In this study, the effects of hydrogen addition on the engine performance were investigated using spark ignition engine fueled gasoline with a compression ratio of 15 at an air excess ratio (lambda) of 1.8 and above. At lambda = 1.8, the indicated thermal efficiency at the spark timing of the knock limit reached the maximum level under the conditions in which the hydrogen fraction was set to 4% of the heating value of the total fuel. Based on a heat balance analysis, the best hydrogen fraction was found as a balance between the improvement in the burning efficiency and the increase in heat loss. The lean limit was extended when the hydrogen fraction was increased from lambda = 1.80 to lambda = 2.28. The hydrogen addition achieved the maximum indicated thermal efficiency at spark timing of the knock limit was obtained at lambda = 2.04, where the hydrogen fraction was 10%. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.