Wednesday, April 23, 2008

Auto Tech: Advances in automatic transmissions

By Jim

When it comes to increasing fuel economy and performance, most people think of engine improvements, hybrids and alternative fuels, but not everyone. There are some out there actively designing new automatic transmissions for vehicles that can make dramatic differences in economy and performance. In the past few years, we have seen vehicles change from three- and four-speed automatic transmissions to five- and six-speed automatics. We have also seen CVT's (Continuously Variable Transmissions) enter the marketplace in significant numbers and the introduction of the automatically-shifted manual transmission, such as Volkswagen's DSG gearbox. Each auto manufacturer seems to be following its own direction with regards to transmissions. Mercedes uses a seven-speed automatic transmission on many of their vehicles, while Lexus has introduced an eight-speed automatic on their LS series Luxury sedans. Ford and General Motors dabbled in CVT transmissions but have now put their focus on building six-speed automatics for both front and rear wheel drive applications as the best compromise between cost of production and maximum efficiency. Why so many gears? With more gears inside the transmission, the gear ratio spacing can be increased. Low gear is now a lower ratio for more multiplication of engine torque. This gives quicker vehicle launches away from a stop with less throttle application. Because the engine isn't working as hard, fuel economy improves. A smaller displacement engine can also be used to provide the same overall performance. Ford claims their new transmission technology delivers four to six per cent improvement in fuel efficiency on average compared with typical four- and five-speed gearboxes. With more gears to drive the vehicle, top gear can now be a higher ratio, giving more fuel economy during highway cruising. The more gears used between low and top gear, the smoother the shifting and the easier it is to keep the engine operating in an optimum r.p.m. range for the best fuel economy. CVT transmissions do just that all the time - automatically. CVT transmissions use two metal pulleys inside the transmission linked by a metal drive belt. The width of the pulleys can be varied, allowing the belt to sit lower or higher on the pulleys. This changes the leverage or gear ratio of the drive. Because the belt can be positioned variably on the pulleys, gear ratios are continuously variable. These transmissions are simple when you look at the number of parts compared to a six-speed automatic, but the technology is complex. The metallurgy of components has to be exact and the control system requires a lot of programming to match engine characteristics. Chrysler has CVTs on several of its smaller vehicles and Audi has offered it on select models, but in my opinion, Nissan has to be the leader in CVT technology, using it in their passenger car lineup and small SUV's like the Nissan Rogue. The programming in the Rogue is excellent, providing the correct gear ratio for all driving conditions instantly and smoothly. These CVT's can be shifted manually, which tells the computer to move the belt position in steps rather than continuously, but maximum fuel economy is achieved in automatic mode. A CVT can achieve the same economy as a manual transmission, without the hassle of shifting. So what does the future hold? I think it will be computer-controlled manual transmissions, such as the DSG transmission Volkswagen already offers. This transmission design uses two clutches instead of a torque converter and synchronized gears like a manual transmission instead of clutch packs as found in a conventional automatic transmission. The computer engages one of the clutches when the driver applies the gas pedal, and the car moves off. The next gear is engaged by releasing the first clutch and engaging the second clutch. While the transmission is in second gear, the computer shifts first gear into third gear position, ready to engage the first clutch again for the next shift. This is a computer-controlled manual transmission and shifts are quick and smooth. Porsche first used it in racing. Now Audi and VW are taking advantage of this design, but others are following. Ford has a new dual-clutch transmission called Powershift, which is already delivering diesel fuel economy improvements of 10 per cent to the Focus in Europe. Powershift is planned for future vehicles in North America. Chrysler's new dual-clutch transmission developed in partnership with Getrag, makes its debut this spring. The new technology will be available in international markets on the all-new 2009 Dodge Journey, the 2009 Dodge Avenger and the 2009 Chrysler Sebring. Vehicles are complex pieces of machinery. Every part contributes to the whole, and the new automatic transmission designs are an integral part of a vehicle's performance and economy.

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