Mean Value Modeling and Analysis of HCCI Diesel Engines With External Mixture Formation

Homogeneous charge compression ignition (HCCI) is a promising concept for internal combustion engines that can considerably decrease $NOx$ and soot emissions in part-load operations without penalizing fuel consumption. The HCCI combustion can be implemented in direct injection diesel engines without major modifications by introducing a specialized fuel injector in the intake port. This decouples the homogeneous mixture formation from the traditional in-cylinder injection, thus providing two fueling systems that can be used to optimize exhaust emissions and fuel consumption over the engine operating range. However, understanding and controlling the complex mechanisms and interactions driving the HCCI combustion process is still a difficult task. For this reason, it is essential to identify the most important control parameters and understand their influence on the auto-ignition process. The current work analyzes HCCI combustion with external mixture formation through experimental investigation and the definition of a control-oriented model. An extensive testing activity was performed on a passenger car diesel engine equipped with an external fuel atomizer to operate in HCCI mode. This provided an understanding of the process as well as experimental data to identify a mean value model of the system and its parameters. The model includes a thermodynamic combustion calculation that estimates the heat release, cylinder pressure, and the relevant variables for combustion control. The tool developed was then validated and used for analyzing the system behavior in steady state conditions. Finally, a description of the HCCI system behavior in transient operations is presented.