Audi unveiled a hybrid version of its quattro transmission at the Frankfurt motor show in September — and it could be used in future Bentleys.Audi engineers have fitted an electric motor inside the eight-speed tiptronic transmission housing, saving weight and bulk. In many current hybrid designs, the motor is sandwiched between the engine and the transmission.The quattro hybrid can operate as a purely electric vehicle driving all four wheels, or the electric motor can kick in to assist the petrol engine, providing extra performance. Audi says it will run at 30mph for almost two miles purely on battery power. This is important, because many car makers fear that European cities will adopt emissions-free zones within the next few years.When the car is travelling using the petrol engine, the motor can act as a generator, charging the boot-mounted battery pack. The whole package adds just 100kg.The set-up on display hooked up a 2.0-litre turbo engine with 210bhp and 221lb ft to the hybrid transmission. Audi says the electric motor adds 44bhp and another 170lb ft of torque, offering potentially huge performance as well as up to 30 per cent better economy.While this layout will almost certainly find its way into the next-generation Flying Spur and Continental, Bentley could choose to adapt the hybrid system being developed for the Porsche Cayenne and Audi Q7. That system is basically a rear-drive format, with a power take-off and propshaft to drive the front wheels.A simplified rear-drive version of this system could underpin the next-generation Arnage. Adoption of a hybrid transmission could make it much easier for the firm’s classic 6.75-litre V8 to meet Euro 6 emissions regulations, as well as offering very rapid acceleration.