Porsche Cayenne hybrid news - Cayenne with a conscience

Terrified it's going to be socially outcast as a carbon-spewing dinosaur, Porsche has been taking measures to protect its reputation as a good corporate citizen by unveiling prototypes of a Cayenne with hybrid drive. You won't actually be able to buy the thing until about 2009, but by unveiling it now, Porsche is showing it won't be left behind in the PR race. I mean the technology race.

The hybridised Cayenne isn't the full-nasty V8 but the 3.6-litre V6 version. In other words, like a Prius it's a hybrid aimed at saving fuel rather than at boosting performance like a Lexus LS600h. However, because there's a torque boost from the electric motor, the overall acceleration is better than the common-or-garden V6. Final numbers aren't in yet, but expect a 0-62 time of about 8.0 seconds.

And fuel economy is about 25 percent better than the V6. Current prototypes are getting about 29mpg in the EC combined cycle. By the time it's on-sale (when it'll have a more efficient battery and low rolling-resistance tyres among other tweaks) the target is 32mpg.

The system uses a donut-shaped 34kW (45bhp) electric motor sandwiched between the engine and the standard six-speed autobox. It won't work with a manual transmission, by the way. Between the electric motor and the V6 is a clutch, operated automatically by the system's enormous brain, to allow the engine to stop while the electric motor carries on.

The battery - twice the size and power of a Prius's, but a similar nickel metal hydride type - is under the boot floor, in the spare wheel well. There's also a transformer/inverter wedged under the bonnet, but Porsche has been careful to make sure it'll still fit on RHD cars as well as the LHD prototypes. Probably that's to please the hybrid-frenzied Japanese, rather than us little Brits.

Porsche boasts the system doesn't reduce the Cayenne's SUV prowess. It'll still wade to 50cm deep, it'll still tow 3.5 tonnes and it'll still trundle through the rough. Not like the Lexus RX400h, which has a sticker saying you're not supposed to go off-road.

Porsche's hybrid principle of operation is actually pretty simple compared with Toyota's. The electric motor turns at the same speed as the V6, except for the considerable periods when the clutch is open and the V6 is stopped. If there's enough charge in the battery the car can go along at 30mph or more on electric power alone, but the V6 cuts in when charge gets low, powering the car and - because the motor becomes a generator when torque is applied to it - charging the battery at the same time. For max acceleration, the motor and engine both push the car in unison.

The clever bit is that the control electronics can start the V6 by engaging the clutch, without a huge thud. To do that it has to engage the clutch while disengaging the torque converter lock-up, while adding fuel and spark to the engine, then dropping the power to the electric motor to compensate for the incoming torque from the V6. It doesn't sound simple, and it's nowhere near as simple as it sounds.

As with all full hybrids, the motor also acts as a generator when you brake, charging the battery. This regenerative braking effect is strong enough in the Cayenne that driving smoothly through town you'll hardly use the disc brakes at all. Clever electronics integrated in the ESP system split the braking effort between discs and regeneration, so all you need do is press the pedal as normal. Regeneration is the 'free' energy that a full hybrid recovers, instead of using the brakes to turn kinetic energy through friction on the discs into wasted heat.

Although it's Porsche that has unveiled the hybrid system, VW and Audi are also putting engineers into the development effort because the same system will also be used in the Touareg and Q7. In fact, it'll go to other cars as well because the components are modular - they can be fitted into different platforms. So we'll also see it on the Porsche Panamera four-door saloon, and I'd guess the Phaeton and A8 too.

We don't know yet what the added cost will be, but it ain't a simple system. Interestingly, it doesn't only add cost but weight - about 150kg. Which is about the same as the extra weight of a diesel SUV over a petrol. For about the same fuel saving...

But Porsche says diesels have nasty black smoke and don't rev like a Porsche should. So Porsche insists on petrol engines. And the obvious way to get an economy jump on a petrol is to make it a hybrid. The other way to cut consumption is to make the car lighter, but in something as congenitally porky as the Cayenne, that might be too much effort.