The awesome Mercedes-Benz F 700 Research Car

The first step to less consumption is to downsize the engine. Instead of a big-volume, naturally aspirated engine, a compact, turbocharged power plant is used. This leap backwards in cylinder size and displacement reduces internal friction in the engine. The smaller unit increasingly runs in a more efficient combustion range. The drive system in the research car is a four-cylinder with a displacement of a mere 1.8 liters. At first, this is somewhat of a surprise for a vehicle of the luxury class, where up to now at least six cylinders and three liters displacement are the standard.

The sensation of effortlessly superior driving in the luxury class nonetheless is retained. The DIESOTTO drive gets additional power from a two-stage turbocharger system. A larger low-pressure turbocharger and a high-pressure turbocharger of smaller diameter are connected in series and together optimally cover the complete engine speed range. The complex system makes for good responsiveness from low revs, provides high torque and pulling power and delivers high peak output. For driving off, the internal combustion engine additionally gets assistance from the electric motor of the hybrid module.

The maximum output of the internal combustion engine is 175 kW (238 hp); the electric motor develops another 15 kW (20 hp); the maximum torque of the system is impressive 400 Newton meters. This guarantees performance commensurate with the luxury class, as the time of 7.5 seconds for the 0 to 100 km/h (62 mph) sprint documents.

The hybrid module with its integral starter-generator helps to even further reduce the excellent fuel consumption mainly in urban stop-and-go operation. The internal combustion engine always is shut off when it is not needed. On moving off, the combination of internal combustion engine and torquey electric motor is a tandem that ensures a powerful but silky smooth start. Besides that, during coasting and braking the electric motor recovers energy which is stored in a special high-voltage battery and used for the next starting operation. This starter-generator is integrated in the housing of the 7G-TRONIC automatic transmission.

Direct injection and variable valve lift

The next, decisive element in the DIESOTTO system is direct injection. With the CLS 350 CGI, Mercedes-Benz has pioneered second-generation spray-guided gasoline direct injection. Compared with conventional manifold injection this technology provides a consumption advantage of some ten percent, among other things due to lean-burn operation at partial load.

In the DIESOTTO engine direct injection is used differently than in the CGI engine, which in lean-burn operation needs an ignitable mixture cloud around the sparkplug. DIESOTTO operates with a lean mixture, but needs direct injection for a homogeneous mixture in the cylinder and for the free control of the injection cycles.

Another component is variable valve control. The valve opening times can be adjusted over a large range, and the valve lift can be altered too. To achieve this, the camshaft is provided with two cams, a smaller one and a larger one, for each valve. The large valve lift is needed for full-load operation and high engine speeds, the small valve lift mainly for CAI at partial load.

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