Mercedes-Benz OM654 diesel engine

Daimler's new 2L inline four-cylinder modular diesel engines

2018/02/01

Summary

Since the VW emissions scandal in 2015 there is public concern about the harmful emissions of diesel engines. In response, Daimler announced its plans to develop a new modular diesel engine family, emphasizing its commitment to continue making diesel engines in the future. This report introduces the 2L inline 4-cylinder OM 654 engine, which is the first in its new family of modular diesel engines.

Outline of Daimler new modular engine

Daimler is developing a family of new generation internal combustion engines (ICE). The engine line-up will include 5 models: 2L four-cylinder inline gasoline & diesel engines, 3L 6-cylinder inline gasoline & diesel engines, and a 4L V8 gasoline engine.

The advantages of modularization includes the common use of components and the ability to share assembly lines, but one major objective is the reduction of development costs and time. By using a common bore diameter and stroke length it is possible to conduct a single test for time-consuming development tasks such as combustion analysis, emissions, and fuel efficiency testing. As a result, Daimler expects to introduce the five engine models to market more quickly.

Generally speaking, when the balance between output and fuel consumption is considered, the displacement per cylinder is 400 to 500cc for the gasoline engines and 500 to 600cc for the diesel engines. It is likely that Daimler decided on an engine displacement of 500cc to share components on both the gasoline and diesel engines and to share assembly lines.

A key characteristic of the new modular engine, the main variant for mass-market production, is that the engine displacement has been downsized from 3L to 2L. Another characteristic is that Daimler reverted to using an inline engine instead of a V-shaped 6-cylinder engine. There are two reasons for this. First, the overall length of a conventional inline 6-cylinder engine is long, so a V6 needs to be adopted for safety reasons in the event of a collision. However, the length of the engine can be shortened by decreasing the bore diameter to improve thermal efficiency, adopting a linerless aluminum cylinder block, and shortening the bore pitch, making it possible to use an inline six-cylinder engine configuration. Secondly, it is easier to develop a modular design for an inline 6-cylinder engine than it is for a V6 engine. In an inline 6-cylinder engine, it is only necessary to add two cylinders to an inline 4-cylinder engine to use a common layout for the auxiliary equipment. Conversely, in a V6 engine, the number of parts such as two pieces of cylinder-heads, increases the complexity and tightness of the auxiliary equipment layout because of shorter and wider characteristics of V6 engine.


Engine exterior (Source: Daimler)
New 2L inline four-cylinder diesel engine

 

Outline of the OM 654 engine

In the spring of 2016 Mercedes-Benz released the E220d Coupe equipped with a 2L four-cylinder inline diesel engine. The OM 654 engine was first engine of its new family of modular engines to be released, succeeding the predecessor OM 651 engine (2.2L displacement). The engine can be installed in both the longitudinal and transverse mounting positions and for 4WD vehicle applications. By reducing the engine displacement by about 10% and using an aluminum cylinder block, the packaging dimensions can be miniaturized and the weight of the engine can be reduced by about 35 kg. The exhaust aftertreatment system complies with the current Euro 6 emission standard, with a fuel consumption (CO2 emissions) of 102-112 g/km. Also, the front and upper sides of the engine are enclosed with a shielding plate to suppress engine noise.

The new 2L engine was designed to increase the potential to achieve a high specific output performance of up to 90 kW/L. In addition, the new diesel engine was designed to comply with the Worldwide harmonized Light vehicles Test Procedure (WLTP) cycle established in 2017 to measure real-world emissions and meet future Real Driving Emission (RDE) legislation and to make the engine smaller and lighter to improve fuel economy.


Engine exterior (Source: Daimler)
OM 654 engine with a cube-like compact layout

 

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Basic specifications of OM 654 engine

The engine is an inline 4-cylinder DOHC four-valve engine + turbocharger, with a 2-axis secondary balancer shaft. Fuel is supplied via a common rail direct fuel injection system. The compression ratio of 15.5 is slightly lower than the 16.2 of the previous engine, reducing combustion noise. NOx, HC, and PM engine emissions are reduced by changing the shape of the piston crown and the adoption of steel pistons.

Compared to the previous OM 651 engine, in the OM 654 engine, emissions are reduced by about 10%, maximum output improves by 14%, and fuel consumption improves by 15%. This performance has been achieved by reducing the bore diameter by 1 mm, shortening the stroke by 6.7 mm, and shifting the stroke-to-bore (S/B) ratio slightly higher speed type from 1.19 to 1.12.

The OM 654 engine is the first diesel engine to adopt an aluminum cylinder block, reducing the weight of the engine by about 35 kg as well as a rationalized exhaust aftertreatment system.
Output performance curve (Source: Daimler)
Although the displacement has been reduced by 10%, the new engine delivers a maximum output of 143 kW, representing a 14% increase from previous engine. It is worth noting that the engine is able to maintain a maximum torque equivalent to 400 Nm. However, engine speed (rpm) at maximum torque is 200 rpm slower due to the reduced displacement of the new engine versus its predecessor, and is 400 rpm faster at low operating loads.

 

Engine specifications

Source: Created based on Daimler materials


Engine block

The aluminum cylinder block consists of deep cylinder bank skirts and a boxed construction to minimize deformation of the cylinder block during combustion. By using a cast iron bearing cap, the metallic rattling sound during engine warm-up is suppressed due to the wider play/gap with the crankshaft. NANOSLIDE coating (*1) is applied to the cylinder surfaces and steel pistons are adopted to optimize the clearances between the two during engine cooling and operation. In addition, the center of the crankshaft is offset 12 mm toward the intake side from the centerline of the piston cylinder (*2). These enhancements make it possible to eliminate the cast iron liner, reduce weight, and make the engine more compact by shortening the bore pitch and the deck height of the block, as well as making the engine run quieter and reducing side force friction between the piston and cylinder wall by about 50%.

 

*1) NANOSLIDE coating
A coating method used to apply a metal alloy at high temperatures to the inner surfaces of the cylinders using twin-wire arc spraying. This technology results in a mirror-smooth surface of the cylinder, making it lighter and more compact than conventional cast-iron liners. The linear steel used is an iron-carbon metal alloy wire with a coating thickness of 0.1 to 0.15 mm. The coating features low friction and high durability. Prior to applying the NANOSLIDE coating, the cylinder surfaces are roughened using a high-pressure 3000 bar water jet to improve the ride of the coating.

*2) Crankshaft offset
Crankshaft offset is a technique used to reduce the side force (lateral force) of the piston by offsetting the centerline of the piston and the centerline of the crankshaft. Since the lateral thrust force of the piston during the power stroke is smaller than during the exhaust stroke, the center of the crankshaft is offset toward the anti-thrust side to lower the angle of the connecting rod, reducing friction during the exhaust stroke.

The cylinder head is made of aluminum alloy, and the camshaft holder is made separate to improve the productivity of the casting and to reduce weight. This design also solves the problem resulting from the camshaft overlapping with the head bolt. The camshaft bearings are supported at 5 points, and the camshaft is driven by gears and chains on the engine rear side.

The two intake ports for each cylinder are independent and Siamese ports are used on the exhaust side. The engine cooling circuit layout is designed so that the coolant circulates sufficiently around the combustion chamber and the exhaust ports to handle the heat loads common in the harsh combustion environment of diesel turbo engines.

 

Aluminum cylinder block (Source: Daimler) Cylinder head and valve train components (Source: Daimler)
Aluminum casting was adopted for the first time in a Mercedes-Benz diesel engine. The aluminum cylinder block is likely contributing for about 30% of the weight reduction of 35 kg in the new engine.
The steel pistons have been shortened by more than 15 mm, reducing the distance between the connecting rods and the height of the cylinder block.
The cylinder head is made of aluminum alloy. The cam holder is made separately to improve productivity and reduce weight.
Four camshaft support bearings are located at the center of each cylinder (between the valves) and one at the camshaft drive gear.


Main drive system

By adopting steel, which is stronger than aluminum, for the pistons it was possible to minimize the dimensions of the ring land and the skirt to that of gasoline engines. As a result, the crevice volumes of the combustion chamber were greatly reduced, suppressing the generation of HC and PM during combustion. In addition, by shortening the piston compression height by more than 15 mm it was possible to reduce the deck height of the cylinder block, contributing to a more compact engine configuration.

A stepped-bowl piston geometry has been adopted for the crown of the steel pistons to eliminate local hot spots and evenly distribute temperatures over the piston surface, thereby suppressing generation of NOx pollutants during combustion.

Steel piston (Source: Daimler) Steel piston (Source: Daimler)
The photo above shows the new steel piston on the left and a conventional aluminum piston on the right. The use of steel dramatically improves piston strength and makes it possible to drastically shorten the land size. As a result, fuel can be prevented from entering the cylinder crevice to mitigate the formation of HC and PM pollutants due to unburned fuel. The photo above shows the cooling channel at the rear of the piston ring groove. The oil in the cooling channel cools the ring groove to prevent the ring from sticking. The photo also shows how the fuel injected from the injector at the top of the piston enters the back side of the flange shaped piston. The glow plug arranged diagonally to the piston ignites the air-fuel mixture during cold start.

 

The connecting rod and the crankshaft are made of high-strength forged steel. The height of the engine has been lowered by shortening the stroke and reducing the distance between the centerlines of the connecting rods while keeping the connecting rod ratio (l/r) equivalent to that of the previous engine.

A standard 4CW crankshaft is used to reduce weight and twisting vibration. With currently available high performance metal bearings, it is more advantageous to use fewer counterweights to reduce weight than decreasing the load of the journal metal by using full counterweights.

By locating the helical gear between the #4 pin and #5 journal of the crankshaft, the balancer shafts on both sides of the crankshaft are driven at double speed to cancel secondary inertia force. This also allows the use of a scissors gear to suppress backlash. The drive chain sprocket is attached to the rear end of the crankshaft.

A crank damper is located at the front end of the crankshaft to suppress the crankshaft's peak level of torsional vibration due to combustion input.

4CW Crankshaft (Source: Daimler) 2-axis secondary balancer shaft (Source: Daimler)
Full counterweights are not used to give priority to weight reduction even though the load to the journal metal may increase.
The plate attached to the rear of the drive plate is the crank angle signal plate used to detect the crankshaft position and top dead center (TDC).
The secondary balancer shaft is built into the oil pan.
The helical gear before the crankshaft #5 journal turns the two balancer shafts on the right and left at double-speed.


Valves and valve train system

The air intake and exhaust valves are arranged at a 10 degree angle and the high pressure fuel injectors are located at the center of the combustion chamber. The valve train is driven by a 4-valve DOHC camshaft via an aluminum alloy swing-type rocker arm at the outer fulcrum.

The hydraulic rocker arm pivot keeps the valve clearance always at 0. The roller rocker arm is attached to the cam lobe to reduce friction at low speeds.

The cam shaft assembly uses thin-walled hollow steel for the shaft and an iron-based sintered alloy for the cam lobe, which is friction welded to the shaft.

The camshaft is driven by a chain in the first stage and a helical scissors gear in the second stage.

Valve train system layout (Source: Daimler) Camshaft drive system (Source: Daimler)
The valve train is driven in the space at the rear end of the engine. Two secondary balancer shafts and the oil pump are built into the oil pan. The camshaft drive gear and the high pressure fuel injection pump are driven by a chain connected to the crankshaft. The oil pump is driven by a chain connected to a sprocket coaxial with the sprocket driving the high-pressure fuel injection pump.


Intake and exhaust system

Air taken in from the fresh air intake duct, located at the front center area of the engine, enters the air cleaner located on the left side of the engine and circulates to the rear of the engine before being led to the turbocharger intake compressor on the right side of the engine. The air supercharged by the compressor recirculates around the rear of the engine and is cooled by the water-cooled intercooler on the left side before being guided to the intake port.

Engine Exterior (Source: Daimler) Engine Exterior (Source: Daimler)
The photo above shows the DOC in the center and the SDPF on the left. The low-pressure EGR passage is the pipe extending from the rear of the SDPF system, passing through the rear of the engine and heading toward the intake side.

 

The turbine side of the turbocharger faces the engine front, and the exhaust gas from the turbine flows through the DOC (oxidation catalyst), located below the turbocharger, before entering the SDPF (a DPF with SCR-coating: a selective catalytic reduction method with a coating to filter particulate matter).

The variable geometry type turbine vane is throttled to increase exhaust flow velocity at low speeds to improve response. In addition, the compressor wheel and turbine rotor have been downsized to help improve response.

Furthermore, the turbocharger housing is water-cooled to prevent caulking of the bearings when the engine is stopped.

The swing valve actuator has good response and is electronically controlled to prevent any deterioration in response.

Engine Exterior (Source: Daimler) Turbocharger (Source: Daimler)
In the photo above, the turbocharger, DOC, and SDPF are compactly arranged on the right side of the engine. In the photo above, the exhaust turbine is on the left side and the air intake compressor is on the right side. The variable geometry vane of the exhaust turbine increases flow velocity at low speeds to improve response.
The swing valve actuator is electronically controlled.


Fuel supply system

The camshaft chain drives the high pressure fuel pump to both compress the fuel and send it to the common fuel rail. The fuel is distributed from the common fuel rail to the high pressure fuel injector of each cylinder to be directly injected (DI) into the cylinders. Piezo type high pressure fuel injectors are used, with a maximum pressure of 2050 bar. Fuel is injected at a maximum of 6 times during each stroke.
Common rail system  (Source: Daimler)
Fuel compressed by the high-pressure fuel injection pump passes through the common rail to be injected by the fuel injector into each cylinder. The fuel injectors are controlled by the ECU, which signal the injectors to spray fuel several times into the combustion chamber through the injector.


Accessories and accessory drive system

The accessory drive basically turns the alternator and air conditioner compressor with a serpentine belt on the engine front side. On the engine rear side, the high pressure injection pump and the oil pump are driven by the chain driving the cam shaft.

The gear type oil pump is located in the oil pan and chain-driven via a sprocket coaxial with the sprocket of the chain driving the camshaft.



Exhaust aftertreatment system

In the previous aftertreatment system, exhaust gas passed through the oxidation catalyst, the diesel particulate filter (DPF) and then the SCR catalytic converter installed in the middle of the exhaust system. In the new system, it is possible to shorten the exhaust gas purification path by directly attaching the integrated SDPF catalytic converter (DPF with SCR Coating: diesel particulate matter filter coated with a selective catalytic reduction coating) onto the engine itself. This configuration not only prevents a reduction of purification efficiency due to lower exhaust gas temperatures, but also results in smaller packaging and weight reduction of the system. As can be seen from the engine exterior all of the elements of the systems are configured on the engine itself.

The AdBlue urea needs to be replenished every 25,000 km and the inlet is located beside the fuel injection port.

Exhaust aftertreatment system (Source: Daimler) AdBlue Inlet (Blue Cap)
The top figure above is the previous exhaust aftertreatment system and the bottom figure is for the new engine. Although a large system previously, the new system is more compact by configuring it on the engine, adopting an SDPF, a DPF with SCR coating. Urea (AdBlue) is replenished every 25,000 km.


Reference material: Evolution of the diesel engine

Evolution of 4-cylinder premium diesel engines over the past 80 years
(Source: Daimler)
Test schedule close to real-world fuel consumption
(Source: Daimler)
Compared to its predecessor, the new engine improves output by 14% and lowers fuel consumption by 15%. The diagram above compares the previous NEDC cycle test pattern and the new WLTP cycle introduced in 2017. The new mode has a longer cycle time and a higher maximum speed.


Reference material: Specification comparison of the new OM654 engine and the previous OM651 engine

Engine specifications

Engine New OM654 Predecessor OM651
No. of cylinders/Arrangement In-line four cylinder engine
Displacement (cc) 1950 2143
Bore×Stroke (mm) 82×92.3 83×99
Bore/stroke ratio 1.12 1.19
Bore pitch (mm) 90 94
Crankshaft offset (mm) 12 to the intake side None
Compression ratio 15.5 16.2
Valve arrangement DOHC 4 valve
Rated output/rpm (kW/rpm) 143/3800 125/3000-4200
Peak torque/rpm (Nm/rpm) 400/1600-2400 400/1400-2800
Supercharger Single Geometry Vane 2-stage Twin
Fuel supply system Common-rail
Aftertreatment system DOC+SDPF DOC+DPF+SCR
Engine weight(kg) DIN 168.4 203.8

Engine block

Cylinder block material Aluminum alloy deep skirt Cast iron deep skirt
Cylinder bore surface treatment Surface Coating (NANOSLIDE) None (Honing)
Block top deck shape Closed
Bore-to-bore cooling method Drill hole processing Water passage
Bearing cap Cast iron
Cylinder head material Aluminum alloy
Valve pinch angle 10deg 6deg
Fuel injection valve position Center of combustion chamber
Camshaft holder Separate, 5 bearings Single body, 5 bearings
Camshaft supporting position Between valves
Cylinder block lower case Aluminum die-cast
Oil pan Back bulge, Resin
Proximity shield plate Engine top and front side Engine top

Main drive system

Piston material Steel (2 split welds) Aluminum alloy
Skirt: Molybdenum disulfide coat Attached (thrust / anti-thrust)
Cooling channel Attached
Top ring trimmer None Attached
Crown shape Stepped bowl (flanged shape) Ωtype
Piston pin support Full float
Piston ring 3 (Top, 2nd, Oil)
Connecting rod material Forged steel
Distance between center of connecting rods(mm) 140 144
Link ratio (l/r) 3.03 2.91
Crankshaft material Forged steel
Counterweight number 4CW 8CW
Balancer shaft drive gear position # 5 before journal
Camshaft drive sprocket position Between ♯5 journal and rear flange
Damper pulley Attached (torsional vibration)
Balancer shaft Dual axis, quadruple reduction helical gear drive

Valves and valve train system

Camshaft drive Two stage deceleration (single chain / helical gear) One stage reduction (double chain)
Camshaft drive position Engine rear side Engine front side
Camshaft Assembly type
Shaft Material / Cam Lobe Material Hollow steel pipe / ferrous sintered alloy
Intake valve / exhaust valve 2 valves / 2 valves
Valve spring Single
Rocker arm Swing arm type
Pivot External fulcrum, hydraulic type
Roller rocker Attached (to the cam lobe)

Intake and exhaust system

Air cleaner Mounted on engine intake side (left side) Mounted on engine exhaust side (right side)
Supercharger Single turbo, exhaust side variable vane type 2 stage twin turbo
Swing valve control Electronic actuator Mechanical actuator
Intercooler Water cooled engine mounted on the left rear side Air-cooled type
EGR retrieval Turbo upstream (high pressure) and downstream (low pressure) Turbo upstream (high pressure)

Fuel supply system

Fuel supply system Fourth Generation Common Rail Type
Fuel injection system DI (in-cylinder direct injection)
Fuel injection valve / fuel injection pressure Bosch piezo type / maximum 2050 bar Bosch piezo type / max 2000 bar

Accessories and accessory drive system

Accessory drive system Serpentine belt type
High pressure fuel injection pump Chain drive Gear drive
Alternator Serpentine belt drive
Air Conditioner Compressor Serpentine belt drive
Oil pump Gear pump (oil pan built in)
Oil pump drive Coaxial chain-driven with high pressure injection pump Gear drive

Exhaust aftertreatment system

System configuration DOC+SDPF DOC+DPF+SCR-Box
DOC mounting position Turbo wake, engine right front Turbo wake, engine right rear bottom
SDPF mounting position DOC wake, engine right side -

 

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Keywords
Daimler, Mercedes-Benz, Modular engine, Diesel engines

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