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The C4 with a 6-speed electronic gearbox system


The six-speed electronic gearbox system combines a manual six-speed transmission with electronically controlled electro-hydraulic actuators.
This system reduces response times and proves optimal control for faster and more comfortable gear changes.
In addition, the permanent exchange of information with the Electronic Stability Program (ESP) ensures a speedy response of the gearbox for perfect safety when the ESP is active.

Changing gears
The six-speed electronic gearbox system features electro-hydraulic controls of the clutch and the gear changes. As a result, there is no clutch pedal and the gear lever – which remains present – has no mechanical link with the gearbox.
The gearbox is managed by a control unit that controls the two actuators: one – the gearbox actuator – ensures the selection and engagement of the gears, and the other – the clutch actuator, controls the clutch.
Gear changes are made as follows:

1. The driver requests a gear change:

  • either directly in manual mode by issuing a gear change command with the shift paddles or the shift lever,

  • or indirectly in automated mode by depressing the accelerator, the brake or as a function of the gradient of the road, engine speed, vehicle speed and grip conditions.

2. The gearbox control unit detects the request and processes it.
3. The gearbox control unit takes control of the engine controller and the clutch actuator.
4. Engine torque is reduced and the clutch is opened progressively to avoid jolting.
5. When the clutch is open, the gearbox control unit activates the gearbox actuator.
6. The selector fork is disengaged, the new gear is selected and synchronized, the selector fork is reengaged.
7. The gearbox control unit progressively closes the clutch and controls torque increase to complete the gear change without jolts or rebounds.
These steps are performed in rapid sequence to disrupt the power flow during acceleration or deceleration as briefly and imperceptibly as possible. When upshifting, between 0.4 and 1.2 seconds are required to go from step 1 to step 7.

The mechanical part
The automated manual gearbox of the electronic gearbox system is based on a manual 6-speed gearbox specially designed for being automated. It is characterized by its:

  • Short response times
    The actuator control system is mounted on the gearbox housing. This integration of the actuator inside the gearbox reduces the length of the lever arm and the play, and shortens the time required for changing gears.

  • Reliability
    The actuator control system is assembled at the Valenciennes factory to integrate the actuators into the gearbox. Thus, all gearboxes are tested before being shipped to the C4 assembly plant at Mulhouse.

  • Quiet reverse
    The reverse gear is very quiet during shifting and operation, which is due to the use of helical (and not straight) bevel gears, just as for the other gears.

  • Compact form factor
    The architecture of the gearbox is a 2.5-shaft architecture with a primary shaft connected to the engine through the clutch, a secondary shaft with a drive pinion positioned on either side of the bearing, and a very short half shaft used only for the reverse gear, which makes it possible to reduce the total length of the gearbox.

Electro-hydraulic control system
The mechanical unit is managed by an electro-hydraulic control system which ensures:

  • Smooth gear changes
    The gearbox control unit, which controls the two actuators, exchanges information with the engine control unit in order to manage engine speed and torque during the gear shifting process. It adapts the torque increase as a function of the differences between the requested torque and the actual torque, thereby optimising the comfort of changing gears.

  • Lightning-fast gear changes
    The gearbox actuator is equipped with a so-called S-CAM system that improves gear change times. With the activation of the S-CAM brake, changes from even to odd gears (N to 1st, 2nd to 3rd, 4th to 5th and vice-versa) are made diagonally, in a single movement and as easily as changes from odd to even gears (1st to 2nd, 3rd to 4th, 5th to 6th and vice-versa), which occur inline.
    This results in very short upshift times of 0.8 seconds in manual mode and 1.2 seconds in automated mode.

  • Good dosage for smooth manoeuvring
    The clutch actuator has a concentric clutch release bearing with a position encoder that transmits its position to the control unit every two milliseconds. The transitions between gears are perfectly controlled and the car manoeuvres very smoothly.

  • Optimal vehicle behaviour in all situations
    The gearbox control unit also exchanges information with the ESP control unit to ensure optimal efficiency when the ESP, which is standard with this engine, is active (control of the brakes, the engine speed and the selected gear).

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