Fiat Reveals Its Dual Clutch Transmission
The automobile industry has known for quite some time that Fiat Powertrain (FPT) was working on a dual clutch transmission for volume applications. It was believed that the transmission would use dry clutches, but its exact specification and configuration remained uncertain despite the revelation that BorgWarner was to contribute control system expertise to the project.
With evident pride and a real sense of excitement, Fiat Powertrain engineers have at last been able to unveil the fruits of their labors, choosing the fitting environment of December's CTI Symposium on Advanced Transmissions in Berlin for the occasion.
The compact, high torque density dual dry clutch transmission (HTD-DDCT) that was
unveiled at FPT's booth surprised and impressed delegates on several counts.
Part of the all-new C635 transmission family, also incorporating manual and four
wheel drive versions, the DDCT will be part of a series built in very large
numbers - up to 800,000 a year within a few years. Industrially, this is an
important development.
Elements of the DDCT’s specification are also impressive: the fact that it can handle torque inputs of up to 350 Nm, making it the highest-torque dry clutch application so far; its weight of just 81 kg, complete with oil and transmission control unit; its compact three-shaft architecture, especially axially, allowing it to be installed in a wide range of B- and C-segment vehicles.
The team, including program engineers Dr-Ing Constantinos Vafidis, director of transmissions and hybrids at FPT research and technology, and Grad-Eng Francesco Cimmino, chief engineer for non-manual transmissions, decided on a common layout for the new transmission family. This ensured that it could be compact enough to be packaged within a small B-segment vehicle, such as the Punto, yet also have the torque capacity to serve in larger and more powerful models, especially those with high-torque turbocharged diesel engines. This pointed to a high torque density layout, said Vafidis, a factor especially important in the DDCT version likely to be used in premium and sports derivatives.
"Volume is important for our company," explained Vafidis. "So, four years ago, when we decided to update our medium range manual transmissions, we took the opportunity to develop the manual and the dual clutch together."
This, in turn, led to significant commonalities, including the three-shaft layout, the casings, the synchronizers, and even the shift forks. All of the transmissions are manufactured on a single production line in Verrone, northern Italy.
Options
When weighing the various options for the new generation of transmissions, the
FPT team noted the trend toward vehicles becoming heavier, but with better
aerodynamics. Wheel diameters were also increasing, as were all wheel drive
(AWD) applications. On the engine side, the engineers observed a trend toward
power units with smaller displacements but greater horsepower, suggesting a
narrower operating speed range.
Transmissions were seen as a key element in Fiat’s drive to reduce fuel consumption and CO2 emissions. While manual transmissions offer the greatest mechanical efficiency, they cannot easily be controlled or integrated with the powertrain for peak overall efficiency. AMTs were rejected due to poor public acceptance, while for FPT's purposes stepped automatics were felt to be too high in cost and weight, with low efficiency and a specific architecture that did not fit well with the group’s plans.
Dual clutch transmissions, on the other hand, were seen as offering comfort, high efficiency, easy integration with the powertrain, an intermediate level of cost and the advantage of using a basically manual transmission architecture, thus enabling a streamlined manufacturing process. "DCT is probably the ultimate solution for the future," proclaimed Vafidis.
Having opted for DCT and manual transmissions on a common architecture, the team’s next big decision was whether to go for a dry or wet clutch system. The choice eventually was a dual dry clutch system for its greater efficiency, lower cost, stop-start capability and greater commonality with manual transmissions. Downsides were a reduced capacity for automatic-style creep and, of course, the space needed for the larger-diameter dry clutch assemblies.
"When we decided on the DDCT the main challenge was to package the necessary torque capacity in the space available, especially on a B-segment car," explained Vafidis. "This meant we needed to maximize the torque density."
The present transmission is a six-speed with a ratio spread of 6.27, though a seventh ratio is possible. The lower three gears have single-cone synchronizers, fourth and reverse have two, and fifth and sixth rely on single cones. The shaft center distance is 197 mm, though versions will be available from 181 mm on up.
The top priority in the gearbox’s internal architecture was to keep axial length to an absolute minimum to allow packaging in B-segment cars. To accomplish this, the upper secondary shaft is kept short, thus allowing the casing to be tucked in tighter and still leaving enough space on narrower cars to accommodate the longitudinal crash beam structures. An intermediate shaft bearing support plate within the two-piece housing allows the differential group to be positioned closer in to the engine, once again saving precious width.
Gear actuation is by an electrohydraulic pump supplying ATF, rather than transmission oil, to five solenoid valves. A shifter spool actuates the pistons, something which Vafidis says helps maximize safety and minimize the number of expensive components. The system gives stop-start capability and is low on energy consumption, running on a duty cycle of less than 25 percent. A pressure accumulator stores sufficient energy for three to four complete shift cycles, say the engineers.
Clutch Priorities
FPT engineers regard the dual dry
clutch unit as something of an engineering benchmark in terms of its packaging,
and consider this to be the key advance in achieving the overall compactness of
the transmission. The two clutches, by LuK, are concentric and equal in
diameter. The observation that over 50 percent of driving is in the odd-numbered
gears, especially under urban conditions, prompted FPT to choose a
normally-closed status for K1, the clutch controlling launches, the odd gears
and reverse. A normally-open clutch was rejected on energy grounds. "The
operation of the hydraulic power unit to maintain the odd-clutch closed was
considered [energy] inefficient," said Vafidis and Cimmino in a paper
accompanying their presentation.
In addition, say the FPT engineers, the arrangement of a normally-closed K1 and a normally-open K2 allows a more compact layout and can be exploited for greater efficiency in motorway driving on eventual seven-speed versions of the transmission.
Vafidis is particularly proud of the means of actuation of the K1 launch clutch: via a pull-rod running through the shaft. While the even-gear clutch K2 is operated by a concentric slave cylinder within the clutch housing, there was no room for a second cylinder. As the authors explain, "Despite the optimized axial dimension of the dry dual clutch unit, the eventual addition of a second actuator mechanism within the clutch housing would prohibit installation of the transmission in the lower segment vehicles."
The inclusion of a normally-closed launch clutch, which also operates on reverse gear, would in theory eliminate the need for a parking brake. However, regulations still insist on a separate mechanical brake system, and one is integrated in the differential block.
Extensive analysis was made of the thermal behavior and heat dissipation capacity of the dual dry clutch assembly under the most critical driving conditions and applications. The thermal model resulting from this study has been integrated into the control algorithms of the transmission control unit, and forms the basis of the vehicle’s torque transmission and thermal protection strategies.
Transmission Control Strategies
Running on the same microcontroller family as those used to control Fiat’s new MultiJet and MultiAir engine applications, the transmission control unit has a second supervisory control system charged with ensuring the mechanisms default to a safe condition if the main microcontroller fails. The odd-gears clutch, K1, is position-controlled, while K2 is controlled in force by a pressure feedback signal from CAM sensors. Gear engagement actuators and selectors are controlled by both force and speed, with continuous position monitoring as a guarantee of the required safety level.
One of the new functions enabled is creep on brake release. This is a critical situation for any dry clutch system. In the case of the C635, control is provided by a programmed modulation of engine-generated torque levels. The creep function is integrated with the hill-hold function and the stop-start mechanism. FPT claims this offers a user-friendly interface, restarting the engine when the brake is released.
In terms of launch and gearshift strategies, much can be tailored around the requirements of the vehicle and the brand where the C635 DDCT is installed. Three automatic shift patterns are possible, along with two manual modes. Each, says Cimmino, is accompanied by specific control strategies and calibrations on the engine side.
FPT believes that the most important step forward in performance was obtained through the implementation of so-called "PB" (power-based) powertrain driveability strategies. Cimmino explains, "The cooperation between vehicle systems is a well-known concept for achieving superior vehicle performance. With the PB concept, while launching or gear shifting, the desired engine torque is defined by the TCU and implemented by the engine control unit according to the concept that, in pressing the accelerator pedal, the driver requests a certain power at the wheels. It is up to the TCU to define the optimal engine torque/speed mix in order to achieve that."
Vehicle Implementation
Production models fitted with the new DDCT have yet to appear, but the system is expected to be in place soon on top versions of the Fiat brand, as well as its premium Alfa Romeo and Lancia nameplates. Likely early ambassadors for the system could be the Alfa MiTo and the new Giulietta; these models occupy market slots at the top ends of the B and C-segments, respectively. Fiat Powertrain is an independent company looking to maximize its sales to third party organizations, so the DCCT system is likely to go not just to Chrysler-Jeep, which Fiat now controls, but to outside customers, as well.
Vafidis and Cimmino say they have successfully implemented the PB strategies in a first C635 DDCT application with the innovative MultiAir 1,4 liter gasoline turbo engine. They state that, "The application is now in its final development phase, and the results achieved so far in terms of fun-to-drive in Dynamic mode and fuel economy in Normal mode are very promising."
Story Filed: 2/2/2010
By Tony Lewin, managing editor DCTfacts.com