High-Speed Optical Diagnostics of Misfire Limits in a Spark-Ignited Heavy-Duty Hydrogen Engine

This study investigates the ignitability of hydrogen in an optical heavy-duty SI engine. While the ignition energy of hydrogen is exceptionally low, the high load and lean mixtures used in heavy-duty hydrogen engines lead to a high gas density, resulting in a much higher breakdown voltage than in light-duty SI engines. Spark plug wear is a concern, so there is a need to minimise the spark energy while maintaining combustion stability, even at challenging conditions for ignition. This work consists of a two-stage experimental study performed in an optical engine. In the first part, we mapped the combustion stability and frequency of misfires with two different ignition systems: a DC inductive discharge ignition system, and a closed-loop controlled capacitive AC system. The equivalence ratio and dwell time were varied for the inductive system while the capacitive system instead varied spark duration and spark current in addition to equivalence ratio. A key finding was that spark energy correlated well with ignitability, as long as the spark was sufficiently stable. In the second phase of this study, we employed an intensified high-speed camera to directly view the early flame development process. Two distinct types of misfires were identified: flame kernels could be quenched as they were convected away from the spark plug, or by the spark plug shell and ground electrode. Flame kernels were typically extinguished within 300 μs after the end of the spark, but their lifetimes varied with spark duration in a way that suggests that flame kernels can be extinguished even during an ongoing spark. Furthermore, the heat release of fired cycles could be delayed both by unusually slow flame development and due to quenching effects of the spark plug.