A numerical simulation was conducted to investigate the combustion characteristics and effects of Hydrogen-Hydrogen Oxygen (HHO) addition on performance and NOx emission of a spark ignition (SI) biogas engine. At a given operating condition, indicative work cycle Wi increases with increase of HHO, CH4 concentrations and load rate; but it decreases with engine speed increase and reaches peak value as variation of equivalence ratio and advanced ignition timing. Combustion temperature T and NOx concentration in exhaust gas also increase with increase of HHO, CH4 concentrations, load rate and advanced ignition timing; but it decreases with engine speed increase, and reaches peak value as variation of equivalence ratio. The increase rate of NOx concentration with equivalence ratio is much higher than that of Wi and T. When HHO concentration in biogas is lower than 30%, the gain of Wi is advantageous before the increase of NOx. The addition HHO is more interesting as biogas engine operates with lean mixture. At a given engine speed, optimal ignition timing reduces 6 crankshaft angle degrees as adding 30% HHO to biogas. A compromise between performance and NOx emission can be obtained by appropriate adjustment of operating conditions of the engine fueled
with HHO enriched biogas.