AS surely as night follows day, hydrogen is going to fuel the future of motoring.

Rechargeable, battery-powered electric vehicles are seen by car industry giants as more of a stop-gap measure, as, indeed, are their very own hybrid motors.

Also something of an in-betweenie in the bigger global picture is the car that runs on bio, rather than fossil, fuel, so it's down to hydrogen, the most commonly-occurring element in the universe, to drive the way ahead over the next couple of decades.

And hydrogen is available in near-infinite supply when renewable energies such as solar, wind and wave power are used to produce the gas in liquid form.

Manufacturers will want to recoup some of their investment into the research and production of hydrogen vehicles fairly quickly, so the first examples will be targeted at the buyers who can afford them - at the luxury end of the market.

Even smaller models will attract a price premium, in the same way that petrol-electric hybrid cars do today, until sales begin to take off. BMW already has a large luxury hydrogen-petrol powered saloon on British roads, the Hydrogen 7, and now Honda says it will launch the FCX as a full production vehicle in both America and Japan in 2008.

But the two cars are very different, and it is worth taking a look at each of the firms' current - note that word - individual approaches to what will ultimately become motoring's emissions-free future.

The BMW Hydrogen 7, based on the existing 7 Series, has an internal combustion engine that can run on hydrogen or petrol.

In hydrogen mode the car emits nothing more than water vapour, yet the 260bhp 12-cylinder engine can whizz from zero to 62mph in 9.5 seconds, and has a limited top speed of 143mph.

The Hydrogen 7 driver can switch from hydrogen to conventional petrol power at the press of a wheel-mounted button.

Running on petrol power, though, sees the big BMW emit up to 327g/km of C02.

The car can cruise for 125 miles on hydrogen, and for a further 300 miles in petrol mode.

The BMW's petrol tank holds 74 litres. and the sister tank holds up to 8kgs of liquid hydrogen, kept at a pressure of 3 to 5 bar, and a consistent temperature of -250C.

In the case of the FCX, Honda's V-Flow FC Stack combines hydrogen fuel with oxygen to create electricity via a chemical reaction. This, in turn, powers an electric motor, that delivers drive to the car's wheels. Which makes it a true zero emissions vehicle.

Honda admits that, as the FCX relies on a chemical reaction, and not moving parts, the technology is relatively new and needs further testing.

Mileage range is 354 miles on a full tank of hydrogen, and with only half of the amount of hydrogen that can be carried by the BMW Hydrogen 7, the FCX can cover 250 miles.

The Honda's design strength lies in the fact that the FC Stack can be can be housed anywhere in the vehicle, allowing for a better centre of gravity and improved weight distribution. That facet could eventually change entirely current thinking on car design.

But the BMW Hydrogen 7 can happily run on petrol until a hydrogen outlet can be sourced, and fuel stations are aplenty. The Honda, on the other hand, is limited to areas where hydrogen is available - a whole new network of refuelling points will be needed to support its future.