Jatco_Paper

Technical Innovation in 2-Pedal Transmissions Led by CVTsShigeo Ishida 1) Takashi Shibayama 2) Akihiro Aoyama 3)At JATCO, we put new-generation CVTs on the market between 2009 and 2012. These new CVTs were developed for the purpose of unifying our CVT lineup through the incorporation of various technical innovations. It was deemed difficult to integrate the lineup just on the basis of conventional improvements. The basic strategy defined for the new CVTs was to “transmit high torque with a small transmission” and specific technical innovations were made to achieve the goal of integrating the product lineup. This paper describes the technical innovations embodied in these new CVTs.KEY WORDS: Continuously Variable Transmission with Auxiliary Gear Box, New Belt, New Chain1．INTRODUCTIONPersonal mobility requires global environmental protection through the reduction of carbon dioxide (CO2) emissions and the means for providing individual freedom of movement. In response to these market needs, automatic transmissions began to diffuse rapidly in the 1980s concomitant with worldwide user demands for driving ease as well. Accompanying this diffusion trend, more speed ranges began to be added to the 3-speed automatic transmissions prevalent at that time, and various new types of automatic transmissions also started to appear during that same period, including automated manual transmissions. However, it was predicted that the evolution of these transmission technologies would reach a limit in the future owing to their complex structures and control procedures.At JATCO, we began early on to focus attention on continuously variable transmissions (CVTs) because of their inherent environmental performance and potential for smooth shifting due to the absence of stepped gear ratios. Through the efforts made to expand our CVT products, we successfully developed a full CVT lineup comprising four different series.However, in the meantime the automobile as a whole was rapidly undergoing technological innovation, including downsizing and electronification. We realized that transmissions also required similar innovation and launched development activities toward that end.In 2009, we developed the world’s first CVT that incorporates an auxiliary transmission and then developed a new CVT series in 2012 that can be applied to a wide range of vehicles from 2.0L-class midsize cars to 3.5L-class large vehicles. As a result, these two newly developed CVT series can cover the entire range of vehicle application that was previously covered by our full lineup consisting of four CVT series.This paper describes the concepts defined for the development of these new CVTs and the technical innovations that were achieved in the process. 2. SOCIAL ENVIRONMENT CHANGES ANDTRANSMISSION REQUIREMENTS2.1 Response to Social Environment ChangesMarket requirements for vehicles rapidly diversified from the previous demand for driving ease to include technologies for complying with fuel economy regulations and collision safety standards, as well as electronification accompanying higher levels of vehicle functionality. The approaches that were emphasized for complying with fuel economy regulations included expanding the ratio coverage of transmissions, downsizing vehicles and engines, and enhancing overall efficiency. In the transmission field, more speed ranges were added to stepped automatic transmissions andautomated manual transmissions were developed (Fig. 1).〔Fig. 1.〕, Trends in 2-pedal vehicle & transmission technologiesThe demand for more speed ranges can also be seen in another way as the expansion of ratio coverage, which works to improve fuel economy. Consequently, transmission ratio coverage has been continually expanded since the early 2000s accompanying the further tightening of CO2emission regulations in Japan, the U.S., Europe and China (Fig. 2).〔Fig. 2.〕, Fuel consumption regulations and wider ratio of transmissionsMoreover, as a result of the continued diffusion of vehicles and more complex traffic conditions, there were also rising demands from the latter half of the 1990s for stronger collision safety standards. Seen from the standpoint of the powertrain system, the improvement of collision safety meant expanding the vehicle crushable zones, which necessarily required the downsizing of powertrains at the same time.The first example of the response made to enable vehicles to meet these requirements was the downsizing of engines. Engine displacements began to be reduced from around 2000, which corresponded to the initial period of the expansion of transmission ratio coverage 〔Fig. 2〕. Subsequently, engine displacements were dramatically reduced by the application of supercharging. It is seen in Fig. 3 that engine displacements were downsized by nearly 40% in some cases during that period. The second example that can be cited concerns the adoption of the controller area network (CAN) for in-vehicle communication. As a result of the rapid progress of automotive electronics and the implementation of information technology (IT) in vehicles, it became necessary to have the capability for transmitting larger volumes of information at faster speeds. That could not be done by simply bundling together more ordinary wiring harnesses. Therefore, the adoption of CAN communication made it possible to transmit large volumes of databidirectionally at high speed (Fig. 3).〔Fig. 3〕, Trends in downsizing of engines & CAN network2.2 Future Vision of 2-pedal TransmissionsSuch requirements from the external environment also made it necessary to move ahead rapidly with the development of transmission technologies. Meanwhile, it was predicted that simply improving transmission technologies along the lines of traditional approaches would reach a limit in the future because of the increased complexity of the structures and control procedures.Therefore, we focused attention on our CVT lineup consistingof four series at that time. The overall transmission length is one index that can be applied when downsizing transmissions for use on front-wheel-drive vehicles. An examination of the overall length of our CVTs developed up to that point revealed that they were clustered into a 400-mm class group and a 350-mm class group. Accordingly, for the former group with the longer overall length, we set a target of a 350-mm class, i.e., the length of the latter group, in order to ensure ease of mounting in the engine compartment. For the group with the shorter overall length, we set a target of 340 mm maximum in order to ensure flexibility for use even on minivehicles. We also examined the lineup from a different perspective with regard to the allowable torque band. The results showed that our CVTs used on minivehicles and small cars had a similar torque capacity requirement of 70-180 Nm and those used on midsize and large vehicles had a similar performance requirement of 250-380 Nm. Accordingly, from the perspective of downsizing, we formulated a strategy for integrating our existing CVT models in these two torque capacity ranges into smaller size units, respectively, as the future vision of our CVT lineup (Fig. 4).〔Fig. 4〕, Target of ＣＶＴ integrationThe fundamental idea of this integration strategy was to “transmit high torque with a small transmission.” For the CVT model in the lower torque range, that is, the unit for use on minivehicles and small cars, we defined the concept of developing“technologies that provide wide ratio coverage even with a small transmission.” That was intended to address the issue of the poor power-to-weight ratio of these vehicles. For the CVT model in the higher torque range, that is, the unit for use on midsize and large vehicles, we defined the concept of developing “technologies for transmitting high torque with a small transmission.”The accomplishment of these concepts would require innovation, as it would be impossible to achieve them by simply continuing the evolution of conventional technologies along the same lines. Based on these two development concepts, we created various innovative technologies that enable just two CVT series to cover a fivefold range of torque capacity from 70 Nm to 380 Nm. As a result, we were able to dramatically integrate our CVT lineup for use on front-wheel-drive vehicles.The specific technical innovations that were developed will be described in the following sections.3. CONCEPT AND TECHNOLOGIES OF SMALLER CVT 3.1 Development of an Auxiliary Transmission and Smaller Pulleys The conventional approach to downsizing that has generally been applied to date is the idea of integrating multiple structures or functions into one. However, we took the opposite approach in the development of a new-generation smaller CVT. Specifically; we divided the shift mechanism into one based on the belt-pulley system and one based on a two-speed auxiliary transmission. Fig. 5 compares the structure of the existing base CVT with that of the newly developed CVT which incorporates an auxiliary transmission. The adoption of the auxiliary transmission allowed the pulley size to be reduced by 15%, thereby achieving an overall length of 323.3 mm and reducing the wet weight by 10 kg (Fig. 6).〔Fig. 5〕, Auxiliary transmission for compact sizeMoreover, the incorporation of the auxiliary transmission allows the use of two pulley ratio patterns between High and Low, thereby making it possible to expand the ratio coverage to 7.3, upfrom 6.0 of the existing unit.〔Fig. 6〕, Downsizing of Pulleys3.2 Control Technology Facilitating the New Shift Mechanism Although a distinctive feature of this shift mechanism is that two pulley ratio patterns can be used between High and Low, the shifting of the belt-pulley system must be synchronized with that of the 2-speed auxiliary transmission in order to ensure smooth shift performance. With this CVT structure, fluctuations in engine speed and torque must be suppressed at the time the auxiliary transmission shifts. If such fluctuation is not controlled well, the time difference in the shifting operations would prevent harsh shift that would compromise the smooth shift performance inherent to a CVT. JATCO has a long history of developing stepped automatic transmissions and has accumulated many technologies related to shifting. In addition, we have also developed various technologies for shifting CVTs.By combining these two sets of technologies, we were able to prevent harsh shift as indicated in Fig. 7. Specifically, a control technology was developed for synchronizing upshifts and downshifts, which maintains a constant engine speed for obtaining smooth shift performance. The control system calculates the target total gear ratio, and from that value it calculates the target pulley ratio. The pulleys and the auxiliary transmission clutches are then controlled hydraulically according to the respective target value. In addition, adaptive learning control is applied to learn the contact point of the clutches in order to reduce any harsh shift that occurs when switching between clutches in the torque phase. Similarly, adaptive learning control is applied to learn the clutch torque capacity needed at the time of clutch engagement. These control features serve to overcome pressure fluctuation and structural variation and also maintain the desired performance as the unit ages with use (Fig. 7).〔Fig. 7〕, Innovations in control system technologyThese technologies were embodied in the new Jatco CVT7 that was released in 2009 and provides improved fuel economy and excellent drivability in a compact package.4. CONCEPT AND TECHNOLOGIES OF LARGER CVT 4.1 CVT StructureWith regard to technical innovation for the CVT to be used on midsize to large vehicles, the first step taken was to review the basic structure. The oil pump that generates hydraulic pressure for torque transmission and shifting is positioned under the first shaft and is driven via a chain system from the input shaft connected directly to the engine. This layout was selected with the aim of reducing the overall length of the unit (Fig. 8).However, the size reduction that can be achieved by simply revising the layout is limited. For that reason, technical innovations were deemed necessary, and it was decided to develop a new belt and a new chain.〔Fig. 8〕, Main cross section4.2 New belt and new chainActivities were undertaken to develop the technologies for transmitting high torque with a smaller transmission, which is required of a CVT for use on midsize to large vehicles. At the time,it was said that new technologies for compact, high torque capacity CVT component was necessary. We proceeded with the development of a new belt and a new chain in order to transmit high torque with a small transmission. That was done to provide the optimal drive system matching the ways in which vehicles fitted with the new CVT are actually driven. As a result, a new belt was developed for use on midsize vehicles that is 2 mm narrower in width than the existing belt, and a new chain was developed for use on large vehicles that is 3 mm narrower in width than the existing chain, as shown in Fig. 9〔Fig. 9〕, New belt and new chain(1) New beltThe new belt was developed with aim of achieving a smaller size, and efforts were also made at the same time to reduce friction. Both a smaller size and lower friction were achieved by 1) reducing the width of the bands, 2) optimizing the element sharpness and 3) increasing the ear width. Specifically, reducing the width of the bands was effective in lowering friction between them and the elements. Additionally, optimizing the element geometry reduced friction while also ensuring the desired torque transmission capacity. These measures resulted in the development of a smaller belt with lower friction (Fig. 10).〔Fig. 10〕, Key technical features of CVT8: new belt(2) New chainThe new chain for use on large vehicles was developed basedTorque Converter Oil PumpSecondary PulleyReduction GearFinal GearOutput Gear Idler Gear Forward, ReverseOut put for Tires Chainon an analysis of how such vehicles are actually driven. Previously, it was well known to use crown pulley faces. At JATCO, we aimed to use common pulleys for the purpose of unifying our CVT models used on midsize and large vehicles. Along with adopting straight pulley faces, the geometry of the pin end face of the chain was re-examined, and the optimum face geometry was found for securing high efficiency. This also made it possible to improve massproduction efficiency.〔Fig. 11〕, New chainThe foregoing measures made it possible to reduce the overall length of the new CVT for large vehicles by 30 mm (-8%) and its weight by 5 kg (-5%) compared with the existing CVT. As a result, we were able to unify our CVT models for use on midsize to large vehicles. In 2012, the Jatco CVT8 incorporating these newly developed technical innovations was adopted on the new Nissan Altima that was released in the U.S. market. This new CVT has been highly evaluated for its compact size and contribution to improved fuel economy and drivability.The technical innovations described here facilitated the complete integration of our CVT lineup into two series, one for use on minivehicles and small cars and the other for use on midsize to large vehicles. Each CVT series provides levels of performance that could not have been obtained by simply making improvements toexisting technologies (Fig. 12).〔Fig. 12〕, Comparison of CVT specificationsOur present lineup of new-generation CVTs established on the basis of these technical innovations has been highly praised by our automaker customers. In CY2011, we gained a 55% share of the global CVT market. Work is now under way to develop a hybrid model.5. DEVOPMENT PROCESS INNOVATIONSThe successful development of the new-generation CVTs was due to the adoption of the individual technical innovations described above as well as to innovations made to the development process. Conventionally, development work has been done in a step-by-step manner, but we transformed the development process so that the vehicle, engine and CVT transmission are now developedsimultaneously (Fig. 13).〔Fig. 13〕, Transforming the Development ProcessThis simultaneous development process makes it possible to achieve performance and functionality thoroughly integrated with the vehicle model. It also helps to speed up the development process and provides greater possibilities for attaining the targeted performance.6. CONCLUSIONSTechnical innovations were pursued in line with the concepts defined for developing the technologies of the new-generation CVTs for purpose of unifying the CVT lineup. The following observations can be made based on a review of the results obtained.1) Social environment changes and vehicle requirements havepromoted engine/vehicle downsizing and innovations in CAN communication and other technologies.In parallel, innovations in transmissions also have beenpromoted.2) The basic strategy defined for the new-generation CVTs was to“transmit high torque with small transmission” and specifictechnologies were identified for unifying the CVT lineup. 3) One approach taken was to develop technologies for obtainingwide ratio coverage even in a small transmission. The conceptof an auxiliary transmission and the use of smaller pulleys were pursued to accomplish that.4) Another approach taken was to develop technologies fortransmitting high torque with a small transmission. Attention was focused on the belt as the key component for transmitting torque, and the challenge of developing a new belt and a new chain was undertaken.5) Innovations applied to the development process made it easierto develop the new-generation CVTs.The development of the technical innovations described here has made it possible to contribute to further advances in vehicle performance and simultaneously to dramatically revise JATCO’s CVT lineup. We want to continue to contribute to the ongoing progress of vehicles through further innovations in our transmission technologies.Finally, the authors would like to thank everyone involved for their cooperation with the development of these technical innovations from the initial stage of the development process.*Presented in Parallel Session I at 2013 TM Symposium China (TMC 2013).1), 3) JATCO Ltd., 700-1, Imaizumi, Fuji City, Shizuoka, Japan 417-85852) JATCO Ltd., 560-2, Okatsukoku, Atsugi City, Kanagawa, Japan 243-0192。