The Effect of High-Power Capacitive Spark Discharge on the Ignition and Flame Propagation in a Lean and Diluted Cylinder Charge

摘要

Research studies have suggested that changes to the ignition system are required to generate a more robust flame kernel in order to secure the ignition process for the future advanced high efficiency spark-ignition (SI) engines. In a typical inductive ignition system, the spark discharge is initiated by a transient high-power electrical breakdown and sustained by a relatively low-power glow process. The electrical breakdown is characterized as a capacitive discharge process with a small quantity of energy coming mainly from the gap parasitic capacitor. Enhancement of the breakdown is a potential avenue effectively for extending the lean limit of SI engine. In this work, the effect of high-power capacitive spark discharge on the early flame kernel growth of premixed methane-air mixtures is investigated through electrical probing and optical diagnosis. The capacitance paralleled to the spark gap and the in-line resistance installed in the capacitive discharge loop are varied to produce different levels of breakdown enhancement. The burnt fraction and the ignition success rate of the premixed methane-air mixtures are analyzed to evaluate the ignitability improvement. The results indicated that the increase of the capacitance can increase the discharge current and the transient power. It is observed that the voltage buildup rate decreased because of the addition of the capacitor. Different to the conventional peaking capacitor ignition techniques, which normally employ non-resistor spark plugs, the high-power capacitive discharge ignition described in this paper employed an in-line resistor to lower the transient discharge current which might lower the electromagnet interference associated with the high-power capacitive discharge. With a 200pF parallel capacitance, the addition of a 50Ω resistance cuts off 27% peak discharge current compared with a conventional non-resistor case while maintains a same lean ignitable boundary under low pressure experiment conditions.