Motoring: Under the bland looks lurks Jekyll and Hyde

A car that boasts the frugality of diesel with the punch of a petrol engine? Roger Bell puts the Carisma to the test.

Mitsubishi claims to have unlocked the secret of motoring's Holy Grail: petrol performance with diesel economy. Eureka? Perhaps. The amazing engine that's capable of this technological party trick is found under the bonnet of the unremarkable Carisma, a pleasant, well-made family saloon (or hatchback) that looks and drives much like any other car in the Ford Mondeo class.

The word "revolutionary" does not spring to mind when you slip behind the wheel of the Carisma GDI. Nor does "charismatic". You'd never know from the rather bland driving experience that its petrol engine - basically an ordinary four-cylinder, 16-valver with special tweaks evolved over three decades - breaks important new ground.

The clever bit is that the GDI (gas direct injection) is really two engines in one, capable at one extreme of exceptional frugality (without the gruffness that characterises many diesels) and at the other of considerable verve and muscularity. Because it's so fuel efficient, harmful emissions are also reduced. Mitsubishi claims a 20 per cent reduction in carbon dioxide, the "greenhouse" gas responsible for global warming.

Driven with restraint, avoiding full power, the GDI runs on an air/fuel mixture so lean (lots of air, little petrol) it would cause an ordinary engine not only to misfire but emit excessive amounts of NOx. This "lean- burn" trick is achieved through various clever ploys including injecting the petrol (gas) directly into the combustion chambers, rather than into the passage that feeds them. Electronics and unusual shaping of the ports and cylinder head play key roles, too. Mitsubishi resolved the NOx problem with a special catalytic converter.

Call for full overtaking power and the air/fuel ratio richens, from an economy-first 40 to one to a power- biased 15 to one. Result: plenty of short-term zap at the expense of economy.

You cannot enjoy the best of both worlds simultaneously. Drive the GDI hard all the time and it will slurp fuel as heavily as an ordinary engine. Drive it with restraint, as you would around town or when motorway cruising, and it rewards with diesel-like economy. Fears that the transition between the two levels of performance might be snappily abrupt are unfounded: delivery is seamlessly linear, whether you're ambling or pressing on.

That Mitsubishi's wonder engine fulfils its promise of combining poke with parsimony is not in dispute. It's the extent of its superiority that's open to debate. Using Ford's 1.8 Mondeo (four cylinders, 16 valves, conventional indirect injection) as a yardstick, the new Carisma stacks up pretty well on specifications (Mondeo's in brackets): power 125bhp (115bhp), 0-62mph 9.7 seconds (11.0), combined fuel consumption 45.6mpg (36.2). A clear performance/economy win, then, for the Carisma - and no catches. Other petrol rivals, including 1.8 versions of the Toyota Carina, Vauxhall Vectra, Renault Laguna and Citroen Xantia, are also said to be humbled.

Mitsubishi monitored the consumption of 13 Carisma 1.8 GDIs on a recent press launch and concluded from trip-computer readings (accuracy unknown) that their average consumption was 44mpg, compared with a mean of 36.6mpg for the superseded models with conventional 1.8 engines. Over the same route, diesel Carismas, powered by turbo-motors that are hardly state of the art, averaged 45mpg.

Although Mitsubishi is ahead of the field in lean-burn technology - several of its other models use GDI engines in Japan - it is by no means alone. Rivals are working on similar engines that cut consumption and emissions without penalising performance. The end of the road for the diesel, then? Hardly. Expect the next direct-injection generation to reopen a significant economy advantage, even over the Carisma GDI. One thing is certain: the healthy fuel efficiency war that's looming is good news for consumers, never mind the environment.

Mitsubishi Carisma 1.8 GDI

Specifications

Price: pounds 14,510-16,485. Engine: 1,834cc, four cylinders, 16 valves, 125bhp at 5,500rpm. Five-speed manual, front-wheel drive. Performance: top speed 125mph, 0-60mph in 9.7 sec. Fuel consumption 45.6mpg combined cycles.

Rivals

Audi A4 1.8, from pounds 19,011. Solid, well made, refined ... expensive.

Citroen Xantia 2.1 turbo-diesel SX, pounds l7,645. Elegant, expensive hatch with magic-carpet ride.

Ford Mondeo 1.8, from pounds 14,875. Class best-seller is more entertaining to drive than the Carisma, but not as lively or economical.

Vauxhall Vectra 1.8, from pounds 14,960. Competitive performance but well beaten by Carisma for economy.

VW Passat 1.9TDi 11OS, pounds 16,800. Good looks, roomy, well made. Fast and frugal, but Carisma much cheaper.