Auto-ignition characteristics of methane/n-heptane mixtures under carbon dioxide and water dilution conditions

Abstract

To further optimize the combustion performance of natural-gas/diesel engine under exhaust gas recirculation (EGR) condition, influence of physicochemical impacts of H2O and CO2 on the ignition characteristics of n-heptane/methane mixture (Φ = 0.5/1.0) under various thermodynamic conditions (p = 2 bar/1199 K < T < 1568 K, p = 60 bar/700 K < T < 1200 K) were investigated by shock tube and NUI mechanism. Experiments indicated at p = 2 bar/1199 K < T < 1568 K, CO2 and H2O additions increased and shortened ignition delay times (IDT) respectively. Mixture of CO2 and H2O slightly accelerated ignition. Original and minor modified NUI mechanism well captured the inhibition and acceleration effect of CO2 and H2O on ignition respectively. Both thermal and chemical effects of CO2 (R36:CO + OH = CO2 + H) were responsible for its ignition inhibition impacts at higher temperatures. Whereas thermal effect of CO2 became the dominant factor at lower temperatures. Chemical effect of H2O (H2 + OH<=>H + H2O&O + H2O<=>2OH) promoted OH formation and enhanced whole system reactivity, which suppressed its thermal effect and accelerated ignition process. At p = 60 bar/700 K < T < 1200 K, CO2 addition significantly retarded ignition due to its thermal effect. At 900 K < T < 1200 K, the higher third-body efficiency of H2O promoted ignition (R21:H2O2(+M) = 2OH(+M)&R34:H + O2(+M) = HO2(+M)). At 700 K < T < 900 K, thermal effect of H2O, which suppressed its third-body effect, inhibited ignition progress. These observations implied that the impact of exhaust gases on the ignition of n-heptane/methane depended on the coupled influence mechanisms of physicochemical effects, the composition of exhaust gases and located thermodynamic conditions. At intermediate-temperature high-pressure conditions (typical thermodynamic conditions of dual-fuel engine), raising the concentration of H2O or CO2 in exhaust gases accelerated or delayed the combustion progress of natural-gas/diesel engine at EGR condition by enhanced third-body effect or thermal effect respectively.