Low emission, high power, good MPG?

Now here's a story to get car drivers happier - we all want (really, we do!) a big petrol engine. We can't afford said engine in terms of outright purchase and the fuel costs to run them are scary. How about, then, an engine that offers you 3.5 V6 petrol performance, but the running costs of a small diesel?

Sounds too good to be true? Well, Mercedes are keeping slightly quiet on this but have developed just that engine. It's called a "DiesOtto" (don't you love it??) and currently, is a 1.8-litre four-cylinder petrol engine with twin sequential turbochargers and direct injection (the "otto" bit). But the clever bit is that Merc engineers have incorporated both the conventional Otto cycle and a sparkless, diesel-style combustion (the "diesel" bit). The breakthrough is that it basically runs at two compression ratios, which is very clever.

The techy bit I'll let Top Gear cover...

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.

Now, the figures - the 1.8-litre engine has 238bhp and 295bhp of torque, with 40mpg in a car the size of an Merc S-Class. Which weighs the same as Norway. Put it in a C-class and you've got a potent little beast. It'll be production ready sometime near 2012, so maybe not the next car, but the one after that.

Just so long as they have learnt how to build them properly by then again...