Study of the heat release and exergy loss of iso-octane self-ignition under engine-like conditions

Abstract

With the severe low-carbon policies, energy utilization from the maximum useful-work exergy becomes one of the breakthrough points towards engine high efficiency. Hence, for constraining the combustion which is the main exergy loss source, the energy analysis from second law is imperative for the further efficiency improving. In this paper, the influence of boundary conditions on the heat release and exergy loss of chemical reaction during the constant-volume combustion of iso-octane at engine-like conditions was studied. It was found that the boundary conditions: initial temperature of 700 K, 900 K and 1100 K, equivalence ratio of 0.5, 1.0 and 1.5, have the same trend effect on the heat release and exergy loss rates of the same chemical reaction. It is impossible to increase the heat release and reduce the exergy loss at the same time, but different equivalent ratios have different degree effects on the reaction. Hence, a parameter α, the ratio of exergy loss on heat release, which stands for the exergy loss of unit heat release was promoted to consider the cost feature of chemical reactions. The results show that enhancing the medium and high temperature reaction during the combustion process is beneficial to reduce the parameter α, and the lean mixture is beneficial to obtain a small α. From the perspective of reaction type, it showed that the α of Fuel Fragment series reaction is the largest and its α is controlled by both temperature and equivalent ratio, the rich mixture should be conducive to reduce its α. The α of H2O2 Loop, H-O and CO-CO2 series reaction are small, and they are mainly controlled by the equivalence ratio, and lean mixture is favourable for reducing their α. Therefore, under the constant equivalence ratio, the best way to optimize the heat release and exergy loss is to control the equivalence ratio as small as 1.0. Under the strategy of the variable equivalence ratio, it is the best to change the mixture from rich to lean in the combustion process, and the reactions would sequentially go through the best α region of Fuel Fragment, H-O, CO-CO2, H2O2 Loop reactions, which would keep the α of whole combustion process maintain at the lowest level.