ABCs of CVT transmissions

Brian Early

SPECIAL TO THE STAR

In terms of how you use it, CVTs are much like a conventional automatic transmission: stick it in Drive and you're off.

But a continuously variable transmission doesn't use separate gears to do the job. Rather, most use special pulleys.

The pulleys are connected by a "variator" (a special steel-link and plate belt or chain), allowing the V-channel pulleys, with their variable diameters, to infinitely shift the drive ratio between their upper and lower limits.

Picture your old 10-speed bike. Using the smallest sprocket on the front and the biggest one on the rear made it easy to pedal, gave you lots of power, but meant that you were pedalling furiously as soon as you gained speed. Eventually, on a nice flat stretch, you could move the chain to the big sprocket in front and the smallest sprocket in the rear, giving you great speed though with high effort.

In simplest terms, the CVT concept is the same. As one pulley spreads apart (the belt riding deeper within that pulley's "V"), the other pulley squeezes together to force the variator farther toward its outter diameter, keeping the variator's tension constant while changing the effective drive ratio.

This is not a new concept – the idea harks back to at least the 19th century, and a CVT was first used in a car in the late 1950s – but it has taken modern engineering and computer controls to make it capable of handling any appreciable amount of torque reliably.

In theory, with a CVT's wider ratio range, the engine can always be at its optimum speed to provide the greatest power or best economy. From an economy standpoint, this tends to work. DaimlerChrysler states that a 6 to 8 per cent increase in fuel economy is realized by using the "second generation" CVT in its 2007 Dodge Caliber instead of a conventional automatic transmission.

Nissan is similarly convinced of the benefits of CVTs. Having used them successfully for years in many overseas models and here in its Murano crossover since its late 2002 introduction, Nissan now plans to equip nearly all of its automatic-shifted front-wheel-drive 2007 passenger cars with CVTs, claiming significant gains in fuel economy and hence appreciable reductions in carbon dioxide, a major greenhouse gas.

As noted earlier, CVTs can potentially improve performance as well. If the motor generates its peak power at around 5500 rpm, it stands to reason that keeping the motor revving at that speed while varying the ratio as the vehicle gains speed will provide the best acceleration.

In practice, the sensation is a bit odd, sometimes even making the vehicle feel more sluggish, since the normal connection between engine speed (via sound) and road speed is lost. It's a lot like riding a snowmobile (they use a variation on the CVT theme).

In the case of the Caliber, there is a tangible detent at the bottom of the throttle's travel, past which the CVT will quickly select a ratio that brings the engine to redline (vehicle speed permitting), perhaps to give the driver the gratification of being able to use the whole rpms range or the illusion of greatest acceleration.

It's hard to see another purpose, since the Caliber's CVT appears to vary the ratio appropriately throughout the rest of the throttle's range anyway.

Even odder is the inclusion of manual ratio selection – outside of the normal low range/engine braking position – in many CVT-equipped models. Giving the driver the ability to shift "gears" in an essentially gearless transmission seems pretty ironic!

Nissan and DaimlerChrysler are but two of the manufacturers offering CVTs in selected models; Audi (A4/A6), BMW (Mini), and Ford (Five Hundred and Freestyle) are among the others.

For the moment, torque capacity, manufacturing costs and perhaps low customer acceptance levels (due to a CVT's unfamiliar behaviour) are keeping CVTs from wider use. Multi-speed automatics can handle higher power levels and be more easily engineered to fit longitudinal (that is, rear-wheel drive) driveline layouts. The latest six-speed units can even match the 6:1 ratio range of current CVTs.

The mechanical simplicity of CVTs is hard to ignore though, particularly in transverse applications, where their design idiosyncrasies are actually a benefit. They're unlikely to supplant conventional multi-speed automatic transmissions altogether, but as time goes on and the hurdles are overcome, CVTs will undoubtedly see greater popularity.

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