F2016-THBC-010

DEDICATED HYBRID TRANSMISSION (DHT) – A SOLUTION FOR INCREASED

PRODUCTION NUMBERS OF HYBRID POWERTRAINS

Brunner, Mario*; Dr. Fischer, Robert; Dr. Küpper, Klaus

AVL List GmbH, Austria

KEYWORDS –Powertrain, Hybrid, DHT, Future-Hybrid, Transmission

ABSTRACT

Driven by legislation or by fun to drive, electrification will influence future powertrain

technology significantly. Electrification is one measure to achieve fuel economy targets. On

conventional powertrains the overall ratio spread and, along with it, the number of gears is

increasing recently. The additional complexity concerning calibration as well as weight and

efficiency objectives will stall this trend, especially as future engines will provide a much

broader range of low specific fuel consumption.

Beyond the portfolio of today’s transmissions evolved with respect to efficiency and comfort,

dedicated transmissions for electrification will gain increasing importance. Transmissions are

the natural place to integrate the second propulsion source. As hybrid propulsion is key to

achieve emission and fuel economy targets, dedicated transmissions designs are favorable with

respect to total cost of ownership and functionality.

The hybridization of the powertrain takes place in the transmission; how far that can go can be

shown at the example of DHTs (Dedicated Hybrid Transmission). The new transmission

category was introduced during the 2015 CTI in Berlin [1]. DHT covers, in addition to the

known categories MT, AT, AMT, CVT, DCT, all hybrid transmissions, which have the power

source for electrical propulsion fully integrated and its functionality depends on the integrated

electrical components. Without an E-motor the transmission cannot fulfill the requirements. In

contrast to DHTs, the category Add-on hybrid transmissions is introduced which build on an

existing conventional transmission. Typically by adding a module or replacing the launch

device by a module the electrification is accomplished. Such transmissions are already

available in the market, e. g. from Honda, Daimler, BMW, VW, etc. There are many different

variants of the Add-on hybrid transmissions already in production, yet each is produced in

relatively small volumes. This actually is the main reason for the modular approach of building

up on existing conventional transmissions, because the volumes would not justify the cost of

development and industrialization of a fully integrated transmission according to the newly

defined transmission category DHT [1].

Within this paper following questions shall be answered:

1. Why do we need new transmission architectures (DHTs)?

2. What are the specifics during the development of such transmissions?

3. What are the solutions that are already available or new on the market?

MAJOR TRANSMISSION LAUNCHES

Figure 1 shows that in the beginning MT and AT dominated the transmission world. CVT was

a completely new category, but was not successful after its introduction.

Figure 1: Major transmission launches before 2000

In the 90tys TOYOTA surprised with the Prius the transmission world and even the powertrain

world: No clutches and two e-motors as central elements (Figure 2). This transmission could

not be placed into one of the existing transmissions categories.

Figure 2: Toyota Hybrid System (THS) Generation 1

Most likely driven by the fuel consumption concept of TOYOTA, the developments in the

transmission world increased significantly after its introduction. The number of gears were

increased and with the DCT (double clutch transmission) a new transmission category was

introduced.

All of those transmission categories have electrified variants, the already mentioned add-on

solutions. Also new DHTs were introduced, Ford was showing a solution, GM, BMW and

Daimler presented the two-mode. GM introduced the Volt as another solution. Figure 3

provides an overview of transmission and DHT launches.

Figure 3: Overview of transmission and hybrid launches

DEDICATED HYBRID TRANSMISSION

DHT is a hybrid transmission using the E-motor(s) to fulfill functional transmission

requirements (e. g. adapting speed and torque of the combustion engine fitting to the vehicle

operating conditions). Simply replacing a launch element by an E-Motor doesn’t make a

transmission a DHT [1].

DHTs have at least two propulsion sources, the ICE and at least one e-motor. Without one of

the propulsion sources the DHTs is not working.

The DHT can provide several operation modes: serial mode, parallel mode, power-split mode,

pure electric or ICE mode.

ADD-ON HYBRID TRANSMISSIONS

Add-on solutions use a modular approach, utilizing components and sub-systems already

available and industrialized. This makes sense for low productions volumes of hybrid

powertrains. And this is the reason, why a multitude of such Add-on Hybrid Transmissions

have been developed and launched by various vehicle manufactures and transmission suppliers,

e. g. Honda, Daimler, VW, ZF, etc. [1]. One good example is VWs DQ400e, see Figure 5. Here

a standard DCT is enhanced by a module positioned between engine and DCT. Within the

module there is a clutch to disconnect the ICE and an E-motor as a second power source. It is a

so-called “parallel-hybrid”. In contrast to a DHT, this transmission has all functionalities of a

conventional DCT even with the E-motor removed.

DHT is a hybrid transmission using the E-motor(s) to fulfill functional transmission requirements.

Simply replacing a launch element by an E-Motor doesn’t make a transmission a DHT.

Figure 4: Definition of Dedicated Hybrid Transmission (DHT)

A second example is ZF’s 8P70H. Figure 6shows the transmission with its bell-housing

extended to carry the E-motor replacing the torque-converter. Strengthening one clutch on the

gear set allows it to be used for launching the vehicle.

Figure 6: Example of RWD Add-on Hybrid: ZF 8HP70 / 8P70H (Source ZF)

Thus if the E-Motor is removed the functionality of a conventional 8-speed transmission is

preserved. The E-Motor is replacing the torque converter, hence the electrified transmission

would be called Add-on.

Which solutions should be preferred, Add-on or DHT?

Several technical aspect lead to different conclusions, for high production volumes the solution

with lower costs with the same or even better performance in terms of CO2 and driveability

will be the preferred one.

To achieve lower costs with a DHT, transmission functionality needs to be taken over by the e-

motor. As an example the reverse gear is removed and reverse driving is done exclusively with

e-motor. The situation is similar for the launching element: For specific applications launching

can be taken over by the e-motor. Additionally the number of gears can be reduced, because the

combination of electric driving at low speeds and low load with new ICE technology, like GDI

Miller, does not require high number of gears. Simply the reduction of components in a DHT

leads to lower costs, weight and provides packaging advantages.

The reduction of the number of speeds in hybrid transmissions must be considered in terms of

product cost. Key for the amortization of development and industrialization is the volume.

Therefore only high volumes would justify developing and launching a 5-speed transmission

for use in hybrid applications (that would be an exclusive use, because the use as conventional

transmission is unlikely and not state of the art anymore). But such a step would still be

Figure 5: Example of FWD Add-on Hybrid: VW DQ400e (Source VW)

challenged by the option to use other existing base transmissions, unless the hybrid volumes are

really high. In those cases a development of a 5-speed transmission could make sense, but to go

one step further and develop a dedicated hybrid transmission (DHT) is probably the better

choice, as the new base transmission would not be used for conventional powertrains anyhow.

This gives additional degrees of freedom for concept and layout of such a transmission. And

with tailored functionalities for use in a hybrid vehicle further cost benefits are possible

(together with other improved properties) [1].

Figure 7 shows the manufacturing costs including amortized non-recurring development and

start-up costs for the base transmission over annual volumes, based on normalized cost analysis

of such transmissions (should-cost / true cost analysis). The first transmission is an 8-speed step

automatic, the second a 5-speed design. Cost are normalized for the 8-speed variant at 200.000

units per year. As expected, at the same production volume the 5-speed transmission is less

expensive compared to the 8-speed transmission. Considering the 8-speed transmission being

already used in higher volumes for conventional propulsion, the 5-speed transmission dedicated

for hybrid powertrains must reach certain volumes to achieve a beneficial business case,

respectively lower recurring costs. Assuming the 8 speed transmission has an annually

production volume of 500.000 units, the volume for the 5-speed transmission needs a volume of

81.000 units annually to reach the same cost [1].

To ensure comparable scenarios, further analysis is based on a hybrid solution with a single E-

motor. Figure 8 shows the cost including the E-motor with 40 kW power-rating without power-

electronics and battery. This allows usage as “plug-in-hybrid” and full electric driving. In

comparison to Figure 7 the additional cost for electrification become obvious.

Additionally to the product cost curves a horizontal line is drawn crossing the curve of the

electrified 8-speed transmission at 500.000 units annually, like in Figure 7. This cost level can

be achieved by a DHT already at an annually volume of about 32.000 units. The additional

benefit of a DHT results mainly from simplification of the base transmission and a system

engineering approach omitting redundancies in the functionality inherent for Add-on hybrid

transmissions. As the volumes of hybrid vehicles are steadily increasing, DHT will become

Figure 7: Base Transmission Cost for 5 and 8 Speeds

Figure 8: Cost Comparison Add-on versus DHT (AVL Future Hybrid Gen2)

very attractive from a product cost standpoint.

With increasing overall volumes for electrification further cost potentials can be expected.

Currently major leaps are achieved for cost improvements of power-electronics. Beyond

volume effects, as the technology becomes mainstream, further potentials resulting out of

integrative system engineering – including the internal combustion engine – become available.

The Toyota Prius took already some of these advantages.

DEVELOPMENT OF A DHT

During the development of a DHT some specifics need to be considered. In parallel to the

development of the 7 mode Future Hybrid, AVL has established the development process for

DHTs. Key for the development of a DHT is the powertrain understanding.

The ICE technology has a significant impact on the DHT architecture and requirements. Highly

efficient TGDI ICE with Miller technology or even a natural aspirated ICE with Miller or

Atkinson cycle can be used in combination with an e-motor. Depending on the ICE technology

the DHT needs to ensure boosting capability for each gear and for example also allow e-motor

operation at higher speeds. In Figure 9 the fuel consumption data of a natural aspirated ICE

with Atkinson cycle and in Figure 10 that of a high efficient TGDI ICE with Miller technology.

Both ICEs provide wide sweet spots and do not require a high number of gears for conventional

combustion engine operation.

Figure 9: Naturally aspirated combustion engine with Atkinson cycle

Figure 10: AVL high efficient TGDI

Electrical driving needs to be ensured for high speeds with low load and for low speeds with

high load, all regions with insufficient efficiency of the ICE. Figure11 shows the traction force

diagram for a NA and for a TGDI ICE. The gray areas represent the required energy during

WLTC driving. The green areas need to be covered with the e-motor to ensure high efficiency

of the complete powertrain. Additionally shifting of load point and recuperation needs to be

ensured.

Figure11: Traction force diagram for NA ICE and TGDI with area of electric driving

To derive possible DHT architectures AVL uses the computer aided synthesis. The standard

synthesis was extended with the parameter of the electrification. Automatic evaluation tools

help to reduce the number of possible solutions. Additionally the manual syntheses is still a

very valuable option to come to new solutions.

All derived architectures need to be evaluated based on a standardized process. Several

evaluation criteria are same or very similar to the criteria for a conventional transmission.

Special care needs to be taken of the e-motor connection to the transmission. Special

requirements of e-motor control are required during shifting between different ICE gears to

compensate the e-motor inertia. Depending on the architecture e-motor torque support is

required for conventional shifting to avoid bad shifting behavior due to e-motor inertia.

Additionally the torques at clutches need to be evaluated to avoid a change of the clutch torque

direction, e.g. during load point shift.

EXAMPLES FOR DHT ON THE MARKET OR IN DEVELOPMENT

Like for a conventional transmission, all criteria need to be weighted based on specific

application requirements. This leads to very different DHT concepts. Examples for different

DHT concepts. Figure 12 shows the GKN DHT concept, Renaults EOLAB, both layshaft

designs and on the other side the most famous Toyota Hybrid System (THS) and the GM Volt

in the second generation, both power-split designs with two e-motors.

Figure 12: Different DHT concepts from the market or currently in development

The GKN Multi-Mode-Transmission was launched with Mitsubishi’s Outlander and is already

available in the market. Two types of propulsion sources propel the vehicle. It provides just a

single ratio for pure mechanical propulsion (the approach “Four are enough”[2] is stretched to

“One is enough”), yet transmission functionality to map the engine’s torque- and power-

characteristics to the vehicle’s need is accomplished by the serial hybrid mode. Additionally a

parallel hybrid mode is provided [3]

Renault’s concept is based on spur gears and uses the E-motor rather than a clutch to launch the

vehicle. There is no other launch possible than by the E-motor [4] and even replacing the E-

Motor by a clutch creates a 3-speed manual transmission which would not be usable for a

vehicle.GM has been working for a long time on DHTs. The vehicle launches in full electric

mode and the system provides additionally parallel and serial hybrid modes. The E-Motors are

essential for the functionality of the system, e. g. for vehicle launch and to connect the ICE to

the wheels.The hybrid system market leader Toyota Motor Company is already in the evolution

phase and introduced 2015 the fourth generation of the Toyota Hybrid System (THS)[5][6].

They continuously improve their THS, and use it on many models and platforms.

AVL FUTURE HYBRID FAMILY

AVL presented the Future Hybrid 7 Mode in 2014; the evaluation of all criteria this evaluation

led to the following design (see Figure 13).

Characteristics of the Future Hybrid 7 Mode: 3 clutches & 1 brake

7 transmission operation modes:

3 speeds for ICE operation

all can be boosted and used for recuperation and load point shift

2 speeds for electric driving

2 torque-split eCVT mode

Figure 13: AVL Future Hybrid 7 Mode architecture

Figure 14: AVL Future Hybrid 7 Mode - Example for the traction force diagram

In Figure 14 the sweet spot area of combustion engine and the typical e-mode areas are

highlighted. The Future Hybrid 7 Mode is built into a demonstrator vehicle (Audi A3, Figure

15). Target of the demonstrator is to show feasibility and functionality.

Figure 15: AVL Future Hybrid 7 Mode demonstrator

Based on the development of the Future Hybrid 7 Mode a new generation of DHT was

developed, the Future Hybrid 8 Mode (see Figure 16). Main target was to increase the

flexibility in terms of vehicle applications and further reduce the components.

Characteristics of the 8 Mode: 2 clutches & 2 brakes

8 transmission operation modes:

5 speeds for ICE operation

all can be boosted and used for recuperation and load point shift

2 speeds for electric driving

1 torque-split eCVT mode

The new AVL Future Hybrid 8 Mode Transmission provides a very compact and modular

design.

Figure 16: AVL Future Hybrid 8 Mode architecture

Figure 17: AVL Future Hybrid 8 Mode - Example for the traction force diagram

Details of the different operating points are explained in [7]. Two very specific design features

of the 8 Mode are a very efficient the standstill charging, shown in Figure 18 and the flexibility

to replace the brake B2 by a launch element. That allows launching with the combustion engine,

which is requested for specific applications (see Figure 19).

Figure 18: Standstill charging of Future Hybrid 8 Mode

Figure 19: 1 ICE gear, B2 can be replaced by launching element

The main advantage of a DHT are cost, weight and/or packaging advantages compared to Add-

on solutions. For the Future Hybrid 8 Mode this is ensured by the consequent reduction of

components. One Ravigneaux with four friction element (2 clutches and 2 brakes) and

packaged in length, width and height of 352x580x421 mm. The design is done for a 1.6l TGDI

Miller ICE with 100kW @ 3500 rpm and 250Nm @ 2000 rpm. The e-motor has a peak torque

of 140Nm and 70Nm constant torque with a maximum speed of 15000 rpm. For this

configuration a spread of 5.17 was chosen as optimum between package/weight and efficiency

for a C-class segment vehicle. With this dimensions a top speed of 220 kph and a pure electric

speed of > 130 kph can be reached.

SUMMARY

To summarize the questions from the beginning are answered.

1. Why do we need new transmission architectures (DHTs)?

For increasing hybrid vehicle production numbers, DHT provide a cost efficient

alternative.

2. What is the specific during the development of such transmissions?

Especially the implementation of the e-motor requires, depending on DHT architecture,

details understanding of e-motor controls and powertrain dynamics to ensure high

driving performance during mode change.

3. What are the solutions that are already available or new on the market?

Several applications are either in development or already in the market. Currently two

trends can be identified: concepts with low product cost and low complexity as the main

driver and power-split designs with two e-motors that ensure different operating modes.

References

[1] R. Fischer, "Dedicated Hybrid Transmissions (DHT) - a new transmission category",

Berlin, 2015.

[2] R. Fischer,"Four are enough!", Berlin: 11th International CTI Symposium Innovative

Automotive Transmissions, Hybrid & Electric Drives, 2012.

[3] T. Gassmann, "Multi.Mode Transmission", Berlin: 14th International CTI Symposium

Automotive Transmissions, HEV and EV Drives, 2015.

[4] A. Vignon, "Alliance Renault/Nissan high performance and low cost PHEV transmission

for EOLAB and some others vehicles...", Berlin: 14th International CTI Symposium

Automotive Transmissions, HEV and EV Drives, 2015.

[5] G. Killmann, "Hybrid technology – towards sustainable mobility", Berlin: 14th

International CTI Symposium Automotive Transmissions, HEV and EV Drives, 2015.

[6] J. De Backer, "Toyota Hybrid System (THS) - dedicated to high volume production",

Berlin: 14th International CTI Symposium Automotive Transmissions, HEV and EV

Drives, 2015.

[7] M. Yolga, "AVL 7- and 8-mode hybrid transmissions - examples for DHT", Berlin: 14th

International CTI Symposium Automotive Transmissions, HEV and EV Drives, 2015.

[8] H. Kohler, "The Connected Powertrain: Further Challenges and Potentials", Graz, Austria:

25th International AVL Conference "Engine & Environment", 2013.

[9] L. Nitz, "e-Motional electrified vehicles - the Gen 2 Voltec drive system", Berlin: 14th

International CTI Symposium Automotive Transmissions, HEV and EV Drives, 2015.

[10] P. Tenberge, "Power split hybrid transmission with compact mechanics", Berlin: 14th

International CTI Symposium Automotive Transmissions, HEV and EV Drives, 2015.

[11] R. Fischer, F. Kücükay, G. Jürgens, R. Najork and B. Pollak, "The Automotive

Transmission Book", Cham Heidelberg New York Dordrecht London: Springer, 2015.

[12] B. Eckl and D. Lexa, "How many gears do the markets need?", Berlin: 11th International

CTI Symposium Innovative Automotive Transmissions, Hybrid & Electric Drives, 2012.

[13] H. List, "The transmission as key for the modern powertrain", Berlin: 13th International

CTI Symposium Automotive Transmissions, HEV and EV Drives, 2014.

[14] R. Fischer, B. Pollak and R. Ellinger, "Direct injection, turbo charging and dual clutch

transmission - hybridization based on latest technology", 5th International CTI Symposium

Innovative Automotive Transmissions, 2006.