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.

An improved model for the prediction of ignition quality of hydrocarbon fuels has been developed using 1H nuclear magnetic resonance (NMR) spectroscopy and multiple linear regression (MLR) modeling. Cetane number (CN) and derived cetane number (DCN) of 71 pure hydrocarbons and 54 hydrocarbon blends were utilized as a data set to study the relationship between ignition quality and molecular structure. CN and DCN are functional equivalents and collectively referred to as D/CN, herein. The effect of molecular weight and weight percent of structural parameters such as paraffinic CH3 groups, paraffinic CH2 groups, paraffinic CH groups, olefinic CH–CH2 groups, naphthenic CH–CH2 groups, and aromatic C–CH groups on D/CN was studied. A particular emphasis on the effect of branching (i.e., methyl substitution) on the D/CN was studied, and a new parameter denoted as the branching index (BI) was introduced to quantify this effect. A new formula was developed to calculate the BI of hydrocarbon fuels using 1H NMR spectroscopy. Multiple linear regression (MLR) modeling was used to develop an empirical relationship between D/CN and the eight structural parameters. This was then used to predict the DCN of many hydrocarbon fuels. The developed model has a high correlation coefficient (R 2 = 0.97) and was validated with experimentally measured DCN of twenty-two real fuel mixtures (e.g., gasolines and diesels) and fifty-nine blends of known composition, and the predicted values matched well with the experimental data.

No abstract

The effect of ethanol blended with three FACE (Fuels for Advanced Combustion Engines) gasolines, I, J and A corresponding to RON 70.3, 71.8 and 83.5, respectively, were compared to PRF70 and PRF84 with the same ethanol concentrations, these being 2%, 5%, 10%, 15% and 20% by volume. A Cooperative Fuel Research (CFR) engine was used to understand the blending effect of ethanol with FACE gasolines and PRFs in spark-ignited and homogeneous charge compression ignited mode. Blending octane numbers (BON) were obtained for both the modes. All the fuels were also tested in an ignition quality tester to obtain Blending Derived Cetane numbers (BDCN). It is shown that fuel composition and octane number are important characteristics of all the base fuels that have a significant impact on octane increase with ethanol. The dependency of octane number for the base fuel on the blending octane number depended on the combustion mode operated. The aromatic composition in the base fuel, effects blending octane number of the mixture, for fuels with higher aromatic content lower blending octane numbers were observed for ethanol concentration.

No abstract