Audi is first with stop-start in volume market automatics
Dual clutch transmission has allowed Audi to steal a march over its competitors by
offering CO2-saving stop-start on its compact models fitted with automatic transmission.
While engine stop-start systems have been widely adopted by European automakers in search of lower CO2 emissions, their application has been generally confined to cars with manual transmission. But, by common consent, carmakers have been slow to provide the same benefits to drivers of automatic vehicles: to date, just one model at the top end of the luxury segment – the Porsche Panamera four door sports sedan at over $90,000 - has succeeded in combining stop-start with an automated transmission.
The Porsche uses a dual clutch transmission from ZF, which is able to get around the problem faced by the familiar planetary automatics that totally dominate the market for luxury vehicles. These conventional transmissions cannot shut down at idle as they need to have the engine running in order to drive the fluid pumps that provide hydraulic pressure for gear selection and other functions.
ZF's newly-developed eight speed planetary automatic is the first to make provision for stop-start operation, which can cut fuel consumption and 2 emissions by 5 to 7 percent in the European driving cycle. The ZF unit incorporates a pressure accumulator which stores hydraulic pressure ready for use when the vehicle restarts and the transmission needs to re-engage gear for launch.
BMW's new V12-engined 760i luxury sedan is the first production application for the new ZF eight-speed automatic; however, says BMW, the stop-start system will not be added until 2010. BMW's 5 Series GT and the Rolls-Royce Ghost will have the same transmission, but neither company has yet stated whether stop-start will be included in the specification.
Mercedes-Benz R&D director Thomas Weber told DCTfacts.com that Mercedes was updating its seven-speed automatic to improve economy and add stop-start functionality. He did not say when these revised transmissions would reach the market.
The difficulties faced by automakers tied to planetary automatics makes Audi's policy of promoting a wide variety of transmissions, including dual clutch, seem even more opportune. Indeed, it is claiming a world first in its application of stop-start technology to versions of the compact A3 fitted with its seven-speed S-tronic dual clutch transmission.
Stop-start is being initially applied to the two most economy-oriented models in the A3 lineup – the 1.4 liter turbo gasoline TFSI and the 1.6 liter diesel TDI. The gasoline model shows clearly how new technologies are pulling down CO2 emissions: the standard six-speed manual emits 139 grams per kilometer, which falls to 133 with the seven-speed DCT fitted. A further 9g is trimmed by the stop-start system.
Similarly equipped, the 1.6 TDI version, with 105hp, puts out just 109 g/km CO2, making it one of the lowest-emitting premium automatic models. Extension of this stop-start philosophy to other Audi model lines will however not be immediate: the DQ200 seven-speed DCT is configured for transverse engine applications, while Audi's larger ranges have inline engines. Instead, the beneficiaries are likely to be the smaller Volkswagen, SEAT and Skoda models, all of which have transverse engine placement. Competing in the volume segments, these brands will be able to leverage the lower CO2 emissions of their stop-start equipped automatics as an important marketing tool.
Story Filed: 11/24/2009
By Tony Lewin, managing editor DCTfacts.com