Mercedes Benz
OM 646, OM 647, OM 648
Manufacturer:	Mercedes Benz
Production period:	2002-2010
Style:	Inline four-cylinder OM 646 Inline five-cylinder OM 647 in-  line six-cylinder OM 648
Engines:	2.2 liters (2148 cc)  2.7 liters (2685 cc)  3.2 liters (3222 cc)
Previous model:	OM 611 (4-cylinder)  OM 612 (5-cylinder)  OM 613 (6-cylinder)
Successor:	OM 651 (4-cylinder)  OM 642 (6-cylinder)

The OM 646 by Mercedes-Benz is a four cylinder - diesel engine with common-rail direct injection . Vehicles with this engine type carry mostly "200 CDI" and "220 CDI" in the sales name. The engine type represents a further development of the OM 611 .

The article also covers the OM 647 ("270 CDI") five cylinder engines and OM 648 ("280 CDI", "320 CDI") six-cylinder engines, also in- line , which are technically closely related to the OM 646.

These engines were mainly installed in the years 2002 to 2010 in several series, including the C-Class and the E-Class , as a six-cylinder in the S-Class and in vans such as the Sprinter .

Technology 

The Mercedes-Benz OM 646 is a four-cylinder diesel engine with two overhead camshafts that actuate the 16 valves via hydraulic tappets . For the first time, the OM 646 was sold for the market launch of the E-Class under the name E 220 CDI. In a direct comparison with its predecessor, consumption has been further reduced and the engine performance can be increased. This is due to modifications to over 80 components . For continuous compliance with the exhaust emission values, an on-board diagnosis was madeInstalled system, which monitors all exhaust-related components in the engine and exhaust system. To improve the smoothness, the more powerful models (220 CDI) were equipped with two Lanchester balancer shafts. They rotate in opposite directions at twice the crankshaft speed and eliminate any mass forces caused by the piston movement.

The mechanical fuel pump has been replaced by an electric version, which performs its service in the tank. In order to ensure sufficient interior heating, one was PTC - heater installed. Compared to its predecessor, not the coolant is heated here, but only the air flowing into the interior. Starting in 2003, a soot particulate filter in conjunction with the Euro 4 emission standard was initially offered as an option, and later as standard .

For the first time, this type of engine uses third-generation common rail direct injection . System supplier is Bosch . The fuel distribution takes place as before via a common fuel rail (common rail)in which the fuel pressure is kept continuously high by a high pressure pump. The system pressure was increased compared to the previous engine. Through the line, the fuel enters the solenoid valve-controlled injection nozzles, from which it is injected at a pressure of up to 1600 bar through seven holes in the combustion chamber. The new injectors distribute the fuel even finer in the combustion chamber. New from this engine type are two-fold pilot injections, by means of which a small amount of fuel is introduced before the actual combustion in the cylinder and burned there. As a result, the combustion chamber heats more effectively, thus lowering the temperature and pressure increase in the main combustion. This ensures an improved running smoothness.

The engine is supercharged by a turbocharger with variable turbine geometry , as in the OMI 611 of the CDI Series II . It has adjustable vanes on the exhaust side , which ensure faster buildup of boost pressure. This improves the cylinder filling and thus the torque. The so-called turbo lag is thereby reduced. The boost pressure controlis achieved by adjusting the vanes. This is electronically controlled unlike the predecessor model. Exceptions are the engines in the Mercedes Sprinter. These are realized in the somewhat higher-powered vehicles via a high and a low-pressure supercharger (two-stage supercharging) by means of negative pressure. It uses a wastegate and a wastegate, which significantly improves engine efficiency and responsiveness.

To cool down the compressed hot charge air and bring more oxygen into the combustion chamber, a charge air cooler is installed.

The intake section of the OM 646 is equipped with the EKAS (intake channel deactivation) , which is already known from the OM 611 . Of the two intake ports per cylinder, one channel, the tangential, can be switched off at low speeds. The sucked air flows now completely through the specially shaped second channel, the swirl duct. The resulting swirl leads to a better mixture formation, whereby a better combustion is achieved. The EKAS is controlled electronically and works steplessly. Unlike the predecessor, the pneumatically controlled exhaust gas recirculation valve was replaced by an electronically controlled version. It reduces the excess air and lowers the temperature by exhaust gasesthe sucked fresh air are supplied. This reduces nitrogen oxide emissions and combustion noise. To regulate the exhaust gas recirculation, a hot-film air mass meter is installed, which allows an accurate measurement of the intake air mass.

OM 646 EVO

In 2006, the OM 646 EVO was presented as part of the facelift for the E-Class . It is an advanced version of the OM 646, in which over 90 components have been modified. The compression ratio has been reduced from 18.0: 1 to 17.5: 1 by installing higher pistons and shorter connecting rods. In addition, a new turbocharger with modified charge air cooling was installed. A structure-borne sound sensor in the injection system allows a more appropriate metering of the fuel. The cylinder head has been redesigned to achieve even more effective engine cooling. The air duct was also redesigned. This reduced pressure loss and noise emissions. For improved engine start and cold running behavior were the glow plugsmade of ceramic instead of metal. They allow higher annealing temperatures. The balance shafts previously known from the larger variants were introduced in all OM 646 EVO engines.

Five-cylinder OM 647 and six-cylinder OM 648

The modifications of the OM 611 to the OM 646, with the exception of the balance shafts, were also adopted for the OM 612 (five-cylinder) and OM 613 (six-cylinder) series. The result was the OM 647 and OM 648 series, which differ structurally from the OM 646 by the number of cylinders; Bore and stroke are the same for all engines of this engine family.  The modifications of the OM 646 EVO (four-cylinder) were the series OM 647 (five-cylinder), and OM 648 (six-cylinder) is no longer beneficial, since these by weaker variants of the following V6 engine OM 642 were replaced.

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