The overall effect of hydrogen addition to methane fuel on the performance of an SI engine is determined for highway and urban driving cycles. Various engine operating schemes are investigated including fixed equivalence ratio and hydrogen fraction with varying load or throttle position (scheme 1), varying equivalence ratio and fixed hydrogen fraction with varying load (scheme 2), and varying equivalence ratio and hydrogen fraction with varying load (scheme 3). scheme 2 is determined to be the best method of operating the engine with optimum hydrogen volume fractions in the range of approx. 11-31 vol% for the highway cycle and 11-38 vol% for the urban cycle. The optimum values are based on maximizing the total distance covered for a full tank of fuel and minimizing the total amount of CO2 produced. It is also shown that except for unburned hydrocarbons, emissions of the other pollutants. CO and NO are substantially reduced below values for the base case of pure methane operating at a fixed equivalence ratio of unity. (C) 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.