Filler R Up - this car runs on nothing but - Air.

If you're looking for the car of the future, look no further than Peter Dearman's rusty, 25-year-old Vauxhall Nova.

A beer keg sits in the messy trunk. Pipes run through the middle of the car, which is littered with wrenches and loose bolts. Under the hood, a red, plastic garbage can holds anti-freeze that spills over the sides and a piece of wood holds, well, everything else together.

But look beyond the homemade at what's not there: no gas, no batteries. The Dearman engine -- which sounds like a high pitched golf cart -- is powered only by liquid air. This may be the greenest car on the planet.

"We're starting to run out of fossil fuels now. We've got to do something," said Dearman, a full-time inventor. "Hopefully this offers a solution."

Dearman's car works like a steam engine, except instead of steam, he uses very cold air. Air turns into a liquid at minus 300 degrees. In Dearman's car, the liquid air is held in the beer keg before it flows into the engine. As it warms up and begins to boil, it expands back into a gas, pumping the pistons.

All of that has been done before. But what Dearman has added is incredible efficiency and a vision for a totally sustainable car that can be both manufactured and driven with almost zero effect on the environment.

"It won't produce any emissions because it's only air we're using," the 61-year-old explained on his makeshift test track, a farm outside of London. "We're not burning anything. We're just using heat from the atmosphere and liquid air."

Starting with a traditional steam engine, Dearman introduced heat exchange fluid -- in this case, anti-freeze -- and has essentially turned a traditional engine on its head. Instead of creating energy inside the engine, his engine absorbs energy from the atmosphere to power the car.

"It's been done before in the past -- many times, actually," he said. "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."

So in his engine, he said, "the liquid air boils and creates a pressure that forces the piston down. As the piston goes down, the gas expands and cools -- but it's able to absorb more heat from the heat exchange fluid, making it much more efficient."

By choosing liquid air, Dearman believes he has created one of the most sustainable cars on the planet. His engine is very light, allowing manufacturers to build a car that could be made cheaply, and, perhaps, out of plastic -- no metal required. And by not using any batteries, manufacturers can avoid using any scarce materials.

Dearman also chose liquid air because it's convenient. Whereas battery powered cars can take hours to recharge, a car powered by air can be refilled in the same time as a gas-powered car.

"Everything that's used in here can be recycled very easily," he said. "Very little energy goes into actually producing the car. So the car is very cheap."

Dearman recently invited an ABC News crew to watch him test the car at a barn near his home, northeast of London. On a cold, grey morning, the car belched what appeared to be steam (it was actually the liquid air expanding back into a gas). It did a few circles at about 20 to 30 miles an hour. Its range is no more than 3 miles.

To turn the car off, he disconnects two wires.

He is a jovial but serious driver, a man who has been inventing in his garage for the last four decades. He said he has come up with hundreds of ideas, but only seen about half a dozen to fruition.

"The car and all that was really an accident," he said, pouring the purest form of liquid air -- liquid nitrogen -- into the beer keg. He was "thinking about geology and realizing that resources were finite and realizing that someone had to come up with an alternative to using fossil fuels."

'We're Taking Energy that Otherwise Would Be Wasted'

In Slough, on the outskirts of London, a tall, white cylinder sits in the middle of a power plant that's about as large as a hockey rink. The words "liquid air" are printed on the front, a tangle of white pipes leading off of it. This is Highview Power Storage -- and it's proof that Dearman's invention will reach far beyond one car.

Dearman originally set out to create efficient power storage, and that's exactly what Highview does -- with Dearman's invention. The plant stores energy created by renewable power sources when supply is high and demand is low -- say, when wind blows through wind farms at night, and local houses aren't using electricity.

Often, storing that "wrong-time" energy can be expensive and environmentally unfriendly, requiring pricey battery farms that use scarce materials. But Highview takes the "wrong time" energy and converts it into liquid air.

The plant then expands the liquid air using the same process as occurs in the car, exporting electricity back onto the grid without any effect on the environment.

"We're taking energy that otherwise would be wasted," said Stuart Nelmes, the engineer team leader at Highview. "The ins and outs are just electricity, and zero emissions."

In the United States, more than $100 billion is earmarked for investment in energy storage in the next 10 years. For Highview, the tower provides energy to a few hundred homes. But soon, Nelmes said, it could provide electricity for 3,000 to 5,000 homes.

"As we integrate the renewables into our system, as that percentage gets larger and larger, then the demand for storage devices such as these will ever increase," he told ABC News. "These systems basically enable renewable power to work. And that can only mean cheaper, cleaner fuel in the future."

'Until the Next One'

In his garage -- which looks as if it's never been cleaned in 40 years -- Dearman shows off a newer version of his engine.

Soon, his homemade invention will be put inside a very professional package.The engineering company Ricardo, which helps design engines for, among others, McLaren race cars, is creating a state-of-the-art version this year.

By next year, Dearman hopes there will be a complete car built around his engine. It will be a large improvement on his Vauxhall Nova.

"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, pouring liquid nitrogen into a small engine on his garage floor, recreating the process that occurs in the car.

After that, Ricardo is looking into combining the air powered engine with a bus diesel engine, creating a gas-air hybrid.

"That would make the liquid air side much more efficient and it would reduce the emissions from the bus," Dearman said. "And we'd be harvesting the heat from the engine and using that to give us a higher temperature of the heat exchange fluid -- and that would make this engine even more efficient."

He showed off one of his other recent inventions -- a resuscitator currently used by British paramedics.

Asked why he has spent 40 years inventing technology that he hopes will help save the planet, Dearman offered a shrug that suggested, "Why not?"

"Everyone assumes someone else can do it, don't they? But that somebody else is someone," he said. "You can't rely on other people to do things because, you know, it might never get done."

Asked if this is the invention he is most proud of, he said, "Yeah, at the moment."

And then he paused and smiled.

"Until the next one," he said.