The need for speeds

Three recent automatic-transmission introductions by Toyota, DaimlerChrysler, and ZF highlight the trend toward greater gear-ratio spreads for better launch performance and cruising efficiency.

The majority of today's automatic transmissions in the U.S. are still four-speed stepped gearboxes, though an increasing number of cars are equipped with five-speed automatics. Engineers at transmission maker ZF believe that current designs will not meet the requirements of the near future. They say that future powertrain concepts will have to better fulfill the requirements of reduced fuel consumption, exhaust emissions, and noise emissions, along with increased performance and driving comfort-all while providing greater differentiation from competitors. Next-generation automatic transmissions are employing six and even seven speeds, first on high-end vehicles, then trickling down the range, while CVTs (continuously variable transmissions) that started on lower-output models are becoming increasingly capable of handling the higher output of mid-range vehicles.

The preferred solution for rear-wheel-drive cars and light trucks is currently the six-speed automatic transmission with torque converter and planetary gearset design because of its driveline packaging advantages. Differentiation from the competition in terms of comfort, performance, spontaneity, and agility, as well as agreements made by the ACEA(Association des Constructeurs Européens d' Automobiles) to reduce corporate average fuel economy and governmental regulations, have spurred DaimlerChrysler engineers to design automatic transmissions featuring large overall gear ratio spreads with small gear ratio steps and directly controlled shifting elements, while meeting technical goals regarding installation space, weight, quality, and manufacturing costs.