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THe 2010 Nissan Rogue has a CVT Transmission (Petrina Gentile for The Globe and Mail)
THe 2010 Nissan Rogue has a CVT Transmission (Petrina Gentile for The Globe and Mail)

How It Works

Giving them the gears: CVT explained Add to ...

In the constant search for more fuel economy, vehicle manufacturers are looking at everything from fuel delivery and combustion to weight reduction and aerodynamics. With huge improvements in mileage and emissions regulated for the coming years, the hunt for efficiency takes on even more significance.

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One of the factors we can expect to see across the board is an increase in the number of gears in transmissions, especially automatic transmissions.

The idea is to keep engines running in their most efficient speed ranges or sweet spot. Think of it this way: With an outdated four-speed transmission, you've got one gear to supply the torque multiplication necessary for getting under way from rest and one to keep revs in the sweet spot at highway speeds and only two in between to cope with elevation changes, acceleration when climbing hills or passing. It is not unusual for engine speed to constantly vary over a range from 1,500 to 4,500 rpm.

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The CVT - continuously variable transmission - uses a belt or chain between two sets of pulleys that vary in diameter and has a theoretical infinite variety of ratios. CVTs are used by a number of manufacturers to great effect. Vehicles with CVTs deliver better fuel mileage than models equipped with the same engine but a manual gearbox.

Drawbacks are the expense of production, the amount of friction in the belt/pulley arrangement and their inability to handle the large amounts of torque produced by engines destined for towing or high performance. They also result in an unpleasant engine note under hard acceleration as they require the engine to maintain high revs while the belts and pulleys adjust ratios.

The CVT has been a good stopgap measure for low- and medium-power engines during the switch from three- and four-speed units to more suitable multi-ratio transmissions, but industry analysts expect it to fall from favour in the coming years.

The most promising transmission for future gains in efficiency are automated manual units that do away with the power-robbing torque converter and its associated fluids, planetary gear sets and pumps. They replace all this with a pair of shafts and dry clutches and electrically activated shifts.

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One shaft holds the even number gears and the other the odd number gears. With one gear engaged, the other shaft and next gear is prepared and, when the request arrives from the control unit, the first clutch is disengaged and that for the next gear engaged - all in as little as 60 milliseconds. Compact, lighter, less expensive to manufacture and offering performance that the most accomplished driver can't duplicate with a manual gearbox, these are the future.

While traditional domestic manufacturers race to catch up in terms of transmissions, they are chasing a moving target. Ford and General Motors joined forces in 2002 to jointly develop a six-speed automatic transmission. That was followed by a second program to come up with a smaller six-speed for lower-powered cars. Both six-speed transmissions are now used extensively by the two companies. But there are already more than 20 vehicles sold here with eight-speed automatic transmissions and 85 with seven speeds. All are from European or Asian manufacturers.

More gears are only part of the answer. Computer-controlled shifting - with the engine control computer acting in concert with the transmission control unit - allows for almost continual shifting to keep the engine in a very narrow sweet spot.

The driver is unaware of this constant shuffling of gears due to the narrow gap between ratios. And let's not forget, very few purchase decisions are made based on the number of gears in a transmission, but many are based on fuel economy.

Over-the-road big rigs have relied on 13- to 24-speed transmissions for years so their diesel engines can be kept in the narrow 1,000-2,500 rpm range where power is produced. Now even little fuel-sipping subcompacts are following that example.

globedrive@globeandmail.com

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