Dual clutches take the lead
Tom Shelley reports on what seems to have become the dominant technology in high efficiency automatic transmissions
Dual clutch automatic gearboxes as efficient as manual gearboxes are to become available for small cars and hybrids in the USA from 2010.
Already available in Europe for certain mid-sized cars, six speed dual transmission gearboxes make a car up to 9% more fuel efficient than one with a torque converter.
The idea has been around for quite a while having been invented by Frenchmen Andolphe Kégresse just before World War II. But it did not find serious use until appearing in a Porsche LeMans car and the Audi Sport Quattro rally car in the 1980s.
The basic idea is to have two parallel gear trains with a separate clutch in each. One of which is transmitting power while the other is ready to. While one gear is engaged, the system has already preselected the next in the other gear train. Once the appropriate shaft speed has been reached, one clutch is opened while the second is closed simultaneously, so there is no interruption in tractive force.
The clutches on the Ford Focus TDCi diesel are wet running, while the new gearbox will have dry clutches. Doing away with the need for an oil pump and coolers improves mechanical efficiency and reduces weight.
Direct Shift Gearboxes such as the S-Tronic started to appear in Audi passenger cars in 2003 through work with Borg Warner. The Bugatti Veyron has a 7-speed dual clutch transmission developed with Ricardo.
Ford is using forms developed by the German company, Getrag, with configurations developed for rear wheel and front wheel drive. Designs for all wheel drive are in preparation. All Ford Focus models equipped with PowerShift offer a fuel economy of 48.6mpg, with average CO2 emissions of 154g/km while the Ford CMax delivers 47.9mpg and 159g/km. The system uses a standard automatic gear lever in the centre console with the usual P, R, N and D settings, which can be moved sideways to a parallel gate should the driver choose the option to change gear manually. The new Ford dry clutch transmissions will be manufactured in a $500million joint venture plant in Mexico.
In April 2008, Getrag signed definitive agreements with Chrysler for the development, production and supply of PowerShift transmissions for its North American market. Plans to invest $455million in a transmission plant there have been cancelled as a result of the financial downturn.
The idea of incorporating a dual clutch gearbox mechanism in a hybrid is also being investigated by Getrag and Bosch. The gearbox could either go between the engine and an alternator/motor. Alternatively a dual clutch mechanism could engage a motor alternator in the system.
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* Cars with dual clutch automatic transmissions are as fuel efficient as cars with manual transmissions.
* Gear ratios are virtually instantaneous with no loss of traction.
* They seem to be becoming the dominant technology.
* Plans are well advanced into incorporating them into hybrid drive trains.
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