In this study, while a hydrogen + methane mixture in the gas phase (30% H-2 + 70% CH4 in moles) is injected from an intake port by a gas injector, ignition is supplied with diesel fuel sprayed from a diesel injector. The energy content of the sprayed gas fuel is modified to constitute 0% (only diesel fuel), 15%, 40%, and 75% of the total fuel energy content. All tests are conducted at 1500 rpm stable engine speed and in full-load condition. The effects of energy content modification of hydrogen + methane gas fuel on engine performance, emissions, and combustion are observed. Both NO and soot emissions are taken under control with 15% and 40% energy content rates in the gas fuel compared to the diesel-only condition. Although an increase is observed in CO and THC emissions with gas fuel addition compared to the diesel only condition, an improvement is considered compared to the results obtained for only methane fuel usage with diesel fuel. Moreover, gas fuel mixtures reach a high value (75% energy content) as peak cylinder gas pressure decreases. A single-phase heat release was observed, and the diesel combustion characteristic of the engine was converted into gasoline combustion with the addition of 75% hydrogen + methane as the energy basis. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.