Transmission options

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Toroidal transmissions in small numbers

Figure 12. In full- and half-toroidal transmissions, input is transferred via the first torus disc by roller friction and flows over a second sheave to the transmission output shaft. "Twisting" the rollers changes the ratio. Click to enlarge

In contrast to CVTs, toroidal transmissions transfer power via friction in a very limited contact zone. Figure 12 shows full- and half-toroidal transmission models. Input is transferred via the first torus disc by roller friction and flows over a second sheave to the transmission output shaft. "Twisting" the rollers changes the ratio. The fundamental advantage is a higher torque capacity than a CVT, with other advantages being greater driving comfort and faster ratio changes. Disadvantages relate to installation space, weight, and manufacturing costs. In addition, toroidal applications are restricted by traction-oil limitations. Oils currently employed or in development are not fully functional at the temperature extremes of -40 to -30°C (-40 to -22°F) and +140 to +150°C (+285 to +300°F).

Even though the first toroidal transmissions went into mass production at GM in the 1930s, the concept has not really caught on. In 1999, Nissan developed the second toroidal transmission application having a 350-N•m (260-lb•ft) capacity, 4.36 gear-ratio span, and 105-kg (230-lb) mass for use in a standard driveline—numbers that are not competitive with those of six-speed automatic transmissions. Due to their greater mass and precise manufacturing requirements, toroidal transmissions will always be at a disadvantage over planetary gearset transmissions regarding mass and manufacturing costs, according to Wagner.

Summary and recommendations
Researchers at ZF evaluated many transmission-system types to determine the preferred applications for different passenger-vehicle driveline configurations. They considered the following parameters: fuel consumption, emissions, noise, performance, driving and shifting comfort, installation space, mass, and manufacturing costs. It was assumed that in the future, manual as well as automatic transmissions should be offered in each application.

The table below shows the results of ZF's driveline configuration evaluations, including the preferred transmission applications, though there will be deviations in case of special applications and isolated cases, Wagner noted.

Longitudinal engines—Five-speed manual transmissions have become the standard for longitudinal engines, though there is a clear trend toward six-speed transmissions in the future. Automated manual transmissions are reserved for the sports car market niche. According to Wagner, there will not be any real breakthrough for AMTs due to their harsh operation and the existing domination of automatic transmissions in this segment. And numerous tests of double-clutch transmissions for longitudinal drivelines have concluded that this technology is not applicable.

In the North American market, a majority of the vehicles with longitudinal drivelines come standard with four-speed automatic transmissions. Due to greater demands for fuel, emissions, and noise reductions, as well as an increased emphasis on driving comfort, four-speed units will be replaced by five- and six-speed automatic transmissions in the next few years. This applies especially to standard drivelines and their derived all-wheel versions. Front- and rear-longitudinal driveline configurations with torque greater than 350 N•m (260 lb•ft) will receive six-speed automatic transmissions.

Click to enlarge

CVTs will not succeed in standard drivelines due to their installation disadvantages and limited torque ratio, according to Wagner. However, the advantages that CVTs offer in regards to installation expenditure, mass, and manufacturing cost will mean an increase in their numbers in applications with front-longitudinal and rear-longitudinal drivelines having less than 350-N•m (260-lb•ft) output.

Toroidal transmissions, whether in full- or half-toroidal configuration, do not have the potential to be produced in significant numbers. Disadvantages with respect to transmission size, weight, and manufacturing cost are so great in comparison to six-speed automatics that the production and distribution of this transmission is not likely.

Transverse engines—The majority of current front-transverse drivelines feature five-speed manual transmissions as standard equipment. In the future, six-speed units will probably remain the exception due to the limited installation space and relatively extensive costs involved with adding a sixth gear.

In cost-sensitive mini, small, and midsize cars with 1.0- to 1.8-L engines, for which the demand for comfort is not as great as in other segments, AMTs offer the possibility for reduced fuel consumption and emissions without considerably limiting the vehicle's performance. Shift-by-wire systems with electric motors will prevail due to installation and cost issues.

Automatic transmissions in applications requiring less than 180 N•m (130 lb•ft) will not be broadly successful for the main reasons of cost and loss of efficiency and performance. For example, a 55-kW (74-hp) vehicle typically sees a loss of about 7 kW (9 hp) due to the automatic transmission at maximum engine speed; thus, about 13% of input power is lost within the transmission. This causes a negative effect on acceleration, maximum speed, and overall performance.

This outlook changes for upper-class vehicles with front-transverse drivelines. To meet demands for further comfort, more automatic transmissions will be used, but this depends on the increased availability of five- and six-speed units. Five-speed transmissions are currently in production, while six-speed units with torque capacities greater than 350 N•m (260 lb•ft) are in development.

CVTs will be applied predominantly in the middle-torque range—between 180 and 350 N•m (130 to 260 lb•ft). They offer optimal driving comfort paired with good fuel consumption and performance numbers. Furthermore, they are a good fit for east-west installations in front-transverse drivelines.

In the future, both six-speed automatic and six-speed manual transmissions will be used on a greater scale with standard drives. The use of CVTs will increase for front-transverse drivelines. For manual-transmission applications, five speeds will dominate front-transverse driveline applications, and the degree of automation will increase. For engine-torque outputs up to 180 N•m (130 lb•ft), preference will be given to the use of automated manual transmissions. The range up to 350 N•m (260 lb•ft) will be predominantly covered by CVTs, while five- and six-speed automatic transmissions will subsist in the segment above 350 N•m (260 lb•ft) for transverse installations.

Information was provided by Gerhard Wagner of ZF Getriebe GmbH.

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