Transmission options
More six-speed manuals
 Figure 8. Automated manual transmission applications include the Selespeed unit of Alfa Romeo's 156. |
Vehicles with front-transverse drivelines use mostly five-speed manual transmissions. However, five-speed transmissions are longer, heavier, and more expensive than those with four speeds since the extra gear requires another gearset. This leads to an installation problem in the engine compartment that is solved only by a modified design, especially in connection with higher-performance vehicles with five- or six-cylinder engines. The demand for shorter overall length in the engine compartment leads to three-shaft transmissions, which, compared with two-shaft versions, are larger and usually more expensive—the latter being more of a problem in price-sensitive vehicle segments. Today, six-speed manual transmissions for front-transverse drivelines are rare, and there seems to be no growing demand for this configuration.
Manual operations become automated
An automated manual transmission (AMT) is differentiated from the manual version on which it is based because it does not require clutch actuation and, in some cases, gearshifting by the driver. These functions instead occur by means of a hydraulic system or an electric motor with the help of electronics. The clutch pedal is rendered obsolete, with clutch control depending on the position and movement of the selector lever. The mechanical connection between selector lever and transmission is eliminated and the transmission is controlled electronically via shift-by-wire. This offers more options when designing the selector than with conventional mechanical shifting systems. With the shifting implemented by algorithms in the transmission control unit, an AMT can execute gearshifts automatically.
Compared to an automatic transmission, advantages of the AMT include the ability of its maker to use existing manual-transmission manufacturing facilities to achieve lower production costs as well as its greater efficiency and lower weight. An existing manual transmission can be modified into an AMT by "adding on" the components for automation. However, the expense for automation should not be underestimated. A substantial amount of components are necessary to compensate for the omission of the clutch pedal and mechanical connection between the shift lever and transmission. Even so, the cost for the entire AMT system is slightly less than that for an automatic transmission.
 Figure 9. DaimlerChrysler transmission development activities include this double-clutch transmission. Click to enlarge |
It can be assumed that, in the future (especially with introduction of the 42-V electrical systems in passenger vehicles), clutch and gearshifting control will be handled via electric motors. The electric motor would use energy only during shifting, making it more efficient than hydraulic power. Only in sports cars, which require extremely short shift times, will hydraulic actuation be used.
Due to the additional components, automation adds about 10% to the weight of a manual transmission. Other disadvantages of AMTs are reduced shift comfort compared with automatic transmissions and an interruption of traction during shift actuation. The latter results in vehicle deceleration during shifting, which can be especially severe during "hard" acceleration and when gearshifting is in full automatic mode. In addition, AMTs currently on the market do not have fail-safe mode programs like those of automatic transmissions, so system failures can cause the vehicle to become inoperable.
The first (hydraulic) AMT applications were in motorsports to shorten gearshifting times and to relieve drivers from using a clutch. The gears were engaged by pushing buttons on the steering wheel. Today, production applications include those from BMW (five speeds) and Mercedes-Benz (six speeds) for standard drivelines as well as front-transverse five-speed units in the Alfa Romeo 156 and VW Lupo 3L. Other front-transverse applications are being developed for production in the next few years. DaimlerChrysler's rear-wheel-drive MCC smart features an automated six-speed transmission.
Advancements in control systems have improved automated shifting and their application to layshaft transmissions. AMT applications are predominantly intended for cost-sensitive segments such as light commercial vehicles, mini and small cars, and lower middle models. An exception is the sports car, for which comfort is less of a concern than shifting speed and a sporty image.
Double-clutch transmissions have limitations
The double-clutch transmission is an automatic transmission derived from a manual one. It consists of two, linked layshaft transmissions with two power paths: one for even, the other for odd gear numbers. Figure 9 shows an example of a five-speed transmission system based on DaimlerChrysler development activities. On the input end, each layshaft has a clutch to allow shifting. For the output, both layshafts are linked to the output shaft. Shifting occurs via synchronization of the nonengaged partial transmission, making it impossible to skip two gears and producing unacceptable shifting behavior.
For longitudinal engines, double-clutch transmissions with dry and wet clutches (the latter with hydrodynamic torque converters) have been closely examined and tested by the industry. They did not show a decisive advantage over planetary gearset systems and have not been successful in passenger car applications.
For transverse-engine vehicles, double-clutch transmissions being developed with wet clutches could offer the advantage of compactness in front-wheel-drive applications. Despite the possible packaging advantages, the double-clutch transmission will not succeed in the front-transverse application, according to Wagner, because of its limited ability to skip gears—an essential characteristic of an automatic transmission.
