This is a cutaway drawing of Porsche's PDK transmission showing the dual-clutch mechanism on the left side of the unit. Two shafts that run inside each other connect clutches to the gears, allowing for instant sequential shifting.
UNDATED - 2009 Porsche 911 Carrera equipped with optional PDK, or Porsche double clutch, automatic transmission.
Transmissions have a simple job to do -- transfer the power of the motor out to the tires.
Many different devices have been used since the invention of the automobile to do this, including chains, belts and fluid drives. Current automobile transmissions can be grouped into three basic types: manual transmissions, automatic planetary transmissions and CVT (continuously variable transmissions).
But, in the past few years, another type of transmission has slowly been making its entry into the marketplace. This is the automatic manual transmission, and Porsche's PDK unit has to be the best example of this system yet.
PDK, from the German Porsche Doppelkupplungsgetriebe, makes more sense when translated into the English Porsche double clutch. This is a manual, seven-speed transmission with two clutches, no clutch pedal and a computer that shifts everything automatically. This unit can be driven like a conventional automatic transmission by placing the shift lever in Drive, but from there on, it does everything better.
The PDK transmission has two multi-plate clutches, with the larger 202-mm-diameter clutch outside of the smaller 153-mm-diameter unit. Each clutch drives an input shaft, with the larger clutch driving a solid input shaft that engages 1,3, 5, 7 and reverse gear. The smaller clutch drives a hollow input shaft that fits over the solid input shaft so that both shafts are on the same axis. This shaft drives 2, 4 and 6 gear. It sounds complex and the programming for the unit is, but for drivers, it is pure simplicity.
When you move the shifter to Drive, the computer moves the transmission gears to engage first gear but the clutch stays released. When you step on the gas pedal, the computer smoothly applies the larger clutch to make the car move. The more quickly you step on the gas pedal, the more quickly the clutch engages.
As the vehicle pulls away, second gear is already engaged by the other input shaft but that clutch isn't engaged. When it is time to shift, the computer simply releases the large clutch and applies the smaller clutch so second gear is engaged. While you are driving in second, the computer analyzes your driving to determine whether you want an upshift to third or a downshift to first gear and pre-selects that gear in the transmission, ready for the shift.
During most driving, the computer will alternate between the two clutches, shifting sequentially from first through seventh gear smoothly and efficiently. I say efficiently, because this type of transmission offers mechanical efficiency similar to a manual transmission with the convenience of an automatic transmission and superior operation when performance is demanded.
The PDK can shift between gears in as short a time as .42 seconds. This is 60 per cent faster than a conventional automatic and much faster than even a skilled driver using a manual gearbox. Using the paddle-style buttons integrated into the steering wheel, the driver has complete control of the shifting or can rely on the computer to conveniently shift through the gears automatically.
Advantages of the PDK transmission include less weight, improved efficiency because of gear design and less drag due to lower required oil levels, and the improvement in fuel economy due to both the .62:1 gear ratio for seventh gear and the programming to optimize early upshifts in automatic mode.
If you think this technology is really new, very few things are new in the automotive industry -- just improved dramatically. Porsche first introduced the PDK in 1983 for the Porsche 956 race car and had their first win with the Porsche 962 in 1984 at the famous Nurburgring. The transmission contributed to their World Championship at Monza in 1986 and now it can be found in many Porsche models from the Boxster to the 911 Carrera 4S.
I really like to drive a manual transmission, but Porsche's PDK is the first transmission that gives me the same control, with better performance, and yet still have all the conveniences of an automatic when I want it.
To diehard performance aficionados, it may seem sacrilege, but I would pick the PDK over a conventional manual gearbox any day.
Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists' Association of Canada.
kerr.jim@sasktel.net
