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 |  |  | | | | 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.
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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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