The Way We Live: At last, a clean petrol car. So why isn't everyone making them?

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It just sounds far too good to be true - a car which cuts petrol bills by a fifth without any sacrifice in performance.

Nicholas Schoon, Environment Correspondent, assesses the green, high economy claims being made for a revolutionary engine.

Today the Japanese car maker Mitsubishi unveils a new kind of petrol engine in Britain. Boasting the fuel economy of a diesel or better, it will go on sale here in the company's Dutch-built Carisma car next month.

The GDI engine - gasoline direct injection - produces a fifth less global warming carbon dioxide gas than a conventional engine of the same capacity and power. Nor does the 1.8 litre engine produce any extra smog-causing pollutants such as oxides of nitrogen (Nox).

To date, the debate about the need for cleaner, greener cars has centred on gas powered or electric vehicles, or on the need for radical changes in the formulation of petrol. Mitsubishi, Japan's third largest car-maker has brought a greener car to market which is more conventional and much more likely to appeal to Mr Average Motorist - since it is much cheaper to run.

But if it can be done, why are other manufacturers not working on GDI engines? They are, but for the most part it goes on behind the scenes. Toyota already markets a GDI in Japan, while in Germany Audi leads the field.

Mitsubishi has been working and spending on the technology for decades, and protected its technology with hundreds of patents. Now, with a new GDI version of its uncharismatic Carisma, it is hoping for a European breakthrough.

Reviewers have found the Carisma reliable but dull. It has roughly the same style, size and performance as the Ford Mondeo and Vauxhall Vectra but it is a little cheaper. Mitsubishi says the new pounds 14,500 GDI version will consume about a third less fuel than these mass-selling rivals with the same 1.8 litre size of engine.

An engineer with Ricardo, Britain's leading consultancy for engines, said: "Basically, they've stolen a march on everyone and taken them all by surprise."

The main problem with GDI engines is that they produce much higher levels of Nox, one of the most important smog pollutants, than ordinary power plants. Thus they need a different kind of catalytic converter - the device found on the exhaust pipes of all new cars which removes the major pollutants.

Another problem is that petrol in Europe contains much higher levels of sulphur than it does in Japan, and sulphur tends to "poison" catalytic converters and stop them working.

The Ricardo engineer said: "In Europe, the major car manufacturers are not confident that they can produce a GDI engine which will meet tailpipe emission standards over the car's lifetime."

Mitsubishi insists that it can. Today, as the company invites motoring correspondents to try out the new engine, it claims that if all cars in Britain had similar technology the savings in fuel would stop nearly 10 million tonnes of climate-changing carbon dioxide being emitted each year.

But with the Government set to carry on its predecessor's policy of raising petrol duty each year by well above the rate of inflation, saving money could be a stronger sales pitch than saving the earth.

how it works

The GDI engine achieves its very high fuel savings compared to conventional power plants when it isn't working hard -- when idling, or cruising along with a fairly low power output. Those conditions prevail for much of the time when a car is being driven.

Petrol is squirted directly into the top of the cylinder next to the spark plug at high pressure - just when the cylinder is at the top of its stroke and compression is at its highest. Ordinary fuel injection engines do not do this - the petrol is injected outside of the cylinder.

This enables the engine to burn a very "lean" mixture of air and petrol - as much as 40 parts of air to one of petrol, compared with 14.7 to one in an ordinary engine. And that allows it to save large amounts of fuel when it is operating at low power. The reason is that less energy is wasted sucking air and petrol into the cylinders.