Chemical Kinetic Insights into the Octane Number and Octane Sensitivity of Gasoline Surrogate Mixtures

Abstract

Gasoline octane number is a significant empirical parameter for the optimization and development of internal combustion engines capable of resisting knock. Although extensive databases and blending rules to estimate the octane numbers of mixtures have been developed and the effects of molecular structure on autoignition properties are somewhat understood, a comprehensive theoretical chemistry-based foundation for blending effects of fuels on engine operations is still to be developed. In this study, we present models that correlate the research octane number (RON) and motor octane number (MON) with simulated homogeneous gas-phase ignition delay times of stoichiometric fuel/air mixtures. These correlations attempt to bridge the gap between the fundamental autoignition behavior of the fuel (e.g., its chemistry and how reactivity changes with temperature and pressure) and engine properties such as its knocking behavior in a cooperative fuels research (CFR) engine. The study encompasses a total of 79 hydrocarbon gasoline surrogate mixtures including 11 primary reference fuels (PRF), 43 toluene primary reference fuels (TPRF), and 19 multicomponent (MC) surrogate mixtures. In addition to TPRF mixture components of iso-octane/n-heptane/toluene, MC mixtures, including n-heptane, iso-octane, toluene, 1-hexene, and 1,2,4-trimethylbenzene, were blended and tested to mimic real gasoline sensitivity. ASTM testing protocols D-2699 and D-2700 were used to measure the RON and MON of the MC mixtures in a CFR engine, while the PRF and TPRF mixtures' octane ratings were obtained from the literature. The mixtures cover a RON range of 0−100, with the majority being in the 70−100 range. A parametric simulation study across a temperature range of 650−950 K and pressure range of 15−50 bar was carried out in a constant-volume homogeneous batch reactor to calculate chemical kinetic ignition delay times. Regression tools were utilized to find the conditions at which RON and MON best correlate with simulated ignition delay times. Furthermore, temperature and pressure dependences were investigated for fuels with varying octane sensitivity. This analysis led to the formulation of correlations useful to the definition of surrogates for modeling purposes and allowed one to identify conditions for a more in-depth understanding of the chemical phenomena controlling the antiknock behavior of the fuels.

Chemical Ignition Characteristics of Ethanol Blending with Primary Reference Fuels

“…The highest regression coefficient (R 2~0 .8, adjusted R 2~0 .79) between known RON values and calculated IDT was obtained for T=750K and P=40 bar. The T-P values were similar to those suggested in previous literature [4,27]. The endgas ahead of the flame front was compressed and heated; this T-P was representative of the endgas before auto-ignition.…”

“…Sarathy et al proposed that the RON of PRFs can be correlated with IDT calculated at po of 20 atm and To of 835 K [26]. Badra et al further expanded the correlation for toluene reference fuel (TRF), and toluene primary reference fuels (TPRF) [27] with po of 50 atm and To of 850 K. Singh et al utilized up to six components for their formulation, correlating RON and IDT with high regression, at po of 25 bar To of 750 K [4]. Westbrook et al used experimentally obtained pressure data to find ignition delay times corresponding to RON and MON values of pure and compound mixtures [28,29].…”

Autoignition of straight-run naphtha: A promising fuel for advanced compression ignition engines

“…Simulated hydroxyl time-history profiles during the oxidation of stoichiometric mixtures of the three surrogates (PRF 60, TPRF 60, and MCS) are presented in Fig. 7 at 20 bar and 650 K. Previous studies by Merchant et al [46] and others [41,47,48] have shown that OH radical profiles provide insights into the chemical kinetics of ignition processes. The initial growth of OH radicals is similar between all the fuels, but at ~5 ms the OH radical growth for MCS slows compared to that of PRF and TPRF.…”

“…Sensitivity analysis also shows that the heat release and OH concentration are sensitive to the isomerization of cyclopentyl peroxy radical (CPTO2J) to QOOH radical (CPT1Q3J). Our previous studies on cyclopentane oxidation [49,50] and mixtures comprising cyclopentane [41,47] have explained its ignition characteristics. Furthermore, hydroxyl (OH) and hydroperoxyl (HO2) rate-of-production analyses were performed for the multi-component surrogate, Fig.…”

Three-stage heat release in n-heptane auto-ignition

“…Complex low-temperature termination, propagation, and branching pathways forming alkenes, cyclic ethers, and ketohydroperoxides (KHP) [9,10], respectively, control heat release rate, raise system temperature, and advance formation and decomposition of H2O2. The chemical kinetics of low-temperature oxidation [11] is strongly related to fuel molecular structure, and can be linked to interesting combustion phenomena such as cool flames [12], negative temperature coefficient (NTC) behavior [13], and fuel anti-knock quality (i.e., octane numbers) [14].…”