Mercedes-Benz Introduces the Mixed-Mode DiesOtto Engine in the F 700 Research Car

11 September 2007

The F 700 with DiesOtto drive delivers the performance of a S-Class with a 3.5-liter V6 but using a downsized 1.8-liter mixed-mode gasoline engine with hybrid module. Click to enlarge.

The introduction of the F 700 research car at the Frankfurt Motor Show marks the debut of Mercedes-Benz’ new mixed-mode DiesOtto engine—a 1.8-liter, turbocharged, four-cylinder gasoline direct injection engine that supports a switch to controlled auto ignition (CAI) under certain operating conditions. (Earlier post.)

A hybrid module assists the internal combustion engine mainly in stop-and-go operation. The maximum output is 175 kW (238 hp), the electric motor pitches in another 15 kW (20 hp), and the system’s maximum torque is about 400 Nm (295 lb-ft).

The F 700 achieves the performance level of a current S-Class car with a 3.5-liter naturally aspirated V6 gasoline engine or the 3.0-liter V6 turbodiesel. The F 700 accelerates from 0 to 100 km/h (62 mph) in 7.5 seconds, with a top speed (limited) to 200 km/h (124 mph). Fuel consumption of the F 700 in the standard cycle is about 5.3 l/100km (44 mpg US)—equivalent to CO₂ emissions of 127 g/km.

Our strategic objective is to make the gasoline-powered car as economical as the diesel, and the diesel as clean as the gasoline engine.

—Dr. Thomas Weber, Board member of DaimlerChrysler AG responsible for Group Research & Mercedes Car Group Development

CAI versus spark plug ignition. Click to enlarge.

The DiesOtto engine. The DiesOtto engine brings the benefits of diesel technology to the gasoline engine: in CAI, mode the air-fuel mixture ignites itself the same as in a diesel engine. The DiesOtto makes no special demands on fuel quality: the engine runs on normal gasoline.

The DiesOtto engine uses spray-guided gasoline direct injection, but applied differently than in the direct injection CLS 350 CGI engine, for example. The CGI engine’s lean-burn operation requires an ignitable mixture cloud around the sparkplug. The DiesOtto engine also operates with a lean mixture, but need direct injection for a homogeneous mixture in the cylinder and for the free control of the injection cycles.

A variable valve control system can adjust valve opening times over a large range, and alter the valve lift as well. 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.

To meet the different requirements for partial and full-load operation, the DiesOtto engine is fitted with a variable crank mechanism with which the geometric compression can be continuously adjusted.

The fuel consumption advantage results mainly from the relatively lean mixture and high exhaust gas content, and from adaption of the compression ratio depending on operating point. Low emissions of NO_x result from homogeneous combustion at a constant, relatively low temperature.

For further emission control a common three-way catalytic converter is adequate for the DiesOtto drive—the F 700 meets Euro 6 specifications.

CAI is suited for the most frequently used driving modes, at low and intermediate speeds as well as for intermediate load requirements. At higher loads, DiesOtto switches to conventional combustion within a single power stroke. In this case it breathes using the large valve lift, fires using sparkplugs and runs as a direct-injection unit in Lambda1 mode.

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 F 700 serves as a research platform for several other innovations, including a new PRE-SCAN suspension that uses two laser sensors in the front headlamp units as “eyes”. The sensors deliver a precise image of the condition of the roadway. Based on the image supplied by the laser sensors and the information on the vehicle state, the control unit figures out a specific strategy which the high-pressure hydraulics translate into precisely calculated fluid flows and pressures for each single wheel.