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How much do DCTs cost?
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The manufacturing cost of a DCT is driven by the four elements:
- gear train;
- actuation system;
- transmission control unit (TCU); and
- dual clutch system
1 is equivalent to the cost of a same size manual layshaft transmission.
#2 (either hydraulic or electromechanic system) is equivalent to a torque converter
valve and solenoid body control system.
#3 is the same as a TCU.
#4 is very much dependant on type (wet or dry), size (torque capacity) and manufacturing
volume.
The expectation is that with wider distribution of DCT technology and the establishment
of a wider supply base for the key systems, DCTs will be cost equivalent to t/c
automatics.
GETRAG has decided not to respond with any specific figures or values. GETRAG and
all other DCT manufacturers recognize that the DCT technology has to persist not
just the technological but also the commercial competition with other transmission
concepts.
All things being equal, the design costs of a DCT are comparable to a modern torque
converter automatic transmission. In fact, if start-stop function is included, DCTs
have the advantage of not requiring a separate device to maintain the hydraulic
system pressure (auxiliary pump or pressure reservoir).
At the current state, however, the direct comparison of DCT vs. torque converter
automatic cost difference does not take into account that volumes have not reached
the projected level, and that the supply base is still limited, i.e., the effect
of market competition in a mature supply base has not taken full effect. With the
observable growth and penetration of DCTs in the market, this disadvantage will
disappear.
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What is the cost variance between an automated transmission and a DCT?
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Compared to an automated manual (AMT), this again depend on the type of automation
system (bolt-on like Magneti Marelli or integrated), and the manufacturing volume.
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What specific areas/subsystems of DCTs have the best potential for future cost reductions?
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Future cost reductions will follow two routes:
- Economies of scale through further proliferation of DCTs; this will affect components
unique to DCTs, like the dual clutch systems.
-
Simplification of design (e.g., synchronizer systems, integration of control systems
and sensor clusters)
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In your opinion, torque converter transmissions would be replaced by DCT in commercial
vehicles.
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There is definitely an interest in the commercial vehicle industry in DCT technology
due to the fuel savings opportunity. Of course, the design criteria specifically
for dual clutch thermal capacity-cooling, protection strategies, etc,-need to be
adapted to the requirements of CV. Again, this will look different comparing road
trucks and construction trucks.
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What is the outlook for application of DCT for commercial vehicles, for example
city buses, off road vehicles?
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DCT technology will be introduced to commercial vehicles-, starting with vans. DCT
technology has been introduced to SUVs, e.g., Mitsubishi, PSA, Audi, already.
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Is it possible to expand DCT technology to class 6-8 trucks? What are the challenges?
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Yes, it is possible to expand DCT technology to class 6- 8 trucks. The package requirements
and the clutch thermal capability are the challenges.
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How far away are you from a DCT design for heavy-duty vehicles?
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Wet DCT clutches have already demonstrated capability for engine torques up to 1000
Nm. We would imagine this capacity would suit many heavy duty applications.
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What are the possible benefits/future of DCTs in large, commercial vehicles? Is
it needed?
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Starting with vans, DCT technology will be introduced to commercial vehicles. On
large, commercial vehicles AMTs are an advantageous technical solution, too.
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How reliable are DCTs for a heavy duty commercial vehicle application and what type
of DCT is recommended?
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Due to the heavy duty operation, only wet clutch systems will meet the requirements
because of their ability to control the cooling flow to the clutch system, depending
on the heat dissipation needs in clutch slipping mode.
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How would this technology apply to replacing a torque converter in a construction
machine application using a torque converter to manage torque at zero speed?
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Torque at zero speed would be applied via controlled clutch slip similar to a stalling
torque converter. What will be missing is the torque multiplication effect of a
stalling converter; this can be compensated for by a lower first gear ratio using
the wider gear ratio spread of the layshaft gear train principle. The clutch cooling
flow control needs to be designed for this purpose, of course.
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Do you see a future for DCTs in the off road market segment? What about automotive
racing?
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When DCTs were introduced, their use in off road vehicles was excluded due to uncertainty
about how specific off-road driving maneuvers would thermally stress the clutch
system. With volume production of DCTs now in the sixth year, knowledge about the
system boundaries and adequate design solutions have improved constantly. This year,
you will see the first SUV type vehicles with DCT on the road (namely Mitsubishi's
Outlander). Admittedly, these are still what the hardcore off-road enthusiast would
call "soft-roaders," but development for a real hardcore off-road brand is already
underway.
Talking about racing: GETRAG has now matched Volkswagen by fielding a Mitsubishi
EVO X with 350 HP and a series produced GETRAG PowerShift DCT in the demanding Nuerburgring
24h Race. The car finished 32nd out of 170 vehicles that started the race and 4th
out of 16 in its class (2.0L turbocharged vehicles) without any incident - just
fuel up, change the tires and clean the screen!
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Of the 20% fuel economy improvement on the FTP cycle that was mentioned, what is
the share of the direct loss improvement (box efficiency) as opposed to the indirect
influences (e.g., ratio spread)?
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The first volume application of the GETRAG PowerShift transmission was launched
in 2007with a 2.0L diesel engine in compact and midsize cars. The certified fuel
economy and CO2 emissions of this application, compared to the outgoing model and
to a wide cross-section of competitor vehicles, all with 2.0L diesel with 6-speed
t/c automatics, indicated a f/e benefit of 16-20%.
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I am still unclear about the fuel economy advantages. What is the % improvement
in fuel economy for a DCT compared to a 6-speed manual? And compared to a 6-speed
automatic?
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The contribution to this benefit is roughly split between: Flexibility of gear ratio
selection and adapted shift scheduling - 6%,
Neutral in idle- 8%
Torque converter losses - 3%
Mechanical losses - 2-3%
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Does the DCT have any advantage in fuel economy over a properly driven manual transmission?
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In comparison to the equivalent 6-speed manual transmission, these first applications
were homologated at f/e values approximately 3% higher. Note that the first VW DCTs
were rated 8-9% above the equivalent MT!
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Today, two different fuel efficiency percentages were provided for a DCT versus
an automatic(15% and 20%). When comparing a DCT to today's most sophisticated 7-speed
automatic, what percentage in fuel economy can one expect?
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The continuous development of DCT technology, including 7-speed architectures and
minimizing auxiliary energy consumption, allows us to homologate DCTs at better
f/e than the respective MT application!
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What is the fuel economy improvement of a typical DCT?
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Even when equipping cars with "shift-up" indicator lights, it requires a lot of
the driver's concentration to keep an MT car at the best f/e - whereas a DCT will
do this automatically!
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How does this compare to a 7-speed automatic with torque converter?
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To answer all the detailed questions about f/e benefits and comparisons: it can
be expected that the development of t/c automatic transmissions will continue to
improve f/e as well, so the most reliable benchmark is always the equivalent manual
transmission car (if it exists).
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How is it possible for a 6-speed dry DCT (no stop start) to have improved fuel efficiency
over a typical 6-speed MT with equal spin loss and ratios and a driver that would
emulate the shift schedule that would be used on the DCT?
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The main reason is, that in the European (NEDC) homologation cycle the choice of
gear is prescribed by the drive procedure when driving a manual transmission car,
whereas any automatic will follow the shift schedule. The same applies to the Japanese
cycle. In other cycles (e.g. the US Metro-Highway), with a lot of transient conditions,
it will be far more difficult for even an experienced driver to always stay in the
f/e sweet spot. which an automatically shifting transmission will do for you. This
is particularly true if staying in the best f/e window would require skip shifts
over two or more gears.
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Is there a fuel efficiency difference between a wet and dry DCT in a small vehicle
and a large vehicle?
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For both, small and large vehicles, the dry DCT is the more efficient solution regarding
fuel economy. On dry DCTs the pump can be designed for high pressure only, because
no clutch cooling oil flow needs to be delivered. On the other hand, the thermal
capability of dry running clutches is lower compared with the capability of wet
running clutches. Regarding efficiency, next generation DCTs, using wet running
dual clutches, will be very near to DCTs using dry running dual clutches.
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What are the typical efficiency numbers for DCT with wet and dry clutches, manual
and CVT?
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Depending on temperature, power and specific gear, there are many tables of efficiency
values per transmission. MTs have highest efficiency values, followed by dry DCTs.
Wet DCTs offer higher efficiency than CVTs can. Average efficiency values of MTs
are roughly 94%.
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How do DCTs differ from conventional automatic transmissions during idle and when
coasting to a stop? (Fuel economy in these situations)
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DCT transmissions are able to actively control the drag torque level as required
during coast down or during idle. Control quality down to 1 Nm is achievable as
the clutches have sufficient capacity. In automatic transmissions this ability is
very much depending on the type of planetaries and the individual clutches and bands
associated to them. Furthermore, the typical response of the torque converters is
a limiting element. The impact on fuel economy is very much dependent on the drive
cycle and the engine type that is used. In NEDC, the impact is up to 2 %.
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How does the idle assist sail-on feature improve fuel economy over engine stop?
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The impact on fuel economy is very much dependent on the drive cycle and the engine
type that is used. In NEDC, the impact is up to 3 % for idle assisted sailing. For
high speed stop start, there is a 3 to 8 % f/e Impact.
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What are the components that help the efficiency of a DCT?
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Small and efficient pump, splash loss reduced oil-management and minimized friction
losses are the main contributors to efficiency increase. In addition, strategies
enabling frequent engine operation near to its specific fuel-consumption best-point
increase the fuel economy.
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What are the advantages and disadvantages of DCT vs. AMT especially with respect
to fuel efficiency and cost?
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AMT has the advantage over DCT because of less parasitic losses due to not having
the dual torque path and only one clutch. All things considered equal, an AMT delivers
a slightly better f/e than a DCT. Major disadvantage is the torque flow interruption
during the shift, which is a major source of customer dissatisfaction, and the reason
why AMT proliferation is still very small compared to t/c AT and DCTs
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What fuel savings can be expected with a DCT in a city cycle and on a highway cycle?
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DCTs combine the advantages of conventional ATs and MTs: A "free choice" of ratios
with much more flexibility than planetary ATs because they are built upon the same
layshaft technology as manual gearboxes with the same strategy-determined shift
points as ATs. Fuel savings of up to 7% compared to MTs resp. >10% compared to conventional
ATs are possible in the NEDC-cycle. Highway fuel consumption is comparable to AMTs
and MTs.
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Are there any hybrid DCT's planned in the next few years? If so, can you describe
the location of the electric motor in the DCT?
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The GETRAG Corporate Group has concrete plans to offer DCTs with parallel electric
motors as a hybrid power pack. The technical realization is adapted to the individual
required level of hybridization - reaching from a coaxial crank mounted alternator,
a coaxial electric motor connected to transmission input shaft, and parallel motor(s)
connected to the input shaft(s).
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If we look at hybrid or electric transmission, what is the DCT advantage from them?
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DCTs offer a fully functional "conventional backbone" in combination with hybrid
systems. They remain almost fully drivable when the battery is empty. For electric
powertrains, a 2-speed system can improve powertrain matching by optimum size of
the components.
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If I have 300 volts onboard, how can I enable transmission improvements, innovations?
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With high voltage and a power system environment on board, powershift transmissions
can be extended with electric motors in different manners, for instance, in the
clutch housing as a starter alternator arrangement. Parallel to the layshafts of
the transmission, almost any arrangement is possible with the connection of electric
motors. (parallel, serial hybrid)
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Where do you see DCT in future hybrid integration and also what percentage will
the DCT have of the hybrid market vs. MT, AT and other transmission technologies?
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DCTs will be found in hybrid powertrains. There are three different options: torque-split
hybrid, parallel hybrid and axle-split hybrid. The percentage value will depend
on the various markets. The Japanese market prefer hybrids based on CVT technology
while the European market prefer hybrids based on DCT technology.
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In the US market, is the DCT going to be accepted as a replacement for a torque
converter planetary automatic? If so, what percentage of market penetration do you
see over the next five years?
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DCTs are already available in the US market and are very well received. We assume
that the market share of DCT will rapidly grow over the next several years, but
the conventional converter will still remain in the market.
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Will DCTs be produced in US, and by whom, within the next five years?
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There are firm plans to launch a DCT in a volume application in the US market soon.
Since the affected product has not been released for sale, confidentiality prevents
me from disclosing any further details.
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What is your expected cumulative average growth rate (CAGR) of DCTs in North America
in the next five to seven years?
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We believe that the DCT take rate in North America will grow rapidly over the next
severalyears. A percentage number is difficult to mention, as every manufacturer
has its own marketing and investment strategy. If vehicles were available today
in sufficient quantity, we believe that in five to seven years you would see more
DCT than conventional AT coming into the market.
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What is the most important thing consumers should know before DCTsarrive in the
mainstream market?
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It is important to understand that a DCT is a fully automatic transmission that
delivers all the convenience and comfort of a conventional torque converter automatic,
but provides superior fuel economy and a more "connected" feel, (which is important
for driving enthusiasts.) It is by no means any "cheaper" or a less sophisticated
product and most importantly, it is not a manual transmission with two clutch pedals!
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Any estimates of the expected penetration of DCTs in North America?
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That is very difficult to estimate. Despite the f/e benefits, it will probably take
some time to replace the available installed capacity for torque converter automatics
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Over what range of vehicle platforms do you anticipate DCT being widely used? Commercial?
Heavy truck? Off-road? Agricultural? Construction?
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The use of DCTs is likely in passenger cars, SUVs, - and light commercial vehicles.
Surprisingly, a lot of benefits can be provided to heavy-duty and construction site
vehicles, based on manual or automated TMs, as well. The advantage appears obviously
once you compare an automated manual T< in a truck with a DCT, which gives you shifts
without traction force interruption. The advantage is huge, due to not decelerating
during a gear shift on gradients or in heavy sand.
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What is the DCT market in Europe?
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Market penetration in Europe is growing faster than in any other region. The main
reason is that Europe is a traditionally manual transmission dominated market (up
to 90%). Since manufacturing of DCTs can use a portion of the MT manufacturing infrastructure,
and since they deliver nearly equal f/e, the take rate of DCTs as the automatic
option has risen dramatically in those product lines offering DCTs as an auto option.
VW for example is reporting a 30-40% auto take rate with DCT (compared to approx.
10% with t/c auto); similar numbers are available for the affected Ford and Volvo
vehicle lines.
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In a wet DCT, oil quantity is high: windage loss results in high drag torque which
effects shift and efficiency. How would you design the syncs and improve shiftability
and efficiency?
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The drag torque of the mechanical gear train is comparable to a 3-shaft manual transmission.
Oil level is kept low enough by case design and/or some oil flow management features
to avoid unnecessary splashing. Synchronizers are equivalent to high performance
parts for manual transmissions, designed to withstand the high slip energies of
skip shifts (multi-step shifts).
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Do you feel that wheel motors will replace DCTs?
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DCTs, like all multi-step or CVT transmissions, fulfill the need to link a driving
motor with limited torque over a speed band matching the available torque band to
the torque requirements of the total desired speed range. As long as this requirement
exists, DCTs will be the favorable alternative in the field of multistep or CV transmissions
for f/e and performance.
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What changes are needed to existing manual transmission gear manufacturing infrastructure
in order to produce DCT's?
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Machining: In addition to the conventional machining and
heat treatment infrastructure for gears, synchronizers and shafts and case machining,
high precision aluminum machining is required for the valve and solenoid bodies
(higher accuracy requirements compared to torque converter auto or CVT hydraulic
system machining).
Assembly: Assembly of hydraulic control units (regardless
if low or high pressure systems) preferably under clean room conditions. Supplier
parts to be delivered under cleanliness requirements.
Function Testing: Functional check of pre-assembled sub-systems
like the mechatronic unit is advisable. Transmission EOL testing needs to be capable
of cycling through all operating modes and connecting to the transmission control
unit. Some control system designs require teach-in capability to teach in and store
specific values like clutch stroke vs. pressure signal.
Quality System: Full traceability of critical components
and sub-systems is required to allow backtracking of potentially defective parts
to individual transmission serial numbers and VIN to avoid costly customer notification
programs in case a defect is detected in a batch of safety critical parts.
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With very little damping in the system, to assure great fuel economy, won't there
be NVH issues with the DCT?
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NVH sensitivity of DCT is similar to manual transmissions. Unlike MTs, DCTs have
the advantage to allowing micro-slip driving to achieve additional NVH damping in
critical driving conditions. Of course, special care must be taken to optimize damper
design and tuning for any specific application.
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Does DCT have lower gear noise compared to conventional transmissions?
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The DCT gear noise is equivalent to the MT’s gear noise, using hard-finished gears.
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All mechanical systems will eventually require repair and maintenance, in comparison
to tradition MT and ATs are there any special issues with DCT's?
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Repair and maintenance of DCT is comparable to AT’s, if they use a wet clutch and
a hydraulic actuation system. Therefore, in repairing DCT’s, the same care has to
be taken for cleanliness and parts handling. Also some teach-in procedures have
to be performed after repair, which can be done with the service tools at the dealer
garage. For DCTs with electric actuation, repair and maintenance is similar to that
of automated MT’s.
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What part of the DCT causes the most premature failures?
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As with any other automatic transmissions, all components and sub-systems of a DCT
undergo rigorous development and production validation tests. There is no specific
component that is a "weak link" in the system. Premature failures are more likely
a result of a manufacturing unit’s lack of familiarity with a piece of new technology
or equipment, but not directly a result of design weakness.
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Why has the VW DCT had so many problems with the valve body?
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Unfortunately we do not have the necessary information from Volkswagen available.
All that we can say is that VW and GETRAG have different valve body component suppliers.
GETRAG manufacturing vertical integration includes machining and assembling the
hydraulic units (valve- and solenoid body) to have full control of all quality parameters
for this sensitive system.
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What parts, if any, have failed in the field?
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As with any other automatic transmissions, all components and sub-systems of a DCT
undergo rigorous development and production validation tests. There is no specific
component that is a “weak link” in the system. Premature failures are more likely
a result of a manufacturing unti’s lack of familiarity with a piece of new technology
or equipment, but not directly a result of design weakness.
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Is there anything special about the shifter used for a DCT compared to the shifter
for an AT?
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You can use any commonly known shifter from an all mechanical to a fully drive by-wire
system. For park-by wire you need some additional actuation device like that in
production for the BMW M3.
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Can you elaborate more on the sensor needs for DCTs?
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In general, a DCT requires more, and more accurate, sensors than an A/T:
Oil/TCU temperature: 2, standard resolution/accuracy Input/Output
Shaft Speed: 2 or 3, high resolution/accuracy (need good control of clutch as launch
element)
Clutch pressure: 1 or 2, high resolution/accuracy (need
good control of clutch as launch element) Shift fork position: 4, high resolution/accuracy
(need good control of shift fork position)
PRNDL shifter position: 1, high resolution/accuracy and
more redundancy (need good control of PRNDL position at an increased safety level)
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What is presynchronisation with respect to DCTs and what are the various presynchronisation
strategies used currently?
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Shifting in DCTs is always a gear change and a clutch torque handover. Preselection
of gears in the DCTs is used to make shifts faster. With the gear engaged, the shift
time is reduced. Different pre-selection strategies are implemented that consider
the driving situation, the performance, fuel economy and comfort requirements.
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What determines the use of a wet or dry DCT?
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The usage is mainly driven by the clutches’ thermal capability. The thermal capability
of wet running clutches is higher.
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Is the function of the dual clutch mainly for increased torque input capacity or
is there some other function?
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The dual clutch enables shifts without any torque-break, so-called powershifts.
DCTs contain two transmission paths.The even gears are linked to the even clutch.
The odd gears are linked to the odd clutch. If one clutch is engaged, the other
clutch is disengaged simultaneously.
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Would a shaft torque sensor provide better information than a fork position sensor?
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A torque sensor would help to control the clutch torque control better. The gears
could not use this torque sensor, so a fork position sensor still would be required.
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How are skip shifts achieved smoothly on DCTs?
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The double-shifts are performed as sequential shifts, giving you traction force
of the "old gear-1" (5th for a 6-4 shift) while the engine revs up. Quad-shifts
are supported by a lower gear, i.e., 5 or 3 while shifting from 6-2. For maximum
response, as in all stepped transmissions you can rev up the engine free of traction
forces to achieve maximum shift speeds.
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What impact will DCT technology have on the use of plastics in transmission control
systems, such as in valve components?
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Plastic parts in transmission control systems become more and more important. The
sensors are clustered on plastic tubes. Aluminum parts will be replaced by plastic
components.
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Can DCT replace CVT?
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Yes, DCTs can replace CVTs. Audi and Daimler introduced CVTs. But now DCT's, covering
the same torque range, have been developed by both companies.
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CVTs are seamless and DCTs will soon be too. How about consumer reactions to seamless
shifting?
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The customer reactions are different. Shifts with a clear feedback to the driver
are preferred for sportive applications. Smooth shifts are preferred for more comfortable
vehicles. The DCT technology is the best choice enabling different characteristics,
depending on the selected drive mode, to the same vehicle.
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Is there anything special about the shifter for a DCT compared to the shifter for
an AT?
|
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You can use all commonly known shifters from an all mechanical to a fully drive
by-wire system. For park-by wire you need some additional actuation device like
in production for the BMW M3. The design requirements are the same for the DCT as
it is for the AT. Although, having the ability to tap shift would be a plus.
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Can you comment on the gear quality level requirements in DCT compared to those
required in an automatic transmission?
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GETRAG has been hard finishing (Reisshauer grinding and, more recently, power honing)
due to NVH and durability requirements for all new products (DCT and M/T) since
2000. Compared to planetary automatic transmissions where a number of gears are
finished green, the production of DCT gear sets requires tighter tolerances.
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Discuss the durability of the DCT as compared to current transmissions.
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As with any other automatic transmissions, all components and sub-systems of a DCT
undergo rigorous development and production validation tests. There is no specific
component that is a “weak link” in the system. Premature failures are more likely
a result of a manufacturing unti’s lack of familiarity with a piece of new technology
or equipment, but not directly a result of design weakness.
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What different control strategies are used with dry clutch for launch?
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Specific control strategies are used to minimize the heat increase. For example,
the stall speed during launch phase can be varied depending on the actual clutch
thermal condition.
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How do you compare launch characteristics of DCTs with torque converter launched
transmissions?
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The comfort of the launch can be calibrated and is reproducible with DCTs. There
is no degradation in comparison to T/C vehicles. On 0-100 with the same first gear
ratio, the DCT is always faster. The DCT offers additional features like the race
start and high-altitude and cold climate compensation, enabled by a calibrated launch
device instead of fixed hydraulic behavior. To compensate for the boost effect of
the torque converter, you can apply the principle that you need about 10% shorter
1st gear ratio to achieve a good comparison for the initial launch phase. After
the first 0 to 5 seconds, you benefit from the lower inertias.
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Please describe the DCT launch strategy.
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The comfort of the launch is able to be calibrated and reproducible with DCTs. There
is no degradation in comparison to T/C vehicles. On a 0-100 with the same first
gear ratio, the DCT is always faster. The DCT offers additional features like the
race start and high-altitude and cold climate compensation, enabled by the able
to be calibrated launch device instead of the fixed hydraulic behavior. To compensate
the boost effect of the torque converter, you can apply the principle that you need
a about 10% shorter 1st gear ratio to achieve a good comparison for the initial
launch phase. After the first 0,5 seconds, you benefit from the lower inertias.
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How do you address the lag at the initial launch, which is a bit annoying?
|
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To compensate for the boost effect of the torque converter, you can apply the principle
that you need about a 10-15% shorter 1st gear ratio to achieve a good comparison
for the initial launch phase. After the first 0 to 55 seconds, you benefit from
the lower inertias.
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Will the hybrid designs help mitigate the DCT launch shortcomings?
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There is only one "shortcoming" in the launch of a DCT which is the lack of torque
multiplication of the torque converter in stall mode. Modern control algorithms
allow customization of the launch torque curve individually to engine torque characteristics
to achieve a smooth and consistent traction resp. acceleration. This can be enhanced
by using the parallel electric motor during launch to amplify the launch torque
and/or to tailor the launch torque characteristics.
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How will the launch judder that is typical of dry clutches be received by consumers
who anticipate the launch vibration associated with a torque converter.
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Any judder in a DCT is treated as an error state, and the closed loop control algorithms
used in DCTs - wet or dry - are designed to respond to early indicators for judder
to avoid or suppress it.
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Has shudder been a problem in current wet DCT designs? What is the cause?
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Any judder in a DCT is treated as an error state, and the closed loop control algorithms
used in DCTs - wet or dry - are designed to respond to early indicators for judder
to avoid or suppress it.
As in all slip controlled friction systems, judder is the result of the friction
coefficient declining with increasing relative slip speed. This needs to be addressed
by using the right friction material specification and geometry, in combination
with adequate friction modifier additives in the oil. This explains the critical
importance of transmission fluid in DCTs
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For small A/B cars with small displacement engines, what do you see as the optimum
# of DCT gear ratios? How about overall ratio spread?
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5 or 6 gears are sufficient for most of the applications. In several applications,
even the ratio spread of a 5-speed derivative cannot be used completely.
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Do you see a practical limit to the ratio-spread for a DCT design?
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The ratio split is limited by the center distance of the shafts. In general: The
bigger the transmission, the higher the gear ratio spread capability. A spread of
8-9 is feasible in the mid-range torque for passenger cars (300-450Nm).
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Where is the point of diminishing benefit with respect to the number of transmission
forward speeds in a DCT vs. planetary automatic for passenger vehicles applications?
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Any increase in number of forward gears also increases the parasitic losses of the
transmission gear train (and also the clutch set in a planetary auto). In DCTs,
the increase step is higher for each additional gear that would require another
synchronizer unit. That explains why moving from 6 to 7 gears provides a measurable
fuel economy benefit, whereas the additional hardware necessary to go from 7 to
8 eliminates a major part of the gains. Independent of the physical limits of improving
f/e by number of gear ratio steps is the question of driveability or shift busyness.
A transmission that is constantly shifting gears to stay in the best fuel window
can be perceived as annoying. A shift strategy that tries to avoid that by exercising
skip shifts in specific driving condition will sacrifice the f/e opportunities resulting
from the number of gears. Generally speaking, for wet clutch DCT’s the limit is
6 to 7 forward ranges. Additional ranges at this point do not improve fuel economy
and make the transmission shifting very busy. For dry clutch, an additional clutch
is needed for clutch thermal and wear characteristics.
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How is hill hold clutch temperature creep handled in dry DCTs?
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In principle, the strategy is similar to the one used in wet DCTs. A temperature
model of the clutch monitors the dissipated slip energy in the clutch until a safe
warning limit is reached, which triggers warning signals and eventually the releasing
of the clutch.
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What are the results when both clutches engage, either by error or component failure?
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Engagement of both clutches with high torque is prevented by three levels within
software strategy:
- the standard slip control algorithm opens at least one clutch if the desired clutch
slip cannot be measured;
- the level 1 software diagnostic function detects too high clutch pressure and reacts
by opening one or both clutches;
- the independent level two software function detects all possible hazardous driving
situations and reacts independently from the level one software with the corresponding
actions.
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What is the limp home strategy of a DCT versus a step gear automatic?
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The DCT does not have a single limp home strategy. There are several different limp
home strategies implemented. The DCT tries to provide highest possible limp home
quality for the driver and it has a big advantage, because in principle it has two
transmissions. So if there is a defect at one transmission path, the limp home strategies
disables one path and goes on with the other path.
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What happens to DCT heat when the driver uses the gas pedal to hold vehicle on steep
hill?
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DCT’s as well as AT’s will be able to hold the vehicle for a while on a hill. First,
the generated heat will be stored in the transmission and then the cooling system
will help to lower the transmission temperature. Depending on how steep the hill
is and if the heat dissipated by cooling is greater than the generated heat the
vehicle can hold on the hill for quite awhile.
During hill hold the transmission temperature will increase. The transmission software
detects the temperature increase and reacts with several degradation modes. The
initial degradation modes should not be recognizable by the driver. The following
modes like light wheel torque oscillations or slow clutch opening will force the
driver to use the brake. The final mode will open the clutches and ensure a relaunch
after a defined procedure.
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What about towing? Boat launch for SUV?
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Typical requirements such as stem pulling, snow plowing or boat pulling can be fulfilled
with DCT transmissions.
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Is the operating temperature of DCTs any higher than automatics?
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No, the DCT oil sump temperature is lower due to higher transmission efficiency.
As DCTs are typically more efficient than ATs, during long uphill drives or in high
speed ranges, DCT will reach limits later. In general, the operating temperature
limits are set by the transmission control units and the installation area. Integrated
transmission control units will have same temperature limits.
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Do DCTs allow two-cone synchronizers at high torque gears or are three-cone synchros
still preferred?
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2-cone-synchronizers with the right design are absolutely sufficient for high torque
gears in DCTs.
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How do the synchronizers differ from those in a manual transmission?
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The worst case load conditions for a synchronizer unit are so-called ski shifts
(e.g. direct 5-2 downshift) which are an essential part of any automatic transmission
(and DCT) shift strategy. Therefore, the DCT synchronizers need a high(er) capability
to deal with slip speed deltas.
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