Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine

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Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
DEVELOPMENT                           New Engines

       Subaru Boxer Diesel
       First Four-Cylinder Horizontally-
       Opposed Diesel Engine
       The integration of the horizontally-opposed engine and the functional principle self-ignition is a
       technical challenge. With the series production of the first boxer diesel engine for passenger
       cars Subaru has achieved a technological breakthrough. While producing maximum power of
       110 kW and maximum torque of 350 Nm, the carbon dioxide (CO2) figure of the 2.0 l four-cylinder
       diesel engine is 148 g/km. Low noise, low vibration and compactness are among its features.

  38   MTZ 09I2008 Volume 69
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
The Author

1 Introduction                                  The core-technology is not possible with-                Kenji Harima
                                                out a boxer engine: Its low height ensures               is Staff Manager,
Subaru, the automotive brand owned by           a low centre of gravity and a nearly ideal               Engine Design Depart-
Fuji Heavy Industries, has delivered hori-      weight balance. The horizontally-opposed                 ment at Fuji Heavy
zontally-opposed petrol engines to the          diesel engine is provided to European                    Industries Ltd. in Tokyo
market since 1966. In 1989, the second-         markets since March 2008 on the models                   (Japan).
generation horizontally-opposed petrol          Legacy and Outback.
engines were developed and installed se-
quentially on Legacy, Impreza and Forest-
er. These engines have convinced with           2 Aim of the Development
driving pleasure and comfortable touring
space, as well as driving performance re­       The aim of the development of boxer die-
presented in the World Rally Champion-          sel engine was to create an environmen-
ship. The Subaru boxer diesel engine has        tally-friendly engine with specific Subaru-
been newly added to the line-up, Figure 1.      performance, driving dynamics and low
It integrates the technologies of fuel injec-   CO2 emissions. With the latest common
tion, supercharging and exhaust gas after-      rail system, a variable geom­etry turbo-
treatment, together with the technology         charger and a large Exhaust Gas Recircu-
of horizontally-opposed engines. Clear          lation (EGR) cooler, it responds to the re-
target was the development of a diesel en-      quests in Europe for environmental pro-
gine without sacrificing the benefits of        tection and engine performance. In order
the Symmetrical All Wheel Drive (AWD).          to make use of the features of horizontal-

Figure 1: Subaru
boxer diesel engine

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Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
D evelopment                                            New Engines

Table 1: Specifications of the engine

                                                               EE20 Boxer Diesel    EJ20 (Petrol)                    EZ30 (Petrol)
 Displacement                                      cc                1998               1994                              2999
 Maximum Power                            kW (PS)/rpm            110 (150)/3600    110 (150)/6000                    180 (245)/6600
 Maximum Torque                         Nm (kgfm)/rpm           350 (35.7)/1800    196 (20.0)/3200                   297 (30.3)/4200
 CO2 Emission                                  (g/km)           148 (Saloon MT)    202 (Saloon MT)                  286 (Saloon MT)
 Fuel Economy                                l/100 km           5.6 (Saloon MT)    8.6 (Saloon MT)                  12.1 (Saloon MT)
 Compression Ratio                                                   16.3               10.2                              10.7
 Bore x Stroke                                   mm               86.0 x 86.0        92.0 x 75.0                       89.2 x 80.0
 Bore Pitch                                      mm                  98.4               113                               98.4
 Bank offset                                     mm                  46.8               54.5                              46.8
 Deck height                                     mm                   220               201                               202
 Diameter of Journal                             mm                   ø 67              ø 60                              ø 64
 Diameter of Pin                                 mm                   ø 55              ø 52                              ø 50
 Con-rod Pin Distance                            mm                   134              130.5                             131.7
 Diameter of Piston Pin                          mm                   ø 31              ø 23                              ø 22
 Compression Height                              mm                   43                33.5                               30
 Fuel System                                                      Common Rail           MPI                               MPI
 Turbocharger                                                    Variable Turbo          –                                 –
 EGR                                                             Liquid Cooling          –                                 –
 DPF                                                               Open Type             –                                 –
 Engine Length                                   mm                  353.5             414.8                             438.4

ly-opposed engines, the development                     –	reduced weight                      3 Specifications of the Engine
placed emphasis on the followings:                      –	compactness
–	low noise, low vibration and high live-              –	high rigidity                       Table 1 shows the boxer diesel engine’s
   liness                                               –	low centre of gravity               major specifications in comparison with
–	direct and sporty response                           –	fuel economy.                       petrol engines. Figure 2 shows a cross-sec-

Figure 2: Cross-section of the Subaru boxer diesel engine

    40     MTZ 09I2008 Volume 69
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
tion of the engine. In order to fulfil the             Table 2: Comparison of inertia force and moment
aim of the development the following
technologies have been adopted:                                                                                                   H4 engine
–	compact design                                                                           1st-order                                      0
                                                        Unbalanced inertia force
–	low friction and low inertia moment                                                     2 -order
                                                                                             nd
                                                                                                                                           0
–	lightweight and highly-rigid cylinder
                                                                                            1st-order                                      0
      block and crankshaft                              Unbalanced moment
                                                                                           2 -order
                                                                                             nd
                                                                                                                           – 2mrec ω rs1 / λ cos2θ
                                                                                                                                       2
–	common rail system with 1800 bar
      fuel injection pressure                                                                                                     L4 engine
–	lightweight, compact and dedicated                                                       1 -order
                                                                                             st
                                                                                                                                           0
                                                        Unbalanced inertia force
      solenoid injectors                                                                   2nd-order                         4mrec ω2r1 / λ cos2θ
–	variable geometry turbocharger
                                                                                            1st-order                                      0
      mounted underneath the engine                     Unbalanced moment
                                                                                           2nd-order                                       0
–	exhaust gas aftertreatment system
      ­installed downstream of the turbo-               mrec: reciprocating weight; ω: crank angle speed; θ : crank angle;
       charger.                                         r: crank radiator; λ: connecting-rod lenght / crank radiator; s: bank offset
Owing to its mechanical system proper-
ties, a horizontally-opposed engine does
not produce any unbalanced inertia forc-
es from the reciprocating piston system         gine, the turbocharger is placed under-                   conditions, the oxidation catalytic con-
weight. Table 2 shows the unbalanced in-        neath the front right side of the engine.                 verter generates nitrogen dioxide (NO2)
ertia forces and unbalanced moments –           In addition, the variable geometry turbo-                 which oxidise the collected diesel par-
first- and second-order – compared be-          charger enables efficient supercharging                   ticulate inside the DPF. In addition, an
tween a four-cylinder horizontally-op-          in all operating ranges, controlling the                  EGR cooler is adopted for Nitrogen oxide
posed (H4) engine and a four-cylinder in-       exhaust turbine’s vane opening angles                     (NOx) reduction to comply with Euro 4
line (L4) engine [1]. Note that engine’s        according to the operating conditions.                    emissions standard.
rolling moments are not included in                Placed closely downstream of the tur-                      The fuel system adopts a common rail
­Table 2, both for H4 and L4.                   bocharger is the exhaust gas aftertreat-                  system with maximum fuel pressure of
       The second-order unbalanced inertia      ment converter with an oxidation cata-                    1800 bar [2]. It consists of a high-pressure
 force of an L4 diesel engine is large, more    lyst and an open Diesel Particulate Filter                fuel pump and eight-hole solenoid injec-
 than 10 kN at 4000 rpm. On the other           (DPF) inside. This layout has shortened                   tors placed at each cylinder, capable of
 hand, the second-order unbalanced mo-          the warm-up time of the catalyst to acti-                 multiple injection events. Compact-size
 ment of the boxer diesel engine (H4) is        vate in a cold condition, enhancing the                   solenoid injectors were developed exclu-
 around 250 Nm at 4000 rpm, having              emissions conversion performance in                       sively for the boxer diesel engine, in or-
 ­little impact on engine vibration. Thanks     wide ranges. The oxidation catalytic con-                 der to make the engine width equivalent
  to such good engine vibration character-      verter has been positioned below the en-                  to that of petrol engines.
  istics, the boxer diesel engine does not      gine together with the turbocharger. It                       The combustion chamber is of a re-
  need a balance shaft, and realises low in-    separates unburnt fuel into water and                     ­entrant type. Together with atomisation
  ertia moment, low friction and reduced        CO2. The unit has been made compact                        characteristics and swirls in the combus-
  weight, Figure 3.                             enough to be activated soon after the en-                  tion chamber, it has allowed the fuel
       For achieving good diesel combus-        gine has been started. If the temperature                  economy, engine performance and emis-
  tion, the piston stroke of the boxer diesel   rises to 250° C under certain driving                      sions performance to be satisfied at the
  engine has been extended compared to
  that of an H4 petrol engine. The engine
  overall width is kept equivalent to the H4
  petrol counterpart, ensuring the mount-
  ing capability to existing vehicles. With a
  shortened bore pitch, the engine overall
  length is 61.3 mm shorter than that of
  an H4 petrol engine. This has made the
  boxer diesel engine more compact than
  its H4 petrol counterpart.
       For better performance of exhaust gas
  purification and a lower centre of gravity,
  the turbocharger layout has been revised.
  In an H4 petrol engine, the turbocharger
  is placed above the back right side of the
  engine. In the Subaru boxer diesel en-               Figure 3: Comparison of the inertia moment on the crankshaft system

                                                                                                                            MTZ 09I2008 Volume 69    41
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
D evelopment                                     New Engines

                                                                                                    4.1.2 High Balance
                                                                                                    Performance by Boxer Layout
                                                                                                    A boxer engine balances out the inertia
                                                                                                    force of reciprocating parts. Therefore, a
                                                                                                    secondary balancer system is unneces-
                                                                                                    sary. Compared to an L4 engine with sec-
                                                                                                    ondary balancer system, the second or-
                                                                                                    der engine vibration is low, from low- to
                                                                                                    high-speed ranges, Figure 5.

                                                                                                    4.1.3 Low Vibration by
                                                                                                    Highly Rigid Crankshaft
Figure 4: Study of combustion chamber shape by CFD                                                  The crankshaft of a boxer engine is short-
                                                                                                    er in overall length, and thus more rigid,
                                                                                                    compared to that of an L4 engine. When
                                                                                                    the rigidity is low, the crankshaft causes
same time. The compression ratio is              mass f lywheel clutch system is em-                resonant torsional vibration in high-speed
16.3:1, and the maximum combustion               ployed to reduce noise and vibration in            range of actual driving, which results in
pressure is 18 MPa. For the study of             the drive­train. In many of the front-en-          engine vibration. On the contrary, with a
­combustion chamber shape, Compu­                gine rear-drive vehicles, the rubber cou-          boxer engine, the crankshaft’s resonant
 tational Fluid Dynamics (CFD), Figure 4,        pling mechanism of propeller shafts                torsional vibration does not occur in high-
 has been utilised.                              absorbs noise and vibration in the driv-           speed driving, Figure 6. Therefore, the inte-
                                                 etrain. This mechanism, however, re-               rior noise is clear even at high speed.
                                                 duces the rigidity of the drivetrain. Even
4 Characteristics of the Engine                  if a dual mass flywheel clutch system is           4.1.4 Reduced Combustion Noise
                                                 employed, many resonance points are                For reducing combustion noises that are
4.1 Low Noise, Low Vibration                     generated, which would cause the                   specific to diesel engines, dedicated
and High Liveliness                              booming noise of 100 Hz or less. On the            acoustic shields are employed for the cyl-
The combination of the Symmetrical               contrary, the Symmetrical AWD system               inder block, intake manifold and trans-
AWD and the boxer principle has maxim-           can absorb the diesel engine’s large               mission housing case, all of which are
ised the potential of the engine that works      torque fluctuations with its four wheels.          main sources of radiation. This has de-
favourable to noise and vibration perform-       Therefore, the vibration at each part              creased the combustion noise of the
ance, and has successfully decreased the         that suspends the drive­train is small. In         ­power unit as a whole, Figure 7, and real-
noise specific to diesel engines. This real-     addition, since it is a highly rigid drive-         ised a quiet diesel engine at idling and in
ises small noise and vibration in all ranges     train system that does not use rubber               all speed ranges.
from idling to high speed.                       coupling, there are only a few resonance
                                                 points that cause the booming noise of             4.2. Direct and Sporty Response
4.1.1 Highly Rigid Drivetrain                    100 Hz or less. Thus, a pleasant interior
by Symmetrical AWD                               noise is realised from the very start of           4.2.1 Engine Output Performance
To deal with the large torque fluctua-           the driving, with low vibration and no             The engine develops a maximum output
tions of diesel engines, a dedicated dual        booming noise.                                     of 110 kW at 3600 rpm and a maximum

Figure 5: Engine vibration (second order)                               Figure 6: Engine vibration (sixth order)

    42     MTZ 09I2008 Volume 69
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
Figure 7: Engine noise (octave sound pressure level 2 kHz)                   Figure 8: Engine output performance

torque of 350 Nm at 1800 rpm. While the              4.2.3 Acceleration Performance                   and is superior to those of other passen-
torque curve gradually starts to descend             The developers of the boxer diesel en-           ger car diesel engines of the same dis-
in the latter half of 2000 rpm, the output           gine focused on intensive tuning for             placement.
keeps increasing, even after the peak of             controllability, aiming at an accelera-
3600 rpm, the engine will revolve briskly            tion feel linear to the acceleration pedal       4.3 Compactness and High Rigidity
up to the rev limit of 4750 rpm, Figure 8.           stroke. The acceleration time in second
This is characteristic for the Subaru box-           and top gear is the evaluation index for         4.3.1 Basic Configuration of the Engine
er diesel engine.                                    the engine muscularity in low- and mid-          The basic configuration of an H4 engine
    Compared to an L4 engine, the crank-             speed ranges frequently used in Euro-            enables a compact, lightweight and
shaft is 50 % lighter in weight and the              pean roundabouts and Autobahns. The              highly rigid design compared to an L4
rotational inertia is reduced by 18 %. Con-          acceleration performance of the boxer            engine. It offers a good noise and vibra-
sidering the elimination of the balance              diesel engine outweighs that of a natu-          tion behaviour, engine performance and
shaft, the inertia moment is reduced by              rally-aspirated petrol engine of Subaru,         fuel economy. In order to make the
34 %. With such low inertia, the boxer
diesel engine has achieved good response
to the accelerator pedal position.

4.2.2 Turbocharger
A variable geometry turbocharger is em-
ployed, which is capable of an optimum
control of the exhaust gas flow at the
turbine inlet according to the operating                     Figure 9: Variable
range, Figure 9.                                             ­geometry turbocharger
   While the engine extensibility at
high speed was already sufficient with
the engine specification in the mid-de-
velopment stage, emphasis was put fur-
ther on the turbine size and the specifi-
cation of variable vanes in order to real-
ise the engine flexibility, responsiveness
and driving feel for controllability at
low speed. Thus, the torque at 1800 rpm
and below by a maximum of 20 Nm
could be improved, as well as the tran-
sient characteristics.
   An intended low-speed performance
has been realised, while maintaining the
maximum output and other high-speed
performance.

                                                                                                                   MTZ 09I2008 Volume 69   43
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
D evelopment                                   New Engines

                                                                     Figure 10: Motoring               bility with a narrower width of the bear-
                                                                     friction                          ing compared to an in-line engine.

                                                                                                       4.3.4. Cylinder Head and Valve System
                                                                                                       High strength cylinder heads have been
                                                                                                       used to withstand the high combustion
                                                                                                       pressures. Compact and low friction end
                                                                                                       pivot type roller rocker arms have been
                                                                                                       used in combination with a Double Over-
                                                                                                       head Cam system. The diameter of the
                                                                                                       intake valves has been optimised for en-
                                                                                                       hanced breathing performance and swirl
                                                                                                       ratios, resulting in improved combustion
                                                                                                       efficiency. The combination of an intake
                                                                                                       swirl port system and optimised intake
                                                                                                       valve diameter results in ample swirl per-
stroke longer than that of an H4 petrol        H4 petrol engines. The block design uses                formance. A highly durable chain system
engine while keeping the engine overall        the semi-closed deck type that has proven               has been used to drive the camshaft to
width equivalent to a petrol engine, the       its durability in the turbocharged petrol               handle the variations in torque produced
service hole in the cylinder block, which      models. This increases rigidity around                  by the diesel engine.
served as a guide in assembling the pis-       the head gasket mating areas.
ton pin and the connecting rod, was                                                                    4.4 Fuel Economy and Engine Friction
eliminated and instead a diagonally frac-      4.3.3 Crankshaft                                        For less CO2 emissions, the friction loss is
ture-split connecting rod employed,            The deflection of the crankshaft under a                kept at a low level. Since a horizontally-
whose cap is assembled from the oil pan        high combustion pressure is smaller in a                opposed engine counteracts the second-
side. This minimised the increase of the       horizontally-opposed engine than in an                  order acceleration of reciprocating iner-
cylinder deck height.                          in-line engine, since the span of the main              tia weights such as pistons, and thus al-
    The large ends of the connecting rods      journal receiving the load is shorter.                  lows the absence of the balance shaft
feature an asymmetrical profile, which         Therefore, the load at the bearing end is               and the system that drives it, the friction
increases precision during assembly and        reduced, where the oil film thickness is                loss is reduced. The roller rocker arm is
in roundness of the surface connecting         at smallest. This has led to higher relia-              adopted in the valve system and also low-
the crankpin for reduced friction. It has                                                              viscosity lubricating oil is employed. All
also contributed to minimise the rota-                                                                 together, a friction of the boxer diesel en-
tional path thus enabled to employ an                                                                  gine could be achieved, which is among
extended piston stroke inside the compact
                                                 Krupp ahead of Subaru?                                the lowest level among the 2.0 l class
                                                 The world’s first boxer diesel engine for a pas-
cylinder block. Moreover, the compres-                                                                 cars, Figure 10.
                                                 senger car, which was described as a “revolu-
sion height has been reduced by adopting                                                                  The specific fuel consumption has
                                                 tion” by Subaru, does in fact have a predeces-
the pistons made of high-strength alu-                                                                 been lowered in all speed ranges, by re-
                                                 sor, although not in a car. Exactly 75 years ago in
minium alloy and by lowering the piston                                                                ducing friction in the engine hardware,
                                                 1933, Krupp launched a boxer diesel for light
temperature with forced oil cooling using                                                              and by calibrating the engine to satisfy
                                                 trucks. Like the Japanese engine, Krupp’s ver-
cooling channel. In addition, the cylinder                                                             both fuel economy and performances in
                                                 sion also had a cast aluminium crankcase and,
head height has also been reduced. All to-       to save weight, the designer Adolf Roth (1892-
                                                                                                       a most balanced way. As a result, the CO2
gether the new boxer diesel engine is            1966) used air to cool the engine. The result was     figure of the Legacy is 148 g/km, which is
compact with an overall width equivalent         the industry’s lightest engine (325 kg, compared      among the lowest in 2.0 l class cars, de-
to that of an H4 petrol engine.                  to Subaru’s 160 kg, wet) and a power-to-weight        spite the fact that it is an AWD saloon.
                                                 ratio of 6.5 kg/PS (Subaru 1.07 kg/PS).
4.3.2 Cylinder Block                             A further highlight was the fact that the diesel      4.5 Overview of the Control System
The cylinder block in a horizontally-op-         engine could be easily converted back into its
posed engine is lightweight and highly           original form as a petrol engine (1932) by replac-    4.5.1 System and Hardware
rigid. For higher strength and rigidity all      ing a few parts. Turbocharging, which gives the       The Engine Control Unit (ECU) employs
five main journals in the cylinder block         Subaru engine an extremely agile response,            the supplier’s standard ECU hardware,
incorporate metal matrix composite               was not introduced by Roth until 1934 – for the       allowing a significant cost reduction.
journals which are inserted during the           two-stroke version, which had a specific output       Systems other than the fuel injection
casting process, resulting in higher lev-        of 20 PS/l (Subaru 75 PS/l). Although the figures     system, such as the communication sys-
els of quietness due to its high rigidity        prove that a good deal of evolution has taken         tem, the air conditioner, the radiator
and similarity in the thermal expansion          place, Subaru can count itself lucky that Krupp       fan and the cooperative control with the
ratio to that of crankshaft. This is a tech-     never built cars.                  Erik Eckermann    Antilock Brake System and the Vehicle
nique that has already been validated in                                                               Dynamics Control systems, are shared

    44    MTZ 09I2008 Volume 69
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
injection pattern change, the correc-
                                                                                                 tion control of cylinder-to-cylinder dis-
                                                                                                 persions and the learning control for
                                                                                                 micro injection quantity as described
                                                                                                 above was adopted.

                                                                                             5 Summary

Figure 11: Multiple injection events                                                         Subaru has launched the series produc-
                                                                                             tion of the first boxer diesel engine for
                                                                                             passenger vehicles and has begun to pro-
                                                                                             vide it to European customers in 2008 on
with the current petrol engine vehicles          written into the ECU during vehicle         the models Legacy and Outback. In addi-
so that revisions are minimised and the          assembly to conduct the correction of       tion to the high engine performance and
original ECU potential suited to the Leg-        injection quantity.                         responsiveness, the low noise and vibra-
acy is maintained.                            –	Learning control of micro-injection         tion and the fuel economy have been bal-
                                                 quantity: It is important to enhance ac-    anced at a high level. Although Subaru
4.5.2 Controls                                   curacy of pre-injections and after-injec-   was a latecomer in diesel engines, the de-
The injector is capable of multiple injec-       tions. To fulfil this, injectors undergo    velopers focused on the proprietary tech-
tion events, up to five injections in one        learning controls. During stable idling,    nology, which led to the creation of an
stroke, Figure 11. It is important for the       the dispersion of injection is measured     unprecedented horizontally-opposed die-
injector to inject the fuel in high accura-      for each injector of all four cylinders,    sel engine that possesses sufficient com-
cy so that low noise and vibration of the        and then used for correcting the injec-     petitiveness and product performance
boxer diesel engine is realised. There are       tion amount. Since this learning con-       for success in the international markets.
two measures taken for this purpose:             trol is an essential factor for the prod-
–	Injector Qick Response (QR) Code writ-        uct performance of the vehicle, it is
   ten into ECU: The dispersion data of          conducted at the time of shipment           References
   each injector’s properties are printed        from the plant, and also on regular ba-     [1]	Society of Automotive Engineers of Japan, Inc.:
   in the form of a QR Code (a two-di-           sis when the customer is driving the             Jidosha Gijutsu Handbook (Handbook of
   mensional bar code) at the top of the         vehicle. Based on the precise setting            Auto­motive Engineering Handbook). Tokyo:
                                                                                                  Society of Automotive Engineers of Japan, Inc.,
   injector. In the engine plant, the code       and calibration of fuel injection pat-
                                                                                                  2004
   reader reads the QR Codes for all cyl-        tern, the tailing process, the gradually    [2] Tanaka, Y.; Nagata, K.: 1800 bar Common Rail
   inders of the assembled engines. Later        changing of the injection quantity at            System for Diesel Engines. In: DENSO Technical
   in the vehicle plant, these data are          each injection event at the time of fuel         Review (2006), Vol. 11, No. 1

                                                                                                              MTZ 09I2008 Volume 69         45
Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine Subaru Boxer Diesel First Four-Cylinder Horizontally-Opposed Diesel Engine
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