Date of Award

3-19-2024

Publication Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

early flame development;effective energy release;gas flow;ignition improvement;on-demand control;spark ignition

Supervisor

Ming Zheng

Abstract

Modern spark ignition (SI) engines can contribute to the decarbonization process significantly via energy efficiency improvement, renewable fuel adoption, and powertrain electrification. The ignition process in high-efficiency engines is increasingly subjected to a fuel-lean or inert-gas diluted mixture of considerably high density, which is challenging for the ignition process. Furthermore, high-efficiency engines commonly introduce extensive turbulence to accelerate flame propagation, despite the apparent difficulties in forming and sustaining a flame kernel under such conditions. The main objective of this research is to improve the ignition via effective spark energy release. The breakdown and glow phases were characterized under sub-microsecond and millisecond time domains, respectively. The present study investigated the breakdown voltage under various gas species (inert gases and combustible mixtures), gas densities and temperatures, and spark gap sizes. The findings can be used to detect flame front arrival and departure, i.e., the active plasma probing method. This method provides more prompt and reliable flame detection compared with conventional ion current sensing, especially under extremely fuel-lean conditions. The correlation between the spark energy release and plasma resistance of the glow discharge was investigated empirically. Compared with conventional transistor coil ignition (TCI) systems with a discharge current of approximately 80mA, the discharge current amplitude was extended up to 3.5A in this study. It was observed that under quiescent conditions, the increase of discharge current amplitude had minimal impacts on the spark energy release, because of the significant decrease in plasma resistance. Under flow conditions, the stretched plasma channel contributed to higher plasma resistance, which improved spark energy release. However, higher flow velocities led to more frequent restrike events that disrupted the discharge process and constrained energy release. The boosted current was proved effective in sustaining plasma stretching, reducing restrike tendency, increasing spark energy release, and improving early flame development. The term, specific plasma channel resistance (R/l) was applied to investigate the factors that affected plasma resistance and spark energy release. It was discovered that discharge current had the dominant influence on the specific plasma channel resistance, while the pressure and flow velocity showed minimal impacts. Furthermore, a high-frequency pulsed current boost strategy was implemented to enhance plasma stretching and energy release, concurrently reducing the overall energy consumption of the ignition system. An on-demand control strategy was proposed based on real-time voltage measurements to predict plasma restrike and enhance the effectiveness of the boosted current system. The early flame development was investigated under quiescent and flow conditions via various ignition strategies. A high transient current strategy delivering up to 18J of spark energy within 30μs showed ignition improvements under quiescent conditions. In contrast, under flow conditions, the boosted current of the glow discharge contributed to sustaining the plasma stretching and improving ignition.

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