[Carmagazine]
Advanced engineering wrapped in ‘aquatic’ styling made the Mercedes F700 concept car one of the stars of the Frankfurt motor show in September 2007, and CAR has been itching to put its technologies to the test ever since.
Flip-over, cork-faced seats and a navigation system run by a virtual assistant called Gloria are novel enough, but we’re really here to find out about the revolutionary DiesOtto engine and Pre-Scan suspension.
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.
And Pre-Scan, what’s that about?
Pulsed laser sensors scan the road surface up to 7m in front of the car, detecting bumps and feeding the information back to the F700’s Active Body Control suspension. As a bump approaches, ABC uses its hydraulics to raise the body slightly, then releases the hydraulic pressure to allow free wheel movement. In effect, it lengthens and softens the springs as a bump approaches, to provide what Mercedes likes to call a ‘flying carpet’ ride.
Does it really ride like a flying carpet?Mostly, yes. Roll and pitch are all but eliminated, transverse ridges in the road surface are magically erased, and minor ripples are smoothed for a calmer, quieter ride. Speed bumps are more heard than felt: Mercedes says production versions of the system will need ‘environment detection’ so the car will know when it’s in an urban area, and won’t let you race over speed bumps…
And does a 1.8-litre four-pot really work in a luxury saloon?
That was one of the questions Mercedes aimed to answer by building DiesOtto as a four-cylinder engine: small, light fours are bound to be more environmentally friendly that hefty multi-cylinder motors, but can they provide the performance and refinement an S-class customer expects?
Performance isn’t in doubt. At 1700kg, the F700 is no featherweight (though it is light for a full-size saloon) yet the combination of 1.8-litre DiesOtto engine and the electric motor deliver brisk performance.
In some ways the engine is remarkably refined: the trickiest part of getting DiesOtto to work is to manage the transition between HCCI and spark-ignition modes, which Mercedes has done incredibly well. An almost imperceptible difference in engine note is the only indication. But ultimately this is still a four-cylinder engine: it needs to be smoother and quieter for a big, pampering luxury car. Merc’s current S-class has it licked for refinement.
Verdict
You know what? We were mightily impressed by this tech fest of a concept car. Mercedes’ Pre-Scan suspension system works amazingly well, while the DiesOtto engine delivers good performance, exceptional economy and clean emissions.
If the F700 is a fascinating glimpse of the next generation of executive car, then it appears your next S-class will be able to square green credentials with the need to pamper, cosset and let the business world know you’ve finally arrived.