In this work, effects of direct-current (DC) electric fields on the flame propagation and combustion characteristics of premixed CH 4 /O 2 /N 2 mixtures were experimentally investigated at excess air ratios of 0.8, 1.0, and 1.2, room temperature, and atmospheric pressure. Results show that the existence of the DC electric fields significantly affects the flame propagation and combustion properties. Specifically, the flame shape becomes a prolate spheroid, with the major axis in the electric field direction as a result of the movement of positive ions by the electric body force, and a further increase in the applied voltage distorts the flame front more significantly. Additionally, the flame propagation speed in the electric field direction (S n ) and corresponding unstretched laminar burning velocity (u l ) are increased as the electric field becomes more intense, and this behavior is more pronounced for lean mixtures. Finally, the initial and main combustion durations defined by the pressure evolution profiles are shortened. The peak pressure and peak rate of pressure rise are increased with the increase of the electric field intensity just for lean mixtures. The observation of the laminar burning velocity and pressure evolution behavior substantiates the potential of the electric field in enhancing lean combustion.

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An investigation was conducted of the ion current characteristics at various excess air ratios λ in a spark ignition engine fuelled with natural-gas–hydrogen blends. The Gaussian mathematical method was employed to fit the pure ion current signal without a spark tail generated by the ignition discharge. The cylinder pressure was recorded, and the Pauta criterion static threshold was used to deal with the outlier errors in the testing data for given cycles. The results show that the spark tail and the ion current can be separated by fitting the ion current with the Gaussian method, and the fitted pure ion current includes the flame-front stage and the post-flame stage. The timing with respect to the flame-front peak exhibits an irregular variation with λ. The maximum current in the flame-front stage increases with increasing λ.The timing of the maximum current in the post-flame stage occurs earlier at the stoichiometric ratio than at other air-to-fuel ratios. The correlation coefficient between the ion current in the post-flame stage and the pressure data treated with the Pauta criterion method increases. In addition, there are high correlation coefficients between the ion current and the pressure.

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