Experimental characterisation of the thermal energy released by a Radio-Frequency Corona Igniter in nitrogen and air

Abstract

The Radio-Frequency Corona Ignition System is characterised by a wide initial combustion volume and precursors production, via radical insemination by the streamers, in addition to high released thermal energy. These features lead to faster combustion, a higher tolerance for lean mixtures and EGR dilutions and, in general, more adaptability. The thermal energy released by the igniter to the surrounding medium can help to understand the performance, the behaviour and the application range. This paper proposes a systematic experimental analysis of the thermal energy released by the igniter at room temperature, via pressure-based calorimetry. This analysis, carried out at different pressures (up to 10 bar) and medium type (air or nitrogen), is extended to the whole range of the corona igniter control parameters, namely streamer duration and driving voltage. The latter is proportional to the maximum electrode voltage, as shown in the model here presented, and as confirmed by experiments. The results show, for all the vessel pressures, the high energetic efficiency of the ignition system and the high amount of the released energy. The latter is found to increase linearly with the corona streamers duration and quickly with the driving voltage up to the streamer-to-arc transition threshold. The efficiency tends to reach a defined upper limit. For each tested point, the energy released to pure nitrogen is higher than to air, which evidences the impact of the oxygen presence under streamer exposure.

An Optical Method to Characterize Streamer Variability and Streamer-to-Flame Transition for Radio-Frequency Corona Discharges

“…Such parameters, stored in the ACIS control unit, can be adjusted by means of a dedicated user interface. The driving voltage is proportional to the max voltage at the electrode [21]; the latter can vary up to 70 kV peak-to-peak. The corona discharge is extremely sensitive to V d .…”

“…The choice of V d value is also critical. A higher V d denotes that more thermal energy is released in the combustion chamber [21]. At the same time, a more advanced ignition timing results in lower in-cylinder pressure at the beginning of the discharge; this requires a reduction in the driving voltage to prevent the streamer-to-arc transition [21,32,36].…”

“…These advantages are related to the three combustion enhancement pathways: thermal, kinetic, and transport [17]. Among all the LTP systems, RF corona was intensively studied in recent years [18][19][20][21], especially in optical engines [11,14,22], to characterize its ability to speed up the early flame kernel generation [15].…”

“…Corona discharge is featured with a strong randomness, which can contribute to the overall CCV. Depending on discharge parameters (mainly the electrode voltage [21,32,34]), pressure and turbulence in the combustion chamber, and mixture composition, streaming behavior is different. Phenomena like branching, transition to arc, and lack of streamers might occur.…”

“…Among all the streamer features, the parameter chosen to investigate the repeatability of the discharge was the max penetration inside the combustion chamber. A higher penetration denotes a higher amount of air-fuel mixture involved in early flame generation (volumetric effect), which is crucial to ensure a robust combustion especially with lean mixtures [11]; there is evidence that links the streamer penetration to the thermal energy released inside the combustion chamber [21] and to the production of radicals and excited species [36]. Thus, a methodology to assess corona discharge variability, in terms of streamer max penetration, is reported.…”