Understanding the dynamic evolution of cyclic variability at the operating limits of HCCI engines with negative valve overlap.

ABSTRACT An experimental study is performed for homogeneous charge compression ignition (HCCI) combustion focusing on late phasing conditions with high cyclic variability (CV) approaching misfire. High CV limits the feasible operating range and the objective is to understand and quantify the dominating effects of the CV in order to enable controls for widening the operating range of HCCI. A combustion analysis method is developed for explaining the dynamic coupling in sequences of combustion cycles where important variables are residual gas temperature, combustion efficiency, heat release during re-compression, and unburned fuel mass. The results show that the unburned fuel mass carries over to the re-compression and to the next cycle creating a coupling between cycles, in addition to the well known temperature coupling, that is essential for understanding and predicting the HCCI behavior at lean conditions with high CV

Development of sequential and fully integrated CFD/multi-zone models with detailed chemical kinetics for the simulation of HCCI engines.

Modeling the Homogeneous Charge Compression Ignition (HCCI) engine requires a balanced approach that captures both fluid motion as well as low and high temperature fuel oxidation. A fully coupled CFD and chemistry scheme would be the ideal HCCI modeling approach, but is computationally very expensive. As a result, modeling assumptions are required in order to develop tools that are computationally efficient, yet maintain an acceptable degree of accuracy. In the first part of this dissertation, KIVA-3V is used to investigate the mixing process in HCCI engines prior to combustion, particularly for operation with high levels of residual gas fraction. It is found that insufficient mixing of the hot residuals with the fresh charge can lead to the presence of significant temperature and composition nonuniformities in the cylinder. Then, in order to investigate the effect of temperature and composition stratification on HCCI combustion, two modeling approaches are explored. The first approach is a sequential fluid-mechanic - thermo-kinetic model. The KIVA-3V code is initiated before the exhaust event and operated over the gas exchange period, until a transition point before TDC. The three-dimensional computational domain is then mapped into a two-dimensional array of zones with different temperature and composition, which are used to initiate a multi-zone thermodynamic simulation. In the second approach, KIVA-3V is fully integrated with a multi-zone model with detailed chemical kinetics. The multi-zone model communicates with KIVA-3V at each computational timestep, as in the ideal fully coupled case. However, the composition of the cells is mapped back and forth between KIVA-3V and the multi-zone model, introducing significant computational time savings. The methodology uses a novel re-mapping technique that can account for both temperature and composition non-uniformities in the cylinder. Validation cases were developed by solving the detailed chemistry in every cell of a KIVA-3V grid. The new methodology shows good agreement with the detailed solutions. Hence, it can be used to provide insight into the fundamental effects of temperature and equivalence ratio distribution on ignition, burn duration, and emissions in HCCI engines.Ph.D.Applied SciencesAutomotive engineeringMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124994/2/3163984.pd