Compact Disconnect System forElectrified Drivetrains

Knowledge LibraryKnowledge LibraryCompact Disconnect System for Electrified
Drivetrains
Recently, powertrain electrification has gained more importance for OEMs aroundthe world due to more stringent emissions regulations with ambitious future CO 2emissions targets, which are difficult to achieve with conventional powertrains using
internal combustion engines only. In an attempt to further reduce fuel consumption
and CO2 emissions, an increasing number of vehicles using a wide range of systems
such as stop/start systems, mild hybrid systems, plug-in hybrid systems and battery
electric systems have been introduced.
By Alexander Moser, Supervisor Advanced Product Engineering, BorgWarner Drivetrain Engineering GmbH

Introduction

One option that can help OEMs to comply withemissions regulations is brake energy recuperation. To make the most of it, the kinetic energy
must not be lost in the form of heat at the brakes.
Simulations conducted by BorgWarner haveshown that a mild hybrid system with an electric
power range of 1 0 to 25 kW is most suitable foradaptation since it offers a relatively low systemcomplexity combined with high functionality. On
the basis of the New European Drive Cycle(NEDC) and the CO2 emission target of 95 g/km,
which will be obligatory for new passenger car
registrations as of 2020, using a mild hybrid
system is the best choice in terms of the recu-peration potential of the braking energy. It allows
the lion’s share of the braking energy of a normal
passenger car to be recuperated with an electricmotor featuring a power rating of about 1 5 to25 kW.
For this reason, BorgWarner developed a mild
hybrid drivetrain concept that can maximize thisrecuperation potential, permitting disconnectionof the engine without the use of an additional
disconnect clutch. This innovative cost-efficientconcept uses a one-way clutch (OWC) in combination with a dual-clutch transmission (DCT),
see Figure 1 , and a small electric motor. The
advanced setup allows pure electric driving or

Figure 1 . Highly efficient one-way clutch (OWC) in combination with a dual-clutch transmission(DCT).
BorgWarner Knowledge Library 201 61

Figure 2. Hybrid powertrain with OWC placed
between the engine and the clutch for evengears of the dual-clutch module.

sailing while maximizing the recuperation potential, thus making it interesting for mild hybrid and
48 V systems.

One-way clutch powertrain concept

Disconnection of the combustion engine is thekey enabler for pure electric driving and themaximization of the recuperation potential. A
disconnect clutch allows engine shut-off in pure
electric driving mode and offers maximum flexibility in terms of electric driving in all gears aswell as the option of shifting between gears
without any torque interruption whatsoever.
The drawback to using a disconnect clutch
which transmits the maximum powertrain torqueis that it requires additional components, actuation and usually also extra space between theengine and the transmission.
For this reason, BorgWarner’s solution makes
use of an OWC instead of a disconnect clutch.
The OWC overruns when the engine is shut off
so that it is disconnected from the powertrain
while the vehicle moves along using the electricmotor. In addition to extremely low drag losses,
OWCs provide high torque density which facilitates compact packaging, especially in comparison with disconnect clutches, and they do
not need any additional controls or actuation.
Extensive testing of numerous layout combinations has led to a concept for mild hybridsthat features an OWC which is placed between
BorgWarner Knowledge Library 201 6

the engine and the clutch for even gears of thedual clutch module, as seen in Figure 2. The
electric motor is placed between OWC and evenclutch, which permits the electric motor to be
used for pure electric driving in all even gears
and in reverse gear and allows extremely compact
packaging. Besides being able to boost or recuperate in most gears, the combustion engine
can operate in all of them.
The electric motor provides the braking torqueduring deceleration, hereby regenerating thebraking energy and charging the battery. In thispowertrain concept, engine braking is realized
by connecting the appropriate odd gear via therespective clutch if the battery is fully charged.
As this setup is intended for a mild hybrid with
an electric power of about 25 kW, the only
limitation compared with the setup including adisconnect clutch is the fact that it is not able tooffer pure electric driving in odd gears. Sincepure electric driving in even gears would be
sufficient for the vehicle speeds possible withthis power configuration, this disadvantage isnegligible.

Modes of operation

BorgWarner extensively investigated the functionality and safety of this concept specifically withregard to the main driving modes to be expectedfrom a hybrid vehicle, which are driving with a
combustion engine and pure electric driving aswell as boosting and recuperation using theelectric motor. By selecting the correct gears andthe appropriate clutch engagement, the OWC
configuration facilitates all of these driving modes.
In addition, electric boosting or regeneration in
one of the even gears with a higher gear ratio
than the odd gear selected while using the combustion engine for driving in odd gears is possible
when both clutches are engaged.
Moreover, BorgWarner simulated various driving
2Figure 3. The one-way clutch (OWC) offers significant packaging benefits as well as high torquedensity and excellent comfort.

situations, including transitions between different
driving modes, to test the functionality andperformance of this hybrid powertrain. The
initial acceleration of the vehicle with the helpof the combustion engine is followed by cruising
at a constant speed. While cruising, the combustion engine is switched off, as the electricmotor alone can provide the power required forsustaining the constant driving speed. In thisscenario, the OWC is in freewheeling mode andthe engine is disconnected. When the engine
is switched on, the OWC engages smoothly
as the vehicle moves along. Due to the excellentcontrollability and thermal robustness of a wet
dual clutch, the engine can be started conventionally by using the engine starter, or it can beactivated by means of controlled engagementof the odd clutch while the electric motor is still
driving the vehicle via the even clutch. Engagement and disengagement of the OWC are usuallynot noticeable, thus increasing comfort for thedriver.
BorgWarner Knowledge Library 201 6Conclusion

BorgWarner’s innovative and cost-efficient concept for a hybrid module including an OWC as
the engine disconnecting device helps OEMs
meet the increasingly stringent emissions regulations and improve fuel economy. Replacing
the engine disconnect clutch and its actuation,
the OWC, as seen in Figure 3, reduces the complexity for clutch control and offers significant
packaging benefits while still supporting all
driving features such as electric driving, boosting,load point shift, coasting, engine braking andrecuperation.

Contact

Email: technology@borgwarner.com
For more information please visit
borgwarner.com
3