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UBC Theses and Dissertations

The effects of combustion chamber design on turbulence, cyclic variation and performance in an SI engine Tippett, Esther Claire


An experimental program of motored and fired tests has been undertaken on a single cylinder spark ignition engine to determine the influence of combustion chamber design on turbulence enhancement in the achievement of fast lean operation. Flow field measurements were taken using hot wire anemometry in the cylinder during motored operation. On line performance tests and in-cylinder pressure data were recorded for the operation of the engine by natural gas at lean and stoichiometric conditions over a range of speed and loads. Squish and squish jet action methods of turbulence enhancement were investigated for six configurations, using a standard bathtub cylinder head and new piston designs incorporating directed jets through a raised wall, a standard bowl-in-piston chamber and an original squish jet design piston. A non squish comparison was provided by a disc chamber. Peak Pressure and Indicated Mean Effective Pressure (IMEP), two parameters characterizing performance and cyclic variability, showed that enhanced turbulence by combustion chamber geometry is effective in improving performance at lean operation. The single jet action directed towards the spark was most effective in improving the efficiency at high speed and lean mixtures. The addition of jets to the single jet, or jet channels to the main squish action of the bowl- in-piston chamber, reduced performance and increased cyclic variability. Mass fraction burn analysis of the cylinder pressure data showed that squish action was most effective in the main burn period. Configurations with large squish area and centrally located spark produced the greatest reduction in both the initial and main burn periods. The potential for the squish jet action to improve engine drivability and increase the knock limit was exhibited in reduced coefficient of variance of IMEP and reduced ignition advance requirements. Directions for further research to exploit this potential for engines operated by alternative fuels are identified.

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