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Fiat Dual Clutch Transmission (DCT): C636

inside the Fiat DDCT

dual clutch automaticThe six-speed C636 Dual Dry Clutch Transmission (DDCT) was to be used with Chrysler’s 2.4 liter engine during the 2011 model year, in the Chrysler 200.

Fiat’s C635 dual clutch transmission was unveiled in 2010, with front wheel drive, all wheel drive, and manual versions. It can handle up to 258 lb-ft of torque, and weighs under 180 lb (including computer and oil). It is compact, and can be used in Fiat’s subcompacts as well as compacts like the Dodge Dart.

The six-speed transmission has a ratio spread of 6.27; a seventh gear can be added in the future.

The Fiat-designed, electro-hydraulically activated transmission eliminates the torque converter and pumping losses associated with wet clutch transmissions. It operates like a manual transmission, with two clutch discs driven independently by a common flywheel assembly. Odd numbered gears (1, 3 and 5) are located on one shaft assembly while even gears (2, 4 and 6) are on the other. Two gearboxes running in parallel, each with its own clutch, allows for the selection and engagement of subsequent gears, while the previous gear is still engaged.

Gear changes are gradual rather than abrupt, ensuring a continuous delivery of engine torque and traction. The gear is anticipated and pre-selected. As one clutch is opened, the other is closed, allowing shifting without torque interruption, resulting in quicker acceleration and near seamless shifting.

With the lay-shaft arrangement of gears, there is flexibility to optimize gear selections for peak performance and fuel economy. Ratios have been spaced to help provide smooth transitions, with virtually none of the torque transfer generally associated with gear shifts in traditional automatic transmissions.

The automatic and manual versions have the same cases, synchronizers, shift forks, and triple-shaft layout; both are made in Verrone, Italy. They use a dry clutch (rather than wet) because it’s cheaper, more efficient, also used on manuals, and can be integrated with a stop-start system— that is, a system that lets the engine stop when the car is temporarily stopped, and then instantly restarts it when the driver hits the gas. The main disadvantage of a dry clutch system is the loss of the familiar “creeping” one gets with an automatic, where the driver can use the brake pedal to keep the car moving very slowly.

Synchronizers are single-cone for all gears except fourth and reverse. To reduce width, an intermediate shaft bearing support plate was placed in the housing, so that the differential can be closer in to the engine.

Gears are engaged using an electrohydraulic pump, which pushes automatic transmission fluid to five solenoid valves. Pistons are actuated by a shifter spool to reduce the number of parts; to accommodate stop-start systems, a pressure accumulator stores enough energy for at least three complete shift cycles. To conserve energy, the clutch controlling odd gears (including first) and reverse was set up as normally closed, since that is the most-used clutch; the second clutch, controlling even numbered gears, is normally open. That arrangement should provide even greater efficiency on the highway when a seven speed version is developed.

ddct fiatThe clutches are opened differently; the odd-gear clutch is controlled by a rod, while the even-gear clutch is opened by a slave cylinder within its housing.

The “parking pawl” (used to stop the vehicle when in Park) is in the differential block, though even without it, the transmission would normally have first gear engaged at rest.

The transmission computer includes a fail-safe controller which brings the clutches to a predetermined point if the main processor fails. Both force and speed are used to control gear engagement, with constant monitoring of their positions. The system controls “creep” (movement in drive when the engine is idling) by altering the engine torque; it’s integrated with a hill holding feature and, on stop-start enabled cars, with that system, so that the engine is started automatically when the brake is released.

To customize the transmission’s behavior to each car, there are three sets of launch and shifting strategies for automatics and two for manuals; each requires different engine controls as well. The transmission was designed to “cooperate” with the engine and appears to require “drive by wire” (where instead of the gas pedal directly and mechanically opening the throttle, it sends a signal to the computer which then controls the throttle).

The program engineers were Dr. Constantinos Vafidis, director of transmissions and hybrids at Fiat Powertrain, and Francesco Cimmino, chief engineer for non-manual transmissions. Information in this article comes from a story by Tony Lewin, managing editor of DCTfacts.com

Also see the story of Chrysler’s own DCT/automated manual transmission, which has not seen the light of day.

fiat ddct

 

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