Analysis of RF Corona Ignition in Lean Operating Conditions Using an Optical Access Engine

Radio Frequency Corona ignition systems represent an interesting solution among innovative ignition strategies for their ability to stabilize the combustion and to extend the engine operating range. The corona discharge, generated by a strong electric field at a frequency of about 1 MHz, produces the ignition of the air-fuel mixture in multiple spots, characterized by a large volume when compared to a conventional spark, increasing the early flame growth speed. The transient plasma generated by the discharge, by means of thermal, kinetic and transport effects, allows a robust initialization of the combustion even in critical conditions, such as using diluted or lean mixtures. In this work the effects of Corona ignition have been analyzed on a single cylinder optical engine fueled with gasoline, comparing the results with those of a traditional single spark ignition. The impact of Corona on combustion stability has been studied focusing on stable, near-limit, and unstable conditions, increasing the air-fuel ratio starting from stoichiometric conditions. The natural luminosity of the premixed flames has been recorded by means of a high-speed camera, synchronized with the corresponding cycle-indicating analysis system. The high-speed imaging allowed to observe the early flame development, providing information not attainable by means of the indicated pressure. The analysis of the cycle-resolved equivalent flame radii highlights a significant increase of the flame speed in all the operating conditions when Corona ignition is adopted, in addition to a lower standard deviation of the flame radius. The indicating results show an extension of 0.25 lambda units of the engine stable lean limit, with consequent reduced emissions of nitrogen oxides, in the investigated part-load operating point.