Audi is on a roll. Its turbocharged engines have allowed greater horsepower and torque to come from smaller, lighter powerplants, enhancing both fuel economy and driving pleasure at the same time. Now it's working on transmissions. The fwd A4 with a Continuously Variable Transmission was introduced last year. If offers all the convenience of an automatic and the control over torque at the wheels that comes with a manual transmission. To an engineer, direct drive means he can meet fuel economy objectives without sacrificing luxury and performance items in the rest of the car.
Audi has called on Volkswagen to bring similar advantages to the TT. In addition to developing the 3.2L version of the VR6 engine as found in the R32, Volkswagen has eliminated the clutch pedal from its transverse 02M transmission, using electronic control to achieve even more seamless operation than the best automatic with all the advantages of a manual transmission.
Unlike BMW's SMG, which is electro-hydraulically actuated but internally little changed from its manual counterpart, Audi's Direct Shift Gearbox was significantly revised. The key is the basic architecture of the 02M, which has two pinion shafts instead of one. In the original configuration, adjacent gears use the same selector; moving a shift fork from one end of its travel to the other changes from, for example, first to second gear. DSG uses two input shafts, one inside the other, and rearranges the gears so ratios adjacent in sequence are on different input shafts. If manually shifted, the DSG box's pattern would look like this:
Each input shaft has its own clutch, allowing one gear on each input shaft to be engaged simultaneously. Let's follow this with the diagram shown above. It shows second gear engaged, which is on the outer input shaft, colored green for clarity. As acceleration continues and the electro-hydraulic ("mechatronics") control module detects an approaching need to shift into third gear, it moves the shift selector to engage third, on the inner input shaft (colored red), while leaving the clutch disengaged. At the moment of the shift, the clutch for the outer input shaft is gradually and smoothly released as the clutch for the inner input shaft is simultaneously engaged and the throttle is manipulated to bring the engine to the correct speed. Power to the wheels is never interrupted.
A similar sequence occurs for each shift up or down through the ratios. Unlike a mechanical sequential shifter, however, DSG has the ability to skip ratios. If the car is cruising on the highway under light throttle in sixth gear, and the gas pedal is pressed to the floor, best acceleration may result from dropping to second gear. With sixth and second both driven by the outer input shaft, that would seem to be a problem. DSG works around this by momentarily shifting into fifth gear and then shifting into second, following the usual sequence. Power is uninterrupted, and the entire process is completed in just 0.9 sec.
There are countless details over which to geek out. The DSG is built in the same factory at Kassel as the manual five- and six-speed boxes that have been in all quattro TTs. The shorter shafts of the dual pinion shaft design are inherently strong and allow more ratios to be stacked into a given length, especially important for a transverse, fwd installation. Many parts, such as the synchros and shift sleeves, are shared in common between DSG and the manual, and the basic architecture is identical.
DSG uses a dual-mass flywheel, but power is transmitted to the clutch housing assembly through splines. The drive shaft for the oil pump runs through the center of both input shafts, and the oil pump lives on the end of the transmission case, where fifth gear was found on older transmissions. The clutch assemblies are splined to their respective input shafts. One clutch is smaller than the other, nested in the center. The larger clutch is used for the inner input shaft to better withstand moving away from a stop in first gear. The clutch housing has oil passages to cool and lubricate the clutches, as well as actuate them.
The oil pump has a filtered pickup inside the case. Oil flows through the pump to a valve body, from there to a cooler mounted on the transmission case, through an external, replaceable filter, and to a spray bar that lubricates the gears. Intermediate pressure regulator stages divert oil as needed to the control systems, and a valve can divert the majority of the flow through the clutch if needed for cooling.
The transmission control module (TCM) lives on the forward side of the gearcase. It is a sandwich with solenoids, valve body and the control unit, which includes speed sensors, temperature sensors and position sensors.
Shaft speed sensors pick up magnet wheels on each input shaft and the output shaft. There are multiple stages of pressure regulators, control valves and a multiplexing circuit that allows four valves to control eight sets of hydraulic pistons for the shift forks.
The gear tooth design on both DSG and the manual version of this transmission has been changed for 2003, but their ratios are closer to each other than to the 2002 manual. Progression of the gear spacing is more even in both the new transmissions, eliminating a jump between third and fourth. Volkswagen is saying at this time that the clutches should last the life of the vehicle.
Off-the-shelf performance modifications will be limited. Official literature states torque capacity is the same as for the manual transmission, 242 lb-ft. The differential can't be replaced with a traction-aiding unit already developed for the manual transmission, because DSG uses a parking pawl integral to the differential housing. Racers seeking to mix and match gear ratios will best stick with the manual versions of the transaxle. Audi's programming is conservative, but hacking the TCM for competition use could be productive.
DSG is, like all the cool technologies, surprisingly simple and elegant. Thanks to Audi's cleverness, being shiftless no longer requires one to be a lazy bum as well.