Abstract
In recent years, radio-frequency corona igniters have been extensively studied for their capability to ensure an effective ignition also in lean or diluted mixtures. Corona ignition is volumetric, with streamers coming from a star-shaped electrode. During the discharge, many radicals and excited species, able to speed up the combustion onset, are generated. At the same time, corona igniters are able to release a considerable amount of thermal energy inside the combustion chamber. The correct determination of such energy is crucial to evaluate the effectiveness of the ignition. In this work, corona discharge is experimentally evaluated inside an optical vessel. In this apparatus, the released thermal energy is measured by means of pressure-based calorimetry, and at the same time the natural luminosity of the streamers is recorded with a high-speed camera. The goal is to find a relationship between thermal energy release and streamers luminosity. Tests are performed using nitrogen as medium, at different pressure levels inside the vessel. The peak electrode voltage is varied to characterize the igniter behaviour in different operating conditions. The results of this work can be used to quantify the corona ignition capabilities to involve a wide amount of medium while releasing a high amount of thermal energy. A repeatability evaluation of streamer evolution is investigated as well.
Highlights
Internal combustion engines worldwide regulations have become more and more stringent, forcing to decrease the amount of pollutant emissions and, at the same time, to increase the thermal efficiency [1]
The quick generation of a considerable amount of excited species, e.g. atomic oxygen, during the corona discharge is fundamental to explain the high capability of this system to speed up the combustion onset [17]
Calorimetry tests showed that the thermal energy released during the corona discharge can be higher than the one released by a conventional spark igniter [18,19]
Summary
Internal combustion engines worldwide regulations have become more and more stringent, forcing to decrease the amount of pollutant emissions and, at the same time, to increase the thermal efficiency [1]. As for spark ignition (SI) engines, engine research community and manufacturers agree in considering a 45% brake thermal efficiency a feasible goal [5], ascertained that boosting, lean combustion and high EGR dilution are mandatory. Instead, interesting technologies to improve the stability of combustion initiation for boosted engines in lean and diluted environments are the pre-chamber ignition [10] and the low-temperature plasma ignition [11]. Anyway, studies on RF corona igniters have only been aimed at determining the combustion effectiveness [22], or the repeatability and the randomness of the streamer discharge [23], or the amount of the sole thermal energy [18]
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