Combustion Behavior of an RF Corona Ignition System with Different Control Strategies

It has been proved that Radio Frequency Corona, among other innovative ignition systems, is able to stabilize combustion and to extend the engine operating range in lean conditions, with respect to conventional spark igniters. This paper reports on a sensitivity analysis on the combustion behavior for different values of Corona electric control parameters (supply voltage and discharge duration). Combustion analysis has been carried out on a single cylinder PFI gasoline-fueled optical engine, by means of both indicating measurements and imaging. A high-speed camera has been used to record the natural luminosity of premixed flames and the obtained images have been synchronized with corresponding indicating acquisition data. Imaging tools allowed to observe and measure the early flame development, providing information which are not obtainable by a pressure-based indicating system. At different air-fuel ratio values, ranging from stoichiometric to lean limit, combustion stability has been evaluated for each combination of supply voltage and corona duration. Varying these parameters, it has been possible to assess the operating range of the corona igniter. In some limiting instances (far from normal operating range) not all the Corona igniter electrode tips were able to generate a discharge, worsening combustion onset: a statistical analysis of this phenomenon has been carried out. Cycle-resolved equivalent flame radii development and flame probability at fixed equivalent radius have been investigated for each condition. Finally, a suitable dataset of supply voltage and corona duration values, which guarantee a fast combustion onset and stable conditions, has been determined.