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Tech Briefs
GM Powertrain manufactures products for General Motors cars and trucks. It also sells engines and transmissions to manufacturers such as AM General, Mercury Marine, Jaguar, Volvo, BMW, Daewoo, and Isuzu. "We see developing improvements in fuel economy as the next impetus in the evolution of automatic transmission technology," said Harvey Won, GM Powertrain Engineering Director - Transmissions. The concept of a CVT is not new, but durability issues have presented a roadblock to making mass-volume production a reality. Since the late 1800s, some engineers have believed that the optimal way to match the performance characteristics of an engine to a drivetrain is with a CVT. Designers have since then unsuccessfully tried to develop one that can match the torque capacity, efficiency, size, weight, and manufacturing cost of step-ratio transmissions. Ironically, the mechanics of a CVT are deceptively uncomplicated when compared to a conventional transmission, with basically three moving parts. Usually a CVT consists of a segmented, steel V-belt wrapped around two variable-diameter pulleys. One of the pulleys is connected to the engine, the other to the drivetrain, to transfer the necessary power from the engine to move the drive shaft. In essence, a sensor reads the engine output and then electronically increases or decreases the distance between pulleys, and thus the tension of the drive belt. The continuously changing distance between the pulleys—their ratio to one another—is analogous to shifting gears. Thus, while a conventional transmission has a limited number of gear ratios, a CVT can essentially access an infinite number of drive ratios, over a wider overall ratio range. "There have now been sufficient advancements in metal-belt technology and control system sophistication to allow for low-torque applications, and we see the industry moving in this direction," Won said. Metal-belt CVT applications are planned for some General Motors vehicles in 2002. "While this will not be the first CVT on the market, it is significant that we will be bringing this technology to volume production," Won said. "The CVT's ability to operate at an infinite number of gear ratios allows the engine to run at its most efficient speeds without sacrificing performance, and improves fuel economy. In our testing of actual CVT hardware, we've been able to achieve more than 7% fuel economy gains compared to a four-speed automatic transmission." GM Powertrain last year began producing its first five-speed automatic transmission, the Hydra-matic 5L40-E, in its Strasbourg, France, facility. Engineers added more ratios beyond four speeds to improve performance in higher torque applications and improve fuel economy. The rear-wheel-drive, electronically controlled 5L40-E currently is available on BMW 3- and 5-Series models and is planned for a GM application by 2002. "Instead of packaging it in a vehicle that already exists, the five-speed transmission will be on an all-new redesign of a GM vehicle, which will give us the best opportunity to really integrate it with the vehicle and make the right decisions to balance customer requirements," Won said. "And beyond 2002, we will be bringing out other five-speed transmissions." According to Won, GM Power-train approaches the automatic transmission and engine as an integrated powertrain, and then integrates the powertrain with the vehicle system. This integrated approach enables GM Powertrain to take advantage of electronic controls, not only to precisely control the transmission, but also to sense in a more effective way what is occurring in a vehicle at a particular moment and respond accordingly. Jean L. Broge |
