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See Inside January 2006

No More Gears

A car's engine can rev wildly, but the vehicle will not move until the driver puts it in gear. For a century, a box of gears called the transmission has transferred engine power to the wheels. In manual designs, the driver shifts the gears to deliver different torques. In automatics, hydraulics or servomotors do the work. A radically different scheme known as continuously variable transmission (CVT) has lurked in the background for 50 years, however, and has recently made inroads in small and midsize cars and in hybrid vehicles.

Several CVT configurations exist, all of which replace gears entirely. The dominant scheme utilizes two pairs of movable cones, under computer control, connected by a tough steel belt. Instead of a fixed number of gear ratios provided by a typical five-speed manual or four-speed automatic, CVTs offer continuous change in the degree of torque transfer--the equivalent of an infinite number of gear ratios. As the car accelerates, the driver feels no lurch because no step occurs from one ratio to the next. The variable transfer speed also allows the engine to operate near its optimum output range over a variety of wheel speeds, improving fuel efficiency. The gearstick has positions for park, reverse, neutral and drive. That's it....

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