Comparative Analysis between a Barrier Discharge Igniter and a Streamer-Type Radio-Frequency Corona Igniter in an Optically Accessible Engine in Lean Operating Conditions

Among plasma-assisted ignition technologies, the Radio-Frequency (RF) corona family represents an interesting solution for the ability to extend the engine operating range. These systems generate transient, non-thermal plasma, which is able to enhance the combustion onset by means of thermal, kinetic and transport effects. Streamer-type RF corona discharge, at about 1 MHz, ignites the air-fuel mixture in multiple filaments, resulting in many different flame kernels. The main issue of this system is that at high electrode voltage and low combustion chamber pressure a transition between streamer and arc easily occurs: In this case transient plasma benefits are lost. A barrier discharge igniter (BDI), supplied with the same RF energy input, instead, is more breakdown-resistant, so that voltage can be raised to higher levels. In this work, a streamer-type RF corona igniter and a BDI were tested in a single-cylinder optical engine fueled with gasoline. Combustion behavior was characterized at stable, near-stable and unstable conditions (depending on IMEP coefficient of variation), increasing the air-fuel ratio starting from a near-stoichiometric mixture. For each test point the maximum allowable electrode voltage for both the igniters was used: In the streamer-type corona case it had to be limited because of the arc transition. Both indicating and imaging analysis were carried out, the latter by means of a high-speed camera that records the natural luminosity of early flames. Raw exhaust gas analysis was carried out as well. Results in terms of cycle-to-cycle variability, combustion duration, pollutant emissions, flame radii evolution and flame growth speed, as well as flame probability presence, are presented. The outcomes of this research can improve the knowledge on streamer-type corona and BDI and can be used to assess the operating range for the control parameters of these igniters.