Optical study of oxygen enrichment on methane combustion characteristics under high compression-ratio conditions

Abstract

Natural gas is a promising fuel for IC engines with low carbon emissions, whereas its poor lean combustion performance at low load and low engine power output at high load remain a challenge for automobile manufacturers. Oxygen-enriched combustion can provide more opportunities for collisions between fuel and oxygen molecules; thus, methane combustion performance can be improved. In this work, using a high-compression-ratio optical engine, the effects of oxygen enrichment on methane combustion were comprehensively investigated, addressing the similarities and differences between high load and low load conditions. High-speed direct photography and simultaneous pressure were performed to capture detailed combustion evolutions. The results show that high oxygen concentration can greatly increase methane flame propagation speed under high energy density condition (high load). As a result, advanced combustion phase and high peak pressure are obtained by more concentrated heat release, which in turn are indicators for improvised thermal efficiencies. Under low energy density condition (low load), oxygen enrichment mainly acts as a promoter of the initial flame formation, which in turn are indicators for improvised combustion stability. A detailed comparison of flame characteristics confirms the different effects of oxygen enrichment on methane combustion under different energy density conditions. Besides, oxygen enrichment can significantly promote initial flame formation with a higher rate and lower cyclic variation, and a low degree of oxygen enrichment can also have a great effect. The current study shall give insights into the strategies for oxygen enrichment on methane engines’ performance at different loads.