Experimental comparison of performance and emissions of a direct-injection engine fed with alternative gaseous fuels

Abstract

Performance and emissions of a direct-injection (DI) spark-ignition engine with high compression-ratio (15.5:1) fed with hydrogen, hydrogen-rich reformate, and methane are discussed. Differences in the particle formation with hydrogen and the reformate containing 75% mol H2 and 25% mol CO2 are investigated for the first time and compared with methane. The total particle number (PN) concentration with the reformate was the highest at all loads because of its significantly longer injection duration. This new finding explains why the elevated PN emission was measured in a DI engine fed with the reformate in contrast to the previously published data for hydrogen and the reformate PFI engines. Hydrogen combustion resulted in higher particle formation compared to methane under high engine loads. The combined effect of the flame quenching distance, fuel carbon content, and injection duration defines the PN formation in a DI ICE fed with gaseous fuel. The study findings demonstrated the appearance of an optimal hydrogen injection strategy enabling the minimization of PN emission. Hydrogen combustion results in the highest engine thermal efficiency. However, when the exhaust waste heat utilization in the engine with the reformate combustion is considered, the overall system thermal efficiency above that of the hydrogen engine is achieved. Negligible and the lowest, in comparison to hydrogen and methane, NOx emissions are found with the reformate-fed engine. CO2 contained in the reformate acts as an inherent EGR enabling almost full mitigation of NOx formation even at the highest engine loads.