Compositional effects on the ignition of FACE gasolines

Abstract

As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40 atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270 K. Results at temperatures above 900 K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900 K demonstrate a stronger negative temperature coefficient behavior for the FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex KAUST multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two-and three-component mixtures composed of n-heptane, iso-octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the KAUST multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition.

Ignition delay measurements of a low-octane gasoline blend, designed for gasoline compression ignition (GCI) engines

“…It was also shown that these discrepancies in IDT at low temperatures can result in significant differences in the combustion phasing of a CI engine operating at low loads [28,34]. [29], FACE A & C from [35], HSRN from [28], GCI blend from this work.…”

“…Even though molecular structure is a very important indicator of the fuel reactivity, matching the molecular structure of all components of the real fuel can be challenging and can lead to a relatively large surrogate mixture [29] for which detailed or reduced chemical kinetic models may not be available. Computational cost will also be high for surrogates containing too many components.…”

“…2 compares IDTs of GCI blend (RON 77) with those of straight-run naphtha (RON 60) [28], FACE A/C (RON 83.5 and 84.7) [35] and FACE F/G (RON 94.4 and RON 96.8) [29]. Detailed comparison of these fuels is available in It may be concluded from the experimental data of the GCI blend and the two surrogates that a multi-component surrogate with non-paraffinic content is needed to match the reactivity at low temperatures where PRF surrogate tends to be too reactive.…”

“…The FACE gasolines mechanism by Sarathy et al [29] and the gasoline surrogate mechanism by Mehl et al [38] were used to perform simulations in Chemkin-Pro with a homogeneous batch reactor. The gradual increase in pressure (dP/dt) behind reflected shock waves was considered by employing a corresponding volume profile in shock tube simulations.…”

The influence of n -butanol blending on the ignition delay times of gasoline and its surrogate at high pressures

“…However, 4-component surrogates involving TRF/1-pentene and TRF/2-pentene have also been formulated and investigated by e.g. [14][15][16], as well as complex multicomponent surrogates for various non-oxygenated gasolines [17,18]. The reasonable extent of agreement between TRF and gasoline, in addition to the availability of well validated chemical mechanisms of its oxidation pathways, makes it currently a more feasible surrogate for gasoline in terms of ignition delay modelling compared to more complex surrogates [19] and blending effects, and hence it is used in this work.…”