The early flame kernel initiation and development is essential to a successful combustion process in spark ignition engines. In order to achieve the stable combustion process during engine operation, lean burn and EGR diluted combustion strategies demand more ignition energy to establish the initial flame kernel, as compared to the conventional stoichiometric combustion. Multiple ignition sites strategy has shown promise to secure the flame kernel initiation process under extreme engine operation conditions. Two factors are accoutable for the enhanced ignition capability, i.e. the higher ignition energy and the multiple initial flame kernels. Engine test results have shown that the multiple ignition sites strategy provides better engine performance even with less total spark energy, as compared to the traditional spark strategy. However, the mechanism why the multiple ignition sites help combustion is less understood.

In this work, the impacts of the ignition energy distribution as well as the multiple flame kernel sites on the flame inception process are investigated in a constant volume combustion chamber. A multi-coil ignition system, along with a spark plug with three electrodes, was used to conveniently adjust the amount of ignition energy delivered to the spark gap, as well as the way that the ignition energy is distributed. The ground electrodes of the spark plug were removed to minimize the impact of the heat loss. Instead, a single needle-shape ground electrode was placed opposed to the three high-voltage electrodes. When using three identical ignition coils to energize three electrodes simultaneously, a bigger initial flame kernel was formed in the vicinity of the spark plug, as three electrodes discharged to the same ground electrode. The ignition energy from three coils were distributed evenly to each of the spark gap. On the other hand, when using three coils to energize one of the three electrodes at the same time, only a small flame kernel was formed within one of the spark gap, while the ignition energy distributed to this spark gap was as the same as the big flame kernel in the three-electrode discharge case. Combustion tests of using air-methane mixture under both quiescent and moderate cross-flow conditions help reveal how the initial flame kernel size and the ignition energy distribution affect the flame initiation and propagation process. A spark energy sweep under the varied background pressure and air-fuel ratios was conducted to find the boundary condition that a bigger initial flame kernel size with multiple ignition sites was more effective for combustion improvement.


Effect of Discharge Energy Distribution on Flame Kernel Development

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

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