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Author Aaron RobinsonNotes From the Road

I Want My CVT (Or Do I?)
By Aaron Robinson

ABS (anti-lock brakes), ESP (electronic stability control), GDI (gasoline direct injection). When it comes to naming a new technology, the auto industry really loves its TLAs (three letter acronyms). Here’s one you may or may not have heard: CVT.

CVT stands for “continuously variable transmission.” The goal of the CVT is to reduce expense and complication in your car and give you better fuel economy. Nissan figures its V-6 powered Murano SUV gets 10 to 12 percent better fuel economy with its standard-equipment CVT than it would with a conventional automatic transmission.

So why doesn’t every car use a CVT today? Let’s find out.

The CVT’s CV
The CVT is a type of automatic transmission. The first CVTs were developed by General Motors in the 1930s, but the technology went on the shelf because the fluids and control devices needed to make it reliable weren’t available yet. Subaru takes credit for introducing the first CVT to America with its 1989 Justy GL ECVT, a small hatchback. In the past decade, interest in the CVT has really picked up. GM launched its own production CVT in 2001 on its Saturn Vue sport utility. Honda put CVTs on the option sheets of its Civic subcompact (1996) and Insight hybrid (2000). Ford came to market last year with an available CVT in its new Freestyle wagon and FiveHundred sedan. Since 2003, Nissan has installed one called the Extroid CVT on its Murano SUV and sells several models in Japan with CVTs. Audi started offering CVTs on its A4 sedan in 2002. This year alone, over 50,000 vehicles equipped with CVTs will be put in American garages.

How does it work? You may already know that a transmission transmits power from the engine to the wheels. An engine isn’t flexible enough to operate at every wheel speed needed for regular driving. (Otherwise they’d bolt the wheels directly to the crankshaft.) Some kind of ratio device is required to allow the engine to operate in its limited speed range but to generate a broader range of wheel speeds. That device is the transmission.

Most passenger cars use a step-gear transmission with anywhere from two to six speeds (though Mercedes-Benz recently introduced a seven-speed transmission). Each “speed” is actually a different gear ratio created by varying the sizes of the gears relative to each other. A small gear turning a big gear has a lot of mechanical advantage, but must turn faster than the big gear to maintain a speed. A big gear turning a small gear has less mechanical advantage, but it gets to turn slower to maintain the same speed. That’s a conventional transmission. The ratios of the transmission – or the flexibility it has – is always limited by the number of speeds.

Now, picture a multi-speed bicycle. It has a transmission more like a CVT. You have a chain sprocket at the pedals. As you shift to higher gears, the chain moves from a smaller sprocket to a larger one relative to the rear sprocket. You pedal slower, the bicycle goes faster, but it also gets harder to pedal. Likewise, as you downshift, say to go up a hill, the chain moves from a larger sprocket to a smaller one. The bicycle goes slower, you pedal faster, and it becomes easier to pedal.


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