Lean combustion analysis using a corona discharge igniter in an optical engine fueled with methane and a hydrogen-methane blend

The robustness of combustion initiation is one of the main issues of actual spark-ignition engines, especially for highly-diluted or lean mixtures. In this work, the effects on combustion stabilization obtained by the usage of a radio-frequency corona igniter were evaluated on a single-cylinder optical engine. The comparison with a conventional spark igniter was carried out using pure methane fuel and a blend of hydrogen and methane. For each combination of fuel and igniter, the combustion stability was explored at different air-fuel ratios, from stoichiometric conditions to the lean stable limit (up to lambda = 2.0 with the corona igniter and the hydrogen-methane mixture). The combustion analysis was carried out by using the synchronized indicating and imaging data. The latter is essential to estimate the contribution of the corona igniter, which was found to be considerable only before the 5% of mass fraction burned. The corona effect igniter, with respect to a conventional spark igniter, was able to extend the lean stable limit of about 0.15 lambda units with methane fuel, and about 0.10 lambda units with the hydrogen-methane blend in the tested engine point. Early flame analysis confirmed the capability of corona igniter to improve combustion onset speed and to obtain a more stable and repeatable flame kernel. The findings of this study can help for a better implementation of corona ignition with gaseous low-carbon fuels, and in particular to achieve a higher lean limit extension without the drawback of a performance decay given by a substantial hydrogen enrichment.