The next generation of TR58 has been developed while requirement from the market on fuel economy, comfort and driving dynamics is becoming higher level. The fuel economy of the next generation is improved by expanding ratio spread and reducing agitation resistance of oil. The whole transmission power loss is approximately 16% less than that of the current model. Comfort is improved by lower CVT radiation noise with a short pitch chain and optimum radiation characteristics of transmission housings. New shift strategy “Auto step shift” is adopted that improved linearity between engine speed and vehicle speed and behaved with fixed ratios like a conventional AT in order to countermeasure rubber band effect. The new shift strategy does not have only theoretical simple fixed ratios, but also optimized shift ratios which make engine operation efficiency and engine-vehicle speed balanced depending on the driving conditions.
This paper describes the unique concept of SUBARU CVT (called LINEARTRONIC®) and the architecture of TR58 next generation (Fig.1).
Fig.1: The next generation of SUBARU LINEARTRONIC® (TR58)
SUBARU was facing a task of new drivetrain strategy around 2000 to meet the CO2 regulations which were becoming stricter year by year. As a result of studying successor of old-fashioned 4AT among candidates like 6AT, dual clutch transmission and CVT, it came to a conclusion that CVT should have the best for fuel economy performance for up to 300Nm class in which SUBARU had main product range. Placing its core strategy toward 2020 on CVT, the LINEARTRONIC® was developed along the following key concepts as the world first chain-type CVT longitudinally integrated into AWD configuration.
- Optimum unique architecture which can be installed into the SUBARU symmetrical AWD platform
- Class-top ratio spread with its basic architecture and configuration to match the SUBARU engine portfolio.
- Comparable power loss to that of conventional step ATs.
- Excellent drive ability by removing rubber band effect out.
The first LINEARTRONIC® TR690 / 400Nm capacity was introduced with 5th LEGACY in 2009, then, TR58 / 250Nm capacity was launched with 4th IMPREZA in 2011. Good reputation on significant fuel economy improvement, driving performance and comfort, was obtained. Considering the trend for wider ratio spread and bigger number of shift to meet the stricter CO2 emission requirement and growing customer demands on high balance among fuel economy performance, ride comfort and driving dynamics, SUBARU was developing the next generation TR58 for more fuel efficient, quieter and more driving fun to accelerate SUBARU brand strategy- “Enjoyment and Peace of Mind”. In the following sections, its unique concept as well as improvement of the next generation which is going to be launched this year, are described.
3. Unique concept of LINEARTRONIC®
The packaging requirement of SUBARU symmetrical AWD platform was one of the most critical issues for the variator specification because of the limited space inside floor tunnel left in the longitudinal configuration. Transmission architecture which determined shafts layout was a key issue by keeping installation compatibility with the former conventional step AT. The variator inside of the floor tunnel while keeping passenger foot space, layout of front differential gear set, propeller shaft, and shift control cable were placed by keeping compatibility with the former AT(Fig.2).
To achieve the target of torque capacity and ratio spread in the restricted packaging space, the chain-type variator was chosen with the reason of the advantage of power density by comparing to push type belt (Fig.3).
Fig.2: SUBARU symmetrical AWD and packaging requirement
Fig.3: Torque density comparison on CVT belts
4. Highly efficient variator System
Minimizing clamping pressure and the consumption of oil volume during the shift duration are important to reduce the variator loss.
In the conventional CVT, the hydraulic pressure for primary and secondary pulleys are defined by clamping force requirement and controlled by oil supply / discharge for each cylinder individually. The hydraulic system of LINEARTRONIC® has double cylinders for primary pulley, the one of them works for the ratio control. The other cylinder is connected to secondary pressure circuit and has common pressure on both pulleys, called “Common hydraulic chamber” (Fig.4). It helps to downsize the oil pump significantly because oil amount discharged from one side can be supplied to the other side through the circuit to reduce necessary oil volume when shifting. The advantage of that system locates in reducing rotational loss torque (better fuel consumption) with smaller size of oil pump. Optimization of clamping pressure was achieved by optimum surface texture on pulley cone surface and optimum friction coefficient of CVT fluid for increase metal-to-metal friction coefficient between a rocker pin end surface and a con surface, therefore, transmittable torque can be maintained under lower clamping pressure (Fig.5).
Fig.4: Hydraulic system of variator
Fig.5: Torque capacity improved by CVT-Fluid
5. Improving Fuel Economy
Continuous CO2 reduction is a top agenda for vehicle manufacturers. SUBARU set the development target with the highest priority in fuel economy improvement. The next generation TR58 aimed to reduce CVT loss and to optimize engine operating efficiency as follows:
- Spin loss reduction by minimizing agitation resistance of oil.
- Engine operating efficiency improved by increasing ratio spread up to the top class of latest competitors of CVT and AT.
5-1. Minimizing agitation resistance
The agitation resistance of secondary pulley was one of important issues in the LINEARTRONIC® configuration. A baffle plate wrapped around the secondary pulley was installed to reduce it. The baffle plate was enlarged and optimized its shape for reducing drag torque by analysing dynamic oil behaviour with a transparent housing (Fig.6). Furthermore, a hypoid bevel gear welded onto a differential gear carrier also contributed to reduce stirring loss (Fig.7).
Fig.6: Analysis of dynamic oil behaviour in a transparent housing
Fig.7: Oil baffles
Fig.8: Welded hypoid bevel gear
5-2. Increasing ratio spread
The recent transmission trend goes to multi steps or wider ratio spread in order to correspond to downsizing engines or engine down speeding. A smaller diameter shaft and a shorter pitch chain was designed for enlarging ratio spread by keeping the pulley centre distance. Smaller running radius of chain was reduced in accordance with smaller shaft diameter from φ47.5 mm to φ45.0 mm.
The stiffness of pulley shaft was decreased by reducing shaft diameter from φ47.5mm to φ45mm. Since decreased stiffness of pulley shaft lead to lower durability performance of chain or noise, cross section profile of pulley sheave being optimized to enlarge stiffness, geometry of rocker pin end face was optimized as well. Moreover, chain width was enlarged from 28.58 to 30.77 mm to secure strength.
Fig.9: Stiffened pulley shape and chain
The result of all improvement of mentioned above, the total power loss was reduced 16% from the first generation TR58 (Fig.10).
Fig.10: Power loss comparison
6. CVT noise improvement
Specific CVT noise was improved by reduction of excitation force with shortened chain pitch and optimization of link plate sequence, as well as suppression of string vibration by guide rails and optimized characteristics of sound radiation from transmission housing and insulator (Fig.11). Those helped it to realize top class quietness among competitors (Fig.12).
Fig.11: Counter measures of CVT noise
Fig.12: Positioning of quietness
7. Optimizing driving comfort and dynamics
One of the biggest advantages CVT has is that it can generate optimum traction in every road condition. However, if it controls shift to generate optimum one according to driver operation, it could also generate traction delay by certain shifting period and poor controllability like rubber band effect due to keeping maximum constant traction (Fig.13).
Fig.13: Typical shifting behavior of CVT
SUBARU worked on improving control accuracy of shift (stability and responsibility) and on eliminating unfavorable feeling by non-harmonization between change of engine rotation speed and change of sound in order to remove uncomfortable feeling out to realize linear acceleration.
7-1. Ratio stability and shift Response
In the former i-CVT, shift was controlled with hydraulic balance by controlling each pressure in primary and secondary pulley with pressure control valves. That configuration of hydraulic control valve was rather simple, however, it had to have “slow” feedback on hydraulic control to stabilize ratio due to hydraulic pressure affected by transmitting torque on pulley, and it had to sacrifice response (Fig.14). In LINEARTRONIC®, a new ratio control strategy with flow-control valve being adopted on primary pulley control, shift stability can be secured even in high shift response or speed by enlarging robustness of pulley ratio control under torque fluctuation (Fig.15).
Fig.14: Hydraulic circuit of ratio control
Fig.15: Shift response comparison
7-2. Improvement of drivability and feeling
In calibrating a vehicle, by estimating real driving situation, optimization was made to synchronize driver’s expectation, engine sound, and acceleration feel when it accelerating. In concrete, continuously variable ratio change can be done when smaller throttle being opened to realize smoothness, on the other hand, auto step shift when bigger one being opened (Fig.16).
Former variable ratio control made engine revolution speed kept at high range constantly when accelerating, and it made unexpected feeling with non-harmonization between constant bigger engine noise and acceleration. The auto-step shift does not realize simple fixed ratios, but also step shift control by restraining engine sound kept at high range, and harmonization of sound & acceleration is improved (Fig.17).
Fig.16: Step shift pattern in different mode
Fig.17: Harmonize vehicle acceleration behaviour with gas pedal opening
Furthermore, together with the auto-step shift, adaptive control was also updated. When driving on a curvy road, shift-down at braking or ratio hold control are activated to realize high torque response when re-acceleration after deceleration. Additionally, manual mode is available to enhance driver wish. Ratio step was also optimized to realize quick and crisp feeling and continuous traction.
- The next generation SUBARU Lineartronic® has realized its target- enhanced fuel economy performance and harmonization of Comfortableness and driving dynamics.
- Fuel economy performance has improved by reduce power loss and enlarged ratio spread.
- Enlarged ratio spread has been accomplished by updating the variator design and adopting the new shortened pitch chain.
- Shortened pitch chain also contributes to CVT noise reduction.
- Thanks to the shift control system which has better response, realizing shift properties for good driving comfort and dynamics which can enhance driver expectation well.
Masami Oguri, Hironaga Itou, Naoyuki Akiyama, Fuji Heavy Industries LTD., Tokyo