The six-speed dual-shift gearbox ("DSG") in Ford's 2011 Fiesta has a four-shaft "gear pack" from Getrag and a dual-clutch assembly from LuK. However, Ford did more than wrap a housing around these components.
Integrating the DSG, which Ford has branded PowerShift, into a new powertrain that can satisfy domestic Ford buyers was a major human-factors engineering effort because the DSG is different, both in componentry and operation.
The DSG has no oil pump, oil cooler, hydraulic controls, or torque converter. Micro-slipping the clutches provides the torsional damping. All shifting is done with electromechanical actuators. Two actuators switch from one clutch to another to instantly effect gear changes with no loss in torque. Others operate forks for gear selection.
DSG's close relationship to a manual transmission means it delivers a fuel economy benefit of about 8 percent vs. a conventional four-speed automatic, Ford claims. U.S. EPA highway fuel efficiency is estimated at 40 mpg for the Fiesta. The transmission is built in a Ford-Getrag joint-venture plant in Mexico. Piero Aversa serves as Ford's PowerShift engineering manager.
According to John Rich, Powertrain Integration Manager during Fiesta development, Ford's objective is a transmission control strategy that warns while gradually trying to guide the driver from low-speed abusive operation into patterns that maintain reasonable clutch assembly temperatures for durability but without annoyance or causing noticeably poor driveability.
This contrasts with the simpler approach of flashing a warning and if the driver doesn't respond appropriately, causing the transmission to shudder while shifting, and at a critical point, throwing the gearbox into neutral.
It might seem most effective to use a temperature sensor to monitor clutch assembly temperatures, but "sometimes sensors have their downsides," Rich explained.
Ford has four primary instrument-panel message center alert levels plus several possible actions the control module may take. These require not only sensor precision throughout the temperature range but for the rated life of the transmission itself. That overall requirement is better served by a virtual sensor—an algorithm that looks at many inputs, including a measurement of electrical energy to each of the two clutch actuators, as well as road speed, engine rpm, ambient temperature, etc.
In fact, there is no transmission temperature sensor, not even in the fluid, as the DSG gear pack contains no clutches and bands. The algorithm was developed in conjunction with LuK with assistance from Getrag, Rich said. Ford started with a simulator program in which "civilian" Ford employees tested a wide range of control strategies.
The simulator provided feel of road grade, including rollback on hills, and the "drivers" faced an instrument panel similar to that on the Fiesta. They were videotaped, including their foot positions, to see how they reacted to different IP messages, so the human factors engineers could see what worked and what some "drivers" tried to resist.
The warning levels start with "Transmission Hot-Stop or Speed Up" plus a warning chime. The message may repeat with a slightly louder chime. If heat buildup continues, the next message is "Transmission Hot-Stop Safely."
If there's no suitable change in driver technique and temperature of the clutch continues to increase, the computer can induce some shift shudder and then display "Transmission Hot-Stop. Wait One Minute" (or subsequently, the same message with more minutes). If the peak temperature level is breached, that message may be accompanied by the DSG going into neutral.
But multiple warnings and then shudder come first. Neutral only occurs when the temperature peak is exceeded, after the multiple warnings. So the declutching is always within a gradual sequence, never abrupt, Rich said. When clutch assembly temperature drops sufficiently, "Transmission Ready" is displayed.
With a properly functioning transmission, excessive heat buildup won't occur during normal or even aggressive driving, Rich claimed. Various forms of abusive low-speed/high-load operation, however, can heat up the DSG clutch. The most severe buildup occurs if the driver uses the gas pedal to hold the car on a relatively steep upgrade, or even creep slightly uphill at perhaps 2-4 mph, Rich said.
The virtual sensor can detect a "hold" and change clutch application torque slightly, so the car will start to roll back (although a warning message precedes that).
The objective is to induce the driver to apply the brakes, which disengages the clutch (producing neutral idle), cooling the clutch.
If the driver attempts to launch repeatedly in a 2-4 mph crawl-along and the clutch assembly is hot, the computer will inhibit launch to allow some cooling. Ford tested its algorithm in Mexico City, Rich said, where drivers tend to keep close to the car in front, and the clutch assembly didn't overheat. The PowerShift does have the creep function that drivers expect, however, along with a hill-hold, in which the brake pressure holds briefly as the driver switches to the gas pedal.
One algorithm that took particularly careful tuning can back off the throttle when the driver has his foot on the brake pedal. A common trait is left-foot braking, with the foot continuously resting on the brake pedal while the right foot is pressing on the gas pedal—a situation for which the clutch assembly temperature algorithm was developed.
However, the control module has to instantly differentiate between that and a motorist trying to stop the car with a stuck throttle. On the Fiesta, a brake pressure transducer in the antilock brake module is monitored, so the computer can distinguish a modest change perhaps because of driver fidgeting from an emergency move.
AEI tested this algorithm during the new Fiesta's U.S. media preview, and we could maintain the throttle with our foot resting even moderately on the brake pedal. But from that point, even a modest but positive press on the pedal brought the engine back to idle.