Glossary automatic transmissions

ATF: Abbreviation for Automatic Transmission Fluid, which is the lubricating and hydraulic oil used in the automatic transmission.

Belt: On a belt-type CVT, the belt is stretched over two pulleys to transmit torque. There are belts that incorporate steel elements, as well as those that come in chain formats.

CVT: CVT refers to Continuously Variable Transmission. This is a power transmission mechanism that continuously changes the gear ratio using mechanisms other than the gear. The transmission mechanism is made up of a belt that connects the pulleys on the input and output sides. There is an increasing number of CVT that employs a mechanism in which the contact radius between each pulley and the steel belt is changed without steps.

Lock-up: Although the torque converter transmits power through fluids, loss is unavoidable due to the viscosity and slippery property of liquids. Therefore, in areas where gear shift operations are not necessary, the lock-up is a mechanism that is applied in order to enhance transmission efficiency by directly connecting the input and output shafts through a mechanical clutch. In addition, improvements were achieved in fuel efficiency for automatic transmission and vehicles equipped with the system.

Pulley: On a belt-type CVT, the belt is stretched over two pulleys to transmit torque. Changing the groove width of the pulley in turn changes the contact radius with the belt, in a stepless way, causing the gear to shift.

Range of gear ratio: The gear ratio is also known as the reduction ratio, and refers to the ratio of the number of revolutions for the engine that is having its speed reduced through the transmission gear. For the same number of engine revolutions, larger gear ratio provides greater power but lower speed, while conversely, smaller gear ratio provides greater speed but less power. The larger the range of gear ratio, from low to high, the better the acceleration performance during start-up, and the better the fuel efficiency.

Step AT: Automatic transmission (AT) is the overall term for transmission that is equipped with a function that enables automatic changing of the gear ratio in response to the speed of the vehicle and the speed of engine revolution. In recent years, in order to distinguish between AT and CVT, the industry has also begun to apply the term “step AT” to automatic transmission. Step AT refers to a stepwise transmission system for switching gears.

Toroidal CVT: Two discs are placed parallel to one another on the input and output sides, and several power rollers (resembling tops) are wedged powerfully between the two discs. Changing the degree of inclination of the power roller changes the ratio of the number of revolutions of the two discs correspondingly, thereby allowing for variable transmission.

Torque converter: The torque converter is a machine that makes use of the effects of fluid dynamics in order to behave as a mechanical clutch. Unlike fluid coupling, which is a similar device, torque amplification occurs as a result of the difference in revolution between the input and output sides. As such, the automatic transmission, which uses the torque converter, has capability with respect to acceleration during start-up, and is able to start-up powerfully even on an slope.

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    Romain Nicolas

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