An engineer has promised that within a year he will start selling a car that runs on compressed air, producing no emissions at all in town.

The OneCAT will be a five-seater with a fibre-glass body, weighing just 350kg and could cost just over £2,500.

It will be driven by compressed air stored in carbon-fibre tanks built into the chassis.

The tanks can be filled with air from a compressor in just three minutes - much quicker than a battery car.

Alternatively, it can be plugged into the mains for four hours and an on-board compressor will do the job.

For long journeys the compressed air driving the pistons can be boosted by a fuel burner which heats the air so it expands and increases the pressure on the pistons. The burner will use all kinds of liquid fuel.

The designers say on long journeys the car will do the equivalent of 120mpg. In town, running on air, it will be cheaper than that.

"The first buyers will be people who care about the environment," says French inventor Guy Negre.

"It also has to be economical."

Major savings

Mr Negre has been promising for more than a decade to be on the verge of a breakthrough. Independent observers are more convinced this time because he recently secured backing from the giant Indian conglomerate Tata to put the finishing touches to the engine.

The compressed air is stored in carbon-fibre tanks

"This will be a major saving in total emissions," he says.

"Imagine we will be able to save all those components travelling the world and all those transporters."

He wants each local factory to sell its own cars to cut out the middle man and he aims for 1% of global sales - about 680,000 per year.

Terry Spall from the Institution of Mechanical Engineers says: "I really hope he succeeds. It is a really brave experiment in producing a sustainable car."

But he said he was interested to see how the car would fare with safety tests and how much it would appeal to a public conditioned to expect luxury fittings adding to the weight of the vehicle.