Purity isn’t always the best approach to problem solving. Engineers who love digital technologies for their accuracy and repeatability will generally acknowledge there are times when a simple analog device will do a better job. Mixing old and new technologies can often create systems that offer more than the sum of the various parts.
Toyota has been a big proponent of that hybrid strategy, becoming a dominant player in hybrid-electric vehicles (HEVs) for more than a decade through its Prius product line. Now that company’s engineers are announcing a new hybrid technology – though this one doesn’t use electric drive at all; it’s a continuously variable transmission (CVT) with a standard first gear.
Engineering experts are calling the transmission a head-smacking innovation – a development so obvious that they can’t believe CVTs have been around for decades (see racing technology story, pg. 32) but no one else thought of doing this.
CVTs use a pair of pulleys to establish gear ratios instead of fixed gears. That infinite range of pulley positions means the transmissions can match engine output more precisely than fixed gears, improving fuel economy. However, there’s a catch. CVTs are highly efficient at cruising speeds and not so responsive at startup. When driving CVT-equipped vehicles for the first time, many drivers complain of a lag between hitting the accelerator and the car responding. That lag comes from the CVT altering its pulley distances to create the equivalent of a low gear.
“The wide range operation means lower efficiency at the extremes. CVTs are generally most efficient at 1:1. At 2:1 and 0.5:1, they are much less efficient,” says Andrew A. Frank, a professor of engineering at the University of California – Davis, and inventor of the plug-in HEV (http://efficientdrivetrains.com/).
Adding a fixed gear for low-speed operations makes vehicles more responsive at startup, and it limits the range of gear ratios that the CVT has to simulate, keeping that portion of the transmission in its sweet spot.
“If you restrict the range of operation to 1.5:1 to 0.67:1 then the CVT part is more efficient,” Frank says. “They could have added an overdrive gear instead of a low gear, but I am sure they compared the alternatives and decided to go for the low gear.”
Mechanical Engineering Professor Wai Cheng, director of the Massachusetts Institute of Technology’s Sloan Automotive Lab (http://web.mit.edu/sloan-auto-lab), agrees, saying, “Adding a first gear does make a better design for the CVT, which then does not have to cover the low speed range. The better designed CVT could be an improvement in mechanical efficiency.”
Toyota engineers say adding the low gear improves driving performance and improves fuel economy by 6%. Also, without the need to handle low speeds, the transmission can offer a wider range of gear ratios at higher speeds. The belts and pulleys in the CVT are smaller and more responsive, lowering transmission weight and accelerating shifting speed by 20%.