Analysis of the impact of selected fuel thermochemical properties on internal combustion engine efficiency

In this study we model the effects of 23 different fuels on First and Second Law thermodynamic efficiencies of an adiabatic internal combustion engine. First Law efficiency is calculated using the lower heating value (LHV), while Second Law efficiency is calculated with exergy, which represents the inherent chemical energy available to perform work. We find that First Law efficiency can deviate by as much as 9% between fuels while Second Law efficiency exhibits a much smaller degree of variability. We also find that First and Second Law efficiencies can be nearly the same for some fuels (methane and ethane) but differ substantially for other fuels (hydrogen and ethanol). The differences in First and Second Law efficiencies are due to differences in LHV and exergy for a given fuel. In order to clarify First Law efficiency differences between fuels, as well as the differences between LHV and exergy, we introduce a new term, the molar expansion ratio (MER), defined as the ratio of product moles to reactant moles for complete stoichiometric combustion. We find that the MER reflects an important part of the physics behind fuel-specific efficiency differences as well as differences between First and Second Law efficiencies. We also discuss how First and Second Law efficiencies are affected by two other fuel-specific thermochemical properties, the ratio of specific heat and extent of dissociation in the reaction products following combustion.