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Continuously variable transmissions on the rise worldwide

Making a case for the use of continuously variable transmissions in IC and hybrid powertrains

Tom Callow, Bosch

 

Driving up a winding mountain road can be a tiresome task. None of the gears seem to be right. Sometimes they are too high, sometimes too low. With continuously variable transmission, it\'s a different story. As its name suggests, this automatic transmission works without any fixed shifting point. The result is a smooth drive at constant traction and engine speed.
"With its smooth and dynamic acceleration, continuously variable transmission makes driving easy and enjoyable," says Stefan Seiberth, the president of the Bosch Gasoline Systems division. "CVT especially comes into its own in urban stop-and-go traffic. It can reduce fuel consumption by as much as 7 percent, since the engine is constantly kept at its most efficient operating point." Bosch manufactures the push belts that are a central component of CVT. They make it possible for the transmission to drive engines with a torque range between 60 and 450 Newton meters and an output of more than 300 horsepower (220 kilowatts). Using an electronic control, a number of different settings are possible, from economical to sporty.
In addition, the continuously variable transmission is extremely compact, since it comprises just a few components. The control unit, for example, can be integrated directly into the system. The compact construction keeps the manufacturing cost down, and also means that this transmission fits into small urban vehicles. This also explains why CVT is especially popular in Japan. However, its market share is also growing in China and North America. Currently, every fifth automatic in the world is equipped with CVT. In just a few years, this share will grow to one-quarter. The technology is currently available in some 300 production models.

Flexible, yet as rigid as a solid steel rod
Bosch is also benefiting from this increase in market share. In 2012, the automotive supplier manufactured its 25 millionth CVT push belt. The component comprises hundreds of specially punched steel elements, stacked together in a high-alloy steel ring package. This design makes the push belt very flexible, but at the same time as rigid as a solid steel rod. This means that the Bosch component is very adaptable. It does not have to be developed separately for each vehicle, but often requires some adjustment only.
The way a CVT works is always the same. It closely resembles a set of bicycle gears: when the transmission ratio is higher, the diameter of the belt is larger on the drive shaft and smaller on the driven shaft. When the transmission ratio is lower, the situation is reversed, with a smaller belt diameter on the drive shaft and a larger one on the driven shaft. This stepless adjustment is made possible by the movement of the push belt. On both the drive and driven shaft, it moves between two cones that face each other. Depending on engine speed and torque, these pairs of cones alter the diameter travelled by the belt. In this way, there is always an optimum balance between required torque and engine speed.
Continuously variable transmissions are equally suitable for diesel, petrol, and hybrid powertrains. And while this technology saves fuel in classic internal-combustion engines, it can also increase hybrid powertrains\' electric range. This is because the CVT allows the internal-combustion engine to run at a higher speed, close to the optimum operating point. As a result, part of the energy released can be used for forward propulsion, while the rest can be stored in the rechargeable battery.

 

8 November 2013

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