Experimental study on the effects of gas additives on uncatalyzed partial oxidation of methane in a spark-ignition engine

This study investigated the effects of gas additives (H2, CO, N2, H2O, and CO2) on the uncatalyzed partial oxidation of methane in a spark-ignition engine. The combustion phases, exhaust compositions, and performance outcomes of each additive were compared. The dilution components (N2, H2O, and CO2) impeded methane combustion owing to their thermal effects, with CO2 exhibiting the most pronounced delay in combustion. The addition of CO2 and H2O led to increased CO and H2 yields, respectively, owing to shifts in the water gas shift reaction equilibrium. In contrast, fuel additives (H2 and CO) enhanced combustion by increasing the flame initiation and propagation speed, resulting in reduced unburned CH4 and increased work. Negligible soot emissions were detected for all the experimental cases, whereas NOx emissions increased significantly in the CO additive experiments, which was attributed to accelerated NOx formation reactions. This comprehensive analysis provides insights into the combustion and reforming characteristics of methane-based mixed gases, such as natural gas and biogas, during partial oxidation using internal combustion engines.