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

Abstract

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 NO_x emissions increased significantly in the CO additive experiments, which was attributed to accelerated NO_x 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.