Comparative exergy models for naturally aspirated gasoline and hydrogen fuelled spark ignition internal combustion engines were developed according to the second law of thermodynamics. A thorough analysis of heat transfer, work, thermo mechanical, and chemical exergy functions was made. An irreversibility function was developed as a function of entropy generation and graphed. A second law analysis yielded a fractional exergy distribution as a percentage of chemical exergy of the intake. It was found that the hydrogen fuelled engine had a greater proportion of its chemical exergy converted into work exergy, indicating a second law efficiency of 41.37% as opposed to 35.74% for a gasoline fuelled engine due to significantly lower irreversibilities and lower specific fuel consumption associated with a hydrogen fuelled ICE. The greater exergy due to heat transfer or thermal availability associated with the hydrogen fuelled engine occurs due to a greater amount of convective heat transfer associated with hydrogen combustion. However, this seemingly high available thermal energy or thermal 'exergy' is misleading due to the higher cooling load which decreases the power of a hydrogen fuelled ICE. Finally, a second law analysis of both hydrogen and gasoline combustion reactions indicate a greater combustion irreversibility associated with gasoline combustion. A percentage breakdown of the combustion irreversibilities were also constructed according to information found in literature searches. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.