Investigation of Transient High Current Ignition for Lean Mixture in Spark Assisted Compression Ignition

Lean burn combustion is an effective strategy to reduce the in-cylinder temperature, hence reduce NOx emission and increase the thermal efficiency of the system. One essential aspect of successful combustion is the flame kernel initiation and development. However, as the fuel-air mixture becomes leaner, challenges arise in achieving a stable flame kernel initiation. This empirical research aims to investigate the impact of the transient high current strategy on early flame initiation in lean mixtures of Dimethyl Ether (DME). Additionally, with the onset of auto-ignition, the impact has been further investigated for the overall combustion period. In this work, a rapid compression machine is utilized to create an engine-relevant conditions, pressure of 16.4 bar and temperature of ~620K, with DME as a volatile and highly reactive fuel. Prior to auto-ignition, spark-assistance is applied to enable a spark-assisted compression ignition mode. The spark event is initiated by a transient high current ignition system, which includes a traditional transistorized coil ignition and an in-parallel high-voltage capacitor for boosting the transient current enabling high discharge energy (upto 8.3J). The combustion process is qualitatively assessed with high-precision pressure data acquisition along with high-speed images and quantitatively processed through image processing. Test results indicate that as spark energy increases, an early flame kernel formation is observed resulting in a faster flame propagation speed in lean DME mixture and a shorter time requirement for reaching the auto-ignition phase. As the flame kernel propagates, the local pocket auto-ignited leading to a high intensity burning as observed in the chamber pressure history.