Novel Method to Estimate the Octane Ratings of Ethanol–Gasoline Mixtures Using Base Fuel Properties

Abstract

Ethanol blending into gasoline yields a wide range of octane rating responses, most frequently synergistic (i.e., greater than expected by linear blending), but also linear or antagonistic. A new approach to modeling ethanol octane blending, applicable for both research octane number (RON) and motor octane number (MON), is proposed, which predicts different ethanol blending responses in base fuels of different compositions and properties. The new model adds an interaction term to the linear molar blending model with a coefficient, Z, that quantifies the synergistic/antagonistic blending: ONblend = (1 – x e)­ONg + x eONe + Zx e(1 – x e), in which x e is the molar ethanol fraction and ONg, ONe, and ONblend are the octane numbers of the base gasoline, ethanol, and their blend, respectively. Fuel property and hydrocarbon composition data for 299 ethanol–gasoline blends and their 90 complex base fuels were collected from the literature, primarily for market gasolines, blendstocks for oxygenate blending (BOBs), and research fuels. Correlations of octane blending parameters for several model approaches were highest for base gasoline octane sensitivity (OSg = RONg – MONg) or saturate and aromatic fraction (Satg, Aromg). Multivariate forward-step linear regression used these same properties to predict the octane blending response in different base fuels over a wide range of ethanol content. For example, the two equations for RON blending are as follows: Z RON = 15.0 – 1.76OSg + 11.3x e and Z RON = −47.8 + 64.3Satg + 24.6Aromg + 12.0x e. These models provide greatly improved predictions as compared to generic models that do not utilize base fuel information.

Refining Economics of Higher Octane Sensitivity, Research Octane Number and Ethanol Content for U.S. Gasoline

“…The blending octane values (RON and MON) of ethanol in finished gasoline, which vary with ethanol content and the properties of the BOB (or of the finished gasoline), were estimated using one of several new methods published by Anderson and Wallington . These methods improve on prior models using linear molar blending for ethanol–gasoline blends, , as they incorporate information about the hydrocarbon portion of the fuel.…”

“…The present study differs in several important respects from our previous refinery study, 25 which similarly investigated gasoline with higher RON and ethanol content in the U.S. context. Most importantly, the present study includes OS as a constrained fuel parameter, incorporates an improved ethanol octane blending model, 22 and investigates the use of merchant-produced iso-octene as a potential refinery blendstock. To manage the expanded set of parameters, the current study models the aggregate U.S. refining sector rather than PADD regions.…”

“…Ethanol has a high RON (109 RON, neat ethanol) and a higher volumetric blending RON in gasoline (∼120–140 RON), depending on the characteristics of the BOB and the volume fraction of ethanol added (SI section 2.1). Ethanol also has high octane sensitivity (OS = 19, neat ethanol) and can have much higher volumetric blending OS values, particularly when added to gasoline with low OS …”

“…Ethanol has a high RON (109 RON, neat ethanol) and a higher volumetric blending RON in gasoline (∼120−140 RON), depending on the characteristics of the BOB and the volume fraction of ethanol added (SI section 2.1). 22 Ethanol also has high octane sensitivity (OS = 19, neat ethanol) and can have much higher volumetric blending OS values, particularly when added to gasoline with low OS. 22 Essentially all U.S. gasoline now is E10 (10 vol % ethanol), with small volumes of E0, E15, and E85 (the latter two sold mostly in the Midwest), largely because of the renewable fuels mandates in the U.S. Renewable Fuel Standard (RFS2), part of the Energy Independence and Security Act of 2007.…”

“…22 Ethanol also has high octane sensitivity (OS = 19, neat ethanol) and can have much higher volumetric blending OS values, particularly when added to gasoline with low OS. 22 Essentially all U.S. gasoline now is E10 (10 vol % ethanol), with small volumes of E0, E15, and E85 (the latter two sold mostly in the Midwest), largely because of the renewable fuels mandates in the U.S. Renewable Fuel Standard (RFS2), part of the Energy Independence and Security Act of 2007. 23 The RFS2 mandates were intended to reduce GHG emissions, promote energy independence, and benefit rural economies.…”