Energy analysis and optimization of iso-octane and n-heptane combustion process

Abstract

The energy analysis and optimization were conducted under different boundary conditions for iso-octane and n-heptane constant volume self-ignition process from the view of energy (HR: Heat Release) and exergy (EL: Exergy Loss) of chemical reactions, which were classified into five series: LTO (Low Temperature Oxidation), H2O2 Loop, Fuel Fragment，H–O and CO–CO2. The results indicate that the HRR (Heat Release Rate) and ELR (Exergy Loss Rate) increase with initial temperature as well as the equivalence ratio increasing before a certain turning point of themselves, however, the decrease of HR and EL are mainly attributed to the shortening of the ignition process. From the view of temperature correspond to the HR/EL of reaction series, the T10 (the combustion temperature of 10% HR/EL released) and T50 of EL of each reaction series are always advanced those of HR, which means there is a conflict between enhancing HR and reducing EL. Considering the cost-effective reaction series, the α, a ratio of the quantity of EL under per unit HR of H2O2 Loop，H–O and CO–CO2 is smaller than that of Fuel Fragment or LTO. Hence, a RSC (Reaction path Segmentation Control) method is proposed for HR and EL control in terms of shifting equivalence ratio between 1500 and 2000K and leading the reactions to take along the optimal α from Fuel Fragment turning to H2O2 Loop，H–O and CO–CO2. The result of RSC method proves the small α by enhancing the high-temperature reactions of H2O2 Loop and CO–CO2 series to make up the reduction of Fuel Fragment series reactions, which results in a lower exergy loss of chemical reaction and turns the reduced part into thermomechanical exergy. In terms of RSC in engine, the process of spray combustion in engine involves the phenomenon that the equivalence ratio varies with the combustion. According to RSC, reducing the global equivalence ratio in the cylinder to strength the RSC transition of the mixture from rich to lean promotes the HRR of equivalence ratio 0.5–1.5 and extends its area, which improves the indicated thermal efficiency.