Possibly one of the most significant advantages that hydrogen may have as a fuel is its potential for increased second-law efficiency, due to fundamental differences in the mechanism of entropy generation during combustion with respect to the usual hydrocarbon-based fuels. A computational investigation of this effect is pursued for the case of mixtures of hydrogen and natural gas combusting in a diesel engine cylinder. The terms of the availability balance during engine operation are studied as a function of hydrogen content of the fuel and the operating parameters of the engine. Of particular importance is the confirmation of results provided in earlier work by the authors that combustion irreversibilities during hydrogen combustion can be drastically reduced. A single-zone computational model of the engine operation is used and the hypothesis of chemical equilibrium is invoked for combustion calculations. For the description of engine processes, such as fuel preparation and heat transfer, computational models established for hydrocarbon fuels are used so that a comparison is performed under the assumption that hydrogen combustion will be feasible in conditions that do not depart exceedingly from current engine configurations. (C) 2007 Elsevier Ltd. All rights reserved.