Mercedes Benz
OM 651
Manufacturer:	Daimler AG
Production period:	since 2008
Style:	Four-cylinder in-line engine
Engines:	1.8 liters (1796 cc)  2.1 liters (2143 cc)
Previous model:	OM 646 (in-line four-cylinder)  OM 642 (V six-cylinder)
Successor:	OM 654

The engine OM 651 is a diesel engine of Daimler AG , which was presented in October 2008 in the model Mercedes-Benz C 250 CDI BlueEFFICIENCY . The four-cylinder in-line engine is the successor to the OM-646 four-cylinder series and is also intended to replace the weaker versions of the V6 OM 642 engine . The OM 651 is not a revised version of the OM 646, but was completely redeveloped.

The engine is pre-assembled at the Stuttgart plant , completed by MDC Power in Kölleda (Thuringia). The crankcase is supplied by the iron foundries Fritz Winter and Halberg .

Technology 

The engine has a crankcase made of gray cast iron , a light metal - the cylinder head and the cylinder four V-shaped at an angle of 6 ° overhead valves ( four valves per cylinder ), two overhead camshafts and roller cam, which on resting lash compensation elements are mounted.

The drive of the camshafts sits at the rear end of the engine (output side) to keep the height at the front end as small as possible. This allows for a longitudinally installed engine a flat and rearward rising hood in favor of aerodynamics and pedestrian protection. In order to reduce the length of the engine, the drive is in two stages : first through gears , as they need less space between the engine and transmission, and in the second stage via a single roller chain . The gears also drive the high-pressure fuel pump and two needle-bearing balancer shafts ( Lanchester compensation ).

The displacement is 1796 cc or 2143 cc with a bore of 83 mm and a stroke of 83 or 99 mm, with a cylinder spacing of 94 mm. In the 2.1-liter version (with a connecting rod length of 143.55 mm), the engine has a much longer stroke than its predecessor, which is favorable in terms of higher peak pressure. The compression is 16.2: 1 slightly lower than its predecessor.

The more powerful versions 220 CDI BlueEFFICIENCY and 250 CDI BlueEFFICIENCY two provide turbocharger from BorgWarnerin two-stage arrangement for better cylinder filling. Both turbines and compressors are connected in series: the exhaust gas flows first through the high-pressure turbine, which drives the high-pressure compressor, and then through the low-pressure turbine. The intake air in turn is first passed through the low pressure and then the high pressure compressor. The boost pressure is regulated by an adjustable turbine bypass (similar to a wastegate), which directs the exhaust gas, bypassing the high-pressure turbine, directly to the low-pressure turbine. At high engine speed, the charge air is passed around the high-pressure compressor, so as not to overload the high-pressure supercharger. The boost pressure for the 150 kW version is just under 3 bar (absolute).

The weaker units up to 105 kW have a single turbocharger with variable turbine vanes (VTG loaders) from IHI . All versions have a charge air cooler (Intercooler).

There is a cooled exhaust gas recirculation (EGR) with pre-cooler and main cooler. Depending on the operating point of the engine, the main radiator is bypassed by an exhaust gas recirculation (EGR) valve manufactured by Handtmann to control the temperature of the recirculated exhaust gases.

Injection 

The common-rail injection works with a fuel pressure up to 2000 bar. The weaker engine variants have solenoid-operated, the stronger ones, however, piezo injectors in which the nozzle needle is operated directly, so that up to five individual injections are possible per combustion process. The closed design of these injectors also eliminates the leakage line.

The high-pressure fuel pump is throttled on the suction side to adjust the flow rate as needed. This reduces the heating of the diesel engine so that it does not require a radiator and also reduces the pump drive power.

Cooling and lubrication 

The engine has a demand-controlled water pump . The controller can shut off the flow of coolant through a pneumatically controlled gate to firstly consume less power to drive the pump, and secondly, to heat the combustion chambers faster when the engine is cold. The setpoint temperature of the coolant is normally 100 ° C, but is reduced at high load and also up to 80 ° C depending on the driver's behavior.

The oil pump is a vane pump with pressure-dependent delivery rate. It regulates the pressure in the main oil passage after the oil filter to reduce the drive power of the pump at low filter load. The piston cooling via oil spray nozzles is switched on or off depending on the map. On the one hand, this reduces the drive power of the oil pump when no piston cooling is necessary at low load. On the other hand, the hydrocarbon emission is reduced when the engine is cold due to the higher piston temperature.

Versions 

The OM 651 is currently available in car models in ten different performance levels:

OM 651 DE 18 LA red.

OM 651 DE 18 LA 

OM 651 DE 22 LA red. 

OM 651 DE 22 LA 

According to Mercedes-Benz, the engine has not yet reached its performance limit. At least 162 kW are still possible. The torque of the engine could be increased so far that it could be quite a replacement for the current OM 642 in the 320 CDI with 165 kW and 540 Nm of torque in terms of its performance data.

All versions comply with the Euro 5 emission standard and the basic design of the engine already meets the Euro 6 emissions standard that the ML 250 BlueTEC will meet from 2011 onwards.

The engine was initially offered in the C-Class class 204 in late summer 2008 in a Frontrunner series called "PrimeEdition" (5000 pieces). Out of use in the following GLK and since May 2009 in the E-Class 212 series / 207 model series . Since 2010, the remaining series have also been equipped with the new machine. This engine is even offered in the S-Class since the end of September 2010 in the highest expansion stage . Since November 2011, he is also incorporated in the B-Class, but here across. The displacement was reduced to 1.8 liters.

Technical problems 

Shortly after the launch of the engine, quality issues became known with the piezo injectors supplied by Delphi . Of these, the two more powerful expansion stages with 125 kW and 150 kW were affected, but the lower-power variants were not, since they have magnetically controlled injectors. The problem usually occurs at a mileage of 50,000 km. In the event of damage, the affected engines only have limited power , as the engines automatically switch to the emergency program to prevent further damage . The vehicle can be so at low speedcontinue. Some customers admitted that Mercedes-Benz was not able to replace all injectors of the defective series in the affected vehicle due to capacity bottlenecks at the supplier's , but only individual failed injectors. As a result, customers had to visit a garage up to four times to replace all four injectors. In the further course, however, all injectors were replaced at once. Altogether, according to various sources, between 2,800 and 50,000 vehicles were affected, elsewhere, there is talk of up to 300,000 vehicles. Due to the high failure rate only a few engines with Delphi piezo injectors are built. The performance of the Delphi magnetic injectors has been increased to 145 kW, so that also the 220 CDI and 250 CDI models equipped with it. In addition, occasionally Bosch magnet injectors in the four-wheel and M-class variants are used.

Another problem is the water pump. This is leaking in many cases.

The engine is equipped with a simplex timing chain (instead of duplex chain). Increased wear of the timing chain and / or the chain tensioner are recorded at higher mileage. The replacement of the timing chain is quite complex, since the timing chain is installed on the rear side of the transmission and thus, in contrast to front timing drives, the expansion of the engine and transmission is required to repair the timing drive.

At the end of November 2014, problems with the chain tensioner became known again. After that failure, the chain tensioner and possibly leak engine oil. All vehicles produced between February and November 2014 have been recalled by the manufacturer.

Exhaust emission.

The diesel engine OM 651 is according to media reports  is based on the suspicion that the engine contains an inadmissible defeat device, whereby the exhaust levels on the test bench are better than in actual use. This concerns more than one million vehicles of different vehicle classes, which were sold in the years 2008 to 2016 in Europe and the USA with an inadmissibly high pollutant emission.