Improvement in Oxy-fuel Combustion of SI Engine Fueled by Natural Gas

Oxy-fuel combustion (OFC), in which fuel is burnt with pure oxygen, is a promising method by which to establish a CO2 recovery system from engine exhaust. The ideal exhaust gas for OFC consists of only CO2 and H2O at a stoichiometric mixture ratio, which can be easily separated by cooling the exhaust. In OFC, exhaust gas recirculation (EGR) is applied in order to avoid an extreme increase of the combustion temperature by increasing the heat capacity. In the present study, in order to improve the OFC of a natural-gas spark-ignited engine, the effect of the operating conditions on the combustion characteristics was investigated using a rapid compression and expansion machine. The effects of the compression ratio, ignition timing, and CO2 concentration in the mixture were examined while keeping the fuel-oxygen mixture at the stoichiometric ratio. The obtained results show that by increasing the compression ratio from 10.1 to 17.5, the gross indicated thermal efficiency of OFC can be increased to the same level as air-fuel combustion by selecting the optimal ignition timing. In addition, it is observed that a lower CO2 concentration produces rapid heat release but decreases the gross thermal efficiency due to increased cooling loss. Furthermore, the effect of ozone addition to an oxy-fuel mixture is also examined.