The thermodynamic relationship between reducing exergy destruction and improving thermal efficiency in internal combustion engines

Abstract

In this study, the effects of various overall equivalence ratios and injection strategies on the heat release rate were conducted adopting the exergy analysis method, and provides a detailed explanation of the thermodynamic relationship between reducing exergy destruction and improving thermal efficiency. The results revealed that exergy destruction during the combustion process is reduced by approximately 400 J, when the overall equivalence ratio decreased from 0.96 to 0.62, and thermal efficiency improved from about 35.0 % to 47.0 %. The equivalence ratio was further decreased to 0.38, the exergy destruction was reduced by 380 J, while the thermal efficiency was only marginally improved by less than 2 %. This indicates that the relationship between the reduction of exergy destruction and thermal efficiency is not linear. In addition, the indicated thermal efficiency can be improved by reducing the overall equivalence ratio and utilizing two-injection. The method of reducing the overall equivalence ratio changes the high-temperature stratification of the heat release, leading more mixture to enter the lean high-temperature area for reaction, while reducing the exergy destruction, which is a way to optimize the combustion “quality”. The two-injection approach modifies the equivalence ratio stratification, allowing more mixture to reach the low equivalence ratio region, where the thermomechanical exergy (It represents the work potential of the combustion process) released per unit volume of the mixed gas is high, and the exergy destruction increases, which is a method to increase the “quantity” of thermomechanical exergy. Therefore, reducing exergy destruction is a sufficient and unnecessary condition for improving thermal efficiency.