In order to increase engine transient performance of 20% hydrogen-enriched natural gas, research including the comparing of ETC cycle emissions in different catalysts, different enrichment rates, and data was taken for both 20% HCNG and pure natural gas. The exhaust composition and efficiency was tested using three different oxidization catalysts: ECOCAT type I, a Domestic catalysts, and ECOCAT typeII. Using these three catalysts, the results show that increasing catalytic efficiency results in increased exhaust resistance, reduced engine power and increased BSFC. All of the three catalytic converters reduce the exhaust emissions, therefore, allowing hydrogen-enriched natural gas engines to more easily achieve Enhanced Environmentally Friendly Vehicle (EEV) standards. By increasing the enrichment rate, the engine's torque increases, which results in increased emissions, especially NOx. According to the data taken using 20% HCNG without a catalytic converter emissions values, compared with no HCNG, for NOx, CO, NMHC, CH(4) were 51%, 36%, 60%, 47% the emissions of pure CNG, respectively, with the BSFC at 7%. But as research shows, by using a catalytic converter, hydrogen-enriched natural gas has the potential to achieve European III standards. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.