Petrol or diesel? Why choose?

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Frankfurt show: Mercedes F700

Hey, man in the street: have you ever been torn between petrol and a diesel? Wanted the performance of a petrol engine but the economy of an oil-burner?

Sound familiar? Well, the solution to all your problems just might be on the way, in the shape of... the Mercedes DiesOtto engine!

OK, enough of the American-accented advertising spiel. But this DiesOtto could be a biggie.

Due to premiere in Frankfurt inside a large luxury concept called the F700, Merc reckons the DiesOtto could give the responsiveness of a 3.5-litre V6 petrol engine with the frugality of a tiny diesel-burner.

But how? Hold on, here comes the technical bit. The DiesOtto is, at heart, a 1.8-litre four-cylinder petrol engine with twin sequential turbochargers and direct injection.

But the clever bit is that Merc engineers have incorporated both the conventional Otto cycle and a sparkless, diesel-style combustion

On start-up, the engine runs in standard petrol mode, with spark plugs igniting petrol injected directly into the cylinder. But once the engine is warm and cruising, it switches to a more efficient 'diesel mode', raising the compression ratio and deactivating the spark plugs.

And because it runs on petrol, the DiesOtto boasts lower emissions than an equivalent diesel engine.

Merc is keeping quiet about exactly how it has managed to create a variable compression engine, but the results are impressive.

The 1.8-litre engine puts out 238bhp and 295bhp of torque, and Merc reckons it'll return 40mpg in a car the size of an S-Class. Wedge it into a hatchback and it could return 70-odd mpg.

Merc says the DiesOtto could see production in 2012. Petrol or diesel? Enjoy the choice while it lasts, good sir.