Mercedes F700 news - Frankfurt show: Mercedes F700

One of the undoubted stars of the Frankfurt show isn't a car at all, but rather an engine: the DiesOtto in the Mercedes F700.

Not that the F700 is anything less than imposing (and more about that in a tick). But if the DiesOtto's performance and economy turns out to be even close to what Merc is quoting, well, it could revolutionise the combustion engine.

In brief, the DiesOtto is a 1.8-litre, four-cylinder petrol engine. That might sound small in a car the size of the F700 - which is longer than an S-Class - but Merc reckons that it'll produce comparable performance to the current 3.5-litre V6 in the S: 238bhp and a 0-60mph time of 7.5 seconds.

And all that with consumption of just 44.3mpg and CO2 emissions of 127g/km - astonishing figures for such a big car.

It's all down to the DiesOtto's ingenious ignition system, which combines the best bits of a petrol and diesel engine. 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.

Clever stuff, and it's all sitting in a clever car. The F700 is described as a 'research car' that could preview the next-generation S-Class and, with its giant pointed grille and 21-inch wheels, it's a big beast.

The right-hand rear door is a suicide effort, opening backwards to reveal a rear-facing seat (a bit like a London taxi, but without the inane conversation) and an enormous TV screen. Equally oversized is the driver information panel, which stretches across the dash behind the squared-off steering wheel.

There's loads of clever technology on board, too, including a road scanning device housed in the headlamps that feeds information back to the F700's hydraulic systems.

But the big news is the engine, which Merc reckons could make it to production within five years. Just imagine how economical it'd be in something the size of a C-Class...