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• Influence of exhaust gases on the ignition of CH 4 /n-C 7 H 16 mixture were revealed. • Thermal, chemical and third-body collision effects are qualitatively compared. • CO 2 suppressed ignition due to its thermal and chemical effect. • H 2 O accelerated ignition due to its third-body and chemical effect. • The addition of CO 2 /H 2 O mixture slightly fastened auto-ignition of CH 4 /n-C 7 H 16. To further optimize the combustion performance of natural-gas/diesel engine under exhaust gas recirculation (EGR) condition, influence of physicochemical impacts of H 2 O and CO 2 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, CO 2 and H 2 O additions increased and shortened ignition delay times (IDT) respectively. Mixture of CO 2 and H 2 O slightly accelerated ignition. Original and minor modified NUI mechanism well captured the inhibition and acceleration effect of CO 2 and H 2 O on ignition respectively. Both thermal and chemical effects of CO 2 (R36:CO + OH = CO2 + H) were responsible for its ignition inhibition impacts at higher temperatures. Whereas thermal effect of CO 2 became the dominant factor at lower temperatures. Chemical effect of H 2 O (H 2 + OH<=>H + H 2 O&O + H 2 O<=>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, CO 2 addition significantly retarded ignition due to its thermal effect. At 900 K < T < 1200 K, the higher third-body efficiency of H 2 O promoted ignition (R21:H 2 O 2 (+M) = 2OH(+M)&R34:H + O 2 (+M) = HO 2 (+M)). At 700 K < T < 900 K, thermal effect of H 2 O, 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 H 2 O or CO 2 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. [ABSTRACT FROM AUTHOR]

To acquire ignition control methods for dual-fuel marine engine and HCCI engine, ignition characteristics of lean n-heptane/methane mixture under pressure of 2.0 bar and temperature range from 1241 to 1825 K were studied by shock tube and CHEMKIN with LLNL3.1 mechanism. And ignition processes under temperature range from 700 to 1200 K and pressure range from 40 to 140 bar were investigated by CHEMKIN with NUI mechanism. The results illuminate that at low-pressure high-temperature condition, n-heptane's replacement and the increase of n-heptane content obviously reduced ignition delay times (IDT). The reduction degree of IDT decreased when n-heptane content was high. N-heptane's addition also reduced IDT. But this reduction magnitude was less than that of n-heptane's replacement. Methane's addition slightly inhibited n-heptane's auto-ignition. The reaction time of n-heptane was obviously earlier than that of methane. N-heptane decomposition induced radical formation firstly, which triggered subsequent n-heptane's H-abstraction and the advance of methane's oxidation. At ultra-high-pressure low-temperature condition, increasing n-heptane's content enhanced negative temperature coefficient (NTC) behavior. The end time point of complete consumption of two fuels was the same. Low-temperature condition inhibited n-heptane decomposition, with n-heptane's H-abstraction dominating ignition process. • Ignition properties of CH4/n-C7H16 mixture was revealed by shock tube and mechanisms. • Active radicals formed from n-heptane's decomposition advanced methane's oxidation. • Methane's addition slightly retarded the ignition of pure n-heptane mixture. • The results provided ignition control method for natural-gas/diesel dual-fuel engine. [ABSTRACT FROM AUTHOR]