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Fuel efficiency in a transmission

Fuel efficiency depends on many factors, but one that’s extremely important is the transmission.

Fuel efficiency depends on many factors, but one that’s extremely important is the transmission.


Most use gears to transmit the engine’s power to the wheels, but continuously variable transmissions, or CVTs, use belts and pulleys instead.


Conventional transmissions, whether manual or automatic, generally contain between four and eight gears.


As the vehicle speed increases or decreases, either the driver (in a manual) or the transmission (if automatic) selects the gear that’s most appropriate to keep the engine running at its most efficient speed.


But there’s another type, the continuously variable transmission (CVT), which doesn’t have gears. Instead, it uses two pulleys with a metal belt between them, which provides a theoretically infinite number of gear ratios.


“We like to compare the CVT to a 10-speed bicycle,” says Rui Nunes, chief marketing manager for Nissan Canada, one of many automakers that uses CVTs.


“The bike uses a chain that rides on the front and rear sprockets. These sprockets are different sizes and the rider can adjust the gears, depending on the type of driving. The CVT uses a high-strength steel belt that essentially replaces the chain on the bicycle, and that belt rides between two variable pulleys.”


The CVT includes a drive pulley, which runs off the engine, and a driven pulley, which sends power to the driveshaft to turn the vehicle’s wheels.
When the vehicle first takes off, the drive pulley turns faster, sending engine torque to the second pulley.


As the vehicle speed increases, a shift control valve increases the speed of the pulley sending power to the wheels. This keeps the engine working most efficiently.


“It’s a less-complicated transmission, it’s lighter, and it has fewer moving parts,” Nunes says.


“It’s also between 10 and 15 percent more fuel efficient. Nissan introduced the CVT originally back in 1992 on a very small compact car, but in mass production in North America, we introduced it in 2003 on the Murano.”


The initial challenge with CVTs was making them robust enough for higher-horsepower engines. Nissan overcame this with the Murano’s transmission, and now several of Nissan’s vehicles use CVTs.


However, there are still some applications where gears are preferred, which is why both types of transmissions are used.

 
 
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