Mercedes-Benz OM611, OM612 and OM613 engines (1997-06)

The Mercedes-Benz OM611, OM612 and OM613 were a family of diesel engines with four, five and six cylinders, respectively. The OM611, OM612 and OM613 turbo-diesel engines had cast iron blocks, alloy cylinder heads, common-rail injection, double overhead camshafts (duplex chain-driven), four valves per cylinder (actuated by tappets) and exhaust gas recirculation.

Released in 1997, the OM611 engine was the first Mercedes-Benz application of Bosch common-rail direct injection (operating at up to 1350 bar). The system included pilot injection whereby a small amount of fuel was injected into the cylinder and burned prior to combustion to pre-heat the combustion chamber to better maintain temperature and pressure through the combustion cycle.

The OM611 engine was initially fitted with turbocharger where boost pressure was controlled by a wastegate valve. From 1999, the OM611, OM612 and OM613 engines were fitted with a variable nozzle turbine (VNT, also known as a variable geometry turbocharger or VGT). The VNT used a set of vanes which were arranged in the path of the air flow and, by changing the angle of the vanes, the area of the turbine inlet would change. At low engine speeds, when air flow through the engine was relatively low, the velocity of the air flow could be increased by partially closing the vanes, thereby increasing turbine speed.

The OM611, OM612 and OM613 engines were replaced by the OM646, OM647 and OM648 engines.