Mixtures of hydrogen and natural gas are considered viable alternative Fuels to gasoline due to Lower overall pollutant emissions but suffer from problems associated with on-board storage resulting in limited vehicle range. To data, vehicle engines have run on Hythane using a fixed hydrogen-natural gas fraction of 20% and fixed air-to-fuel ratio but it may be possible to leverage greater advantage from hydrogen addition due to its unique lean-burn properties. This study presents the results of a one-cylinder CFR engine test with mixtures of hydrogen in methane of 0, 20, 40 and 60% by volume. Each fuel was tested at speeds of 700 and 900 rpm, full and part loads, and equivalence ratios from stoichiometric to the partial burn limit. These results are used in a driving cycle simulation which is presented in a companion paper. (C) 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.