A comparative study of the combustion of methane in a spark-ignited engine using CO2-diluted oxidizer mixtures

To investigate a potential method to eliminate NOx and facilitate carbon capture in reciprocating engines for stationary power generation, an experimental and numerical study is conducted to understand the effect of CO2 dilution on the performance of oxy-CH4 combustion in an air-cooled SI engine. Experiments are conducted at constant fuel mean effective pressure (fMEP) and at maximum brake torque (MBT) spark timing while the CO2 in the oxidizer is gradually replaced by N2 and the effect on engine performance is quantified. The increased CO2 dilution results in a decrease in the ratio of specific heats (g) and flame speed. Due to the significant thermal quenching effect of CO2, there is an overall decrease in cylinder pressure and temperature leading to a total decrease of indicated efficiency, even when operating in a significantly oxygen-enriched environment. The interplay between turbulent flame propagation and the composition of generalized oxidizer mixtures consisting of O2, N2 and CO2 is further explored using predictive modelling and compared with premixed, turbulent combustion theory. This analysis ultimately demonstrates that the dilution limit of the oxy-CH4 combustion event is captured by the previously developed premixed combustion theory when compared against an experimental measurement of combustion instability, and that this instability is largely driven by the change of thermal mixture properties due to the introduction of large quantities of CO2.