It's taken decades for the message to get through, but at last we are turning to greener motoring. The demand for hybrid cars in the UK is at an all-time high and waiting lists for the proven technology are long.

In Japan, drivers have been shaken so seriously by the oil shocks that they became the first in the developed world to experience declining car ownership. They are also poised to benefit from the biggest experiment with electric cars since the milk float.

This is all terribly green and worthy, but these solutions mean forking out for another car. Wouldn't it be better to adapt our existing vehicles? And while giving up our gas-guzzlers is one thing, what should we do with the billions of old-tech cars already on the road? After all, simply to scrap them means building new cars. As Conor Faughnan of the AA points out: "Most of the pollution associated with cars actually relates to their construction."

In first gear

A Japanese start-up company says it might have the answer. It is proposing to retrofit our existing cars with tiny hydrogen generators that work off the car exhaust and supplement existing combustion.

Makoto Okuda, director of Hrein Energy, says: "Adding about 3% of hydrogen to the intake air results in a lean burn, which has never been made possible with gasoline alone." His company, based in Hokkaido, develops systems to make, store and supply hydrogen. He says "We have improved fuel efficiency by 30% and reduced CO2 emissions by 30% in recent tests." Now working with Japanese carmakers, Okuda says he hopes to have a system on the market in two to three years.

The beauty of the new system is that it employs the otherwise wasted heat generated by car engines - up to 40% in some cases - to convert an easily transportable liquid organic hydride (a hydrogen-storing chemical) into a gaseous state. This hydrogen is then added to the intake air resulting, says Okuda, in reduced emissions.

The emissions are so low, claims the company, that they will conform to the new EU regulations expected on CO2 emissions in 2012. Hrein suggests retro-fitting all cars with the system.

Scientists agree that transporting hydrogen in a liquid state makes more sense than filling up from service stations equipped to handle frozen hydrogen.

"Hydrogen does have fantastic potential, says Faye Sunderland of greencarwebsite.co.uk. "Retail network distribution is always a problem with new alternative fuels. A UK company called ITM recently announced they had developed a home refuelling unit for hydrogen cars. So a solution is forming."

Says Sunderland: "The key problem with hydrogen technology so far has been retail distribution of hydrogen and economic and environmental viability of the technology. So far hydrogen has been held back because the easiest way to produce hydrogen was to burn fossil fuels which meant the well-to-wheel CO2 emissions were actually higher than if you just put the fossil fuels directly into the car. As a result, it was a bit of an expensive non-starter."

Hrein says all its hydrogen production comes from wind-generated sources on Hokkaido, Japan's most northerly island. But although Japan is the home of the hybrid and host to more electric plug-in cars than the rest of the planet put together, as well as being a leader in the number of hydrogen outlets for vehicles, renewable energy is not something Japan has been strong on so far. There are doubts about whether it would be possible to scale up renewable operations to meet the demand necessary for all this hydrogen needed to be mixed with petro-fuel at service stations.

Hydrogen storing technology, however, does have potential, according to papers published on the technology since the 1970s. The system, they generally conclude, "appears feasible".

"Organic hydrides are a high-efficiency high-density storage media for hydrogen," says Masaru Ichikawa of Hokkaido University, who worked with Hrein on the formula. "The end product can be transported alongside petrol in fuel delivery lorries and deposited at petrol stations just like today's conventional fuels."

But if the technology has been available since the 1970s, asks John Turner, a research fellow at the US's National Renewable Energy Laboratory, why didn't the automakers turn to it to provide themselves with an edge over competitors?

Dirty work

"Conspiracy theories aside, the automakers have some very smart engineers and if they see something that will easily give them a 30% boost in fuel economy then they are going to take a hard look at it," he says. "Modifying an existing technology, unless it is mandated by some government agency, will not likely go anywhere."

Hrein, with its take on delivering a little hydrogen to clean up the dirty work of gasoline, may be about to do that, according to the company's representative in the UK. Hrein is talking to the UK government to discuss bringing the technology to the UK. Should Downing Street give the system a green light, it could provide a stopgap in the march towards the zero emission car. It would certainly provide many of us who can only afford second-hand bangers with a cleaner conscience, if not a cleaner engine.