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Operation Strategies for Controlled Auto Ignition Gasoline Engines
Abstract:
Controlled Auto Ignition combustion systems have a high potential for fuel consumption and emissions reduction for gasoline engines in part load operation. Controlled auto ignition is initiated by reaching thermal ignition conditions at the end of compression. Combustion of the CAI process is controlled essentially by chemical kinetics, and thus differs significantly from conventional premixed combustion. Consequently, the CAI combustion process is determined by the thermodynamic state, and can be controlled by a high amount of residual gas and stratification of air, residual gas and fuel.
In this paper both fundamental and application relevant aspects are investigated in a combined approach. Fundamental knowledge about the auto-ignition process and its dependency on engine operating conditions are required to efficiently develop an application strategy for CAI combustion. To develop a comprehensive understanding of the CAI process detailed thermodynamic analysis of CAI combustion, optical diagnostics on a transparent engine and 3D-CFD analysis with reduced chemical kinetics is used. In order to deduce measures for stability and operating range extension, the detailed fundamental information is transferred to a 1D-model, extended by a multi-zone approach describing thermodynamic parameters and incorporating reduced reaction kinetics.
Application strategies for CAI are developed on a single cylinder research engine with a fully variable valve train and direct injection. It is found that control of the CAI operating range can be achieved by realizing stratification of the in-cylinder charge. Stratification control is possible via valve timing and the strategy for direct injection.
Based on the thermodynamic requirements, the necessary variability of the valve train for realization of CAI operation in multi-cylinder engines can be identified. A multi-cylinder engine with a mechanically variable valve train to realize the CAI combustion process is presented.