Transmission designs are changing quickly as drivers demand better control and more speeds.
By Bart Eisenberg
Of all the components in the powertrain, none has launched more design variations and heated debates than the transmission. The engineering trade-offs are several: convenience versus control, acceleration versus fuel economy, manufacturing costs, and — most assuredly — the coolness factor.
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| BMW’s Sequential M gearbox allows for upshift with two paddles mounted on the steering wheel. |
"Things are changing so fast that there's no unanimous consensus on the transmission of the future,” says David Cole, chairman of the non-profit Center for Automotive Research. Short-term, the crystal ball gets clearer. “The move is toward increasing the number of speeds — giving a little more overdrive for fuel economy and closer integration with the engine,” he continues.
How many speeds? While the Lexus LS 460 will feature an eight-speed automatic, many observers think that’s overkill. Cole predicts that “the dominant transmission technology will be a conventional automatic transmission based on torque converter principles, with six speeds that can be easily integrated into an all-wheel drive train."
Shifting to Six-Speeds
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BorgWarner’s dual-clutch technology employs
separate clutches for odd and even gears. |
General Motors is gearing up to produce a new family of six-speed automatics at its Willow Run facility in Ypsilanti, MI. The transmissions will make their debut in several of the company's rear-wheel drive models, including the 2006 Chevy Corvette, Cadillac STS-V, and XLR-V, as well as in some of GM's 2007 full-size SUVs.
"The biggest benefit of a six-speed transmission is that we increase the overall ratio spread — the first gear ratio divided by the top gear ratio — yielding a 6.04 overall ratio,” says Jeff Baran, GM's chief engineer for six-speed rear-wheel drive automatic transmissions. “By contrast, most four- and five-speeds are in the 4-4.5 range. The wider ratio provides very good engine operating speed for highway cruising, with improved first gear launch ratios over today's transmissions.” Baran says that driving 60 mph at the top gear, engine speed is reduced by 9 percent to about 1,500 rpm, increasing fuel economy and decreasing cabin noise.
Shifting Priorities for Manual Transmissions
To clutch or not to clutch? Prince Hamlet might well be asking that question were he motoring around Ellsinore. “The manual transmission still sets the standard for efficiency, if it is used efficiently,” says David Cole of the Center for Automotive Research. “But while some people believe that the fun of driving is in pushing the clutch and shifting gears, the automatic transmission is becoming ever more popular, even in Europe.” Europe has been the manual transmission’s stronghold. Continental drivers have tended to associate manual transmissions with youth and vigor, for added control on narrow roads and better fuel economy to counter high gas prices. But among North American car enthusiasts, manual shifting is in danger of becoming a lost art. “I tend to be a purist and believe that a manual transmission is essential to the full driving experience,” wrote one participant in an online forum. While some have called the manual transmission an antique technology for luddites, others have questioned the manhood of anyone who would drive anything else. ADVERTISEMENT
But lest you think this is a matter of testosterone, consider the findings of Art Spinella, president of CNW Marketing Research, which conducts extensive surveys at the retail end of the automotive business. He says that for the past three years, women, not men, have driven the manual transmission market. In last year’s survey, 14.3 percent of women versus just 8.5 percent of men were shopping for a stick shift. In 1985, the numbers were dramatically reversed: 4.4 percent of women versus 52.8 percent of men. Spinella says that the selling point for women is economy: better mileage, and cheaper to buy. “The resistance to learning how to drive a manual transmission is long-gone — a remnant of the 1950s and 1960s, a time when automatic transmissions were aimed directly at women.”
As for men, Spinella believes that traffic jams are the overriding consideration. The other factor is the male fascination with gadgets. “Most guys are more into sound systems than into driving a stick shift.” Other distractions also beckon: cell phones, MP3 players, global positioning systems, and cappuccinos — all of which require the occasional free hand. |
Baran says that GM has reduced the cost of production through a lower parts count and a modular design — the first for GM’s Hydra-Matic family. Four six-speed RWD variants in 25 models are planned by 2007, with up to 47 percent of the parts common to all.
GM has reduced mechanical complexity by employing just five internal clutches — fewer than on many four-speeds. “We've done that through clutch-to-clutch shifting technologies that eliminate all but one of the free-wheelers,” says Baran “And even with that free-wheeler, which is used for the 1-2 upshift, we re-use one of the existing clutches.”
Clutch-to-clutch shifting is done by disengaging one clutch and engaging another, with control via an internal microprocessor. Baran says that in traditional free-wheeler designs, additional engine braking clutches are needed when the driver manually downshifts to slow the vehicle. "Free-wheelers can't do that because, by design, they don't transmit torque in the opposite direction. With clutch-to-clutch technology, the free-wheelers are gone and engine braking is available all the time.”
The GM six-speed RWD was a global engineering effort with engineers involved from GM Powertrain's European Technical Center in Strasbourg, France and an engineering team at GM Powertrain Engineering in Ypsilanti. The company uses Unigraphics tools for powertrain design, and, like many manufacturers, has reduced physical testing through computer analysis.
Dual-Clutch Designs
BorgWarner is promoting a less conventional dual-clutch technology, which is primarily found in the Volkswagen Group’s DSG (Direct Shift Gearbox) transmissions. The technology employs separate clutches for odd and even gears while eliminating the torque converter. “The design has excellent fuel efficiency, beating both planetary automatics and constant velocity transmissions,” says Bernd Matthes, president of BorgWarner Transmission Systems. “And because they substitute two clutches for a torque converter, dual-clutch transmissions are also more responsive because of the lower drag on the transmission. This responsiveness directly translates into being fun to drive, with no torque interrupt between shifting.”
Another advantage: the transmission can be “tuned” via software, thereby changing the personality from sporty to limo-comfortable. Matthes predicts that that the technology will account for 25 percent of the European market by 2015.
Industry observers are generally less enthusiastic about the long-term prospects for the continuously variable transmission (CVT). Matthes, for one, believes that the only long-term CVT market may be in Japan. David Cole says that CVTs are well suited for matching a ratio with the engine’s maximum efficiency point, but also says, “The problem is that they aren’t very amenable to higher-powered engines.” With the commonly used belt-and-pulley design, “you get a little slippage, which takes a hit on efficiency.”
The CVT’s most prominent perch has been on the hybrid Toyota Prius. But while larger hybrid cars will use step transmissions, CVTs will be featured in the 2007 Nissan Sentra and the Dodge Caliber, which replaces the Neon. The Caliber’s CVT, produced by Jatco, is a first for the automaker. "We think CVTs are good for specific markets," says Cole Quinnell, engineering spokesman for The Chrysler Group. He agrees that the technology's reception in the market has been mixed — and the company has looked for ways to make the CVT more familiar.
"We've emulated what people are accustomed to on an automatic transmission," Quinnell says. "One complaint we've heard is that when you go to full throttle, the tachometer goes up to the red line and stays there." Some drivers find that unnerving. “We’ve calibrated the transmission to race the engine to a level below the red line, then slowly ease higher after more speed is gained,” he continues. “So you get the primary benefit of high rpm for acceleration, with a more familiar experience." In addition, drivers can select six gear ratios — thereby transforming a CVT into a pseudo-step transmission.
Indeed, virtually all automatic transmission designs now offer some form of manual control. One of the most notable is BMW’s electrohydraulic Sequential M Gearbox, which provides both clutch-less manual shifting, as well as automatic shifting with multiple personalities. First used on Formula 1 race cars, the technology debuted on BMW’s M3 SMG model. Depending on the model, drivers can upshift with two paddles mounted on the steering wheel — one for upshifting, the other for downshifting — or by using either of two push-pull paddles. BMW has also applied SMG to its automatic transmissions on the 7 Series Sedan. Drivers can control all six forward gears while in manual mode using one of two thumb buttons. "A lot of people, but especially BMW owners, like to have full control of their cars," says spokesman Vincent Kung.
“That assumes, of course, that you are a typical BMW driver,” says Max Rose, an auto mechanic in Southern California who is fond of the brand. “I have one friend whose mother thought the two paddles were volume controls for the radio.”
NEXT STEP: More information on Borg Warner is available at www.borgwarner.com or by calling 248.754.0883. More information on Center for Automotive Research is available at www.cargroup.org or by calling 734-662-1287. More information on The Chrysler Group is available at www.daimlerchrysler.com or by calling 248-576-5741. More information on General Motor s is available at www.gm.com or by calling 800-331-9922.