Study on promotion of lean combustion by oxygen enrichment using a rapid compression expansion machine

While lean combustion in SI engine is one of the promising methods to increase the thermal efficiency and reduce the pollutant emissions, it causes instability of ignition and combustion due to the low burning velocity. Thus, we propose to increase the combustion intensity by incorporating oxygen enrichment with lean combustion. The objective of present study is to investigate the effects of oxygen enrichment on lean combustion behavior in SI engine. Premixed methane-air-oxygen mixture was combusted in a rapid compression expansion machine with varying their concentration. Experimental conditions were determined by identifying the conditions where the laminar burning velocity significantly changes by oxygen enrichment with 1-D simulation of premixed combustion. Histories of pressure were measured to evaluate heat release rate. Luminous flame was captured by a high-speed camera to observe the flame propagation. As a result, laminar burning velocity increased with oxygen concentration, which caused reduction of combustion period and enhancement of the indicated thermal efficiency. On the other hand, this trend was suppressed when the fuel concentration is lower than 6.85 %. In addition, when the fuel concentration is relatively large, flame propagation speed increased with increase in oxygen concentration, which can cause autoignition of end gas. NOx emission decreased or kept almost constant with increase in oxygen concentration, which might be due to the shortened combustion duration.