Transmission options
More gears for automatic transmissions
Today's four- and five-speed automatics for longitudinal engines use planetary gearsets with hydrodynamic torque converters as startup elements. These transmissions dominate the midsize and luxury sedan, SUV, and sports car segments.
The introduction of lock-up torque converter clutches and electronic transmission controls, an increase in the number of speeds, and numerous optimized components have contributed to a significant reduction in automatic transmission power losses. As a result, the fuel consumption gap relative to a manual transmission has been narrowed. In addition, better shift quality and algorithms have allowed for the transmission's adaptation to driving conditions and driver behavior for improved driving comfort.
 Figure 10. ZF's 6 HP 26 six-speed automatic transmission is designed for engine-torque capacities up to 600 N•m (440 lb•ft) in standard drivelines. |
The introduction of five-speed transmissions in the late 1980s, as well as a broadening of functions such as manual shift modes, has strongly added to the lure of automatic transmissions. Increased requirements for lower fuel consumption and emissions; reduced transmission size and weight; and increased environmental safety, driving comfort, and performance have all led to the development of the six-speed automatic transmission. Advantages of six- over five-speed automatics include a 5-6% fuel consumption savings, a reduction in emissions, 5% better acceleration, a 13% reduction in weight, and increased reliability.
Figure 10 shows ZF's six-speed automatic transmission for engine-torque capacities up to 600 N•m (440 lb•ft) and an overall gear-ratio range of 6.05. The 6 HP 26 is designed for standard drivelines in conjunction with eight- and twelve-cylinder engines and will begin production this year. Many industry experts predict that six-speed automatic transmissions will become the preferred solution for vehicles with longitudinally installed engines because of their compact designs and improved performance.
Most current transverse-engine applications use three- and four-speed automatic transmissions, though the first five-speed transmissions are making their way into Volvo, Rover, and VW production cars. Since these designs are based on existing four-speed transmissions, and the lack of space does not permit an enlargement of the transmission, gearsets and clutches are arranged on a countershaft—making for expensive and relatively heavy designs.
Most of today's four- and five-speed planetary gearboxes are installed inline with the engine and cover a torque range of up to 350 N•m (260 lb•ft). Because of higher torque requirements, especially in diesel engines, future transmissions will have to be designed for 450 N•m (330 lb•ft) and higher, Wagner said. Only U-arrangements, in which the engine and transmission lie next to each other transversely in the vehicle, have higher torque capacities—up to 410 N•m (300 lb•ft). These configurations are predominantly used in connection with six-and eight-cylinder engines for the North American market.
For lower-torque applications in association with four-cylinder engines, current automatic transmissions, such as Honda and Saturn four-speed units, have designs similar to manual transmissions with the synchronizer replaced by wet clutches. One of the most compact designs is the Mercedes-Benz A-Class five-speed unit, which is designed for up to 180 N•m (130 lb•ft), with an overall gear-ratio spread of 5.0 and a length of only 316 mm (12.5 in).
Front-transverse drivelines are becoming more common in midsize and upper midsize cars. Demands for greater driving comfort and a trend toward greater power and torque in these applications make it necessary for automakers to offer automatic transmissions with more speeds. The employment of six-speed automatic transmissions with planetary gearsets, similar to those in standard driveline applications, might be necessary to meet demand for reduced fuel consumption and emissions.
CVTs with greater capacities
 Figure 11. In 2000, Audi introduced its VL 300 CVT for a front-longitudinal driveline in connection with a six-cylinder 2.8-L engine. |
CVTs have not been successfully adapted for series production standard drivelines due to packaging. A CVT requires a relatively large center distance, which cannot be accomplished effectively in an inline configuration with longitudinal engines in rear-wheel-drive cars.
In 2000, Audi introduced a CVT for a front-longitudinal driveline in connection with a six-cylinder 2.8-L engine. The VL 300 was the first CVT for high-torque applications—280 N•m (205 lb•ft). It has a wet clutch and chain and an overall gear-ratio span of 6.05 (Figure 11).
CVTs offer advantages for front-longitudinal driveline configurations. The necessary gap between the crankshaft and transaxle unit is bridged by the variator. This results in a relatively straightforward design with a startup element, reversing planetary gearset and incoming gear stage for transmission adaptation, variator, and integrated transaxle. A disadvantage is the still-limited torque capacity. Therefore, Wagner said, these transmissions cannot be used for the entire torque spectrum; for high-output applications, automatic step transmissions are still required.
CVTs have been used predominantly with the front-transverse driveline. In 1999, almost 440,000 CVTs were installed in vehicles worldwide, Japan dominating with 99% of the applications. Current CVTs are installed mainly in mini, small, and lower midsize cars with 1.0- to 1.6-L gasoline engines, though the trend is toward higher engine torque and CVT capacities.
In Japan, mini cars having 0.66-L engines feature CVTs with dry belts having rubber elements, so torque capacity is limited to 65 N•m (48 lb•ft). To enable adequate acceleration from a standing start, these transmissions have a mechanical-gearbox stage corresponding to a first gear. After this first gear ratio, the variator comes into play, and further ratio changes occur continuously. A possibly better alternative to this complicated design for mini cars is an automated manual transmission, according to Wagner.
In 1999, Nissan was the first automaker to introduce a CVT for the front-transverse driveline in conjunction with an engine having greater than 100-kW (135-hp) output. The 2.0-L Primera features engine torque of about 190 N•m (140 lb•ft). The motivation for adopting CVTs in front-transverse drivelines, combined with four- and six-cylinder engines from 1.8- to 2.8-L displacements, is greater fuel economy and lower emissions, as well as fulfillment of consumers' expectations for the comforts of automatic transmissions.
CVTs for front-transverse applications use push belts. Most first-generation units used wet or magnetically actuated clutches for the startup element, though many newer designs use hydrodynamic torque converters. Other differences compared with earlier CVTs lie in the design of the oil pump, variator, and hydraulic control unit, as well as placement of shafts. One of the latest CVTs is the CFT 23 developed by ZF, which is designed for engine torque up to 250 N•m (185 lb•ft) and features an overall gear-ratio span of 5.8.
