The New Mercedes-Benz 3-Cylinder Diesel Engine - coltgalant-info

COVER STORY 3-Cylinder Diesel Engine
You will find the figures mentioned in this article in the German issue of MTZ 1/2005 beginning on page 6.
Der neue Dreizylinder-Dieselmotor
von Mercedes-Benz für Smart und Mitsubishi
The New Mercedes-Benz
3-Cylinder Diesel Engine
for Smart and Mitsubishi
By Steffen Digeser,
Mario Erdmann,
Franz-Paul Gulde,
Thomas Mühleisen,
Joachim Schommers
and Roland Tatzel
For the joint project Smart Forfour and Mitsubishi Colt,
Mercedes-Benz developed a 3-cylinder diesel engine
derived from the A-Class 4-cylinder engine. This engine
allows both high performance and low fuel consumption.
The top version with 70 kW is the world’s most
powerful three-cylinder passenger car diesel engine.
1 Introduction
In mid-2000, Mitsubishi and Smart began a
joint project to develop a four-seater compact
car. Their combined efforts led to the
idea of transforming a four-cylinder A-
Class engine into a three-cylinder diesel engine
and installing it in Smart and Mitsubishi
compact cars. In addition to the cost
benefits resulting from the newly created
engine model series, the large individual
displacement of a 1.5-litre three-cylinder
diesel engine leads to better thermodynamic
ratios compared to the four-cylinder
engine with the same displacement.
As a result, a completely new threecylinder
in-line engine producing 50 and 70
kW of power was developed for the diesel
version of the Smart Forfour and the Mitsubishi
Colt.
1.1 The Concept
The stringent guidelines for exhaust emission
level Euro 4, as well as fuel consump-
tion, performance, comfort and low manufacturing
costs led to a solution with the
following characteristics:
■ transverse, front installation
■ unified combustion chamber as in other
Mercedes-Benz CDI engines going on the
market in 2004
■ four-valve design with built camshafts
and roller cam followers
■ swirl generation through tangential and
spiral intake with infinitely variable swirl
control
■ exhaust gas turbocharging
■ cooled exhaust gas recirculation
■ second-generation common rail fuel-injection
system with 1600 bar and solenoid
valve injectors
■ balancer shaft
■ dual-mass flywheel
■ engine (transmission) control unit (with
Green Oak processor)
■ powertrain designed for peak pressures
of up to 180 bar
■ engine mounting as roll axis system.
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1.2 Key Data
The most important technical data are
compiled in the Table.
2 Engine
2.1 Longitudinal Section and
Cross-Section
When designing the engine layout in the
vehicle, our objective was to make the engine
as compatible as possible with the
petrol (gasoline) engine. At the same time,
we wanted to maintain the production network
with the A-Class four-cylinder diesel
engine. As a result, the petrol and diesel engine
designs differ with regard to the position
of the intake and exhaust sides. This is
primarily due to the cylinder head design.
The resulting exhaust ducts as well as the
compact overall height requirement led to
connecting the balancer shaft on the side of
the crankcase. The shaft is driven via a joint
chain leading to the oil pump.
Figure 1 shows the structural design of
the OM 639.
2.2 Crankcase
The crankcase was designed as a closed
deck and is made of cast iron 26 Cr. A land
width of only 7 mm results from the 90 mm
cylinder spacing and an 83 mm bore,
through which two land coolant bores pass.
The water pump body and an exhaust
return duct are integrated. The flange was
adapted to the transmission shared with
the petrol engine. In contrast to conventional
in-line engines, the starter is located
on the transmission side.
2.3 Powertrain
The A-Class four-cylinder diesel engine production
network determined such key dimensions
as the 92 mm stroke and the 90
mm cylinder spacing. The crankshafts are
lightweight forgings made of 38 Mn S6 BY
with rolled radii, hardened working surfaces
and four counterweights. The pistons,
rings, rods and plain bearing powertrain
components that are identical to the fourcylinder
diesel engine have been described
in detail in the MTZ article 12/04.
Throughout the development process,
one of the objectives was to maintain low
lubricating oil consumption even after long
service lives, Figure 2. Employing a 2 mm
high ventilated spring-loaded oil ring with
tapered mini-lands in the third piston ring
groove proved to be the decisive solution.
At the same time, the flow through the oil
injection nozzles was reduced and the
crankcase was locally reinforced in order to
reduce deformation of the cylinder bores.
Moreover, the flow through the oil injection
nozzles was reduced.
MTZ worldwide 1/2005 Volume 66
COVER STORY 3-Cylinder Diesel Engine
The post-bonded torsional vibration
damper on the crankshaft, which holds the
five-groove belt, is designed for a frequency
of 495-550 Hz.
A dual-mass flywheel (DMFW) was developed
to improve ride comfort. The dualmass
flywheel is equipped with an external
damper with bow springs and a three-stage
internal damper. It almost completely filters
out the torsional vibrations that are
above the natural frequency. Consequently,
the vibrations in the transmission and
the powertrain that occur above the engine
idling speed are reduced to a minimum.
This prevents rattling noises in the transmission
during idling or while driving. Because
the torsional excitation on the powertrain
is reduced, noise interference and
vibrations do not develop at low engine
speeds of up to approximately 2000 rpm.
2.4 Three-Cylinder Diesel Engine
Mass Balancer System
A mass balancer offsets troublesome free
inertial forces of the first order developing
in three-cylinder engines. It sharply reduces
noise emission and prevents vibrations
and, therefore, contributes to a distinct
improvement in ride comfort.
The balancer shaft is designed as a compact
module and is bolted to the side of the
crankcase.
The shaft is driven by the oil pump chain
via a gear with a 1:1 gear ratio. A gear in the
chain drive is integrated in order to reverse
the rotation between the crankshaft and
the balancer shaft. A rubber coating on the
crankshaft gear was chosen to improve the
acoustics. Figure 3 shows the complete
chain drive.
The balancer shaft is mounted using
bearing bushings with a centre oil groove.
Oil ducts that are fed directly out of the
main oil duct of the crankcase supply the oil.
At the same time, the balancer shaft casing
holds the oil filter and the oil/water
heat exchanger.
As a result of extensive FEM calculations
and external tests, an optimum compromise
was found between rigidity (positive
effect on noise and vibrations), strength
and weight for the mass balancer system.
2.5 Cylinder Head and Valve Gear
In addition to the crank assembly and the
crankcase, the cylinder head is the most important
component in the production network
for the four-cylinder engine of the A-
Class. The crossflow concept, duct geometry,
oil circuit and control system are identical.
A new feature is the integration of the
camshaft bearing covers into the cylinder
head cover. This results in advantages for
the height of the engine.
The high peak pressure of 180 bar required
an optimised sealing concept. A
four-layer cylinder head gasket and high
bolting forces ensured the desired positive
results.
The timing chain and the balancer
shaft/oil pump chain have been designed
as single-bush chains. Particularly the design
of the highly stressed chain drive for
the balancer shaft and the oil pump required
detailed analysis and investigation,
e.g. torsional vibration measurements. Precision-blanked
lugs are used to minimize
wear on the guide rails. A non-return valve
in the chain tensioner limits the movement
of the tensioning rail and thus reduces the
dynamic loads on the chain drive.
Both are composite camshafts and are
manufactured using hydroforming.
The exhaust camshaft drives the vacuum
pump while the intake camshaft drives
the common rail high-pressure pump. The
valves are actuated by the roller cam follower
with vertical hydraulic valve play
compensation.
2.6 Oil Circuit
In order to reduce the friction losses, the oil
pump was optimised and the gear width
was designed to 18 mm. Moreover, oil
foaming is minimized by a chain wheel
cover at the oil pump chain sprocket and an
oil deflector under the crankshaft.
The oil/water heat exchanger limits the
maximum oil temperature to 135 °C.
The service interval is variable and is
calculated depending on the driving style
and the oil brand used.
Intervals of between 25,000 and 31,000
km are achieved when low-viscosity oil according
to DaimlerChrysler specification
229.5 is used.
2.7 Auxiliary Units
The auxiliary unit drive is designed as a single
belt drive. A five-groove belt made of
EPDM and a mechanical belt take-up is
used. The automatically tensioned belt tensioner
has tapered bearings and is
equipped with a synthetic friction lining.
An air-cooled 120 A generator with a freewheel
pulley and a type 5SEU9 A/C compressor
are integrated as auxiliary units.
The belt drive without refrigerant compressor
uses a guide pulley at this point,
with the result that only one belt length is
necessary because of the uniform belt layout.
2.8 Charge Exchange and
Exhaust Gas Recirculation
Figure 4 shows the layout of the gas-carrying
components. The outside air flows
through the intake pipe to the vehicle’s air
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COVER STORY 3-Cylinder Diesel Engine
filter and is measured by a hot-film airmass
meter (HFM) upon exiting. The wastegate
turbocharger compresses the air to a
maximum pressure of 1.3 bar. A heat exchanger
located in front of the left wheel
arch cools the boost air. This increases the
air density by 21 %.
The necessary high exhaust return volumes
(up to 40 %) require the relevant scavenging
gradient between the exhaust return
tract and the compressed fresh air
tract. As a result of numerous series of tests
conducted to optimise flow conditions, it
was possible to keep the pressure losses
that occur over the entire return track so
low that there was no need for an additional
throttle valve to generate a pressure differential.
The exhaust heat exchanger has a cooling
matrix with six rectangular pipes and
impressed “winglets”. The turbulent gas
flow generated by this has a self-cleaning
effect on the internal pipes and positively
influences the cooling capacity (DT exhaust
temperature = 180 K), which does not exceed
a power loss of 8 % throughout the
component’s service life.
The pneumatic EGR valve found in the
mixing chamber directly upstream of the
collecting section was designed as a bevelled
poppet valve in order to further improve
the regulating conditions for small
strokes.
The exhaust gas enters the mixing
chamber centrally through an intake pipe
with slots on its sides in order to optimally
supply the exhaust gas with fresh air.
2.9 Ventilation System
A multistage separator directly mounted to
the cylinder head cover separates the oil
from the blow-by gases. It is made up of a
smoothing volume with spiral separation,
a flow separator to protect the HFM and an
integrated pressure-reducing valve. The
separated oil flows back into the oil pan
through the oil level gauge guide pipe via a
freeze-proof hose. The ventilation gas flows
through a cooling water-heated tube into
the intake manifold in front of the turbocharger.
3 Fuel-Injection System
The configuration of the fuel-injection system
used in the three- and four-cylinder
diesel engines is consistent in its design
with the proven common rail fuel-injection
system in second-generation Mercedes-
Benz CDI engines (MTZ 4/2002).For the second
generation, the injection pressure was
raised from 1350 bar to 1600 bar. Due to the
special design of the A-Class engine, the
overall length of the injector had to be
shortened. For the first time, Daimler-
Chrysler integrated a gear drive to run the
flow rate-controlled high-pressure pump.
In order to fulfil the stringent Euro 4
specifications, including good combustion
acoustics and without the need for active
aftertreatment, DaimlerChrysler worked
very closely with Bosch to increase the minimum
volume capability in comparison
with the 1600 bar EU3 solenoid fuel injector
known to date, Figure 5. This was achieved
by reducing the solenoid valve armature
stroke and also by optimising the cross-section
of the control room input and output
throttle.
The reduced hydraulic flow rate of the
mini blind-hole nozzle and the flow-optimised
spray orifice (KS geometry) excels in
its improved injection preparation.
The long-term stability of the fuel injector
was improved by applying a carbon
coating in the area of the nozzle seat, which
in conjunction with the optimised solenoid
group results in consistent fuel-quantity
performance over time.
The injector fuel-quantity compensation
guarantees that the target injection quantity
is maintained precisely as manufactured.
The self-adapting zero fuel quantity calibration
ensures that the small injection
quantities are controlled over the engine’s
lifetime.
In order to generate multiple injections
for “soft” combustion depending on the operating
range, the pressure waves triggered
by the preceding injection and the quantity
changes associated with it are corrected by
a pressure-wave correction.
4 Combustion
From a customer’s perspective, a modern
diesel engine must exhibit high torque and
power performance on the one hand, and
low noise emission and fuel consumption
on the other.
Meeting all the specifications regarding
bulk volume, Euro 4 emission levels and
target costs in addition to customer needs
required a fundamental redevelopment of
the combustion process and the engine application.
The basic principles were derived in conjunction
with the newly developed OM 640
four-cylinder engine for the new A-Class.
Using the 1500 cc displacement of the
derived OM 639 three-cylinder engine, the
50 and 70 kW power variants were designed
with an equivalent wastegate turbocharger.
This makes the top version with
70 kW the world’s most powerful threecylinder
passenger car diesel engine. During
the development phase, the air com-
pressor efficiency and the turbine response
were extensively optimised. In conjunction
with the optimum-flow single-piece exhaust
manifold turbine casing, this technology
achieved high medium pressures,
both steady-state and transient in the lower
engine speed range. The maximum
torque of 210 Nm in the top version is available
over a wide range from 1800 to 2800
rpm, Figure 6.
The turbocharger is controlled by a
pneumatic actuator.
The air-to-air charge-air cooler is located
in the vehicle’s wheel arch. The high air
compressor-dependent charge-air temperatures
caused by the high boost ratio of 2.3 in
the top version are reduced by over 75 K.
The cooled charge air is fed into the cylinder
head via an integrated mixing chamber
with the introduction of recirculated exhaust
gas and a low-volume air intake
manifold on the cylinder head, practically
without effective intake runner length.
The emissions concept is based on a very
low level of engine-out emissions. The
structural design of the combustion chamber
and the duct was developed in conjunction
with the four-cylinder engine for the
new A-Class. The goal in this case was to reduce
the conflicting objectives of specific
power and emissions.
Here, the focus was on the design and
detailed optimisation of all parameters in
the intake duct configuration, made up of
tangential (permanent) and spiral ducts
(can be deactivated) in order to control the
swirl electrically using an infinitely variable
intake duct valve.
The delicate optimisation of the injection
nozzle geometry was an additional focal
point.
In conjunction with a high combustion
chamber proportion in the piston recess
and an optimised recess shape, the combustion
process fulfils all requirements regarding
emissions, fuel consumption, power
and noise.
In particular, it was possible to ensure
the engine’s insusceptibility and long-term
stability to tolerances. Outstanding specific
fuel consumption was achieved despite the
high specific power and internal engine
compliance with Euro 4, Figure 7.
This was managed within the requirement
of a favourable mean friction pressure
in conjunction with an efficiency-oriented
application of all combustion parameters.
In order to meet the high requirements
regarding NVH, dual pilot injection was applied
over a wide engine map range. Information
gained during the parameter tests
on the engine noise test stand was implemented,
taking emissions and fuel con-
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5 Engine Control Unit
Figure 8: Basic structure of EPB modular design Figure 9: View of control unit
sumption into account. In particular, emphasis
was placed on the subjective combustion
noise impression (frequency spectrum)
inside the vehicle and the various vehicle
applications as a basis for decisionmaking.
5 Engine Control Unit
A Bosch control unit designed specifically
for this application is used for the OM639
three-cylinder engine. In addition to the vehicle
functions and the common rail injection
control, the complete transmission
control has been implemented in the control
unit, Figure 8 and Figure 9. Most of the
required sensor signals are directly obtained
and processed by the control unit.
The control unit directly triggers the appropriate
actuators. Data exchange to the other
systems used in the vehicle is achieved
through the CAN and LIN communication
interfaces.
5.1 Hardware
Additional requirements regarding dissipation
of maximum thermal power of the
control unit are caused by the integration
of the manual transmission actuators.
Through extensive evaluation and simulation
for the sequence of the required
current profiles of each actuator, the dimensioning
of the H-bridge final stages
was optimised, thus enabling integration
into the Bosch EPB housing.
In this housing design, the thermal power
loss of the components is released directly
to the environment through cooling
MTZ worldwide 1/2005 Volume 66
banks in the floor plate. A constant thermal
dissipation over the life of the control unit
is ensured by a bolted connection of the circuit
board and by using a heat conducting
paste.
5.2 Software/Application
The control unit software is based on the
EDC 16 platform and is designed as a
torque-driven diesel engine control.
The range of engine related functions
corresponds for the most part to those of
the control unit for the diesel engine in the
new A-Class.
5.2.1 Emulation EEPROM
When the vehicle is running, all sensor and
actuator signals are constantly monitored.
Faults found are stored with additional environment
data for further diagnostic purposes.
Compared to previous engine control
units, data are not stored in an external, serial
EEPROM, but in form of an “emulated”
EEPROM. In this case, emulated means that
the reference data are archived in the control
unit’s flash memory. This is designed
only for a small number of write cycles,
thus requiring a special write strategy,
which ensures that the EEPROM emulated
in the flash is capable of the required number
of write cycles for the entire life of the
control unit.
5.2.2 Synchronization
without a Camshaft Sensor
A special feature of the three-cylinder engine
is the possibility to synchronize fuel
injection and position of the crankshaft
without using a camshaft sensor. As opposed
to other engines, here the synchronization
takes place here with the help of a
“virtually” generated camshaft signal. This
is detected using the irregularity of the
crankshaft rotation as it develops through
the compression processes in the cylinders,
and it is made possible by a high-resolution
rotation signal acquisition.
6 Test Results
At -25 °C, the OM 639 starts almost as quickly
as at start temperatures above 0 °C. For
example, the preglow time is only 3 s at
-25 °C. Endurance runs on engine test
benches amounded to a toal of 30,000
hours. The vehicle endurance runs took
place in part under extreme climatic conditions.
A total of 1.5 million km was
achieved.
7 Summary
The three-cylinder diesel engine designed
by Mercedes-Benz meets the Euro 4 emission
level and allows both high performance
and low fuel consumption to be
achieved. The top version with 70 kW is
thus the world’s most powerful three-cylinder
passenger car diesel engine. The integrated
balancer shaft module and the dualmass
flywheel result in a pleasantly quiet
engine. ■
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