The captured gases can be processed to create a biofuel using genetically modified algae.

Dubbed "Greenbox", the technology developed by organic chemist Derek Palmer and engineers Ian Houston and John Jones could, they say, be used for cars, buses, lorries and eventually buildings and heavy industry, including power plants.

"We've managed to develop a way to successfully capture a majority of the emissions from the dirtiest motor we could find," Palmer, who has consulted for organisations including the World Health Organisation and GlaxoSmithKline, told Reuters.

The three, who stumbled across the idea while experimenting with carbon dioxide to help boost algae growth for fish farming, have set up a company called Maes Anturio Limited, which translates from Welsh as Field Adventure.

With the backing of their local member of parliament they are now seeking extra risk capital either from government or industry: the only emissions they are not sure their box can handle are those from aviation.

CAPTURE RATE

Although the box the men currently use for demonstration is about the size of a bar stool, they say they can build one small enough to replace a car exhaust that will last for a full tank of petrol.

The crucial aspect of the technology is that the carbon dioxide is captured and held in a secure state, said Houston. Other carbon capture technologies are much more cumbersome or energy-intensive, for example using miles of pipeline to transport the gas.

"The carbon dioxide, held in its safe, inert state, can be handled, transported and released into a controlled environment with ease and a minimal amount of energy required," Houston said at a demonstration using a diesel-powered generator at a certified UK Ministry of Transportation emissions test centre.

More than 130 tests carried out over two years at several testing centres have, the three say, yielded a capture rate between 85 and 95 percent. They showed the box to David Hansen, a Labour MP for Delyn, North Wales, who is now helping them.

"Based on the information, there is a clear reduction in emissions," Hansen told Reuters.

"As a result, I'm facilitating meetings with the appropriate UK government agencies, as we want to ensure that British ownership and manufacturing is maintained."

The men are also in contact with car-makers Toyota Motor Corp of Japan and General Motors Corp. of the United States. Houston said they have also received substantial offers from two unnamed Asian companies.

Both Toyota and General Motors declined to comment.

SECRETS

If the system takes off, drivers with a Greenbox would replace it when they fill up their cars and it would go to a bioreactor to be emptied.

Through a chemical reaction, the captured gases from the box would be fed to algae, which would then be crushed to produce a bio-oil. This extract can be converted to produce a biodiesel almost identical to normal diesel.

This biodiesel can be fed back into a diesel engine, the emptied Greenbox can be affixed to the car and the cycle can begin again.

The process also yields methane gas and fertilizer, both of which can be captured separately. The algae required to capture all of Britain's auto emissions would take up around 1,000 acres (400 hectares).

The three estimate that 10 facilities could be built across the UK to handle the carbon dioxide from the nearly 30 million cars on British roads.

The inventors say they have spent nearly 170,000 pounds (US$348,500) over two years developing the "three distinct technologies" involved and are hoping to secure more funding for health and safety testing.

Not surprisingly, the trio won't show anyone -- not even their wives -- what's inside the box.

After every demonstration they hide its individual components in various locations across North Wales and the technology is divided into three parts, with each inventor being custodian of one section.