The chemistry controlling post combustion hydrocarbon oxidation and homogeneous charge compression ignition

Weiying Yang

doi:10.17918/etd-91

Environmental pollution and energy conservation have become of increasing concern over the past few decades. As one of the major sources of energy consumption and urban emissions, automobiles and light trucks (light duty vehicles) are under significant pressure to improve efficiency and reduce exhaust emission levels. While dwindling fuel reserves and greenhouse gas emissions are motivating factors behind long-term interest in increased efficiency, ambient air quality standards and associated public health concerns have spurred the call for lower emissions. In this thesis, both the concerns are addressed in two companion studies: post combustion hydrocarbon oxidation studies address the factors that control HC exhaust emissions; and Homogeneous Charge Compression Ignition (HCCI) studies provide insight on this promising combustion mode, which has the potentials for both increasing efficiency and lowering emissions. The post combustion hydrocarbon oxidation processes consume a significant fraction of the fuel that escapes the primary combustion process and thereby reduces HC emissions. This process is critical in the warm-up period of IC engines before the exhaust catalyst reaches its light off temperature. As a part of the research program and the follow-up work of the engine exhaust port tracer studies, a general 'cutoff temperature' behavior for hydrocarbonxiv oxidation at the exhaust port was detected, and the role of mixing vs kinetics in the exhaust port oxidation was determined. The Homogeneous Charge Compression Ignition studies determined the engine operating range under HCCI mode using selected fuels, provided kinetic and mechanistic information of hydrocarbon oxidation under HCCI conditions (high dilution), conducted HCCI combustion analysis, and exercised existing chemical models under HCCI conditions to identify deficiencies that must be improved in order to predict and to control stable HCCI operation. This work employs a three-stage approach: (1) HCCI mapping studies; (2) detailed speciation studies; and (3) modeling efforts. In the first stage, the stable operating range for selected fuels was identified and the optimized engine operating parameters were selected for the detailed in-cylinder speciation experiments. The effects of real fuel components like alkenes and aromatics, and exhaust gas recirculation components like CO₂ and NO on HCCI operation were also tested. In the second stage, the gas samples as a function of crank-angle degree (CAD) before the onset of autoignition were collected and analyzed by gas chromatography for seven fuels at selected engine operating conditions. The results were used to elucidate the chemical kinetics controlling HCCI operation. In the third stage, a thermodynamics model was employed to conduct the combustion analysis under HCCI condition, and our existing skeletal kinetic model was tested with the information obtained from the detailed speciation experiments and combustion analysis. The results showed that our current kinetic model did a pretty good job in predicting pressure, temperature andcumulative heat release under stable HCCI operation, but the predictions of fuel consumption and CO formation need to be improved.

The chemistry controlling post combustion hydrocarbon oxidation and homogeneous charge compression ignition

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The chemistry controlling post combustion hydrocarbon oxidation and homogeneous charge compression ignition

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