Low Temperature Combustion Using Nitrogen Enrichment to Mitigate $NOx$ From Large Bore Natural Gas Fueled Engines

Low temperature combustion is identified as one of the pathways to meet the mandatory ultra low $NOx$ emissions levels set by the regulatory agencies. Exhaust gas recirculation (EGR) is a well known technique to realize low $NOx$ emissions. However, EGR has many built-in adverse ramifications that negate its advantages in the long term. This paper discusses nitrogen enrichment of intake air using air separation membranes as a better alternative to the mature EGR technique. This investigation was undertaken to determine the maximum acceptable level of nitrogen enrichment of air for a single-cylinder spark-ignited natural gas engine. $NOx$ reduction as high as 70% was realized with a modest 2% nitrogen enrichment while maintaining power density and simultaneously improving fuel conversion efficiency (FCE). Any enrichment beyond this level degraded engine performance in terms of power density, FCE, and unburned hydrocarbon emissions. The effect of ignition timing was also studied with and without $N2$ enrichment. Finally, lean burn versus stoichiometric operation utilizing nitrogen enrichment was compared. Analysis showed that lean burn operation along with nitrogen enrichment is one of the effective pathways for realizing better FCE and lower $NOx$ emissions.