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Technical Details on an Innovative Transmission
Two Clutches X No Clutch Pedal = Optimum Dart Fuel Efficiency
In addition to the standard six-speed manual gearbox, an electronically controlled and automatically shifted version of that gearbox is available on the 2015 Dodge Dart with the 1.4L four-cylinder turbocharged engine. Utilizing a double dry clutch, it combines the comfortable gear shifts of an automatic transmission with the lower operating costs of a manual transmission. In a sense, it’s an innovative manual gearbox that is shifted automatically. There is no torque converter as with a conventional automatic transmission.
C635 DDCT BASICS
The C635 DDCT (Dual Dry Clutch Transmission) is produced by Fiat Powertrain (FPT). The letters and numbers in the name mean the following:
- C = Cambio (Italian for transmission)
- 6 = 6 forward gears
- 35 = 350 N•m of torque
Unlike other automatic transmissions, this unit does not use ATF+4®. Instead, there are two different fluids that are used. First of all, the gearbox uses Mopar® C Series Manual and Dry Clutch Transmission Fluid. The hydraulic fluid used in the slave cylinders is Mopar C-Series DDCT Hydraulic Fluid. Both components are filled for life, but if repair work is performed on the transmission, these specific fluids are mandated. Like most gearboxes, the fluid level is correct when the fluid is at the bottom of the fill port.
Internally, the C635 DDCT and the manual shift, clutch pedal-equipped C635 – its conventional mechanical counterpart – are the same. It’s a three-shaft design that reduces overall unit length, enabling easier vehicle packaging. The extra shaft is an output shaft. Gears 2, 4, 5 and Reverse are on one output shaft, gears 1, 3 and 6 are on the second shaft. Each output shaft has a different final-drive ratio, which is also different from most transaxles. All ratios are synchronized. The following components differentiate the C635 DDCT from the C635:
- Double dry clutches
- Double slave cylinders
- Dual mass flywheel
- Hydraulic power unit
These unique components allow the transaxle to shift gears mechanically without any input from the driver. There is one clutch for the odd number gears (1, 3 and 5) and a second clutch for the even number gears (2, 4 and 6). The clutches operate in the same manner as the manual clutch in the C635, but the actuator arm is activated by the hydraulic power unit instead of the driver’s left foot pushing down on the clutch pedal. The C635 DDCT does not have a clutch pedal.
OPERATION OF THE DDCT
The C635 DDCT is an electronically controlled and automatically shifted transaxle with a dual-clutch torque conversion system. The heart of the transaxle is the main shaft that actually consists of two mutually coaxial driveshafts. Two dry clutches (Figure 1), operated by two separate slave cylinders, are used to transmit torque to the main shaft.
A traditional concentric slave cylinder, located under the gearbox casing, is used to operate the even-number gear clutch. A second slave cylinder, located at the back of the gearbox casing, operates the odd-number gear clutch. These slave cylinders are operated and the gears are shifted by the hydraulic power unit. The hydraulic power unit is controlled by a dedicated transmission control module.
The transaxle is designed to optimize the gear shift points, as if driving in a sport mode with a manual gearbox, while allowing for uninterrupted drive torque delivery to the wheels. An added benefit is smooth gear shifting that is usually associated with an automatic transmission. In other words, it has the benefits of manual shifting without the complexity found in an automatic transmission (remember, there is no torque converter).
Shifting from one gear to the next occurs behind the scenes. As you accelerate, engine speed increases before reaching the point where the transmission should be shifted to the next gear. If you have a manual transaxle, the driver would depress the clutch pedal, move the shift lever to the next position, then release the clutch. With the C635 DDCT, the clutch for the current gear stays engaged, but the hydraulic power unit moves the gears to the next position because the clutch for the next gear is not engaged. In reality, the next gear is being preselected before its clutch is engaged.
At the appropriate time, the clutch for the current gear disengages and the clutch for the next gear engages. This is done without interrupting torque delivery to the wheels, because the disengagement of current gear clutch and the engagement of the next gear clutch actually overlap. The shift from an odd-number gear to an even-number gear, or vice-versa, only requires the clutches to actuate (one engages, the other disengages).
A key component in this transaxle is the hydraulic power unit (Figure 2). It supplies hydraulic energy for performing two functions. First, it selects and engages the gears in the transaxle. Second, it operates the slave cylinders that engage and disengage the clutches. The hydraulic power unit has an accumulator that maintains the operating pressure (435 psi). It works in a similar manner to the accumulator in the anti-locking brake system. A slight pump noise might be heard when the front doors of the vehicle are open. This is the hydraulic power unit priming itself before the ignition is turned to the “on” position.
DRIVING WITH THE C635 DDCT
In a typical drive cycle, the C635 DDCT engages each gear in a sequence; a 1-2 shift is followed by a 2-3 shift and so on. There are certain conditions however, that allow the transaxle to skip gears, such as a 6-4 downshift when passing a vehicle on the freeway. This kick-down capability allows a downshift of one, two or three gears, as necessary, to meet the pedal request of the driver.
If the vehicle picks up speed when traveling downhill with a gear engaged and the accelerator released, the clutch closes automatically (when a preset speed is reached) and provides the engine brake function. The amount of engine braking transmitted through the clutch is variable based on the vehicle operating conditions.
Finally, the C635 DDCT has the AutoStick® function that operates in a similar manner to this option in other FCA US vehicles.