A variety of transmission technologies have emerged for engineers to choose from, including wet- and dry-clutch dual-clutch (DCT), continuously variable (CVT), electrically assisted automated manual (AMT), as well as eight- and nine-speed "conventional" automotive transmissions.
The pace of transmission development is quickening, Dr. Hamid Vahabzadeh, General Director, General Motors Global Powertrain Advanced Engineering, said at an April 12 panel on transmissions held as part of the SAE 2011 World Congress in Detroit. “Starting with the three-speed automatic in 1964, there were about 10 to15 years between each major introduction of a new version of the automatic transmission, up to the introduction of the six-speed in 2005.”
Since then, just a few years separated the introduction of seven-, eight-, and now nine-speeds. “During that time, we have also seen CVTs and DCTs introduced into the market, as well as a number of AMTs," Vahabzadeh continued. "While there are a wide variety of transmission technologies in the marketplace, they do not replace each other. Each one has a unique and specific place in the marketplace. This is exciting because we now have a wide variety of options to choose from.”
He sees automatic, planetary transmissions maintaining the lion's share of the transmission marketplace.
While the number of gears is a factor in transmission efficiency, many other details are important. Thomas C. Brown, General Manager Drivetrain Engineering, Aisin Technical Center of America, discussed a number of these additional factors in his time on the SAE Congress panel. They include more aggressive lock-up schedules to bypass torque converters, high-efficiency pumps, minimization of hydraulics leakage, and high-friction-coefficient clutch packs. He also discussed reducing friction in gears either through grinding, superfinishing, or chemical surface finishing to reduce lubrication requirements. Chemical surface finishing attracted some interest from those in attendance.
Computers and algorithms are just as important as manufacturing technology, he said. “Developing a control system with a shift pattern that matches the highest BSFC regions of the engines will be especially important.” Tight integration between engine and transmission developers is crucial, he added.
Smaller, boosted engines create their own challenges
Steve Thomas, Manager of Transmission and Driveline Research and Advanced Engineering at Ford, discussed the challenges to transmission engineers posed by downsized and boosted engines—a strategy Ford is pursuing with its EcoBoost downsized, turbocharged, direct-injection engines.
“Higher torque per engine displacement and fewer cylinders on downsized and boosted engines drive higher engine torsional excitations," he said. "It also reduces engine torque available prior to full boost, which may adversely impact initial vehicle acceleration.” Developing new transmission technologies will become increasingly important, he noted, because global engine forecasts project growth of gasoline and diesel engines with fewer cylinders. “All global growth is going to three and four cylinders,” he said.
Kiran Gowindswamy, Chief Engineer of Vehicle and Driveline Systems for FEV, agreed with that assessment. “Downsized and boosted engines, coupled with fuel economy pressures, will place challenges on transmission development,” he said.
One of the challenges from these engines is launch performance degradation form reduced transient torque, such as from turbo lag, and use of transmissions without torque converters. Gowindswamy said he anticipates that the increased requirements for launch performance and fuel economy—not to mention driveability—will demand ratio spreads of up to eight for conventional planetary automatics, and up to ten for what he called "converter-less" transmissions.
“These may also need to use a ‘launch gear’ below first gear, which is not noticeable by the driver,” Gowindswamy said. Electrified parallel hybrid transmission architectures are a good fit for converter-less transmissions, he said. Possibilities include integrating electric motors into a hybrid AMT, which could provide launch assist during parallel drive mode and torque fill during shifts to give acceptable shift quality as well as electric drive.
Electric vehicles will need transmissions
Perhaps the most interesting discussion for those steeped in transmissions for ICEs is the growing recognition for transmissions for electric vehicles. Andy Yu, Vice President of Driveline Engineering for Borg Warner, explained that a transmission coupled to an electric motor provides the same benefits as with an ICE.
“It enables a smaller motor and allows that motor to operate in its most efficient region of the torque speed map,” he explained. Transmissions with multiple speeds improve gradeability and top speed as well.
you noted that EV transmissions are simpler compared to their ICE counterparts. Electric motors go to zero speed (unlike ICEs that need to maintain some rpm at idle) and can reverse themselves, eliminating a reverse gear in the transmission. He said a number of companies are developing multi-speed EV transmissions, with a number of technical options under review. “The jury is still out on what is the best technology,” he said.
System integration trumps component optimization
While noting that defining an ideal transmission in terms of gear ratio spreads and speeds is important, panel moderator Mircea Gradu, Director Transmission and Driveline Engineering, Head of Virtual Analysis, Chrysler, emphasized a different priority.
“Independent of the transmission technology we select, [critical] is integrating that transmission and driveline properly in the vehicle architecture," he said. "If the integration is not performed properly and the whole is not approached as a system, then we leave a lot benefits on the table.”
Transmission technologies are major contributors to a vehicle’s fuel economy, performance, and fun-to-drive qualities, Gradu noted. “The general public is becoming more discerning of transmission technologies. As the mass consumer becomes more educated, it is more important for us to provide information about these technologies."