Investigating auto-ignition behavior of n-heptane/iso-octane/ethanol mixtures for gasoline surrogates through rapid compression machine measurement and chemical kinetics analysis

Abstract

Overall ignition delay time (OID) is used as indicator to characterize reactivity and anti-knock quality of fuel. In this work, both experimental measurement and modeling work have been conducted to evaluate OID properties for six ternary blends comprising n-heptane/iso-octane/ethanol. Experimental measurement was performed in a rapid compression machine, while modeling work was based on the gasoline surrogate mechanism. The decoupling study on research octane number (RON)/motor octane number (MON) and octane sensitivity (S) has been carried out by adjusting fuel composition. The thermal conditions of experiments cover low-to-medium temperatures from 640 K to 740 K and pressures from 9.5 bar to 21 bar. Oxygen is utilized as the oxidizer while nitrogen and argon are regarded as the buffer gas to adjust thermal conditions. Combined with chemical kinetics analysis, negative temperature coefficient (NTC) behavior in the experiment is entirely attributed to iso-octane and the timing of the first peak HO2 radical concentration has more significant influence on NTC behavior than peak value itself. Octane number and/or S cannot fully revealed fuel anti-knock quality, which is substantially influence by the property of fuels and thermal conditions. At low temperature (640 K), fuels with higher S exhibit better anti-knock performance. However, this advantage will disappear when temperature falls into NTC region. For engine operating in this area, high RON but low S fuels are recommended. Ethanol extends the beneficial area of high S contents due to its low temperature chemical inertness. The low temperature boundary of NTC for iso-octane can be regarded as the demarcation line to determine whether high S contents should be utilized, which is an important reference for fuel-engine co-optima research.