Up To Standards
- Author: Mike Weinberg, Contributing Editor
- Subject Matter: The rise of the DCT
- Issue: Growing market can’t be ignored
First in a series
We have enjoyed two major designs of transmissions for many years. The manual transmission where the driver disconnected the power flow from the engine to the transmission by stepping on the clutch pedal and manually moving the shift lever from the present gear to the next gear to be selected.
The other is the “automatic transmission,” which utilized a torque converter attached to the crank shaft flex plate to send power to the transmission, which had a series of gear ratios available using a planetary gear system and various clutch packs and bands to achieve different gear ratios. Most of the older transmissions of both types were of two-, three-, or four-speed configurations. The ability of the torque converter to slip at idle speeds permitted the vehicle to be stopped in gear without stalling the engine.
When fuel economy, component weight and stricter emissions began to affect vehicle design, transmission design became much more advanced to meet federal and state requirements. The advanced designs were cost effective and a selling point for the vehicles. Manual transmissions are now six or seven speeds with some having two overdrive gears, and the typical automatic has gone as high as nine speeds in cars to meet the new regulations and consumer demands. Torque converters were designed with lockup functions to eliminate the normal slippage in operation. Very complex computer programs were developed to regulate shift patterns matching engine performance with ratios to create better mileage and reduced emissions.
Other designs also came into the market such as constantly variable transmissions (CVTs) that used a drive belt and a system that varied the distance between the drive and driven pulley to change the transmission ratio without a perceptible shift occurring. The problem with CVTs has always been the coefficient of friction for the belt drive, which limited CVTs to lower torque rated vehicles.
While not really new, the dual clutch transmission (DCT) is now making increased inroads into the market. Essentially the DCT is a manual transmission with the clutches that does not need a driver-actuated clutch setup. There are two input shafts, one that is a hollow tube with the other solid shaft inside of it. One clutch drives the outer input shaft, which is connected to first, third, and fifth gears in the transmission, while the inner input shaft drives the second, fourth, and sixth gears in a typical six-speed transmission. This can be reversed in some designs.
The amazing advances in computer technology have made these transmissions the quickest shifting transmissions on the market, with some units capable of shifting gears in 8 milliseconds, which increases fuel economy dramatically. Another benefit is that these DCT units can preselect the next gear as the power is not routed through the input shaft that is with the clutch pack that is not engaged, so that the next shift up or down can occur just by releasing one clutch and applying the other.
In terms of efficiency, DCT units have eliminated the heavier mass of the torque converter from the crankshaft, which greatly improves fuel economy. It also eliminates the shift overlap or underlap of the typical planetary automatic, which further increases economy and emission control, also eliminating the parasitic drag encountered with multiple clutch packs engaging or releasing, and lessening the rpm drop encountered with both manual and automatic transmissions during the shift events.
These units are available in 2WD, AWD, and longitudinal and transverse designs. They can be shifted automatically through the computer, manually without the need to step on the clutch (there is no clutch pedal), or a bit of both with an automatic shift position or shifts controlled by levers or buttons on the steering wheel. There are models developed using two dry clutches, one dry and one wet clutch, or two wet clutches. At this point in time the dry clutch models have only been used in lower torque configurations while the wet clutch models can handle the torque of the Bugatti Veyron with 922 lb.-ft. of torque.
Looking at history, the DCT was actually invented before World War II, although the inventor never created a working model as the war intervened. In 1980, the AP company in Britain developed the first commercially viable DCT, which was used by the Ford Fiesta Mk1, the Ford Ranger and the Peugeot 205. These units used analog circuitry as computer control was still in its infancy. At this point Porsche began to develop the technology for its race cars, and computers had become small enough to begin to control the necessary functions.
This was the beginning of the Porsche Doppelkupplungsgetriebe, or PDK, for those of you not fluent in German, DCT. In 1983 these units were used in the Porsche 956 and 962 cars that dominated endurance racing from 1983 on and also showed up in the Audi Quattro S1. Racing has brought so many improvements to the passenger car market and in 2003 VW introduced the DCT into the 2003 VW Golf MK4 R32, which was the first production car run of the DCT units.
DCT units will be found in the following vehicle manufacturers products: Volkswagen Group, Lamborghini, Kia, BMW, BYD, Daimler AG (which includes Mercedes and Smart), Ferrari, Fiat Chrysler, Ford Motor Co., General Motors, Honda, Hyundai, John Deere, Lotus, McLaren Automotive, Mitsubishi, Mitsubishi Fuso, Nissan, PSA Peugeot Citroen, Porsche, Qoros and Renault. This is a vast and growing market that cannot be ignored by our industry as it appears to be part of our future. A subject this complex cannot be discussed properly in one article, so consider this Chapter One. The next article plans to address the mechanical issues, and number three will attempt to explore the electronics involved in making this all work. Stay tuned.
