In recent years, from a viewpoint of global warming and energy issues, the need to improve vehicle fuel economy to reduce CO2 emission has become apparent. One of the ways to improve this is to enhance engine thermal efficiency, and for that, automakers have been developing the technologies of high compression ratio and dilute combustion such as exhaust gas recirculation (EGR), and lean combustion. Since excessive dilute combustion causes the failure of flame propagation, combustion promotion by intensifying in-cylinder turbulence has been indispensable. However, instability of flame kernel formation by gas flow fluctuation between combustion cycles is becoming an issue. Therefore, achieving stable flame kernel formation and propagation under a high dilute condition is important technology.
This paper discusses the most suitable discharge specification for stable high dilute combustion based on the mechanism of flame kernel formation in various gas flow conditions, and then shows the expanding effects of dilute combustion limit compared with conventional ignition technology. Engine tests were conducted in various engine conditions. In the high gas flow condition, keeping high discharge current achieved stable flame kernel formation because of the suppressed discharge blow-off, and in low gas flow conditions, expanding discharge duration improved dilute combustion limit because of heat loss reduction. As a result, controlling discharge current to optimal values under each engine conditions could achieve superior engine thermal efficiency. In addition, it achieved ignition system energy savings.
