Mercedes-Benz M256 inline 6-cylinder gasoline engine

Daimler plans modular engine updates, adopts 48V mild hybrid system

2018/09/14

Summary

 Daimler has been gradually introducing its newly designed modular engine family having a 500cc per cylinder displacement since 2016. These engines are developed based on Daimler’s new common design guideline, the MPA (Mercedes Powertrain Architecture).

 Among the family of modular engines, the M256 is to be equipped on the company’s S-class flagship 3L 6-cylinder gasoline engine which is Daimler’s first vehicle to adopt a 48V system for mild hybrids. By electrification of auxiliary components, the drive belts for accessory drives are eliminated, shortening the engine’s overall length and improving the ease-of-mounting components in the engine room as well as improving collision safety. Since Daimler discontinued production of the inline 6-cylinder engine M104 in 1997, it has adopted a V-type 6-cylinder configuration, but now it has introduced the inline 6-cylinder engine as successor unit to the M276 V6 engine. The M256 realizes 20% CO2 emissions reduction and more than 15% power improvement compared to its predecessor, the M276 V6 engine.

M256エンジンフロント側 M256エンジンリア側
M256 engine (front side) M256 engine (rear side)

Source: Daimler

 

Related reports:
2021-2030 CO2 regulations in Europe, backlash against diesel, and electrification (May 2018)
Daimler: all-electric vehicles to account for 15-25% of total unit sales by 2025 (Jan. 2018)
Mercedes-Benz OM654 diesel engine (Jan. 2018)
Mercedes-Benz, BMW, and Audi adopting ADAS and automated driving systems (Aug. 2017)
European automaker technology trends: Electric Vehicles (Nov. 2017)
Research on revolutionary combustion technology(2): Contemplating the limits of innovation (Jan. 2017)

 



Overview of the new modular engine family

 All models of the new modular engine family are equipped with a turbocharger and available in 5 engine variations: 2L inline 4-cylinder gasoline/diesel, 3L inline 6-cylinder gasoline/diesel, and 4L V8 gasoline engines. The exhaust system for all models comply with the Euro6 Phase 2 regulations.
 Since the initial planning phase the 2L and 3L engines that form the core of the series have been designed for combination with a 48V mild hybrid system, realizing a fuel economy performance comparable to that of the diesel engines in the same series. By adopting the BSG (Belt-drive Starter/Generator) and ISG (Integrated Starter-Alternator), in addition to improved launch acceleration performance running on the motor at low speeds, energy is recovered during deceleration to charge the Li-ion battery.

 In 2016 Daimler started with the inline 4-cylinder diesel engine (designated as the OM654) and from 2017 launched the following 4 models.

  • Inline 6-cylinder gasoline engine (M256)
  • Inline 4-cylinder gasoline engine (M264)
  • V8 gasoline engine (M176)
  • Inline 6-cylinder diesel engine (OM656)

 Starting with the M256, the modular family engines are assembled at the Untertürkheim plant in Stuttgart. Major casting parts such as cylinder blocks, cylinder heads, crankshafts, conrods, exhaust manifolds, and turbocharger turbine housings are manufactured in-house at the plant.

 

Main specifications of the new modular engine family

Engine M264 OM654 M256 OM656 M176
No. of cylinders/arrangement Inline 4-cylinder Inline 6-cylinder 90° V8
Displacement (cc) 1,991 1,949 2,986 2,927 3,982
B×S (mm) 83×92 82×92.3 83×92 82×92.4 83×92
Compression ratio 10 15.5 10.5 15.5 10.5
Bore pitch (mm) 90
Valvetrain DOHC4V
Engine weight DIN (kg) 145.1 168.4 - 215.7 -
Fuel type Gasoline Diesel Gasoline Diesel Gasoline
Rated output (kW/rpm) 220/5800-6100 143/3800 320/5900-6100 250 345
Peak torque (Nm/rpm) 400/3000-4000 400/1600-2400 520/1800-5500 700 700
Charge system Twin-scroll
single-turbo
Variable valve timing
single-turbo
Twin-scroll
single-turbo + e-AC
2 stage Twin-turbo
(inside V bank)
After-treatment system GPF+Three-Way Catalyst DOC+SCR filter GPF+Three-Way Catalyst DOC+SCR filter GPF+Three-Way Catalyst×2
Emissions Standard Euro6-2
Mild hybrid option With option Without option With option Without option
48V system components BSG, LIB,
electric water pump
- ISG, e-AC, LIB,
electric water pump,
electric AC compressor
- -
Vehicle Class E Class S Class S560
Start of production (year) 2017 2016 2017 2017 2017

GPF: Gasoline Particulate Filter
BSG: Belt-driven Starter-Generator
LIB: Lithium Ion Battery
e-AC: Electric Auxiliary Charger
ISG: Integrated Starter-Generator
DOC: Diesel Oxidation Catalyst
SCR filter: DPF (Diesel Particulate Filter)+Urea SCR (Selective Catalytic Reduction) (integrated)

 

Inline 6-cylinder engines make a comeback

 So, why did Daimler change from the inline 6-cylinder to the V-type 6-cylinder in the past, and is now returning to the inline 6-cylinder?
 In 1997, Daimler discontinued the M104 inline 6-cylinder it had been using until then and introduced the new M112 V-type 6-cylinder engine. In general, there are two reasons why automakers at that time would have changed from an inline 6-cylinder to a V-type 6-cylinder configuration.

  1. In the US and Europe, there were many cars that had engines mounted transversely in a FF (Front-engine, Front-wheel-drive) layout and it was not economical to produce both the FR vertically mounted inline 6-cylinder and the FF transversely mounted V-type 6-cylinder engines. If the V-type 6-cylinder FF vertically mounted engine can be used as an FF transversely mounted engine, then the number of engine variants can be reduced.
  2. Occupant protection during collisions was becoming increasingly important as a safety measure at that time. An inline 6-cylinder with a long engine overall length mounted inside the engine room has a narrow crumple zone, and in the event of a collision the engine and transmission could be pushed into the passenger cabin causing injuries to the occupants. By changing to the V-type 6-cylinder engine that has a shorter engine overall length, a more sufficient crumple zone in the engine room is made available thereby providing appropriate security to occupants.
FR (Front engine rear drive) inline 6-cylinder engine FR (Front engine rear drive) V-type 6-cylinder engine
FR (Front engine rear drive) inline 6-cylinder engine FR (Front engine rear drive) V-type 6-cylinder engine

 

 At the time, Daimler did not make the FF (Front engine Front-wheel-drive) transverse engine vehicle, and for the above two reasons the inline 6-cylinder engine was discontinued, and the V-type 6-cylinder was selected. In addition, Daimler was already making the V-type 8-cylinder engine, so by setting the bank angle at 90 degrees the V-type 6-cylinder could also be manufactured on the same assembly line.

 So why is the inline 6-cylinder making its comeback from the V-type 6-cylinder configuration? Below are described the 3 reasons for Daimler actively selecting the inline 6-cylinder design.

  1. When modularizing the inline 4-cylinder and 6-cylinder engines, the inline 6-cylinder engine is easier to develop and manufacture than the V-type configuration. For the inline 6-cylinder, it means adding only 2 more cylinders, but with the V-type, component types and the number of components (cylinder heads, camshafts, etc.) increase as well as component and developments costs.
  2. In the V-type 6-cylinder engine, the overall length is too short, and there is a shortage of space for mounting the ever-increasing auxiliary components and exhaust aftertreatment devices.
  3. Especially for the diesel engines with V-type 6-cylinder configurations, exhaust aftertreatment devices had to be split into the right and left banks resulting in higher costs.

 Below are the following technological innovations to resolve problems regarding the need to switch from the inline 6-cylinder to the V-type 6-cylinder configuration.
 ・In the conventional inline 6-cylinder engine, a short stroke design is used to obtain maximum power and water passages between bores required for cooling.
 ・Currently, the ideal S/B (stroke-to-bore) ratio is now known to be at around 1.1 as the ratio to achieve the optimal balance for power and fuel efficiency. Furthermore, sleeveless aluminum blocks are already commercialized. From these reasons, even for engines with the same displacements, by reducing the piston diameter and shortening the bore pitch, the overall length can be shortened to around 50~80mm.
 ・In addition, electrification of the auxiliary components makes it feasible to reduce or eliminate engine driven auxiliary belt drives, thereby making it possible to reduce the overall length of the engine by more than 50mm.
 For the above reasons, even if the inline 6-cylinder is mounted on an FR configuration, the obstacles to improving collision safety have been eliminated.



48V mild hybrid system: BSG, ISG, and electrification of auxiliary components

 In Daimler’s modular family of engines, on the inline 4-cylinder M264 and inline 6-cylinder M256 gasoline engines, a 48V mild hybrid system is adopted with a BSG/ISG and Li-Ion battery (approximately 1kWh capacity) configuration. Although the BSG/ISG functions are comparable, the BSG is mounted at the same location as the conventional alternator. With respect to a conventional system driven by the crankshaft and the accessory belt drive, the ISG adopted on the M256 is mounted between the engine and transmission with the crankshaft directly transferring power, which is the main difference. The inline 4-cylinder M264 has a shorter overall length, and although space for the accessory belt drive was made available, there was no available space for the inline 6-cylinder M256 so that, as described above, the BSG was mounted between the engine and transmission.
 With the BSG during torque assist and power generation by the alternator, an opposite tension force is exerted on the belt so that in conventional belt tensioners slacking occurs which causes the belt to slip. However, in this BSG a new belt tensioner is used to provide stable tension even if the belt is stretching or loosening in the reverse direction, therefore preventing belt slippage.
 In the M264 maximum power during drive assist is 12.5kW and in the M256 is 16kW. During charging and energy recuperation, a maximum of 10kW can be charged to the battery.

ISG system integrated with engine/transmission Powertrain layout
ISG system integrated with engine/transmission Powertrain layout

Source: Mercedes-Benz Japan

 

Functions of the hybrid system

Idle start-stop
Launch start assist (max 16kW)
Low speed driving on motor
Braking energy recuperation (max 10kW)
*Shifting of the load points
Engine off during coasting

*Shifting of the load points: Changing the amount of engine assist depending on the state of charge of the Li-ion battery, maintaining an appropriate state of charge.

 

Electrification of auxiliary components

 In the M256 engine, with the adoption of the 48V-based mild hybrid system, an improvement in power efficiency by high voltage is achieved as well as advancing the electrification of auxiliary components that was not feasible with conventional 12V systems.

48V system components
Component Engine
M256 M264
ISG/BSG
Lithium ion battery
Water pump
Auxiliary compressor ×
A/C compressor ×

 

 Details of the ISG/BSG and Li-ion battery were discussed previously and abbreviated here.

Water pump

 In a conventional engine, the water pump was driven mechanically by the accessory belt drive. With this method the water pump is driven corresponding to the engine speed even at times when there is no need for cooling, resulting in low drive efficiency as well as friction loss from the accessory belt drive. By electrically driving the water pump, it can be stopped during cold start conditions to accelerate engine warm up and improve heater efficiency. Furthermore, electrification enables the circulation of the optimum coolant amount corresponding to engine operating conditions.

Auxiliary compressor

 Although details will be touched upon later in the turbocharger section, the M256 has a large displacement and high power, so single turbo specifications require a large turbo size, causing bad response at low speeds and low loads. To compensate for this problem, an electric auxiliary compressor on the turbine side was added. Based on the 48V system, a high power of more than 5kW is produced.

A/C compressor

 During conditions such as idle start-stop, EV driving, and coasting the engine is stopped. Therefore, if the air conditioning system is being used, the air conditioning system will also cease to function when the engine is stopped. The answer to this issue is to use an electric compressor. By using the electric compressor, the air conditioning system can be operated even during engine stop conditions.



Engine: Cylinder block

 The aluminum alloy cylinder block is a closed deck + deep skirt type block with high rigidity, and the aluminum alloy lower block is also formed in a similar box-shaped structure controlling deformation of the cylinder block due to high combustion pressures. The engine block features a cylinder spacing of 90mm which is standardized on the series engine with 83mm bore diameters on a 6-line Siamese structure. The minimum wall thickness between bores is 7mm and there is no space available for cast iron sleeves. In this modular engine, conventional casting of iron sleeves is not used, but this problem is resolved by using a technology that sprays an iron coating with an approximate 0.2mm thickness on the inner surface of the aluminum bore.
 At Daimler, this technology is known as TWAS Coating (Twin Wire Arc Spraying). The TWAS coating is applied by melting wire made of iron-carbon alloy and spraying the inside of the bore using an electric arc. The melted alloy is sprayed to the inner surface of the cylinders with a gas flow, coating the surface with an extremely thin layer (ultra-fine nano-crystal thin layer). Very fine finishing of the resulting nano-crystalline iron coating creates an almost mirror-like, smooth surface with fine pores, which reduces friction and wear between the piston assembly and the cylinder wall. Lubricating oil seeps into the microporous surface and acts as a lubricant for the bore and the piston.

 By eliminating the cast iron sleeve with the TWAS coating, cooling of the bore is improved and the large amounts of heat generated in the combustion chamber are immediately transferred through the piston ring to the coolant.
 In the case of a cast iron cylinder bore, the thermal expansion of the aluminum alloy piston is approximately twice that of the thermal expansion of cast iron, so that the clearance between the bore and the piston during cold assembly (at approximately 20degC) becomes quite small compared to when the engine is in operation. However, if the clearance between the bore and the piston is wide during cold operating conditions, piston slap occurs; therefore, clearances must not be too wide. As a result, clearances are tight during normal operating conditions and friction increases.
 By adopting TWAS coating technology, a combination of aluminum bore and aluminum piston becomes possible, eliminating the changes in clearances between the two materials during both cold conditions and normal operating conditions. With this technology, piston slap can be prevented, making it possible to maintain the appropriate clearances between the piston, piston rings, and cylinder bore during operating conditions, therefore reducing friction and achieving good NVH. For reference, in the OM654 and OM656 diesel engines, steel pistons are still being used so that the clearance between the steel piston and the aluminum bore becomes wider, achieving better friction reduction (40~50% compared to a conventional engine) than the gasoline engines.

TWASコーティング(Source: Mercedes-Benz Japan) Comparison of conventional cast iron sleeve and TWAS coating(日産広報資料をもとに作成)
TWAS coating(Source: Mercedes-Benz Japan) Comparison of conventional cast iron sleeve and TWAS coating
(Source: Nissan PR material)

 

オフセットクランク(ホンダ広報資料をもとに作成)
Offset crank (Source: Honda PR material)

 In the inline engine of this modular family engine, an offset crank design is adopted. The piston during the compression stroke (piston is ascending) is subjected to side pressure because the conrod is tilted. Likewise, the piston is also subjected to side pressure during the expansion stroke (the piston is descending). Due to the side pressures, the piston is pushed to the cylinder walls which creates sliding friction. The pressure applied on the crown surface of the piston during the compression stroke is small but becomes large when combustion pressure is applied during the expansion stroke. Therefore, if the slope of the conrod during the expansion stroke is made smaller but during the compression stroke it is made larger, the total sliding friction due to the piston side pressure can be decreased. The offset crank mechanism uses this principle. In the Daimler design the crank axis with respect to the cylinder axis is offset by 12mm to the right side of the engine (thrust). Moreover, in the V-type 8-cylinder M176 engine, the offset crank is not adopted.



Engine: Cylinder head and valve train

 The cylinder head is made of aluminum alloy with a separate structure for the cam holder for weight reduction and improved manufacturability. The valve train is a swing-arm type that uses a fulcrum, incorporating a pneumatic adjuster in the pivot, and is maintenance-free. Furthermore, a roller is incorporated in the contact area with the cam lobe which reduces friction. The combustion chamber is a 4-valve pent roof type, with a downsized exhaust valve seat diameter, providing space for a downsized spark plug smaller than the normal M10 screw size plug. A 200bar piezo-injector is located near the center of the combustion chamber. By adopting a downsized spark plug and an exhaust valve with a smaller seat diameter, better coolant flow around the combustion chamber is achieved to ensure heat dissipation during high load operating conditions. In addition to the sodium-filled hollow-drilled exhaust valves, by designing the combustion chamber to have less knocking, the idea is to improve power by adjustment of the ignition timing and air-fuel mixture towards the LMBT (Leaner Side & Minimum Advance for Best Torque).
 Moreover, when the valve lift is small, the intake air temperature is low and combustion conditions are insufficient. To resolve this issue, fuel injection from the piezo-injector is divided into multiple injections and delivered in multiple spark ignitions.

Sodium-filled hollow head (bottom) and Sodium-filled system (top) Intake valve lift amount and turbulence generated: the left figure shows small lift, and the right figure shows large lift amount.
(Top) Sodium-filled system for previous engine
(Bottom) Sodium-filled hollow head for M256 engine
(Source: Mahle product catalog)
Intake valve lift amount and turbulence generated: the left figure shows small lift, and the right figure shows large lift amount.
(Source: Mercedes-Benz Japan)

 

 The camshaft adopts the Camtronic system with variable lift on the intake side. Camtronic performs two types of valve lift adjustments using a variable valve timing mechanism to change the camshaft phase at a wide angle and a 2-stage variable valve lift mechanism.
 The camshaft phase control has a wide adjustment range of 70 degrees (crank angle), done on early intake valve closure to improve fuel economy with the Miller cycle. The phase adjustment is done hydraulically by turning the camshaft adjuster that is located at the front end of the camshaft with the revolution relative to the camshaft timing gear. In the variable valve lift mechanism, cam lobes with different valve lifts are arranged in parallel at the front and back to switch the valve lift by sliding the camshaft back and forth through the hydraulic center actuator. At low load operating conditions, a cam lobe with a lower valve lift is used to reduce pumping loss, and at high load operating conditions a cam lobe with a higher valve lift is used to obtain the necessary intake air for high power output.

カムプロフィール(Source: Mercedes-Benz Japan) カムトロニックシステム(Daimler広報資料をもとに作成)
Cam profile(Source: Mercedes-Benz Japan) Camtronic system (Source: Daimler)

 

Main system

Spray guided combustion chamber(Source: Mercedes-Benz Japan)
Spray guided combustion chamber(Source: Mercedes-Benz Japan)

 The pistons are made of cast aluminum, with a cooling port provided for cooling the crown surface and the ring groove wherein oil is sprayed from the oil jet arranged at the lower deck of the cylinder block. The crown surface shape of the pistons basically follows that of the spray guided combustion chamber design used for the conventional gasoline engines. To improve the spray-guided combustion chamber and to stabilize catalytic converter temperatures during cold starts, the position of the ignition plugs was moved to the exhaust valve side, and the piezo-injector position was shifted 3mm from the center of the combustion chamber. As shown in the illustration, this design realizes an almost spherical flame propagation.
 The crankshaft and the conrod are made of forged high-strength steel that is common across the modular family engine. The stator of the ISG system is directly mounted at the rear end of the crankshaft.

Valve train drive, accessory drive system

 The camshaft is driven by a single chain via the crank sprocket mounted on the rear side of the crankshaft. The high-pressure fuel pump driver and the oil pump driver are integrated at the rear end of the engine. The high-pressure fuel pump is driven by a dedicated shaft of the chain drive system. The oil pump is gear-driven and located inside the oil pan, and is chain driven via a sprocket on the same axis as the sprocket that drives the camshaft. This same sprocket also drives the vacuum pump on the same shaft.

Intake and exhaust system

 The intake port is located on the left side of the engine, and the exhaust port is located on the right side of the engine. The exhaust from the exhaust ports are divided into cylinder groups #1~#3 and #4~#6 and introduced in separate paths from the exhaust manifolds to the turbine housing. The exhaust is guided from the turbine housing to the rear side and flows into the 3-way catalytic converter mounted near the engine and then passes through the GPF (Gasoline Particular Filter) mounted underfloor. The intake air coming from the intake duct mounted on the left side of the engine front passes through the air cleaner and goes through the turbo compressor. The intake air coming out of the compressor passes through the slider adjustment valves arranged in parallel to the electric auxiliary charger (e-AC) that is mounted on the left rear side of the engine, then passes through the intercooler located at the backflow of the throttle valve, and finally flows into the intake ports of each cylinder.



Turbocharger

ターボチャージャーシステム(シングルターボ+補助チャージャー)(Source: Mercedes-Benz Japan)
Turbocharger system (single-turbo + auxiliary compressor)
(Source: Mercedes-Benz Japan)

 In the new modular engine family, the components and the layout of the exhaust after-treatment devices in the engine room are standardized as the space within the engine room is extremely limited. This is the reason why from the outset that the M256 was designed selecting a single-turbo configuration instead of a twin-turbo. However, using a single-turbo configuration requires a larger turbocharger be used to achieve the target power which unfortunately results in a poor response at low speeds. To resolve this problem, the 48V system incorporating an electric auxiliary charger (e-AC) is adopted. The e-AC is a motor on the turbine side that charges the compressor turning on the same shaft, capable of accelerating to a maximum speed of 70,000rpm within 300ms and a maximum pressure ratio of 1.45.
 The M256 turbocharger is a twin-scroll turbocharger with air-gap-insulated exhaust manifold (the exhaust is separated into #1~#3 and #4~#6 cylinders).
 By using this configuration, exhaust efficiency is increased and charging efficiency is improved to obtain excellent turbo response even at extremely low exhaust flows. By using the air-gap-insulated exhaust manifold configuration, the exhaust manifold surface temperatures decrease significantly during high load operation, making it possible to lower the temperature of the engine room during thermal conditions such as engine soaks.

 

Engine performance

 Looking at only engine performance, the M256 maintains 520Nm from 1800~5500rpm, having an extremely flat torque curve. This reflects the effect of the electric auxiliary turbo compressor.
 At regions below 1000rpm, the electrically-assisted torque is at 250Nm. This is equivalent to approximately 50% of the maximum engine torque, providing a sufficiently powerful feel when accelerating at full motor assist from a full stop, etc.

Power Output performance Torque on the ISG System
Power output performance Torque on the ISG System

Source: Mercedes-Benz Japan


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Keywords
Daimler, Mercedes-Benz, modular engine, M256, electrification, HV, hybrid vehicle, mild hybrid, BSG, ISG, e-AC, TWAS

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