The purpose of this study is to investigate the effect of adding small amounts of hydrogen to gasoline air mixtures on the performance and exhaust emission characteristics of a spark ignition engine. Four air fuel ratios are used ranging from stoichiometric to very lean. The amount of hydrogen added is varied from 0% to 2.14%, 5.28%, and 7.74% by volume. The test engine is operated at 2000 rpm, and measurements are made over 1000 consecutive engine cycles. From the experimental observations, the effect of hydrogen addition on (a) thermal efficiency, (b) specific fuel consumption, (c) cyclic variations of the indicated mean effective pressure (IMEP), and (d) emissions of CO, NO and unburned hydrocarbons are analyzed. The cyclic NEP variations are analyzed using the coefficient of variation (COV) and wavelet analysis. An advantage of wavelet analysis is that it can identify the dominant modes of variability and delineate the engine cycles over which these modes may persist. (C) 2011 Elsevier Ltd. All rights reserved.