By Hugh Pym BBC News business correspondent

VW's Polo Bluemotion can travel 700 miles on a tank of diesel Eco-car's designer

It wasn't so long ago that you rarely heard the words green, low emission and cars in the same sentence.

At this year's Geneva Motor Show, which begins this week, they will be speaking of little else.

Car manufacturers are under pressure from the European Commission to deliver more environmentally-friendly models.

They also know that the way to win over new customers is via their wallets.

Cheap and green

So the answer is to come up with a car that is cheap to run and is also clean and green.

Volkswagen's response to this agenda - albeit one which VW says has been in development for several years - is the Polo Bluemotion.

The firm has backed diesel rather than the hybrid petrol/electric technology favoured by rivals.

The Bluemotion, it says, will deliver more than 70 miles to the gallon - a figure which should deliver more than 700 miles from its 45-litre diesel tank.

From Germany to London

But it has to be said when the BBC drove the car, the range was rather less than that.

We took the car from VW's headquarters at Wolfsburg in Northern Germany to BBC Television Centre in West London - a distance of 566 miles - on a single tank of diesel.

The in-car fuel management system had predicted that we would run out at around 630 miles.

VW pointed out that we had been driving constantly on motorways at 70 mph or so, not the most fuel-efficient form of driving.

Big investment

At �12,000 or more, the Bluemotion, due to be launched in the UK later this year, won't come cheap.

But Dirk Pesditschek, one of the design team leaders, believes the low running costs will be the priority for most drivers.

"If the mileage is high enough, you can for sure reduce the total cost of the car with the lowest running costs for this model," he says.

The company says the fuel economy starts with a high-tech turbodiesel engine.

Aerodynamics tested at VW's wind tunnel in Wolfsburg boost the car's efficiency, as do low-resistance tyres.

Hybrid rivals

VW's central claim for its new baby is an even greener performance than the hybrids.

The Bluemotion is said to emit 102 grams of carbon dioxide per kilometre.

This is below the Toyota Prius at 104g/km and the Honda Civic hybrid at 109.

VW's competitors, however, counter with the argument that diesels produce other damaging pollutants, known as particulates.

Honda is putting more eggs into the hybrid basket by developing a lower-cost model.

"By 2009, we will bring out a purpose-built small hybrid - and a smaller hybrid equals an even cheaper hybrid, making the technology more accessible to a wider market," says Honda's John Kingston.

Green debate

And so the green debate will continue.

Hybrids have hogged headlines, particularly the Toyota Prius.

But diesel-powered Volkswagen is coming up in the outside lane.