A solution to the burning question

A new engine boasts the best of petrol and diesel.

Have you noticed how diesels have gone out of fashion? Not so long ago, driving a diesel was touted as a highly constructive way of saving both your pocket and the planet, because diesels ingest little fuel and excrete little in the way of greenhouse gases. Modern diesels are quite quiet, too, and give a decent turn of speed.

Yet people have become increasingly aware that driving behind another diesel car is not pleasant, especially if you're feeling a bit chesty. Frankly, they smell, and even the cleanest of today's hi-tech diesels can be caught puffing out a big black cloud when asked to work hard. Research into whether or not diesel fumes constitute a health risk continues apace. Meanwhile, people are going right off the idea and slipping back into thirstier but faster and much less odorous petrol cars.

But what if we could combine the benefits of a diesel with the performance of a petrol engine? We'll soon be able to do exactly that if claims for the next big engine breakthrough prove correct. In Japan, Mitsubishi has just launched its GDI (Gasoline Direct Injection) engine in an all-new Galant, promising near-diesel economy with petrol-engine performance in terms of smoothness, quietness and ultimate pace.

Here's how it works. In a normal petrol engine, the petrol and the air have to be mixed in just the right proportions before they are sucked into the engine's cylinders, otherwise the mixture won't burn properly. Too little fuel, and the engine will splutter and misfire. In a diesel, however, the fuel is injected into the cylinder itself, just when the air already sucked in is being compressed to its maximum.

It's the heat generated by this compression that ignites the fuel, the expanding gases then forcing the piston down and making the engine go. The fuel starts to burn before it's fully dispersed among the compressed air, so it doesn't matter that there's more air than is chemically "correct".

So how would it be if petrol could be squirted directly into a cylinder, diesel-fashion, just at the microsecond it's needed? That's what happens in the Mitsubishi engine, thanks to advances in electronics that have made control of timing and quantity precise enough to match a higher-revving petrol engine's needs. As in a diesel, there's an excess of air, but clever shaping of the channel through which the incoming air enters the cylinder, and of the top of the piston, causes that air to swirl in such a way as to guide the squirt of petrol towards the spark plug. The plug ignites the petrol and gets the burning under way before the mixture is too dispersed.

Running like this, the new engine is very economical indeed: a true "lean- burn" motor. If its driver demands more power, though, perhaps for accelerating harder or cruising at high speed, the fuel is instead injected with the incoming air, and in greater quantity, to mimic what happens in a standard petrol engine. Even then, Mitsubishi's engine is still highly efficient thanks to that swirling and a high compression ratio.

Theory is one thing, practice quite another, and your sceptical correspondent expected there to be a definite "step" between the two modes of operation. Also, given the lifeless feel of simpler, lean-burn engines to date (Honda Civic VTEC-E, Toyota Carina E), I would not have been surprised if the GDI engine felt as flat as a pancake. Not so: not only does this 1.8 litre engine feel entirely normal all the time, it's also encouragingly muscular and entirely smooth.

The new Galant that I tested, a Rover 600-sized car, comes to the UK soon but not, at first, with the new engine. That's because there's still some emissions work to do on matching it to European driving conditions with their higher sustained speeds. However, both the Mitsubishi Carisma and the Volvo S40, which are built in the same Dutch factory, will get the new engine by the end of the year.

The future, looks likely to lie with Gasoline Direct Injection, so it's hardly surprising that Mitsubishi has taken out 180 patents. Toyota is poised to launch its own version, and others are sure to follow. What price a smelly diesel now?