Now, car that runs only on cold air!

A UK-based inventor claims to have developed the ultimate green vehicle - a car that runs only on cold air - and it can reach speeds of up to 48 km per hour.

Peter Dearman has modified his run-down jalopy - a 25-year-old Vauxhall Nova - to run on nothing but air.

The vehicle by 61-year-old from Bishop's Stortford, Hertfordshire can drive for about five km and reach speeds of up to 48 km per hour, New York Daily News reported.

"It won't produce any emissions because it's only air we're using. We're not burning anything. We're just using heat from the atmosphere and liquid air," the man told ABC News.

Dearman's propulsion system is inspired by a steam engine, except he uses very cold liquid air. He said he chose liquid air as a fuel source because it's light and cheap.

At around -190 degree Celsius, air turns into a liquid that can be stored in insulated, vacuum-sealed containers. Dearman uses a beer keg as a makeshift container for the liquid air.

When the liquid air courses through the engine, it heats up from its cryogenic temperature and boils via a heat exchange fluid (in this case, anti-freeze).

The liquid air expands as it changes back to gas form. In a confined space, this phase change creates enough air pressure to power a piston.

"The secret to [my engine] is that once you warm the liquid air, you have to be able to keep it warm as it expands. If you let it cool, it shrinks and you lose all the efficiency," said Dearman.

Dearman's invention has already caught the eye of engineering company Ricardo, who help design state-of-the-art engines for professional race cars.

"I've done sort of the basic work, and they're going to refine it and bring it onto the next stage for us," he said.