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 emissions 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 and a moderate speed of flame propagation. This empirical research aims to investigate the impact of the transient high current ignition strategy on flame kernel development, flame propagation and auto-ignition timing of lean Dimethyl Ether (DME). In this work, a rapid compression machine is employed at engine-relevant conditions, a pressure of ~15 bar and temperature of ~650K. Spark-assistance is applied at the end of compression to enable a spark-assisted compression ignition combustion 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 (up to 11.3J). 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, faster flame propagation is observed resulting in a shorter time requirement for reaching the auto-ignition in lean DME along with a shorter combustion duration.