal was to create a common engine design that maximized the parts commonality across the engines manufactured by parent company, Daimler.

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).

There are 6 variants in all. Of these, 4 of the engine variants have the same 2.1 L (2,143 cc) swept volume, tuned to different power outputs 120 PS (88 kW) (badged x180 on rear wheel drive models) 136 PS (100 kW) (badged x200 on rear wheel drive models) 170 PS (125 kW) (badged x220) to 204 PS (150 kW) (badged x250). The 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. The lower output versions have only a single turbocharger. There are also 2 shorter stroke "square" variants of this engine with 1.8 litre displacement 109 PS (80 kW) (badged A/B180) 136 PS (100 kW) (badged A/B200 and used only in smaller front wheel drive models like the A-Bclass). These two variants differ only in electronic tune, and have a single turbocharger.

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