An experimental study on the performance and emissions of a spark ignition engine operating on different ratio of lower heat value gas hydrogen mixtures is conducted in a modified single-cylinder engine. The lower heat value gas is composed of CNG (compressed natural gas), N-2 (nitrogen) and CO2 (carbon dioxide). The results show that the engine indicated thermal efficiency increased with the increase of hydrogen and CNG fraction in the fuel blends. Engine power is influenced by both lower heat value and hydrogen fraction. HC (hydrocarbon) emissions decrease, CO (carbon monoxide) and NO (nitric oxides) emissions increase with the increase of hydrogen fraction. An engine operating on lean-burn lower heat value fuel gas hydrogen mixtures is favorable for getting lower emissions. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.