• These are basically the manual transmissions operated automatically hydraulically or electrically. 
  • They do not require clutch actuation and gear shifting manually by the driver. 
  • Thus there is no clutch pedal and no mechanical connection between the transmission and the selector lever. 
  • The transmission is controlled electronically though shift by wire.



Types :-

AMTs can be broadly classified into following types :-

1. Single sided clutch transmission (SSCT) :-

  • This type incorporates actuators with an existing production manual transmission, for operating the clutch and shifting the gears. 
  • Since these type can use a conventional manual transmission without any modifications to its existing shafts and gears, it is the cheapest of all the AMTs available. 
  • However, it has the disadvantage of the inherent power interruption during the gear shifting, which is present in a manual transmission. 
  • However, this type is still better than the manual type because it operates the clutch and moves the shift lever much faster than human being. 
  • Besides, it also adjust the engine speed faster and more accurately to achieve a synchronous shifting.

2. Double sided clutch transmission (DSCT) :-

  • In this type, the double sided clutch allows power from the engine to be transferred between either of the two power paths in the transmission itself. 
  • Due to this the total clutch travel time is drastically reduced, consequently improving the shift quality.

3. Dual clutch transmission (DCT) :-

  • Also called Direct Shift Gearbox, it consists of two, linked layshaft transmission with two power paths, one for even and other for odd number gears. 
  • There is a layshaft as in an ordinary manual gearbox, but in this case, the layshaft actually consists of two concentric shafts, one inside the other. 
  • There are two output shafts that mesh to a third shaft which goes to the differential. 
  • At the input end, each layshaft has a clutch to allow shifting. 
  • Both these layshafts are again linked to the output shaft. 
  • In this the shift quality is better. it can shift gear in 8 milliseconds, which is 50 times quicker than the blinking of an eye which takes for 400 milliseconds. 
  • Moreover one clutch engages gear as the other one disengages, thus minimizing the time the gearbox is not driven under power, which means least fluctuations in the torque output. 
  • However, both in DSCT and DCT, since the shifting is achieved through synchronization of only the none engaged partial transmission, it becomes impossible to skip two gears. 
  • Due to the best shifting characteristics out of the above types, the double clutch type (DSCT or DCT) is finding maximum use in modern transmissions.

Advantages of the Double clutch type of transmissions are :-

1. Improved acceleration, fuel economy and emissions compared to conventional automatic transmissions.

2. Gentle, jerk free gear changes as in an automatic transmission, combined with the efficiency of a manual transmission.

3. Less complicated and lighter since automatic's torque converter, fluid pump and multiple clutches are eliminated, reducing the weight by 30% approximately.

4. Less costly.

The main disadvantage of a Double clutch manual transmission is its limited ability to skip gears, which is an essential characteristics of an automatic transmission.

  • Volkswagen's DSG can be tuned via software, thereby changing its performance from sporty to comfortable. 
  • Moreover, in 2008, they introduced dry clutches in DSG instead of earlier wet clutches, which has resulted in lower weight, higher efficiency and more compact gearbox. 
  • A dry clutch DCT has 1% additional fuel economy benefit compared to a wet clutch DCT due to the elimination of the oil pump and its accompanying losses.
  • Borg Warner is developing a unique new dual clutch transmission for developing markets, like India, and small cars, which would have higher efficiency at lower cost. 
  • Germany's luk is also working on core components of dual clutch transmission, that is the dual clutch actuation system and relevant software. 
  • Ford has also started using double clutch transmission last year, which has been branded as 'Powershift' and is employed initially in the 'Focus' and 'C-max' models. 
  • Chrysler's new dual clutch transmission, developed with Getrag in the 2009 Dodge Journey and Avenger, and Chrysler Sebring claims improved fuel economy and CO2 emissions by reducing parasitic losses by eliminating the torque converter and using synchronizers instead of shift clutches.
  • With legislation worldwide pushing issues like fuel economy and emission reduction and consumers desiring performance and durability enhancement, the use of dual clutch transmissions is bound to increase sharply in the near future.