Title

A TIME-DEPENDENT THEORY OF SPARK IGNITION

Date of Completion

January 1979

Keywords

Engineering, Mechanical|Energy

Degree

Ph.D.

Abstract

A new theory of spark ignition has been formulated which includes for the first time the temporal dependence of the spark energy input. This theory was based on the observation made here that for ignition the spark kernel or hot bubble of gas created by the spark must grow to a critical size before its expansion velocity falls below a critical value. A model of spark kernel growth was developed which related the time varying energy input to the kernel expansion in the high pressure region behind a strong shock wave or in an ambient pressure environment behind a weak shock. The strong shock kernel growth model was developed from a simulation of a time dependent energy release in an ideal gas without transport effects which was modified to reflect real gas behavior. The assumptions that energy loses from the spark kernel due to conduction, radiation or convection were negligible were justified by existing experimental data and observations of spark ignition.^ Comparisons of observed spark kernel growth patterns and ignition energies with prediction made using this theory showed very good agreement for a wide range of pressures, air-fuel ratios and spark durations for both stagnant and flowing gases. The increase in ignition energy and decrease in critical spark duration for flowing gases compared to stagnant mixtures were shown to be due to a distortion of the arc by the flow which makes some of the arc energy unavailable for ignition. The use of turbulent critical radii overpredicted the turbulent ignition energies suggesting that even in turbulent flow the critical kernel size is the same as in laminar flow or stagnant cases. ^

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