Using wavelet-based multiresolution analysis, this study investigates the effect of hydrogen addition on cyclic variability in a natural gas spark ignition engine. The engine is operated at 3000 rpm, and a lean combustible mixture with excess air ratio of 1.4 is used. Three cases are examined: natural gas with no hydrogen added, and natural gas with the addition of 23% and 40% hydrogen by volume. The time series of the indicated mean effective pressure are analyzed over 192 engine cycles. The method of maximal overlap discrete wavelet transform is used to decompose the time series into five levels with different frequency bands, each level consisting of a "detail" signal and an "approximation" signal. The root mean square amplitude of the detail signal at each level is used as a measure of cyclic variability. The results reveal that with the addition of 23% hydrogen, the root mean square value of the detail signal in each of the five bands is less than that for 100% natural gas. When the amount of hydrogen addition is increased to 40%, the root mean square value in each of the five bands is further reduced. In other words, hydrogen addition has a pronounced effect on reducing the cyclic variability of the indicated mean effective pressure. (C) 2011 Elsevier Ltd. All rights reserved.