It could be thought that there is a fight going on among the engineers at Porsche, but the thinking ones who run the hugely successful Stuttgart-based company insist that all is sweetness and light within its walls.

There are sound reasons for these concerns. Until now, those engineers would have created evocative noises to accompany lashings of performance from first-throttle squeeze. But now there is a whispering giant in prospect — the Cayenne hybrid is being developed. With an electric motor working in parallel with its V6 petrol engine, it is a combination designed to bridge the needs of traditional Porsche performance and low emissions.

It can move away from a resting position and trickle through heavy traffic in near silence, propelled by only a 46bhp electric motor. At high speed or when maximum acceleration is needed, the petrol engine wakes up and the “right” Porsche noises return.

The aim is to give it a combined fuel consumption of at least 32mpg and subsequently low emissions. The present Cayenne Turbo puts out 358g/km of CO2.

A brief drive in a hybrid Cayenne development vehicle showed that, apart from eerie silences – up to 75mph on the level – it seemed to be smooth, efficient, quick and sensible, making this big SUV a far more socially acceptable beast. It could be in production by 2009, followed by the still-secret four-door, four-seat Porsche Panamera Gran Turismo in 2010 using similar technology. Porsche is working on lithium-ion battery technology to gain even greater efficiencies.

Before ardent Porsche fanatics shudder at the thought of where all this may lead (a hybrid 911?), it is worth noting that, as a young engineer, Dr Ferdinand Porsche, the founder of the company, developed a wheel-hub electric motor in 1899. So there.

Just down the road from Porsche, Mercedes, which is launching one of its noisiest road cars, the 206mph SLR McLaren Roadster producing CO2 at the rate of about 350g/km, is also busy refining hybrids for production — petrol and diesel — using two electric motors. In future, it will develop only vehicles and engines that can be enhanced with hybrid technology.

Like VW, it has also announced a half-diesel/half-petrol engine: the unfortunately named DiesOtto (DieselOtto cycle). It could bring great benefits, combining the best aspects of the diesel (high torque, low fuel consumption and CO2 emissions) with the best of a petrol unit (high power, low production costs and low overall pollution levels) and, allied to electric motors, create the ultimate hybrid.

“Turning this into reality is not quite so easy, although we are well ahead with our development work,” Professor Herbert Kohler, whose titles include that of chief environmental officer of DaimlerChrysler, said. “To deliver six-cylinder performance, this revolutionary engine requires only four cylinders and a displacement of only 1.8 litres.”

As a hybrid, it could give an SClass size car better than 50mpg economy. Mercedes will unveil the technology in a concept car at the Frankfurt Motor Show next month.

Mercedes petrol and diesel hybrids are expected by 2009. With very low fuel consumption and CO2 emissions, a diesel hybrid is particularly significant, although a diesel engine can be 50 per cent more expensive to build than a petrol, so it is essential to reduce costs.

Peugeot will also have a diesel hybrid concept in Frankfurt likely to be based on the new 308 hatchback. A super-economical, low-emissions production car is targeted for 2009.

All this clever stuff is costly, but there is a piece of effective high technology that is free: our brains. Dr Dieter Zetsche, chairman of DaimlerChrysler and head of its Mercedes Car Group, believes that driver education and awareness is crucial to cutting fuel consumption and emissions. The company trains truck drivers in achieving economy and low emissions, without lowering average journey times.

Car drivers could do the same, particularly if incentives were offered to encourage them to take part in a short training programme “with the possibility of an ecological travel licence”, Zetsche said.

So perhaps the most cost-effective, environment-enhancing hybrid of all may be the combination of brain and right foot. What will they think of next?

Tips for frugal driving

Change into a higher gear as early as possible

Avoid high speeds

Maintain an even speed

Do not brake unnecessarily

Allow the vehicle to coast (but not in neutral)

Do not leave the engine running unnecessarily

Start the engine without the accelerator and drive off as soon as possible

Check correct tyre pressures