Mercedes-Benz OM668 Specifications
| Engine Model: Mercedes-Benz OM668 |
| Engine Type: 4-cycle 4-cylinder DOHC 16-valve turbocharged liquid-cooled diesel |
| Total Displacement: 103.1 cu.in (1.69 L) |
| Rated Engine Power: 60-95 Hp (44-70 kW) at 4200 rpm |
| Fuel System Type: Common rail direct injection |
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The Mercedes-Benz OM668 is a 1.7-liter, inline-four
turbocharged diesel engine that holds a significant position in the history of
automotive powertrain development as the brand’s first small-displacement
common-rail diesel. Introduced in 1997 alongside the first-generation A-Class
(W168), the OM668 was engineered specifically for the unique packaging
constraints of the “sandwich” floor architecture, which required a highly
compact engine capable of sliding beneath the passenger compartment in the event
of a frontal collision. This safety concept dictated a pronounced forward tilt
of 59 degrees, influencing every aspect of the engine’s design.
The OM668 combines a cast iron cylinder block with an aluminum alloy cylinder
head, featuring a dual overhead camshaft (DOHC) valvetrain with four valves per
cylinder - a configuration that was advanced for a small diesel at the time. Its
common-rail direct injection system, operating at pressures up to 1,350 bar,
represented a significant leap forward from the mechanical injection systems
used in previous Mercedes-Benz diesel engines.
The engine was offered in two power variants, designated CDI (Common-rail Direct
Injection), and incorporated a balance shaft and dual-mass flywheel to achieve
the refinement expected of a Mercedes-Benz passenger car. The OM668 powered the
A-Class and its Vaneo derivative throughout their production runs, establishing
the foundation for Mercedes-Benz’s subsequent family of compact common-rail
diesel engines.
The OM668 is a four-stroke, water-cooled, turbocharged diesel engine configured
for transverse mounting in front-wheel-drive vehicle architectures. Its defining
characteristic is the 59-degree forward tilt from the vertical axis, a feature
integral to the Sandwich Concept safety system.
Engine Type: Inline-4, 4-cycle, water-cooled, turbocharged diesel.
Displacement: 1,689 cc (103.1 cu.in).
Bore and Stroke: 80.0 mm (3.15 in) x 84.0 mm (3.31 in).
Compression Ratio: 19.0:1.
Valvetrain: DOHC, 16-valve (4 valves per cylinder) with chain drive.
Aspiration: Turbocharged with intercooler.
Fuel System: Common-rail direct injection (first generation).
Installation Angle: 59° forward tilt from vertical.
Firing Order: 1-3-4-2.
Redline: 4,800 rpm.
OM668 DE 17 A (A 160 CDI)
Maximum Power (early): 44 kW (60 Hp) at 3,600 rpm.
Maximum Power (late): 55 kW (75 Hp) at 4,000 rpm.
Maximum Torque: 160 Nm (118 lb-ft) at 1,500-2,800 rpm.
OM668 DE 17 LA (A 170 CDI)
Maximum Power (early): 66 kW (90 Hp) at 4,200 rpm.
Maximum Power (late): 70 kW (95 Hp) at 4,200 rpm.
Maximum Torque: 180 Nm (133 lb-ft) at 1,600-3,200 rpm.
The cylinder block is manufactured from gray cast iron,
a material selected for its rigidity and durability under the high cylinder
pressures generated by turbocharged diesel combustion. Despite the cast iron
construction, the block was engineered for reduced mass through optimized wall
thicknesses and a compact overall design. The block features a closed-deck
configuration, with the cylinder bores fully enclosed by the water jacket
structure at the top deck surface, ensuring maximum cylinder head gasket
clamping force and resistance to bore distortion.
Cast-in-place cylinder bores are machined directly into the iron casting, with a
finished surface honed to precise tolerances. The block is designed to
accommodate the 59-degree forward tilt installation angle, with the oil pan and
oil pickup configured to maintain proper lubrication under all operating
conditions. A structural aluminum oil pan is bolted to the lower crankcase. The
block incorporates a balance shaft housing integrated into the lower crankcase,
with twin balance shafts rotating at twice engine speed to counteract the
second-order vibrations inherent to inline-four diesel engines.
Block Material: Cast iron.
Bore Diameter: 80.0 mm (3.15 in).
Main Bearing Caps: Cast iron, two-bolt design.
Balance Shafts: Twin shaft system with gear drive, housed in lower crankcase.
The cylinder head is constructed from aluminum alloy
and utilizes a DOHC valvetrain with four valves per cylinder - two intake and
two exhaust. This four-valve configuration was advanced for a diesel engine of
this era, providing improved volumetric efficiency and allowing central
placement of the fuel injector within the combustion chamber for optimal mixture
formation. The valve angle is exceptionally narrow, a design choice made to
reduce the overall width of the cylinder head and accommodate the engine’s
compact packaging within the A-Class engine bay.
The camshafts are driven by a silent-type roller chain from the crankshaft, with
an automatic hydraulic tensioner maintaining optimal chain tension. The
valvetrain employs bucket-type tappets with mechanical valve clearance
adjustment - hydraulic lash adjusters were not used in this design. The
combustion chambers are of the direct injection type, with a re-entrant
bowl-in-piston design that promotes efficient air-fuel mixing and complete
combustion. The cylinder head incorporates integrated intake and exhaust ports
optimized for the turbocharged configuration.
Head Material: Aluminum alloy.
Valve Configuration: DOHC, 4 valves per cylinder with bucket tappets.
Valve Diameter (Intake): 25.0 mm (0.98 in).
Valve Diameter (Exhaust): 23.0 mm (0.91 in).
Valve Stem Diameter: 6.0 mm (0.24 in).
Valve Seat Angle: 45 degrees.
Camshaft Drive: Roller chain with hydraulic tensioner.
The OM668 employs a first-generation common-rail direct
injection system, a significant technological advancement at the time of its
introduction. A high-pressure pump - a radial-piston type - draws fuel from the
tank and delivers it to a common rail accumulator mounted on the cylinder head.
The system operates at pressures up to 135.0 MPa (1,350 bar), substantially
higher than mechanical injection systems, enabling finer fuel atomization and
more precise injection timing control.
Four solenoid-actuated fuel injectors, one per cylinder, are controlled by the
engine control unit (ECU). The ECU calculates injection parameters based on
input from the accelerator pedal position, crankshaft position, camshaft
position, boost pressure, coolant temperature, and fuel rail pressure sensors.
Multiple injection events per combustion cycle - pilot injection followed by
main injection - reduce combustion noise and improve cold-start performance. A
fuel cooler is integrated into the system to maintain stable fuel temperatures.
Injection Type: Common-rail direct injection.
Maximum Injection Pressure: 135.0 MPa (1,350 bar).
High-Pressure Pump: Radial-piston type, cam-driven.
Injector Type: Solenoid-actuated, 7-hole nozzle.
Fuel Requirement: Diesel fuel with minimum cetane number of 51.
Fuel Filter: Replaceable cartridge type with integrated water separator.
The lubrication system is a fully pressurized, wet-sump
design engineered to accommodate the 59-degree engine tilt angle. A gear-type
oil pump, driven by the balance shaft assembly, draws lubricant from a specially
shaped cast aluminum oil pan through a mesh strainer positioned to maintain oil
pickup under all operating conditions. The oil pan incorporates internal baffles
and a windage tray to prevent oil starvation during cornering and acceleration.
Oil is circulated through a full-flow, replaceable oil filter before entering
the main oil gallery. An oil cooler, integrated into the oil filter housing,
uses engine coolant to stabilize oil temperature. The turbocharger bearing
housing receives pressurized oil through a dedicated supply line, with a drain
returning oil to the sump. The balance shaft bearings are also supplied from the
main oil gallery.
Oil Type: SAE 5W-30 or 5W-40 (MB 229.3 or MB 229.5 specification).
Oil Capacity (with filter): 5.0-5.5 liters (5.3-5.8 US qt).
Oil Pressure (at idle): 100 kPa (14.5 psi) minimum.
Oil Pressure (at 3,000 rpm): 300-400 kPa (43.5-58.0 psi).
Oil Pump Type: Gear-type, balance shaft-driven.
The cooling system is a pressurized, liquid-cooled
design engineered to manage the thermal load of a turbocharged diesel engine in
a tightly packaged front-wheel-drive configuration. A compact cross-flow
aluminum radiator with a dual-speed electric fan is mounted at the front of the
engine bay. A centrifugal-type water pump, driven by the timing chain,
circulates coolant through the cylinder block, cylinder head, turbocharger
bearing housing, and the oil cooler.
A wax-pellet thermostat regulates coolant flow to maintain optimal operating
temperature, with a bypass circuit that allows the engine to warm rapidly. The
cooling system incorporates an auxiliary electric water pump in certain variants
to circulate coolant through the turbocharger housing after engine shutdown,
preventing localized overheating. The system is designed for the tight packaging
constraints of the A-Class engine bay.
Coolant Type: Ethylene-glycol based antifreeze (MB 325.0 specification).
Coolant Capacity: 6.5-7.0 liters (6.9-7.4 US qt).
Thermostat Opening Temperature: 82-86 °C (180-187 °F).
Radiator Cap Pressure: 120-140 kPa (17.4-20.3 psi).
Water Pump Type: Centrifugal, timing chain-driven.
Turbocharger: Single-scroll, fixed-geometry
turbocharger with a wastegate control system. The turbocharger is mounted on the
exhaust manifold and provides boost pressure of approximately 0.8-1.0 bar
(11.6-14.5 psi) depending on variant. An air-to-air intercooler (charge air
cooler) is mounted at the front of the vehicle to reduce intake air temperature.
Intake System: Air is drawn through a dry-type paper element air filter before
entering the turbocharger compressor. Compressed air passes through the
intercooler and a swirl-control intake manifold before entering the cylinder
head.
Exhaust System: Stainless steel exhaust manifold with integrated turbocharger
mounting flange. The exhaust system incorporates an oxidation catalytic
converter and a diesel particulate filter on later variants.
Timing Drive: A silent-type roller chain serves as the primary timing drive,
connecting the crankshaft sprocket to the intake camshaft sprocket. A secondary
chain connects the intake and exhaust camshafts. An automatic hydraulic
tensioner maintains optimal chain tension. The balance shaft assembly is driven
by a separate gear train from the crankshaft.
Overall Length: 560 mm (22.0 in)
Overall Width: 620 mm (24.4 in)
Overall Height: 650 mm (25.6 in) (as installed with 59° tilt)
Dry Weight (complete engine): 145 kg (320 lb)
Oil Change Interval: 15,000 km (9,000 mi) or 1 year
under normal driving conditions; shorter intervals under severe conditions. Oil
Filter Change: At every oil change. Timing Chain: Inspection for noise and
stretch; no scheduled replacement interval. Engine Coolant Replacement: 120,000
km (75,000 mi) or 5 years.
Fuel Filter Replacement: 40,000 km (25,000 mi) or as indicated by water-in-fuel
warning. Fuel Water Separator Drain: Every 10,000 km (6,000 mi) to remove
accumulated water. Glow Plug Inspection: 60,000 km (37,000 mi) for resistance
and heating performance.
Valve Clearance Adjustment: Inspect and adjust as required. Intake: 0.20-0.30 mm
(0.0079-0.0118 in) cold. Exhaust: 0.30-0.40 mm (0.0118-0.0157 in) cold. Drive
Belt (Accessory) Inspection: Every 15,000 km (9,000 mi) for tension, cracking,
and glazing. Air Filter Replacement: 30,000 km (18,000 mi) or as indicated by
service indicator. Timing Chain Tensioner: Inspect for proper function at major
service intervals.
Cylinder Head Bolts (M10 x 1.25), 1st pass: 30 Nm (22
lb-ft).
Cylinder Head Bolts (M10 x 1.25), 2nd pass: 60 Nm (44 lb-ft) + 90° rotation.
Main Bearing Cap Bolts (M8 x 1.25): 1st pass: 20 Nm (15 lb-ft) + 90° rotation.
Connecting Rod Cap Bolts (M7 x 1.0): 1st pass: 20 Nm (15 lb-ft) + 90° rotation.
Camshaft Bearing Cap Bolts (M6 x 1.0): 10-12 Nm (89-106 lb-in).
Intake Manifold Bolts (M6 x 1.0): 10-12 Nm (89-106 lb-in).
Exhaust Manifold Nuts (M8 x 1.25): 20-25 Nm (15-18 lb-ft).
Glow Plugs: 15-20 Nm (11-15 lb-ft).
Fuel Injector Clamp Bolts: 20-25 Nm (15-18 lb-ft).
Oil Pan Bolts (M6 x 1.0): 10-12 Nm (89-106 lb-in).
Water Pump Bolts (M6 x 1.0): 10-12 Nm (89-106 lb-in).
Turbocharger Mounting Nuts (M8 x 1.25): 25-30 Nm (18-22 lb-ft).
Crankshaft Pulley Bolt (M16 x 1.5): 150-170 Nm (111-125 lb-ft).
Flywheel / Drive Plate Bolts (M10 x 1.25): 30 Nm (22 lb-ft) + 90° rotation.
Balance Shaft Housing Bolts (M8 x 1.25): 20-25 Nm (15-18 lb-ft).
The Mercedes-Benz OM668 engine was utilized exclusively
in vehicles built on the Sandwich Concept platform, which featured the tilted
engine mounting and raised floor structure for enhanced frontal impact safety.
Mercedes-Benz A-Class (W168): 1997-2004 model years. The OM668 powered the A 160
CDI (44 kW and later 55 kW variants) and the A 170 CDI (66 kW and later 70 kW
variants).
Mercedes-Benz Vaneo (W414): 2001-2005 model years. This compact multi-purpose
vehicle utilized the 1.7 CDI engine in various power outputs.
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