Comparative second-law analysis of internal combustion enginge operation for methane, methanol, and dodecane fuels

A method for both combustion irreversibility and working medium availability computations in a high-speed, naturally-aspirated, four-stroke, internal combustion engine cylinder is presented. The results of the second-law analysis of engine operation with n-dodecane (n-C₁₂H₂₆) fuel are compared with the results of a similar analysis for cases where a light, gaseous (CH₄) and an oxygenated (CH₃OH) fuel is used. The rate of entropy production during combustion is analytically calculated as a function of the fuel reaction rate with the combined use of first- and second-law arguments and a chemical equilibrium hypothesis. It is shown theoretically that the decomposition of lighter molecules leads to less entropy generation compared to heavier fuels. This is verified computationally for the particular fuels and the corresponding decrease in combustion irreversibility is calculated. Special reference is made to the effect of the lower mixing entropy of the exhaust gas of an oxygenated fuel (CH₃OH) as a contribution to the discussion of the advantages and disadvantages of the use of such fuels.