Characterization of the Homogeneous Charge Compression Ignition Combustion Process

Abstract: The main objective of this project was to characterize the HCCI combustion process. Central parts in the project were the progress of the HCCI combustion process, the influence of charge homogeneity on the combustion process and sources of UHC and CO emissions in HCCI combustion. Some of these parts were experimentally explored by applying non-perturbing optical diagnostics to the HCCI combustion process in engines with optical access. Other aspects were examined through regular engine testing while measuring pressures, temperatures and the regulated emissions of CO, NOx and UHC. From the conducted experiments and the discussion regarding these, it can be concluded that due to charge heterogeneities in temperature, air/fuel equivalence ratio or residual gases, the HCCI combustion process starts in many areas simultaneously. The ignition process is slow compared to the main heat release. It features a gradual start of combustion with small gradients between burned and unburned gases. The main heat release is characterized by the rapid growth of burned areas. Towards the end of combustion, the expanding areas of burned gas compress the areas of unburned gases, generating strong gradients between burned and unburned areas. Due to the cool gas along the combustion chamber walls and in the crevices, the combustion in these areas is delayed or not at all initiated. Near-wall combustion and wall quenching was not found to be a major source of UHC. Combustion chamber crevices proved to be major contributors to the emissions of UHC in HCCI engines. To support the experimental observations and provide explanations to the essential features of the HCCI combustion process, a conceptual model is proposed in this work.