The Impact of Spark Discharge Pattern on Flame Initiation in a Turbulent Lean and Dilute Mixture in a Pressurized Combustion Vessel

An operational scheme with fuel-lean and exhaust gas dilution in spark-ignited engines increases thermal efficiency and decreases NOx emission, while these operations inherently induce combustion instability and thus large cycle-to-cycle variation in engine. In order to stabilize combustion variations, the development of an advanced ignition system is becoming critical. To quantify the impact of spark-ignition discharge, ignitability tests were conducted in an optically accessible combustion vessel to characterize the flame kernel development of lean methane-air mixture with CO₂ simulating exhaust diluent. A shrouded fan was used to generate turbulence in the vicinity of J-gap spark plug and a Variable Output Ignition System (VOIS) capable of producing a varied set of spark discharge patterns was developed and used as an ignition source. The main feature of the VOIS is to vary the secondary current during glow discharge including naturally decaying and truncated with multiple strikes. These discharge patterns were studied to characterize the interaction of discharge phases and initial flame formation. High-speed Schlieren optical setup was employed for visualization with synchronous measurement of discharge waveforms. The results showed that multi-strike discharge was able to generate multiple flame kernels whose interactions affect flame initiation. With proper timing of each discharge event the individual kernels merge and lead to propagating flame. However, this flame initiation is highly subjected to the flow field in the vicinity of spark plug. Based on these observations, a mathematical description of the discharge timing requirements is formulated to describe the multi-kernel flame initiation under turbulence.