The Full-Hybrid Powertrain of the new BMW, Grzegorz Płuciennik, Mechanika Samochodowa, Hybrydowy Napęd
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20. Aachen Colloquium Automobile and Engine Technology 2011
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The Full-Hybrid Powertrain of the new BMW
ActiveHybrid 5
Dipl.-Ing. Claus-Otto
Griebel
, Dr.-Ing. Friedrich
Rabenstein
, Dr.-Ing. Manfred
Klüting
, Dipl.-Ing. Frank
Kessler
, Dipl.-Ing. Jan
Kretschmer
,
Dipl.-Ing. Elmar
Hockgeiger
BMW Group, Munich, Germany
Summary
Mit dem ActiveHybrid X6 und dem ActiveHybrid 7 hat BMW bereits zwei Hybrid-
Modelle erfolgreich auf den Markt gebracht. Im Rahmen der Efficient Dynamics
Strategie verfolgt die BMW Group den Weg konsequent weiter, über die Einführung
der Hybrid-Technologie in weitere Fahrzeugbaureihen die CO
2
Emissionen der
Fahrzeugflotte nachhaltig zu reduzieren. Der nächste Schritt steht nun in der oberen
Mittelklasse mit der Serieneinführung des BMW ActiveHybrid 5 unmittelbar bevor.
Wesentliche Zielsetzungen für den Antrieb waren eine signifikante und kundenwerte
Verbrauchsreduzierung, eine markenadäquate Ausprägung des elektrischen
Fahrerlebnisses, sowie eine intelligente Vernetzung der Fahrzeugsysteme zur
optimalen Anpassung der Betriebsstrategie an die Fahrsituation. Mit dem gewählten
Konzept sowie den durch eine gezielte Neuentwickung elektrischer
Antriebskomponenten realisierbaren neuen Antriebsfunktionen konnten diese
Anforderungen in beeindruckender Weise erfüllt werden. Dabei ist ein Fahrzeug
entstanden, das die BMW-typische Fahrdynamik und Agilität mit höchster Effizienz
vereint.
With the ActiveHybrid X6 and the ActiveHybrid 7, BMW has already successfully
launched two hybrid vehicles. Within the context of the Efficient Dynamics strategy,
the BMW Group consequently follows the way of sustainable reduction of CO
2
emissions of the vehicle fleet by introducing the hybrid technology in further vehicle
product lines. With the launch of the series production of the BMW ActiveHybrid 5,
the next step is now directly ahead in the upper middle class.
Essential targets for the powertrain were a significant and customer-relevant
reduction of the fuel consumption, an electric driving experience appropriate for the
BMW brand, as well as the intelligent combination of vehicle systems in order to
optimally adjust the operating strategy to the driving situation. With the selected
concept and the newly developed electric components and powertrain functions,
these requirements have been met in an impressive way. By this, a vehicle has been
created which combines BMW-like driving dynamics with highest efficiency.
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20. Aachener Kolloquium Fahrzeug- und Motorentechnik 2011
1 Introduction
In the development of highly efficient powertrains, the new powertrain of the
ActiveHybrid 5 represents another consistent step in the Efficient Dynamics strategy
of the BMW Group.
Today the customer can choose from various hybrid cars from different
manufacturers. The different concepts mean the customer faces a difficult task in
deciding what powertrain system is the right one for his individual requirements. As a
manufacturer it was necessary for BMW to realise a concept that on the one hand
exhausts the maximum fuel economy potential of the hybrid technology and on the
other offers the powertrain characteristics and drivability typical of BMW also in
electrified powertrains.
The well-known 6-cylinder N55 with the 8-speed automatic transmission is the basis
of the ActiveHybrid 5. The aim was to make all necessary adaptations in this well-
known package for integration of the electric motor. Based on the ActiveHybrid 7 [1]
and the ActiveHybrid X6 [2] the challenge in the development of the parallel hybrid
lays in implementing an operating strategy which enables efficient use of the
electrical energy in line with the electrical driving experience.
In addition, the important core components of the new powertrain system have been
developed from scratch and designed to the requirements of a full parallel hybrid
concept. The lithium-ion battery, developed and produced in-house, the power
electronics and the electric motor result in an electric powertrain system which is
connected via the electronics of the combustion engine to give one harmonious unit.
A starter unit for the combustion engine which was developed especially for this
application ensures that the full electric powertrain energy is available for electric
driving under all conditions.
Another focal point is the intelligent networking of the two drive systems within the
framework of the Connected Drive approach. Here navigation data have influence on
the operating strategy and thus lead to a new dimension in the networking of
systems.
2 Drive concept
The market for hybrid drives is today shaped by a variety of different concepts in
diverse designs. There is a rising number of power-split concepts, parallel and serial
hybrid drives on the market, or provide an outlook of future applications in the form of
concept studies. Also the degree of electrification ranges from mild hybrid solutions
without pure electrical drive and full hybrid applications with electrical distances of a
few kilometres for inner-city speeds and accelerations to plugin hybrids with the
option of managing typical distances such as the daily commute to work by pure
electric drive. There is high dynamism in the development of this segment of drive
20. Aachen Colloquium Automobile and Engine Technology 2011
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technology and it is a challenge for all vehicle manufacturers to develop concepts
which on the one hand optimally use the functional potential of the hybrid technology
and on the other can be realized efficiently in a wide variety of applications.
A fundamental objective in the design decision was therefore to define an
architecture which both satisfies current requirements and also offers excellent
scalability for future designs. The requirement for high synergies with the
conventional drive, as well as the preservation of the typical BMW driving experience
for the hybrid application, also played a decisive role in the selection of the technical
concept for the hybrid drive of the ActiveHybrid 5.
Based on these considerations, a parallel hybrid design was chosen with the
capability of purely electric driving for the powertrain concept of the ActiveHybrid 5.
The transmission is an 8-speed automatic, which bears high synergies with
conventional transmissions. An integrated electrical machine takes the space of the
conventional torque converter and the start-up function is realized by an internal
transmission clutch. A separating clutch is located between the electrical machine
and the combustion engine. This allows the combustion engine to be disconnected
from the drive and thus makes possible purely electric driving with a stationary
combustion engine. For optimal electric vehicle performance, as well as excellent
comfort and response characteristics during start of the combustion engine, the
design has a separate starter unit which has been integrated as an additional starter
motor in the belt drive. Figure 1 illustrates the powertrain architecture of the
ActiveHybrid 5.
Abbreviations:
K0:
Separating clutch: combustion engine – transmission/E-machine
EM:
Electric Machine
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20. Aachener Kolloquium Fahrzeug- und Motorentechnik 2011
IAE:
Integrated start-up element
EME:
High-voltage power electronics with integrated control unit
DC/DC:
Voltage converter HV – 12V
SGR:
Starter unit in the belt drive
EPS:
Electric power-steering pump
ELUP:
Electric pump for brake servo
Fig. 1: Powertrain topology of the ActiveHybrid 5
3 The powertrain components of the ActiveHybrid 5
3.1 Combustion engine
The 3.0 l six-cylinder gasoline engine, which is also used in the non-hybrid vehicle
535i, is used as a combustion engine [3].
The firmly established combination of direct injection with turbocharging has been
complemented in this engine for the first time with the fully variable valve train
VALVETRONIC introduced by BMW and is one of the most efficient combustion
engines of its kind. To achieve a high power output in the naturally aspirated region
of the engine map, as well as a high turbocharged full load, and at the same time
high specific performance values with direct response characteristics, the cylinder
bank separation favourable with the six-cylinder has been maintained through the
use of a turbocharger according to the twin-scroll principle.
The results are excellent full load values with a low-end torque of 400 Nm at an
engine speed of 1200 rpm and maximum power of 225 kW at 5800 rpm in
combination with outstanding response characteristics.
The modifications of the combustion engine are restricted to the adaptation of the
belt drive for integration of the starter unit together with a cancellation of all other
ancillary components.
3.2 Starter system
The comfort of the start of the combustion engine from electric driving is particularly
important for full hybrid electric powertrains. In order to fulfil the requirements for this
engine start an additional starter system has been integrated. This system comprises
a 12V battery, which can be galvanically isolated from the 12V power supply battery,
and a starter motor in the belt drive which has significant advantages over a
conventional starter with regard to NVH quality. The starter motor is a claw-pole
machine with integrated power electronics. Modifications to the belt drive ensure that
the mechanical energy can be reliably transferred to the crankshaft.
20. Aachen Colloquium Automobile and Engine Technology 2011
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3.3 Hybrid transmission
One of the most important strategic objectives within the framework of the
development of the hybridised automatic transmission for the new BMW ActiveHybrid
5 was the implementation of a modular and scalable transmission concept based on
the conventional 8-speed automatic transmissions which can be used in combination
with the entire BMW engine range and in all BMW vehicle product lines.
As a logical next step to the above-mentioned objective it was also necessary to
integrate the hybrid-specific additional components, separating clutch and E-
machine, without the requirement for any additional installation space in comparison
to the conventional transmission. For this purpose a hydro-dynamic torque converter
was omitted in the hybrid transmission. The free installation space arising in the
transmission bell housing is instead used to install the hybrid drive comprising the E-
machine (incl. water cooling shell, rotor position sensor and power connection),
separating clutch and torsional vibration damper with the same spatial requirement
as that required in the conventional basic transmission. The existing geometric
interfaces such as motor flange, gearbox mounting, position of the oil cooler
connections, position of the transmission connector, etc. are identical to the
conventional version of the gearbox.
In order to support the fuel economy benefits of the hybrid technology, in the
development special attention was paid to the efficiency characteristics of the hybrid-
specific changes to the transmission. Care was also taken to ensure that the general
energy consumption of the transmission is optimized.
The typical BMW characteristics of excellent dynamic gearshifts and precision
together with excellent comfort were able to be implemented in the hybridised
transmission as another fundamental contribution to the corporate strategy Efficient
Dynamics. Figure 2 shows the transmission topology, as well as an overview of the
fundamental characteristics of the hybrid transmission.
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