The effects of different intake charge diluents on the combustion and emission characteristics of a spark ignition natural gas engine

• CO2 is the most effective on reducing NOx and CO emission followed by N2 and then Ar.
• NOx and CO emission could be simultaneously reduced by intake charge dilution regardless of the diluents when appropriate dilution extent is chosen.
• Both CO2 and N2 dilution worsen while Ar dilution improves thermal efficiency.
• Thermal effect is a dominant factor for reducing NOx emission.

Exhaust gas recirculation (EGR) is the most common method to control NOx emission of internal combustion engine. The major components of EGR are CO2 and N2, which have different influences on engine combustion and pollutants formation through thermal, dilution and chemical effects. The main objective of this work is to investigate the different influences of CO2 and N2 on engine combustion and emission on a four-cylinder, turbo charged, spark ignition natural gas engine with electronically control unit, simultaneously to separate the thermal effect with the comparison with Ar. It was found that the peak in-cylinder pressure and heat release rate both decreased along with the increase of intake dilution extent regardless of the diluent's type. For each diluent gas, NOx emission decreases while HC emission increases with the increased dilution ratio. However, CO emission firstly decreased and then increased. Results also revealed that NOx and CO emission could be simultaneously reduced by intake charge dilution at a little sacrifice of HC emission. The effects of three diluents are different compared with each other. Among these three diluents, it can be found that CO2 is the most effective on reducing NOx and CO emission followed by N2. However, both CO2 and N2 dilution deteriorates the thermal efficiency while Ar dilution improved it. Besides, when NOx emission was reduced to the same level, the thermal efficiency is the highest and CO emission is the lowest for Ar dilution.