What’s so clever about the Mercedes F700’s engine, then? Mercedes calls it DiesOtto because it combines elements of diesel and petrol (Otto cycle) engines. Centrepiece of the DiesOtto concept is a new combustion system called Homogeneous Charge Compression Ignition (HCCI), which several manufacturers are now developing.

It uses carefully controlled in-cylinder temperature and pressure conditions to ignite the air/fuel mixture without a spark. The result is very clean, efficient combustion, but the drawback is that HCCI struggles to work over a wide range of engine speed and load. So for cold starts and full-throttle acceleration, DiesOtto switches back to spark-ignition mode. So is the F700 a green special, or does it fly too?

With twin sequential turbochargers and a capacity of just 1.8 litres, the F700 DiesOtto generates 238hp. An electric motor adds another 20hp and boosts total torque to 295lb ft. As a result, the S-class-sized F700 has S350 performance. Thanks to the efficiency of the engine, plus an urban start/stop function and regenerative braking, it returns up to 53mpg. Highly impressive for a big limo like this.