Extension of Lean and Diluted Combustion Stability Limits by Using Repetitive Pulse Discharges

A newly developed small-sized IES (inductive energy storage) circuit with a semiconductor switch at turn-off action was successfully applied to an ignition system. This IES circuit can generate repetitive nanosecond pulse discharges. An ignition system using repetitive nanosecond pulse discharges was investigated as an alternative to conventional spark ignition systems in the previous papers. Experiments were conducted using constant volume chamber for CH₄ and C₃H₈-air mixtures. The ignition system using repetitive nanosecond pulse discharges was found to improve the inflammability of lean combustible mixtures, such as extended flammability limits, shorted ignition delay time, with increasing the number of pulses for CH₄ and C₃H₈-air mixtures under various conditions. The mechanisms for improving the inflammability were discussed and the effectiveness of IES circuit under EGR condition was also verified. Moreover, an application of this ignition system to a SI engine was conducted preliminarily and the effectiveness of IES circuit for the real engine was also confirmed. Following previous studies, the present study moves to more realistic development stage using two experimental configurations same as former studies; a constant volume chamber and single-cylinder SI engine. With a constant volume chamber, the effectiveness of IES circuit for Iso-octane (C₈H₁₈) and conventional ignition plug is investigated, and the flame kernel formation process was observed using Schlieren photography with a high-speed camera to make the difference of the ignition mechanism between IES circuit and conventional ignition circuit clear. With a single-cylinder SI engine, two experiments are carried out to qualify the difference of the inflammability between IES circuit and conventional ignition circuit. The operational limit of lean combustion is investigated. The result shows that the lean limit is extended from 20 to 23 in A/F at IMEP 440 kPa by replacing the ignition system from a conventional one. Also, the diluted limit is investigated. The result shows that the diluted limit is extended from 17.5% to 22.5% in EGR ratio at IMEP 630 kPa and that thermal efficiency is improved by 5%. The ignition delay and the combustion duration are decreased and the cycle-to-cycle variation of the ignition timing and IMEP are decreased.