LAUSR

Abstract Increasing discharge energies and employing advanced discharge strategies have been deemed to be effective methods for improving the ignition processes, especially for diluted or lean combustion. However, so far knowledge on the relevant mechanism is far from adequate. In particular, the effects of the plasma produced between the spark plug electrodes on spark ignition processes need to be further clarified. The plasma temperatures are important as they are closely related to the chemical reaction rate. In this study, the vibrational and rotational temperatures of the discharge plasma are quantitatively evaluated by a time series of spectral measurements with different discharge energies and strategies in air under atmospheric pressure, based on the N2 second positive molecular emission spectra. The vibrational and rotational temperatures show a perfect consistent trend with the release rates of delivered energy to the spark plug gap. This indicates that the two temperatures can be enhanced by the higher energy release rates and can be effectively controlled by different discharge strategies. The vibrational and rotational temperatures measured in this study are in the range of 3700–4300 K and 1400–2600 K, respectively. The temperature differences between the vibrational and rotational temperatures exceed 1600 K, increasing with the energy release rate decreasing. This indicates that the spark discharge plasma is in a state of non-thermal equilibrium with the existence of the discharge energies under the non-flow conditions. These results would be a reference to further develop the advanced discharge strategies and improve the ignition stability.