The effects of HHO-enriched air on the combustion process and emission variation of simulated-biogas fueled spark ignition engine

This paper investigates enhancing the operation characteristics and reducing pollutants of a simulated biogas (65 % CH₄ and 35 % CO₂) engine by the implementation of HHO gas. The simulation results showed that the appearance of HHO as an additive in charged air will shorten the ignition delay and combustion duration. HHO combustion lasts 40°, shorter than 44.5° without HHO. The RoHR peaks at 41.68 J/deg at 375 deg CA with HHO, compared to 34.52 J/deg without it. The maximum pressure rise is 1.90 bar/deg for gasoline, 0.97 bar/deg for simulated biogas, and 1.08 bar/deg for HHO-enriched biogas. The peak cylinder pressure was slightly raised from 42.12 bar to 45.73 bar with HHO aid. A system for supplying HHO to the intake manifold of the test engine was devised in a pilot model to conduct experiments. In comparison to the original gasoline engine, the average brake power of the biogas-fueled engine degraded by 39.1 % under full throttle conditions. The coefficient of variation of speed (COV_speed) under idling conditions increased from 0.31 % for gasoline to 1.58 % for the simulated biogas engine. However, as a small volume of HHO was presented in charge air, the biogas-fueled engine's performance and fuel economy improved by 7.86 % and 4.5 % at full-throttle conditions. Engine stability enhanced significantly as the COV_speed reduced from 1.58 % to 0.47 % with the HHO additive. The engine's exhaust emissions changed remarkably when operating with the HHO additive. Specifically, CO was reduced by 20.2 % on average at fully operated throttles and 6.5 % to 19.4 % at a constant speed of 4200 rpm; HC was moderately decreased 14.2 % on average at full throttle conditions and by 18.6 % on average at a constant speed of 4200 rpm; NO_x emissions increased marginally from 9.4 % to 33.4 % at full throttle conditions and an average increase of 35.5 % at a constant speed of 4200 rpm.