Hybrid Power Swap
When it comes to hybrid vehicles, electronics and batteries tend to get the most attention. But it is elegant mechanical engineering that established Toyota as the market leader in hybrids. Now General Motors, DaimlerChrysler, and BMW are applying mechanical engineering to catch up. The three manufacturers have joined forces to develop transmission technology that could beat Toyota’s system–at least on the highway.
Toyota’s patents for their hybrid vehicle focus on the control systems and sophisticated transmission used to shift and share power among the engine, electric motors, and wheels. “There’s a lot of mechanics in the system,” says David Hermance, executive engineer for environmental engineering at Toyota’s Gardena, CA, Technical Center. “Even if you make significant improvements on the electrical side, if you don’t do a good job on the mechanical side you don’t get as much efficiency, and you’re looking to improve efficiency every place you can.”
The first hybrids from GM and DaimlerChrysler were so-called “light hybrids” providing a relatively small efficiency boost. (BMW has yet to release a hybrid.)
Toyota’s hybrid system is notably different because of its power-splitting transmission. To date, competitors such as Honda have integrated electric power by adding motors to more-conventional transmissions. As a result, a hybrid’s engine must be operating for the vehicle to move. In contrast, Toyota’s transmission enables hybrids such as the Prius sedan and the Highlander SUV to start in all-electric mode, leaving the engine off during the low-speed, high-torque regime where mechanical power from the engine is least efficient. The engine comes on only when the driver requests more power than the electric motors can provide or to recharge the vehicle’s battery.
Toyota’s transmission also employs its electric-vehicle (EV) mode to drive its hybrids in reverse, so there’s no need to build in dedicated reverse gears found in conventional transmissions.
Some 500 engineers at GM, DaimlerChrysler, and BMW are developing a new transmission system that delivers EV-mode operation just like Toyota. The transmission will add one more trick: the system can also take the electric side of the drivetrain out of the loop and run in engine-only mode. Their two-mode hybrid transmission, patented in 1999 by GM and currently used in its hybrid buses, swaps out the motors with a set of fixed gears, locking the engine to the driveshaft. The electric motors help make the switch seamless by synchronizing the speed of the two sets of gears, but once the shift is done the motors are out of the picture. “The motors do all the fine-tuning and the clutches just cog over. That’s the big revolution with the two-mode,” says Tim Grewe, GM’s chief engineer for the two-mode hybrid power train.
Taking the electric motors out of the drivetrain delivers two benefits. When moving heavy loads at a steady clip, the electric motors are inefficient because they actually represent a drag on the engine. Taking the electric motors completely offline in such scenarios makes the car’s overall operation more efficient. The more significant benefit, says Grewe, comes in packaging the transmission. By swapping out the motors under high-power operation, the alliance partners get by with smaller motors. The result is a hybrid transmission that is no bigger than a conventional transmission. “We took clutches out and we put electric motors in, and with that we were able to get it to package in a similar volume,” says Grewe. That should simplify the design and assembly changes required to offer hybrid as an option on existing vehicles.
Rich Schaum, former chief engineer at Chrysler and the incoming president of the Warrendale, PA-based Society of Automotive Engineers, says GM has a “very robust” patent position for the two-mode transmission and that is allowing the alliance partners to go their own way with a better technology. “The architecture of the dual-mode technology being pursued by GM, DaimlerChrysler, and BMW is superior to what’s currently on the streets from other manufacturers,” says Schaum.
Of course, warns Schaum, one should not assume that Toyota will stand still. Toyota hybrid engineer Hermance won’t commit on whether Toyota would consider a two-mode system. For one thing, says Hermance, it’s not clear whether drivers actually cruise enough to deliver a significant benefit. “The question becomes, what fraction of the total drive is in that steady-state regime? Until we see actual cycle test data for their system, it’s hard to evaluate what kind of an improvement they may or may not have made,” says Hermance.
Nevertheless, Hermance calls the two-mode concept a “good idea” in principle, especially in full-size trucks–a market segment where Toyota has yet to apply its hybrid technology.
GM will debut the two-mode late next year in the full-size Chevy Tahoe SUV, and DaimlerChrysler will follow in early 2008 with a hybrid version of the equally beefy Dodge Durango. Both manufacturers claim the two-mode system and more-efficient engines will make their SUVs as much as 25 percent less fuel-thirsty than their nonhybrid counterparts–performance that could be worth big bucks if it can reverse the recent sharp decline in high-profit margin SUVs brought on by high prices at the pumps.
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