Engines

Mercedes-Benz OM656 engine (2017-on)



[su_image_carousel source=”media: 52057,52058,52059,52060,52061,52062,52063,52064,52065,52066,52067,52068,52069,52070,52071″]

Introduction

The Mercedes-Benz OM656 is an inline, six-cylinder diesel engine that is part of a modular engine family with:

The OM656 diesel engine effectively replaced the OM642 V6 diesel engine and, according to Mercedes-Benz, consumes around seven per cent less fuel (see table below for comparison). Production of the OM656 diesel engine commenced in 2017 and it was first introduced in the Mercedes-Benz W222 S 350 d and S 400 d. Furthermore, the OM656 is manufactured at Mercedes-Benz’s Untertürkeim plant in Baden-Württemberg.
[su_table responsive=”yes”]

  OM656 OM642
No. of cylinders/arrangement I6 V6
Valves per cylinder 4 4
Displacement per cylinder 488 cc 498 cc
Displacement 2927 cc 2987 cc
Cylinder spacing 90 mm 106 mm
Bore 82 mm 83 mm
Stroke 92.4 mm 92 mm
Rated output Over 230 kW 190 kW
Peak torque Over 650 Nm 620 Nm
Compression ratio 15.5:1 15.5:1
Emissions standard EU6/RDE EU6

[/su_table]

Crankcase

The OM654 engine has an aluminium crankcase with 82.0 mm bores and a stroke of 92.4 mm for a capacity of 2927 cc; like the other OM654, M264, M256 and M176 modular engines, the cylinders are spaced at 90 mm intervals.

Within the cylinder bores, twin-wire arc spraying (TWAS) is used to apply a low-friction coating (‘Nanoslide’) that is based on an iron-carbon alloy. Since the Nanoslide coating results in a microporous surface for the cylinder walls, effective lubrication is achieved without the need for cast-iron cylinder liners. According to Mercedes-Benz, the Nanoslide coating reduces friction between the piston, piston rings and cylinder wall by up to 50 per cent and achieves a mass reduction of several kilograms.

Crankshaft and pistons

The OM656 engine has a forged crankshaft which operates on seven bearings. The OM654 engine features flat steel pistons that have stepped combustion bowls, the first diesel engine for passenger cars to have such as design. According to Mercedes-Benz, the stepped bowl has a positive effect on the thermal loading of critical areas of the pistons and achieves a higher burning rate than the previous ‘omega’ combustion bowl.

While the OM642 engine had aluminium pistons, the OM656 engine has steel pistons. Since steel is stronger than aluminium, the pistons for the OM656 engine have a compact and lightweight design. Furthermore, the lower expansion of steel increases clearance between the piston and the aluminium engine block as operating temperatures rise, reducing friction by 40 to 50 per cent.

Chain drive and accessories

The primary timing chain for the OM656 engine is located on the flywheel side and acts directly on a sprocket attached to the crankshaft. As such, the primary chain drives the high-pressure fuel pump, the oil pump and the camshaft drive gear which, in turn, drives the exhaust camshaft; the exhaust camshaft then drives by the intake camshaft. Beyond this, a second chain track drives the tandem oil pump and the vacuum pump.

For the OM656 engine, a poly-V belt – which is tensioned by a self-tensioning belt tensioner – is driven by the crankshaft belt pulley and drives the coolant pump, the alternator and the refrigerant compressor.

Cylinder head

The cylinder head of the OM656 engine is made of an aluminium-silicon alloy and has a double water jacket. According to Mercedes-Benz, the double water jacket improves cooling and is a more rigid than single jacket designs. Flow openings in the cylinder head gasket between the upper and lower water jackets enable the flow and distribution of coolant inside the cylinder head to be adjusted to achieve optimum operating temperatures. Furthermore, each cylinder has one tangential port and one spiral swirl port which can be switched via the intake port shut-off system.

Mounted in a separate camshaft bearing housing, the two overhead camshafts operate two intake valves and two exhaust valves per cylinder via roller cam followers. The valves are arranged in parallel to optimise the cross-section and strength of the combustion plate. Unlike the OM654 four-cylinder diesel engine, the OM656 engine has Mercedes-Benz’s ‘Camtronic’ variable exhaust valve lift which ‘supports the consumption-neutral heating of the exhaust system’.

Two-stage exhaust turbocharging

The OM656 engine has two exhaust turbochargers – a smaller turbocharger with variable turbine geometry and a larger turbocharger – that are positioned in series. At low loads, only the smaller, low-inertia turbocharger operates. As engine speed increases, however, the larger turbocharger commences operation.

With variable turbine geometry, the guide vanes are moved to a closed position to increase boost pressure since this reduces the flow cross-section between the vanes and increases the inlet speed of the exhaust gases into the turbine wheel. Conversely, the guide vanes are moved to an open position to reduce boost pressure.

Charge air cooler and charge air manifold

Once the compressed and heated air exits the turbocharger, it is cooled by a charge air cooler (or ‘intercooler’) and fed via the throttle valve actuator to the charge air manifold; the air then flows into the combustion chambers. To improve mixture formation, the intake port shutoff actuator motor can open or close air ducts which are integrated in the charge air manifold, since changes in flow rate and increased swirl can produce for more efficient combustion.

Fuel system and injection

The OM656 engine has a fourth-generation common-rail injection system. Initially, fuel is supplied by the low-pressure fuel pump and compressed by the high-pressure fuel pump, while fuel quantity is regulated according to requirements via the quantity control valve. The fuel is then routed to the common rail where the pressure regulating valve regulates fuel pressure in the fuel rail to approximately 2500 bar (based on the signal of the high-pressure fuel pressure sensor). The fuel is then fed via high-pressure lines to the eight-hole piezo fuel injectors.

The MRD1 electronic engine management system calculates the injection period and the fuel pressure on the basis of the fifteen sensors and signals. Furthermore, the injection control system has the following sub-functions –

  • Pre-injection: up to two pre-injections (i.e. before the main injection) to reduce combustion noise and exhaust emissions;
  • Main injection; and,
  • Post-injection: used to increase exhaust gas temperatures, assist the regeneration process of the diesel particulate filter and the conversion process of the exhaust components in the oxidation catalytic converter.

Exhaust and emissions

The OM656 has the following emissions reducing items located downstream of the turbocharger:

  1. A diesel oxidation catalytic converter;
  2. A third generation Selective Catalytic Reduction (SCR) system. The SCR system injects an aqueous urea solution (‘AdBlue’) into the exhaust system to cause thermolysis and hydrolysis reactions which reduce nitrogen oxides in the exhaust gases;
  3. An SCR-coated diesel particulate filter (sDPF); and,
  4. An SCR catalytic converter.

Multi-way Exhaust Gas Recirculation (EGR)

The OM656 engine has multi-way exhaust gas recirculation (EGR) with cooled high-pressure and low-pressure exhaust gas recirculation. According to Mercedes-Benz, the interaction of the low- and high-pressure EGR actuators permits a high EGR rate with no losses in efficiency.

For EGR, the exhaust gas is taken directly from the exhaust manifold, cooled by a heat exchanger that is integrated in the coolant circuit and supplied to the charge air manifold. EGR reduces nitrogen oxide (NOx) emissions in exhaust gases by reducing the oxygen concentration in the combustion chamber and this process is assisted by the reduction in combustion temperature that occurs due to the higher heat capacity of the recirculated exhausts gases (relative to the intake air).

For the OM656 engine, all EGR components are mounted to the engine rather than under the vehicle floor as with the OM642 engine. While EGR is active from idle up to the ‘upper partial-load range’, the low-pressure EGR circuit is only active at coolant temperatures above 60 degrees Celsius and in the idle to mode-rate partial-load ranges.
[su_table responsive=”yes”]

Model Engine Peak power Peak torque
2017 W222 S 350 d 3.0-litre OM656 turbo diesel I6 210 kW at 3400-4600 rpm 600 Nm at 1200-3200 rpm
2017 W222 S 400 d 3.0-litre OM656 turbo diesel I6 250 kW at 3600-4400 rpm 700 Nm at 1200-3200 rpm

[/su_table]


Back To Top