HC4-3: Innovative Multizone Premixed-Diffusion Combustion Model for Performance and Emission Analysis in Conventional and PCCI Diesel Engines(HC: HCCI Combustion,General Session Papers)

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A new multizone premixed-diffusion combustion model was developed, assessed and applied to the analysis of the burning process and emission formation in two different DI diesel engines, one working with a conventional combustion system and the other with a Premixed Charge Compression Ignition (PCCI) one. The combustion chamber was split into a liquid fuel zone, an unburned gas zone, a rich mixture of fuel vapor and unburned gas, with the related premixed burned-gas zone, and several diffusive burned-gas zones. All of these are treated as homogeneous. Basically, according to a combustion mechanism close to the one of Dec, a fuel rich zone is generated first, giving rise to a premixed flame surrounding the fuel-vapor/air mixture at the liquid jet tip. This forms a plume, which entrains the oxygen required to oxidate the combustion products at its periphery, and thus completes its oxidation in a nearly stoichiometric diffusion flame, consequent to an unburned gas induced mass dilution. The computed thermo-dynamic and thermo-chemical properties in the burned gas zones allowed the post-processing analysis of nitric oxide (NO), particulate matter (PM) and carbon monoxide (CO) formation. The model calibration was made by comparing the experimentally determined engine combustion efficiency to the combustion efficiency that was calculated by applying the energy conservation equation to the whole cylinder charge. The model was tested and assessed for two distinct commercial-type 16V, DI Common Rail (CR) diesel engines. For the conventional combustion engine, the model was applied to the heat release and emission formation analysis in a NO - PM trade-off mode, by changing the EGR mass rate. In particular, in order to estimate its effectiveness and robustness, the model was calibrated on the test condition with the highest EGR level and the calibration parameters were kept constant when lower EGR rate conditions were investigated. In addition, the model was used to analyze the combustion process at full load conditions, for different engine speeds. For the other engine, an investigation in PCCI combustion mode was carried out. The transition from conventional to PCCI mode was made by strongly increasing the EGR rate. With reference to NO emissions, the model outcomes showed an excellent agreement with experimental data for all test conditions, and good results were also obtained for the prediction of CO and PM emission levels. It was ascertained that higher local A/F ratios were required in PCCI combustion mode than in the conventional one.
2008-07-28

HC4-3: Innovative Multizone Premixed-Diffusion Combustion Model for Performance and Emission Analysis in Conventional and PCCI Diesel Engines(HC: HCCI Combustion,General Session Papers)

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