Audi has developed a new engine, which can be a V8 for high performance driving and a V4 when cruising, to help reduce fuel consumption and emissions.

It will replace the V10 used in the previous Audi S6 and S8 when new versions of both cars arrive in the UK early next year, and also go into the new S7 Sportback.

Cylinder cut-off - Audi calls it ‘cylinders on demand' - is not a new idea. It was developed by some US automakers after 1970s oil crises caused a spike in petrol prices, was used on the Mercedes S-Class about 15 years ago, is used in some Japanese V6s sold in North America and is again being developed by Mercedes for its new 5.5-litre V8 engine but Audi believes this application is new.

It includes active hydraulic engine mounts to reduce vibration and active noise cancellation. There are also variable camshaft profiles, based on a system in use in other Audis, to isolate four of the cylinders when the car is running under light loads.

The engine takes its readings from the amount of effort the engine is being asked to do and can therefore run as a four cylinder unit over a wide area of the rev band.

"It will be able to run as a four-cylinder at 130 to 140 kilometres an hour (80-90mph) on a flat road with the air conditioning working,' said Axel Eiser, the head of Audi powertrain development. The fuel savings with the new V8 can be close to 20% and it is much less costly that developing a hybrid powertrain. It will also be a big contributor to the weight-saving programme Audi is undertaking with all future models.

There is no reason why the idea could not be carried over to engines with fewer cylinders and Volkswagen is developing the idea for four cylinder units, but the savings would not be as great, he said.

"It depends on the size and weight of the car and its aerodynamics."

Technically, it would also work for diesels, although the exhaust gases would then need more after treatment to meet emissions targets so the advantage would be reduced.

The new V8 develops 415bhp in the S6 saloon, Avant and the A7 Sportback and 514bhp in the S8 but its arrival does not signal the end of 10- and even 12-cylinder engines at Audi.

"We will still have them in future," said Eiser.