Semi-Automatic Transmission

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Semi-Automatic Transmission

    

 

The two most common semi-automatic transmissions are-

1.     Dual-clutch Transmission

2.     Sequential Transmission

Dual Clutch Transmission:

            A dual-clutch transmission, commonly abbreviated to DCT uses two clutches but has no clutch pedal. Sophisticated electronics and hydraulics control the clutches, just as they do in a standard automatic transmission. In a DCT, however, the clutches operate independently. One clutch controls the odd gears (first, third, fifth, and reverse), while the other controls the even gears (second and fourth) as shown in the figure. Using this arrangement, gears can be changed without interrupting the power flow from the engine to the transmission.

 

            A two-part transmission shaft is at the heart of a DCT. Unlike a conventional manual gearbox, which houses all of its gears on a single input shaft, the DCT splits up odd and even gears on two input shafts. The outer shaft is hollowed out, making room for an inner shaft, which is nested inside. The outer hollow shaft feeds second and fourth gears, while the inner shaft feeds first, third and fifth.

Sequential Transmission:

            A sequential transmission is a type of transmission used on motorcycles and high-performance cars for auto racing, where gears are selected in order, and direct access to specific gears is not possible. Cars with SMTs have a manual transmission with no clutch pedal; the clutch is automatically engaged.

            In a race car, the motion of the shift lever is either "push forward" to upshift or "pull backward" to downshift. If you are in a gear and you want to go to a higher gear (e.g. from 2nd to 3rd), you push the shift lever forward. To go from 3rd to 4th, you push the lever forward again. To go from 4th to 5th, you press it forward again. It is the same motion every time.

            To drop back down a gear, say from 5th to 4th, you pull the lever backward. In European mass-produced automobiles, the shift lever, the motion of the shift lever is either "push forward" to upshift or "pull backward" to downshift moves forward and backward to shift into higher and lower gears, respectively.

            In Formula One cars, there are actually two paddles on the sides of the steering wheel, instead of a shift lever. The left paddle upshifts, while the right paddle downshifts. On a motorcycle, you do the same thing, but instead of moving a lever back and forth with your hand, you move a lever up and down with your foot.

Advantages of using Sequential Transmission:

1. It provides a direct connection between engine and transmission, allowing 100 percent of the engine's power to be transmitted to the wheels.

2. The SMT provides more immediate response and ensures that the engine RPMs do not drop when the driver lifts off the accelerator (as happens with an automatic), giving her more precise control over power output.

3. It uses a solid coupling, as opposed to a fluid coupling (torque converter)

4. The sequential shift lever takes up less space in the race car cockpit.

5. The sequential shift is quicker.

6. The sequential shift is consistent. You do not have to think before gear change.

7.  The hand location is consistent; the shift lever is always in the same place for the next shift.

8. The gearbox reduces the risk of blowing up the engine due to mis-shift.


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