Permanently Engaged Starter Systems
with Dry Running One-way Clutch

Knowledge LibraryKnowledge Library

Permanently Engaged Starter
Systems with Dry Running One-way Clutch
Being the future standard in all vehicle categories, stop/start systems shall allow afurther reduction in fuel consumption. However, there is little acceptance of conventional solutions by the end customers up to now due to their operational behaviour.Toimprove the system behaviour and in particular the starting performance of a combustion engine, BorgWarner is developing a compact dry running permanently engaged starter system that can be easily integrated into existing powertrains.
By Richard Bäumler, Alexander Moser and Michael Schäfer, BorgWarner Drivetrain Engineering

Current Conventional and Future
Stop/Start Systems

Besides increasing the efficiency of powertrains in passenger vehicles, it is expedient
to shut off the combustion engine as often as
possible to achieve a further significant reduction of fuel consumption and greenhousegas emissions. For increased acceptance bydrivers, stop/start systems should provide aconstant comfort, and high functional reliability. Future stop/start systems must therefore
have the following system functions and features:- change-of-mind and quick-start capability- repeatability of starting behaviour- toleration of crankshaft backlash at engine
stop- reduction of torsional vibrations during cranking- improvement of NVH behaviour (Noise,Vibration, Harshness)- engine shutoff while the vehicle is moving.
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In a conventional stop/start application with apinion shift starter and the starter ring gear
permanently fixed to the crankshaft, the engine cannot be restarted – especially under
its self-load speed – until the crankshaft hascompletely stopped or is at least rotating atvery low speeds. If the driver requests a restart in this case, there is a start delay which
the driver could misinterpret as an engine
stall. The measurement shown in thefollowing figure shows this “change-of-mind”
situation for a vehicle with an automatictransmission.
The system waits for 350 ms from the moment when the driver releases the brake pedal until the starter can be driven. For
vehicles with automatic transmissions in particular, this start delay is unacceptable for thedriver and prompts him to frequently disable
the system manually. The following figure also shows the NVH behaviour (acceleration1

Conventional stop/start system, “change-of-mind” situation

sensor on the gearbox). The impacts resulting from the engagement and disengagement of the pinion are clearly perceptible, as
is the stimulation due to the gearing during
cranking. The repeatability of the starting
process is poor due to the mechanical engagement of the pinion.
There are other enhanced stop/start technologies besides these conventional systems,
but they require additional installation spaceor show a disadvantageous NVH behaviour.
Starter generator systems have the best performance in terms of NVH and response
time, but they add high costs to the system
and require an additional pinion starter forcold starts. The dry running permanently engaged starter (PES) system presented here
matches the performance of beltdriven systems but has the simplicity of a pinion starterand is suitable for integration into any kind ofpowertrain. As a plug and play system located between the engine and the transmission, it replaces the original starter ring gear
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that was there beforehand and uses existing
interfaces with only minor modifications tothe powertrain. Knowing the load spectrum
allows the PES to be designed as a sturdy
unit. The load spectrum takes the torques into account as well as the number of loadchanges occuring under different operating
conditions such as key start, warm start andchange-of-mind.

DesignFirst permanently engaged starter systems
are already on the market. Here an OWC(one-way clutch) is used to disconnect ringgear and crankshaft [1 ]. In contrast, the dry
running permanently engaged starter systemfrom BorgWarner is located in the dry environment between the combustion engine andthe transmission, see following figure. For
this reason, there are high requirements regarding the resistance of the system against
corrosion and contamination. It can be integrated into a flexplate or flywheel dependingon the powertrain and has interfaces to2

Dry running PES system for flexplate application

crankshaft, crankcase and starting device. At
the heart of the dry running PES system is a
one-way roller clutch between ring gear andcrankshaft. The starter ring gear is mounted
to the crankcase. As the starter only rotates
during engine start and at low speeds, thisarrangement has certain advantages compared with a ring gear supported by a bearing
on the crankshaft. In this case, the bearing
would have to permanently rotate at the same speed as the engine and tolerate the axial and radial vibrations of the crankshaft.
The torque introduced into the starter ringgear is transferred via the inner race of theroller one-way clutch to the outer race, whichis integrated into the flexplate, and finally tothe crankshaft. Once the engine speed
exceeds that of the starter ring gear, theone-way clutch takes over and the starter
motor can be shut off. At a certain crankshaft
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speed below the engine idle speed, the rollers completely lift off the inner race due tocentrifugal forces. The roller one-way clutchof the dry running PES system compensatesaxial and radial movements of the crankshaft
and flexplate as well as misalignment andaxial run-out of the crankshaft flange.
As the starter pinion of a PES system is permanently in tooth contact with the starter ringgear, the tooth geometry of starter pinion andring gear can be optimised with regard to
NVH behaviour and durability, whereas thepinion and ring gear of a conventional starter
system are optimised for easy engagementof the pinion. In contrast, the contact ratio of
the gearing of a PES system can be increased significantly and backlash can be reduced. Also, there is no additional wear due toaxial movement of an engaging pinion. In
conventional systems, in which the starter3

ring gear is permanently fixed to the crankshaft, the areas of the ring gear toothing thattransfer the load during engine cranking arealways the same. In opposition to those systems, the toothing of the ring gear in a PES
system is loaded statistically and thus even.
It is also possible to improve the NVH behaviour still further by using helical gears.
A stop/start system with a permanently engaged pinion therefore provides many possibilities for improving the NVH behaviour of
the ring gear and pinion meshing as well as
the durability of the gear set to sustain an increased number of starts and stops [3].

Vehicle Testing and Operating Performance

The vehicle testing of the dry PES system
has been successfully performed on powertrains with manual transmissions and automatic transmissions. The measurement ofthe key start of a dry PES system shows five
torque peaks which the one-way clutch has
to transfer, see figure below. To accelerate
the engine from a standstill, the starter motor
has to overcome a breakaway torque followed by a virtually constant mean torque. Dueto the torsional fluctuation of the engine during cranking, there are four more torquepeaks once the one-way clutch re-engages.

The total number of load changes, for whichthe one-way clutch must be designed, is derived from the number of torque peaks incombination with the amount of engine
stop/start cycles or based on a load spectrum.
A comparable key start with a conventional
pinion shift starter is shown below. The torque impacts rise steeply and have a highpeak value because of the rigid connection of
the flexplate to the crankshaft. Due to thelower torsional stiffness, the torque impactsof the dry PES system are considerably
lower. This has positive effects on the NVH
behaviour of the starter system during cranking [3] and on the durability of the ring gear.
In addition, smaller torque impacts are transferred through the crankshaft to the accessory drive of the engine during engine start.
Another advantage which can be directly experienced by the end customer is thechange-of-mind capability of the dry PES
system. While the engine is coming to a
standstill after being switched off, the starter
motor can be driven to assist the restart ofthe engine, see figure on the next page. The
one-way clutch engages when crankshaft
and ring gear are both rotating at the same

Key start measurement of automatic transmission: dry PES (left) and pinion shift starter (right)
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speed and transfers the torque from the starter motor to the crankshaft, so that the engine can be started again with fuel injection.
This significantly improves response timeand repeatability of restarts.

Durability, Drag Torque and Wear

The durability tests of the dry PES systemwere carried out on stroker test rigs in particular, with the maximum torque and theequivalent torque of a specific application.
On the basis of these tests, which includedsalt water spray tests, it was possible to demonstrate the durability and overload capacity of the dry PES system in terms of torquerespectively Hertzian stress. Depending on
the application and the load profile specified,the S/N curves, see figure next page, allow
the main components of the dry PES system

to be adequately dimensioned and a suitablechoice of base materials, heat treatments or
surface coatings to be made.
The dry PES system is different from othersystems in that it shows zero drag torqueabove the idle speed of the combustion engine. The roller and spring system is designed
to completely disengage at idle speed(750 rpm in this case) due to centrifugal
forces, and there is no contact between thecomponents rotating at engine speed and thestationary components of the PES system.
For this reason, the dry PES system shows nopower dissipation whatsoever in normal operation of a combustion engine. There is an extremely low drag torque below thedisengagement speed, see figure below. During starting and stopping of the engine, theChange-of-mind start of a dry PES with manual transmission
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S/N curve of the main components of the dry PES

rollers rotating with the outer race have temporary contact with the inner race. The resulting wear is extremely low, however, and isprovided for in the design of the PES by allo-wing for a corresponding number of engine
starts and engine stops [3]. A certain hysteresis occurring between disengagement and reengagement is being considered in the sys-Drag torque measurement of a dry PES system
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tem layout, so that it does not affect thechange-of-mind capability.

Conclusion and Outlook

The new BorgWarner dry running permanently engaged starter system is a solution that allows a very good stop/startbehaviour to be achieved. On the basis of simulations [2], bench test results and measurements made in several demonstrator
vehicles, the benefits of this system were demonstrated. The advantages in the systembehaviour such as response, repeatability,NVH, robustness and the absence of draglosses were demonstrated in bench tests andin real vehicles. The results of this advanced
development are the basis for several customer development programmes of the systems.

Moser, A.; Saxena, V.; Bäumler, R.; Schneider, F.: Permanently Engaged Starter Concept for Comfortable Launch & Excellent
Efficiency. International VDI Congress “Drivetrain for Vehicles”, Friedrichshafen, 201 3[3] Bäumler, R.; Moser, A.; Schäfer, M.: Dryrunning, Permanently Engaged Starter System (PES) for a comfortable start of combustion engines providing further potential toreduce fuel consumption. International VDI
Congress “Drivetrain for Vehicles”,Friedrichshafen, 201 4

BorgWarner’s dry PES system not only provides the above advantages compared topresent stop/start systems; in combination
with a matching engine application, it also
opens up great potential for future technologies in automotive powertrains with combustion engines. The system allows advanced
sailing and coasting with early engine shutoff, even to the extent of enabling an “idlingfree” vehicle, and is able to handle a large
number of engine stop/start cycles, especially in hybrid applications. These technologies
support further fuel savings and increase driving comfort. With their improved operational
behaviour, in particular their starting performance, dry PES systems will ultimately contribute to a wider acceptance of stop/startsystems by the end customers.

References[1 ] Asada, T.; Sakai, M.: The New Toyota
Stop & Start System. 1 7 th Aachen Colloquium
Automobile and Engine Technology, 2008 [2]BorgWarner Knowledge Library 201 4

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