Mercedes-Benz OM656 Specifications
| Engine Model: Mercedes-Benz OM656 |
| Engine Type: 4-cycle 6-cylinder DOHC 24-valve turbocharged liquid-cooled diesel |
| Total Displacement: 181.6 cu.in (2.92 L) |
| Rated Engine Power: 282-362 Hp (210-270 kW) at 4200 rpm |
| Fuel System Type: Common rail direct injection |
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The Mercedes-Benz OM656 is a 2.9-liter inline-six
diesel engine introduced in 2017. Replacing the V6 OM642, it is the most
powerful passenger car diesel engine in the brand's history. It shares its
modular architecture with the four-cylinder OM654, utilizing a clean-side design
where all exhaust aftertreatment is mounted directly to the engine. The OM656 is
an all-aluminum engine featuring a stepped-bowl combustion process and a
near-zero friction cylinder coating.
This powerplant represents a comprehensive engineering effort to reconcile high
specific output with stringent emissions regulations, achieving reductions in
internal friction and thermal losses through the integration of steel pistons,
NANOSLIDE cylinder coating, and variable valve lift technology.
Configuration: Inline-six (I6), transverse or longitudinal mounting.
Displacement: 2,925 cc (181.6 cu.in).
Bore: 82.0 mm (3.23 in).
Stroke: 92.3 mm (3.63 in).
Compression Ratio: 15.5:1.
Firing Order: 1-5-3-6-2-4.
Valvetrain: DOHC, 4 valves per cylinder, roller cam followers, hydraulic valve
lash adjusters, CAMTRONIC variable valve lift on exhaust side.
Fuel System: Fourth-generation common rail direct injection.
Injection Pressure: 2,500 bar (36,259 psi) maximum.
Aspiration: Twin-stage turbocharged with two variable nozzle turbine (VNT)
turbochargers in series.
Engine Management: Bosch EDC17 electronic control unit.
OM656D 29 R (350 d)
Power: 210 kW (286 PS; 282 hp) at 3,400-4,600 rpm.
Torque: 600 Nm (443 lb-ft) at 1,200-3,200 rpm.
OM656D 29 (400 d)
Power: 243-250 kW (330-340 PS; 325-335 hp) at 3,600-4,200 rpm.
Torque: 700 Nm (516 lb-ft) at 1,200-3,000 rpm.
OM656M (Mild Hybrid-450 d)
Power: 270 kW (367 PS; 362 hp) from combustion engine + 15 kW (20 PS) integrated
starter-generator EQ Boost.
Torque: 750 Nm (553 lb-ft) at 1,200-3,000 rpm, plus 200 Nm (148 lb-ft) from
integrated starter-generator.
The cylinder block is constructed from a high-strength
aluminum alloy using a sand-casting method. An open-deck design reduces weight
while maintaining structural integrity. The bores receive the NANOSLIDE coating
- an iron-carbon arc-wire sprayed application directly onto the aluminum bore
surface - which produces a mirror-finished, low-friction cylinder wall with
hardness exceeding conventional cast iron.
A separate aluminum bedplate forms the lower crankcase, bolted to the main
bearing webs to provide a rigid structure for the crankshaft. The forged steel
crankshaft rotates in seven main bearings, each with a diameter of 60 mm (2.36
in). Cracked steel connecting rods measure 146.5 mm (5.77 in) between centers.
High-strength steel pistons incorporate a closed cooling channel to manage
thermal loads from peak combustion pressures reaching 200 bar (2,900 psi).
The aluminum alloy cylinder head employs a cross-flow
design with four valves per cylinder. Dual overhead camshafts are driven by a
duplex timing chain located at the rear of the engine on the transmission side,
reducing overall engine length and minimizing torsional vibration. Roller-type
cam followers with low-friction bearings transmit camshaft motion to the valves.
CAMTRONIC variable valve lift on the exhaust side uses adjustable camshaft
elements to switch between two lift profiles, enabling rapid heating of the
close-coupled aftertreatment system during cold starts.
Number of Valves: 24 total (4 per cylinder).
Valve Diameter (Intake): 32.6 mm (1.28 in).
Valve Diameter (Exhaust): 28.0 mm (1.10 in).
Valve Stem Diameter (Intake): 6.0 mm (0.236 in).
Valve Stem Diameter (Exhaust): 6.0 mm (0.236 in).
Valve Seat Inserts: Powder metal inserts for intake and exhaust seats; exhaust
seats feature high-temperature nickel-based alloy for durability under elevated
thermal stress.
The fourth-generation common rail fuel system operates
at a maximum pressure of 2,500 bar (36,259 psi). A two-piston high-pressure pump
mounted at the front of the engine, driven by the timing gear, supplies fuel to
the rail. Piezo injectors capable of up to eight injection events per combustion
cycle enable precise, low-noise combustion. The stepped-bowl combustion geometry
in the steel pistons optimizes air-fuel mixing to minimize particulate
formation.
Injectors: Solenoid-valve or piezo-actuated depending on production year; piezo
design employed for higher-output variants. Fuel Filtration: High-efficiency
fuel filter with integrated water separator and fuel cooler; filtration rating
of 3-5 microns. Fuel Pressure: Idle: 250-300 bar (3,626-4,351 psi); full load:
2,500 bar (36,259 psi).
A variable-displacement vane-type oil pump is mounted
within the oil sump. The pump is regulated by the engine control unit to adjust
oil pressure according to operating conditions, reducing parasitic power loss.
Oil passes through a full-flow filter housed in a module that also contains an
oil-to-coolant heat exchanger for temperature stabilization. An oil level and
quality sensor continuously monitors dielectric properties to assess oil
degradation.
Oil Pump Type: Variable-displacement vane pump with solenoid-actuated control
ring. Oil Pressure: Warm idle: 1.5-2.0 bar (22-29 psi); at operating speed:
3.5-4.5 bar (51-65 psi); maximum regulated pressure: 5.0 bar (73 psi). Oil
Capacity (with filter): 8.5 L (8.98 US qt). Oil Specification: Mercedes-Benz MB
229.71 or MB 229.72 (low-viscosity, low-SAPS).
The cooling system employs a split-flow architecture
with a belt-driven water pump. Electronically controlled rotary valve thermostat
permits variable coolant temperature regulation between 85 °C and 105 °C (185 °F
and 221 °F) to balance rapid warm-up with high-load cooling capacity. A separate
low-temperature circuit services the charge-air cooler, exhaust gas
recirculation cooler, and turbocharger bearing housing.
Water Pump Type: Belt-driven centrifugal pump with composite impeller; flow rate
regulated by engine speed and thermostat position. Coolant Capacity (total
system): 12.5 L (13.2 US qt). Coolant Specification: Mercedes-Benz MB 326.0 or
MB 325.6. Thermostat: Rotary valve with wax element and auxiliary electric
heating element for dynamic temperature control.
Intake System: Two-stage turbocharging system with
high-pressure and low-pressure VNT turbochargers arranged in series. Charge air
passes through a water-to-air intercooler integrated into the low-temperature
coolant circuit. Boost pressure is managed by high-speed electric actuators on
both turbochargers.
Exhaust Aftertreatment: Close-coupled aftertreatment assembly mounted directly
to the engine comprises a diesel oxidation catalyst (DOC), diesel particulate
filter (DPF), first selective catalytic reduction (SCR) catalyst with upstream
urea injection, second SCR catalyst, and ammonia slip catalyst. This
configuration achieves NOx conversion rates exceeding 95% without reliance on
exhaust gas recirculation dilution.
Exhaust Gas Recirculation: High-pressure EGR circuit with water-cooled EGR
cooler and electrically actuated EGR valve; EGR rate managed by engine control
unit based on load, temperature, and NOx sensor feedback.
Electrical System: 12 V primary electrical architecture; mild hybrid variants
(OM656M) incorporate a 48 V integrated starter-generator with a separate 48 V
lithium-ion battery and DC-DC converter.
Length (front to rear): 690 mm (27.17 in)
Width (including exhaust aftertreatment): 590 mm (23.23 in)
Height: 680 mm (26.77 in)
Dry Weight (without aftertreatment, fully dressed): 202 kg (445 lb)
Engine Oil: MB-Approval 229.71 (0W-20) or 229.72
(0W-20) for optimal fuel economy; 229.51 or 229.52 (5W-30) permissible where
low-viscosity grades are unavailable. Oil Change Interval: 25,000 km (15,500 mi)
or 12 months, whichever occurs first; interval governed by oil quality sensor
output. Coolant Change Interval: 250,000 km (155,000 mi) or 15 years for initial
fill; subsequently 125,000 km (77,500 mi) or 5 years.
Fuel Filter Replacement: 60,000 km (37,000 mi). Air Filter Replacement: 60,000
km (37,000 mi); more frequent under dusty operating conditions. Timing Chain:
Duplex chain with hydraulic tensioner; no routine replacement interval
specified; inspection recommended if engine exhibits timing correlation faults.
Valve Clearance: Hydraulic lash adjusters; no adjustment required. Spark Plugs:
Not applicable (diesel engine). Idle Speed: 700 rpm ± 50 rpm. Engine Oil Quality
Sensor: Integral to oil pan; performs continuous dielectric measurement and
recommends service based on oxidation, contamination, and driving profile.
Cylinder Head Bolts (M10), Stage 1: 40 Nm (30 lb-ft) +
90° + 90°
Main Bearing Cap Bolts (M10): 50 Nm (37 lb-ft) + 90° + 90°
Connecting Rod Cap Bolts (M9): 30 Nm (22 lb-ft) + 90° + 90°
Crankshaft Damper Bolt (M18): 200 Nm (148 lb-ft) + 180°
Camshaft Bearing Caps (M8): 12 Nm (9 lb-ft) + 90°
High-Pressure Fuel Pump Mounting Bolts (M8): 21 Nm (15 lb-ft) + 90°
Fuel Injector Clamp Bolt (M6): 8 Nm (6 lb-ft) + 180°
Turbocharger to Exhaust Manifold (M8): 35 Nm (26 lb-ft) + 180°
Engine Mount to Cylinder Block (M10): 50 Nm (37 lb-ft) + 90°
Oil Pan Bolts (M6): 8 Nm (6 lb-ft) + 90°
Oil Pump Mounting Bolts (M8): 18 Nm (13 lb-ft) + 90°
Water Pump Bolts (M6): 10 Nm (7 lb-ft) + 90°
Flywheel/Flexplate Bolts (M12): 60 Nm (44 lb-ft) + 90° + 90°
Egr Cooler to Cylinder Head (M8): 20 Nm (15 lb-ft) + 90°
Thermostat Housing to Cylinder Head (M6): 10 Nm (7 lb-ft)
S-Class: W222 (S 350 d, S 400 d); W223 (S 350 d, S 400
d, S 450 d).
E-Class: W213 sedan, S213 estate (E 350 d, E 400 d); V213 long-wheelbase (E 350
d).
CLS-Class: C257 four-door coupe (CLS 350 d, CLS 400 d).
G-Class: W463 (G 350 d, G 400 d).
GLE-Class: V167 (GLE 350 d 4MATIC, GLE 400 d 4MATIC).
GLS-Class: X167 (GLS 350 d 4MATIC, GLS 400 d 4MATIC).
GLC-Class: X253, C253 (GLC 400 d 4MATIC in selected markets).
Sprinter: VS30 (Sprinter 350 d, Sprinter 450 d with mild hybrid).
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