An Overview of the Dual-Clutch Gearbox for Educators

The dual-clutch gearbox 

A dual-clutch gearbox has two clutches connected by two separate input shafts. 

• One of the input shafts controls the odd-numbered gears, such as 1, 3, 5, as well as the reverse gear. 
• The other input shaft controls the even-numbered gears. In this example: 2, 4, and 6.

The advantage of this construction is that the following gear can be preselected.

This will allow it to: 

• Change gears quickly and smoothly. 
• Change gears with (almost) no interruption of force transmission. 

Depending on the manufacturer, dual-clutch is also referred to as: 

• DirektSchaltGetriebe (DSG). 
• Dual-Clutch Transmission (DCT). 
• Porsche Doppelkupplung (PDK). 

Overview of components

A dual-clutch gearbox can be divided as follows:

1. Dual-clutch unit 

The dual-clutch unit is used to alternate the driving force of the engine between the input shaft controlling the odd-numbered gears and the input shaft controlling the even-numbered gears. 

As the name implies, the dual-clutch unit consists of two clutches. 

Different kinds of clutches can be used in this unit: 

• Two wet multi-plate clutches. 
• Two dry multi-plate clutches. 

Due to their reliability, manufacturers are increasingly deciding to use wet plate clutches. In general, it can be said that wet plate clutches are often selected for engines with a high torque. In other situations, dry plate clutches will suffice.

Wet multi-plate clutch 

This clutch unit consists of: 

• A housing. 
• The clutch friction plates of clutch No. 1. 
• The clutch friction plates of clutch No. 2. 

On one side, the housing is attached to the flywheel by the tooth profile. The friction plates of clutch 1 & 2 are, in turn, directly connected to the housing.   

When clutch 1 is operated, the friction plates press against the steel plates. These steel plates are directly connected to input (direct) shaft No. 4 by a disc.

In the same way, clutch 2 can be connected to the input shaft No. 3. This shaft is a hollow shaft that is installed on input shaft 4. This means that input shafts 3 & 4 are able to rotate independently of each other. 

The two clutches are controlled by two independent hydraulic pistons. The control unit controls the hydraulic pistons. 

Dry plate clutch  

A dual-clutch unit with two dry plate clutches consists of the following components:   

• 1st clutch lever. 
• 1st clutch release assembly. 
• 1st clutch plate. 
• 1st input shaft. 
• 2nd clutch lever. 
• 2nd clutch release assembly. 
• 2nd clutch plate. 
• 2nd input shaft. 

In most cases, clutch release assemblies are used that ensure that the clutch is not engaged at idle. As soon as the clutch release assembly is operated by the clutch lever, the clutch will engage. This is the polar opposite of a regular clutch release assembly!  

Therefore, when the 1st clutch lever is operated, the 1st clutch release assembly will ensure that the 1st clutch plate engages. In this case, the driving force of the engine is transmitted through the 1st clutch plate and the 1st input axle. 

The same applies to the 2nd clutch. When the 2nd clutch lever is operated, the 2nd clutch release assembly ensures that the 2nd clutch plate engages. In this case, the driving force of the engine is transmitted through the 2nd clutch plate and the 2nd input axle. 

 2. Gear train 

Because a dual-clutch gearbox is a semi-automatic gearbox, the gear train and the gear shift mechanism have many things in common with a manual gearbox. 

For instance, the gears are engaged and disengaged using ‘regular’ shift forks. Depending on the manufacturer, these shift forks are either controlled by an electric motor or a hydraulic valve. The gear wheels also have a ‘regular’ synchroniser mechanism. 

The positioning of the gear train, however, can not be compared to that in a ‘regular’ manual gearbox. A very common arrangement is: 

• 1st output shaft with gears 1, 3, 4, and 2. 
• A split input shaft. 
• 2nd output shaft with gears 5, 6, and reverse. 
• A reverse gear shaft with an extra gear wheel for reverse. 

Both output shafts are directly connected to the differential via two separate output gears. 

3. Gear shift mechanisms 

Shift forks are controlled by the control unit to preselect the gears. Depending on the manufacturer, the shift forks may be hydraulically controlled or controlled by an electric motor. 

Hydraulically controlled shift fork 

A hydraulically controlled shift fork has many things in common with the shift fork of a manual gearbox. The largest difference between them is that both ends of a hydraulically controlled shift fork have been installed into a cylinder with a piston. 

To move the shift fork to the left, the control unit will increase the fluid pressure exerted on the piston at the right end. And vice versa, to move the shift fork to the right, the control unit will increase the fluid pressure exerted on the piston at the left end. A steel ball with a spring ensures that the shift fork is able to remain in the neutral position. 

Shift fork controlled by an electric motor 

This shift fork is controlled by an electric motor, an intermediate gear, and a shift drum. By controlling the electric motor, the intermediate gear will rotate and, eventually, the shift drum as well. This drum has notches that can push the shift fork up, down, and into the neutral position. Therefore, rotating the shift drum will ensure that the gear is engaged and disengaged. 

4. Control unit 

In many cases, the control unit of a dual-clutch gearbox is integrated into the gearbox. It is responsible for the following: 

• Preselecting the following gear so that it is possible to shift quickly. 
• Determining when to shift gears. 
• Driving the actuators to control the shift forks. 
• Driving the actuators to control the two clutches. 

Preselecting the following gear and determining the right moment to shift the gear is a complex decision. Basically, it comes down to the following: 

• The next incrementally higher gear is already preselected when the accelerator pedal is depressed and the vehicle accelerates.    

• The next incrementally lower gear is already preselected when the accelerator pedal is released and the vehicle decelerates. 

Of course, this is an extreme simplification of a complex decision. In reality, the control unit takes the following information into account, among other things, to determine the correct preselection of the following gear, as well as the right moment to shift the gear. 

• Position of the selector (P/R/N/D/S/L, etc.). 
•  Vehicle speed.  
• Engine speed.  
• Position of the throttle valve.  
• Coolant temperature.  
• Gearbox oil temperature (to determine how viscous the gearbox oil is when starting a cold engine).   
• Steering angle (to prevent the shifting of gears when cornering).   
• Information about the brake control.  
• The rotational speeds of shafts No. 1 & No. 2.   
• Inclination angle.   

Using this information, the control unit can control the shift fork actuators and the clutch actuators. 

Power flow

In this section, we will describe the transmission of driving force in a typical dual-clutch gearbox with six gears and a reverse gear. 

3rd gear:

The 3rd gear has already been engaged. To have the driving force actually go through the 3rd gear, the gearbox switches back from the 2nd clutch to the 1st clutch. The driving force of the engine is now transmitted as follows: 

• 1st clutch. 
• 1st input shaft. 
• 1st output shaft. 
• Differential. 

Also in this situation, if the vehicle accelerates further, the 4th gear, which is driven through clutch No. 2 and input shaft No. 2, is already preselected.

4th gear:  

The 4th gear has already been preselected. To have the driving force actually go through the 4th gear, the gearbox again simply switches from the 1st clutch back to the 2nd clutch. The driving force of the engine is now transmitted as follows: 

• 2nd clutch. 
• 2nd input shaft. 
• 1st output shaft. 
• Differential. 

Also in this situation, if the vehicle continues to accelerate, the 5th gear is already preselected. 

5th gear:  

The 5th gear has already been preselected. Also in this case, a switch is only made between the clutches in order to have the driving force actually be transmitted through the 5th gear. The driving force of the engine is now transmitted as follows: 

• 1st clutch. 
• 1st input shaft. 
• 2nd output shaft. 
• Differential. 

Also in this situation, if the vehicle continues to accelerate, the 5th gear is already preselected. 

6th gear:  

The 6th gear has already been preselected. To have the driving force actually go through the 6th gear, the gearbox switches from the 1st clutch to the 2nd clutch. The driving force of the engine is now transmitted as follows: 

• 2nd clutch. 
• 2nd input shaft. 
• 2nd output shaft. 
• Differential. 

Reverse gear:  

And finally we come to the reverse gear. If it has already been preselected, the driving force of the engine is transmitted as follows:

• 1st clutch. 
• 1st input shaft. 
• Reverse gear shaft. 
• 2nd output shaft.