Autoignition and detonation development induced by a hot spot in fuel-lean and CO2 diluted n-heptane/air mixtures

Abstract

Fuel-lean or diluted combustion is widely used in advanced internal combustion engines (ICEs) such as homogeneous charge compression ignition (HCCI) engines, low temperature combustion (LTC) engines, and engines utilizing exhaust gas recirculation (EGR). The thermal efficiency of ICEs is constrained by knock and super-knock due to end-gas autoignition and detonation development. Therefore, the effects of equivalence ratio and CO2 dilution on autoignition and detonation development induced by a hot spot are numerically investigated here. It is found that the decrease of equivalence ratio and increase of CO2 dilution ratio can both greatly increase the excitation time and reduce the total heat release. Under fuel-leaner or more diluted conditions, the interaction between chemical reaction and pressure wave becomes weaker and thereby the propensity of detonation development is lower. Different autoignition modes are identified and quantified. The excitation time is shown to play a controlling role in the chemical-acoustic interaction and detonation development. It is demonstrated that reducing equivalence ratio and increasing CO2 dilution have the same influence on the autoignition mode if the same excitation time is maintained. Furthermore, the detonation development regimes for n-heptane and dimethyl ether at different conditions are obtained and compared. Non-dimensional parameters used to well quantify the detonation development regime are identified and discussed.