Elements of an Automatic Transmission



If you are considering the engine as the heart of any automobile, then I think you won’t have any problem in considering “Transmission” as the arteries and veins. While in the early years of automobiles, we had very few configurations for the transmission system, today we are having a range of transmission systems like Manual Transmission, AT (Automatic transmission), DCT (Dual Clutch Transmission), Clutch-less Transmission and many more.
Each of the above-mentioned transmission types are having their own advantages and disadvantages but in this article, we are going to talk about the AT. Automatic Transmission is something as the name itself suggests in which everything is done automatically. Unlike the Manual Transmission, there is no need of shifting the gears manually in the AT. Just press the acceleration paddle and you go.
AT comes with a range of different mechanisms out of which the CVT and Torque convertors are the common ones. CVT and Torque convertors both can provide n numbers of velocity ratios depending upon their configurations.
CVT
CVT (Continuously Variable Transmission) also known as step-less transmission or twist-and-go transmission can seamlessly provide the required number of velocity ratios while maintaining the constant angular velocity of the input shaft. The most common type of CVT which is belt-driven CVT can provide around 88% efficiency which is lower than the manual transmission but it has got its own advantages. You can tune your CVT to run at the most optimum power to provide the maximum power or the better economy.
CVTs normally consists of the two pulleys- primary and secondary and by varying the diameters of the pulleys, different velocity ratios can be obtained. While most of the CVTs come without any clutch, many manufacturers are providing centrifugal clutches to facilitate the idling or manual reversing.  Various types of CVTs are listed as follows: -

Ø  Variable-diameter pulley (VDP) or Reeves drive
Ø  Toroidal or roller-based (Extroid)
Ø  Magnetic or m
Ø  Infinitely variable transmission (IVT)
Ø  Hydrostatic
Ø  Naudic incremental (iCVT)
Ø  Cone
Torque Converter


Torque Converter is a kind of fluid coupling which transmits the power from the IC engine to the driven shaft. In an AT vehicle, the torque converter takes the place of a mechanical clutch but unlike the normal fluid coupling it not only matches the rotational speed but also multiplies the torque while acting as the reduction gear.
A typical torque converter consists of a turbine, a stator, an impeller, working fluid and the casing. The impeller is connected with the input shaft from the engine while the turbine is connected to the driven shaft of the vehicle and stator is placed in between the turbine and the impeller. The input shaft rotates the impeller which due to centrifugal force and its curved blades forces the fluid towards the turbine where the fluid rotates the turbine  and is again directed toward the impeller. But, here lies the multiplication ability of the torque converter. The fluid before striking with the turbine strikes with the stator which absorbs most of its kinetic energy and then the fluid goes toward the turbine and the cycle keeps repeating.
There are basically three operational phases of the torque converter: -
Ø Stall
This is that phase when the driver has put the engine in the gear but is preventing its motion by the constant application of brakes. In this case, the maximum torque multiplication can be achieved if the enough input power is provided. The resulting multiplication of torque is known as the “stall ratio”.
Ø Acceleration
Acceleration phase refers to the phase when the torque multiplication is lower than the maximum torque multiplication.
Ø Coupling
This is the case when the impeller speed has reached about 90% of the speed of the turbine  and in this case the converter simply woks as a simple fluid coupling.

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