How Car Parts Work: Dual Clutch Transmission

Dual clutch transmission (DCT) uses a set of two clutches to operate the odd and even gear sets separately. One clutch is used to operate even set of gears and the other clutch will operate the odd set of gears. It is also known as semi-automatic transmission.

Working Principle:

When we look at a conventional manual clutch transmission, a clutch pedal is used to first cut off the power supply from the engine to the gearbox and then gear stick shift is used to change the gear and then later on releasing the clutch pedal, the clutch is engaged with the flywheel and the power supply from the engine to the gearbox resumes. Therefore, there is no continuous flow of power from engine to wheels. This results in torque shifts and can result in the passengers thrown forward or backward as gears are changed by an unskilled driver.

In the case of DCT, there is no clutch pedal. The clutches are controlled by suitable electronics and hydraulics mechanism. One clutch will control the odd gear ratios (1, 3 and 5) and the other clutch will control the even gear ratios (2, 4). The trick behind this is to achieve lightning fast gear changes without interrupting the power supply from the engine.

Design of Dual Clutch Transmission:

A dual clutch transmission uses 2 multiplate clutch assemblies. Even single plate clutches can be used instead of multiplate clutches. There are 2 transmission shafts that carry the power from engine to the gearbox. The outer transmission shaft is connected to the outer clutch assembly.

The outer transmission shaft is hollow, which allows us to insert the inner transmission shaft and connect it to the inner clutch assembly. The outer transmission shaft is supplies power to the even gears (2 and 4) and the inner transmission shaft supplies power to the odd gears (1, 3 and 5).

Both the clutches are controlled hydraulically. It consists of a piston assembly placed against a stack of clutch plates and friction discs. When clutch is engaged, hydraulic pressure from the piston forces the clutch plates and friction discs to move against the pressure plate. The entire clutch assembly is locked and rotates together as a single unit. When the clutch is disengaged, the return springs help to pull the clutch plates and friction discs so that the transmission shaft rotates freely.

Working:

To understand the working in a better way, I suggest you to learn how a manual transmission works by clicking on the following link:

How manual transmission works?

The gear shifting is similar to a conventional manual transmission. Dog clutch and synchronizers are used to engage an individual gear to the output shaft.

1^st Gear:

The inner clutch assembly is engaged and the dog clutch engages itself with the 1^st gear on the output shaft. Make a note that the outer clutch will be disengaged during this process. Once we reach a high enough speed in the 1^st gear, the selector rod will automatically engage the dog clutch to the 2^nd gear on the output shaft. As soon as the outer clutch is engaged, the 2^nd gear is achieved in no time.

When the outer clutch is engaged, the inner clutch will be disengaged, hence cutting off the supply through 1^st gear.

2^nd Gear:

The outer clutch assembly is engaged now and since the dog clutch is already connected with the 2^nd gear, instantaneous acceleration is achieved. The inner clutch will be disengaged during this process.

3^rd Gear:

When the speed in the 2^nd gear is high enough, the dog clutch engages with the 3^rd gear in order to be ready to supply the power during gear shift. As soon as the inner clutch is engaged and outer clutch is disengaged again, the 3^rd gear is achieved. The inner and outer clutch engagement cycle continues for every odd and even gear shifting.

4^th and 5^th Gear:

The outer clutch is engaged to achieve 4^th gear and inner clutch is engaged to achieve 5^th gear.

Reverse Gear:

Reverse gear can be controlled by any one of the clutches. In this case, the outer clutch is used to achieve reverse gear. An idle gear ‘I’ is inserted to mesh between the gears ‘H’ and ‘R’ to reverse the rotation of the wheels.

Advantages of DCT:

· Even with automatic engagement and disengagement of clutch, drivers can tell computers when to take action with the help of paddles or gearshift.

· It provides smooth acceleration by avoiding torque shifts or gear shift shocks that are usually experienced in a car with manual transmission.

· Fuel economy can be improved dramatically by up to 10%.

· Can handle high torque demands of high performance cars.

The only main disadvantage of DCTs would be the manufacturing cost of it since they involve two clutches and two transmission shafts.

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