Fiat Dual Clutch Automatic Transmission (DCT)
Fiat’s new C635 dual clutch transmission (DDCT, for dual dry 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 also very compact, so it can be used in the subcompact cars Fiat specializes in 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 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.
The 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
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