Volvo is leading the charge into gas-powered vehicles with its Bi-Fuel, reports Roger Bell.

Natural gas is just as good at powering your car as roasting the Sunday joint. Wishful thinking? Far from it. Volvo's Bi-Fuel will run on compressed natural gas (CNG) just as well as it will on unleaded petrol.

Volvo claims the Bi-Fuel - not an after-market conversion but a factory- built model based on the 2.5 S70 saloon and V70 estate - is the cleanest, least polluting car you can buy in Britain. The idea is that you run on gas (methane) in urban areas, where pollution is worst, and switch to petrol elsewhere. Covered by Volvo's normal three-year, 60,000-mile warranty, the automatic-only Bi-Fuel costs pounds 20,825 - the same as the petrol-only S70 2.5 automatic.

Until quite recently, there was little or no cost incentive to use low- pollution gas in Britain. However, successive cuts in excise duty on CNG and LPG (liquid petroleum gas) have seen its price drop substantially below that of petrol/diesel. British Gas says the equivalent cost per litre of CNG is 43p, against 65p for conventional fuels. The Natural Gas Vehicle Association (members include Ford, Rover and Vauxhall, besides Volvo) reckons that Britain's NGV movement is now ripe for expansion.

According to Volvo, the S70's hydrocarbon emissions are down 88 per cent, comparing gas with petrol. Carbon monoxide is down 77 per cent, nitrogen oxide 20 per cent, greenhouse carbon dioxide by the same amount. Urban bus operators are prime targets for conversion, as gas eliminates the high particulate emissions - black smoke - associated with diesels. CNG engines are also quieter and smoother than diesels, as bus users in Southampton, where there's a fleet of 16 Dennis Darts with roof-top gas tanks, will testify.

With two separate fuel tanks, range between fill-ups is extended - by up to 150 miles in the case of the S70. Fears about safety are dispelled by Volvo. The reinforced gas tank will withstand massive impact, it asserts. And if its lighter-than-air content did escape, it would rise and disperse, not "pool" on the ground. Unlike petrol and LPG, CNG does not need transporting in bulk by road. It's piped to the pumps, using existing underground networks.

There are snags - starting with the cost of conversion to gas, which is normally a four-figure sum. For roving drivers, the number of gas refuelling stations - 17 British Gas ones at the last count, though others are planned - is unacceptably small. The infrastructure needed to make CNG or LPG viable nationally is in a classic chicken and egg situation: no demand, no forecourt pumps; no pumps, no customers. It is this that has so far restricted gas users to urban fleet operators with their own supply. Another deterrent is that the large gas tank occupies much of the boot.

To change from one fuel to the other, you simply flick a switch. Should the gas run out, the engine reverts to unleaded. It also starts on petrol, which ignites more readily than CNG. Performance? Even on petrol, the S70's 2,435cc five-cylinder engine is hardly vigorous. On gas - which cuts power by about 10 per cent - acceleration is sluggish. Performance apart, you couldn't tell which fuel was in use. The engine is smooth and quiet on both.