Mercedes-Benz provided some details on its DiesOtto engine project: a 1.8-liter, turbocharged, four-cylinder gasoline direct injection engine that supports a switch to controlled auto ignition (CAI) under certain operating conditions.
When starting and under full load, the fuel/air charge is ignited by a spark plug, as in a conventional spark-ignition (SI) engine. The switch to controlled auto ignition mode occurs under partial load conditions, i.e. at low and medium engine speeds.
Despite its reduced displacement—downsizing being one of the major factors for reducing fuel consumption —the research engine delivers an output of 175 kW/238 hp and a maximum torque of 400 Nm (295 lb-ft), with projected fuel consumption of less than 6.0 l/100km (39 mpg US). This, Mercedes-Benz said, could power a vehicle the size of its current S-Class.
Our next goal will now be to make the gasoline engine as economical as a diesel. All the preconditions for this are provided by our DiesOtto concept, which incorporates the foremost strengths of both the gasoline engine and diesel engine.—Prof. Dr. Herbert Kohler, Head of Group Research & Advanced Engineering Vehicle and Powertrain; Chief Environmental Officer of DaimlerChrysler
Volkswagen has also earlier seized on the term “DiesOtto” to reflect its Combined Combustion System (CCS) work (earlier post). A number of automakers, engineering firms and universities are exploring ways to combine CAI/HCCI with conventional combustion systems as a way to increase the fuel economy of gasoline-fueled engines. (Earlier post.)
Some of the issues involved in engineering such combined systems include the design of control systems for cycle-to-cycle control, ways to enlarge the operating range for the CAI mode, and transient response.
Features of the Mercedes-Benz DiesOtto engine include:
Downsizing with fewer cylinders and a smaller displacement;
Turbocharging for enhanced performance;
Direct gasoline injection;
Controlled auto ignition;
Variable valve control;
A variable compression ratio; and
An optional microhybrid module with an integrated starter/generator.
The low temperature combustion reduces engine-out NOx. All further emissions control in the DiesOtto engine is by means of a standard three-way catalytic converter.
Mercedes-Benz noted that some of the technologies in the concept engine—such as gasoline direct injection—are already in production, while others will emerge in the mid-term. The company again emphasized that it is working on hybrid options for its models.
(A hat-tip to Rafael!)