WHEN Porsche first introduced the Cayenne it cut new ground for the sports car firm, branching out into the luxury - and lucrative - large SUV market.

But to confound the critics the Cayenne has become Porsche's best selling model clocking up some 280,000 sales, 14,000 of them in Britain.

Now the Cayenne is blazing another trail, this time as Porsche's first hybrid.

The petrol-electric Cayenne Hybrid has near diesel economy with the punch of a V8.

It's a 2.2-ton heavyweight with a 150mph top speed and 0 to 60 acceleration time of 6.5 seconds but can average 34.4mpg with CO2 emissions of 193g/km.

That makes it the leanest and cleanest vehicle Porsche has ever produced while still fulfilling the mandate of plenty of performance.

The hybrid is powered by a supercharged three-litre V6 petrol engine mated to an electric motor with an eight speed tiptronic auto box.

Total power output is a hefty 380bhp - pure Porsche and enough for brisk acceleration if needs be.

But this is an SUV with a difference. Not only can it handle off-road conditions, it can run purely as a zero emission electric vehicle in town or blast along like a sports saloon.

It's clever stuff which will make the hybrid a compelling member of the new Cayenne line up which includes a mean diesel and a very potent V8 turbo.

Priced from £57,609 the Cayenne Hybrid is up against the likes of the Range Rover, Audi Q7 and the Lexus RX 450h - but only the latter is a fellow hybrid and realistically it's a weight below.

As an electric vehicle the Cayenne has a theoretical range of about five miles and a top speed of 37mph. Exceed either and the petrol engine cuts back in, either to reboost the nickel-hydride battery beneath the boot floor or to power it on to cruising speed.

In regular use the electric motor reduces the work the conventional engine needs to do, aiding fuel economy and cutting down on emissions.

However the Cayenne goes a step further than other hybrids and does something Porsche calls sailing.

Throttle off at any speed up to almost 100mph and the petrol engine decouples itself from the transmission and switches off to save even more fuel. It takes stop/start which is also fitted on the Cayenne to a new dimension.

While 'sailing' the Cayenne is really coasting and speed will decay but touch the accelerator or brake pedal and the engine reignites instantly to help braking or give acceleration.

It's an armoury of fuel saving systems and in the real world makes 30mpg-plus easily attainable in very large car.

On a run through the New Forest to the Goodwood motor racing circuit in Sussex, the Cayenne's onboard computer showed 32.1mpg - a little shy of the 34.4 average Porsche claims but creditable nevertheless.

The hybrid set up also means some new instrumentation with extra graphics on the central display screen and powerflow indicators added to the dash.

The electric mode button is clustered along with the switches which vary suspension settings in the centre of a middle console which is now raised between the front seats creating a cockpit feel to the interior.

There's slightly more room inside the new Cayenne than in the original with the back seat now adjustable while leather upholstery is standard on all models.

It looks as dynamic inside as it does out and handling is very sporty for an SUV, although the passengers may feel the ride a little hard whatever the suspension setting.

The hybrid is one of five versions of the new Cayenne with the range starting at £41,404 for a 3.6-litre V6 petrol and a three-litre diesel V6 at £44,178. The diesel actually offers better fuel economy averaging a claimed 38.2mpg but although its CO2 emissions are below 200g/km it can't match the hybrid nor is it as quick.

Both the diesel and hybrid are permanent four-wheel-drive with off-road work now controlled electronically than via a transfer box.

Active four-wheel-drive is fitted to the V8 models with the range topping Cayenne Turbo costing from £81,589 but capable of driving like a supercar.

Around the Goodwood track it proved it was set up to thrill demonstrating crazy acceleration and straight line speed for a car of such bulk.

A 0 to 60 time of 4.7 seconds and top speed of 172mph make it quicker than the original 911 Turbo - and it can handle as well with optional ceramic brakes proving their worth on the circuit.

From hybrid to 500bhp muscle machine the latest Cayenne remains a remarkable piece of Porsche engineering, now spruced up to look as stylish as it can be potent as it can be eco friendly.