Quantitative study on chemical effects of actual/simulated recirculated exhaust gases on ignition delay times of n-heptane /ethanol fuel blends at elevated temperature

Abstract

Three sets of exhaust gas recirculation (EGR) are introduced into shock tube experiments to measure and analyze their influences on n-heptane/ethanol auto-ignition behavior. EGR 1 and EGR 2 are simulated recirculated exhaust gases, and EGR 3 is the field collection of the low pressure and cooled EGR from a single-cylinder engine operating under reactivity controlled compression ignition (RCCI) regime, with major components being N2/CO2/O2/CO/Unburnt hydrocarbon (UHC). CO and UHC are the main incomplete oxidation products (IOP). Ignition delay times of n-heptane/ethanol blends mixed with these three sets of EGRs are measured at the equivalence ratios of 0.5, 1, and 1.5, temperature range of 950–1350 K, pressures of 10 and 18 atm, and EGR ratios of 0%, 30%, and 50%. Experiment results show that the chemical effects of O2 and IOP are weaker compared with the physical effects of EGR; they, however, should not be neglected since these two effects do promote the ignition to some extent. Quantitative analyses suggest that the increase of equivalence ratio and EGR ratio leads to a stronger promotion effect of O2, while the effect of pressure is minimal. The chemical effect of IOP decreases with increasing equivalence ratio, and unnoticeable correlation is observed between pressure and the chemical effects of IOP. Sensitivity analysis has been conducted to identify the predominant reactions during the ignition. The current work is helpful to understand the effects of EGR thoroughly, especially in RCCI combustion regime.