Hydrogen has many excellent combustion properties for improving combustion and emissions performance of spark ignition (SI) engines. This article presents a comparative energy and exergy analyses of a hydrogen-methane blends fuelled direct injection diesel engine. The engine was modified to realize hydrogen port injection by installing hydrogen injectors in the intake manifolds. Engine was used to obtain the experimental data at full load conditions for constant 3100-rpm speed. It is shown that exhaust exergy is inversely proportional to the hydrogen addition as well as bio diesel. After hydrogen addition brake thermal efficiency was rised and reached almost a constant value in a wide range. Regarding emission levels, CO2 and HC emissions were reduced with hydrogen addition but CO emission tended to increase when excess air ratio was approaching stoichiometric conditions. However, when the engine was gradually leaned out, CO emission from the hydrogen-enriched engine was reduced. NOx emissions increased with the increase of hydrogen addition due to the raised cylinder temperature. The data are also very useful for operational changes needed to optimize the hydrogen fuelled SI engine design. Thus it has been shown that hydrogen is a good choice as an alternative engine fuel. (C) 2015 Elsevier Ltd. All rights reserved.