A predictive computationally efficient cycle simulation tool to accurately model all variants of HCCI combustion is presented. The comprehensive system level modeling tool has been used to identify effective combustion control concepts during direct injection diesel HCCI transients, to expand the operating range in the HCCI regime and to explore strategies for mode transitions between conventional combustion.
Depending on the intended use and complexity, three different in-cylinder modeling approaches are used to couple advanced models with a cycle simulation program to capture the physics of HCCI combustion. The approaches vary from simple zero dimensional models to CFD driven models. While the accuracy of these different methodologies varies, they show similar trends and in essence serve to bridge the gap between basic system level and complex CFD modeling techniques.
After steady state validation with available experimental data, the system level tool was used to simulate transient operations. The combustion and emissions tradeoffs associated with different actuators and control strategies during transients are discussed. The impact of various actuating techniques on the overall engine performance is quantified by investigating the amount of actuation required, timing of actuation and the use of a combination of actuators. Modeling results showed that combined actuation of intake valve closure (IVC), injection characteristics, boost and cooled EGR are beneficial to control diesel HCCI transients and combustion mode transitions. While IVC actuation and cooled EGR provide ways to control and improve combustion phasing, variations in intake boost pressure help to reduce wall wetting, provide sufficient oxygen under highly dilute conditions, and maintain combustion stability. Optimal injection characteristics (injection timing, injection pressure) are required to provide the right amount of thermal and charge stratification to minimize carbon monoxide and unburned hydrocarbon emissions.
The coupling of the combustion phenomena to the transient cycles provides a fundamental understanding of the nature of transient emissions. The sources of dominant emissions in relation to critical actuators are identified. The importance of these actuators to minimize transient emissions is discussed.