Effects of oxygen enrichment on combustion instability under super lean and high EGR conditions

Super lean and high EGR combustion have been studied to improve the indicated thermal efficiency in SI engines. The stability of ignition and flame propagation is essential for super lean and high EGR combustion. Therefore, we focused on the oxygen enrichment as a method to enhance combustion. First, in order to examine the fundamental effect of oxygen enrichment on combustion under lean conditions, strain rates at the extinction limit for various oxygen concentrations were investigated in strained flames by using a counter-flow burner. Oxygen concentration was varied from 21% to 30%. Equivalence ratio was varied from 0.7 to 0.8. As the oxygen concentration increased, strain rates at the extinction limit increased for each equivalence ratio. That is, oxygen enrichment extended the extinction limit. Next, the effects of oxygen enrichment on combustion under super lean and high EGR conditions were investigated by using a Rapid Compression Expansion Machine. NOx emission was also measured. The fluctuation of pressure history was suppressed by oxygen enrichment without EGR, while it was suppressed or promoted by oxygen enrichment depending on equivalence ratio with EGR. The amount of NOx emission in oxygen enriched condition was the same as that with air in the order of magnitude. It can be concluded that combustion instability under super lean and high EGR conditions was improved by oxygen enrichment.