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.