Energy Fuels 2010, 24, 6576–6585 Published on Web 11/17/2010
: DOI:10.1021/ef101125c
Octane Numbers of Ethanol- and Methanol-Gasoline Blends Estimated from Molar Concentrations J. E. Anderson,*,† U. Kramer,‡ S. A. Mueller,† and T. J. Wallington† † Systems Analytics and Environmental Sciences Department, Research and Advanced Engineering, Ford Motor Company, P.O. Box 2053, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053, United States, and ‡Powertrain Research & Advanced Department, Research and Advanced Engineering Europe, Ford Motor Company, Spessart Strasse, D-ME/5-B8, D-50725 Cologne, Germany
Received August 20, 2010. Revised Manuscript Received October 7, 2010
When expressed using volumetric concentrations (as is industry practice), the addition of relatively small amounts of ethanol or methanol (e.g., 10% by volume) to gasoline appears to result in disproportionately large, nonlinear increases in research octane number (RON) and motor octane number (MON). As a result, volumetric “blending octane numbers” are of limited value for estimating the octane number of alcohol-gasoline blends because they vary with alcohol content and base gasoline composition. We show that RON and MON increases with alcohol content are approximately linear when expressed using molar concentrations. Moreover, molar-based blending octane numbers are effectively equal to the octane numbers of the pure alcohols for most base gasolines. A limited dependence on gasoline composition was observed, namely, greater-than-predicted octane numbers for ethanol-gasoline blends with unusually high isoparaffin content. We suggest that octane numbers of methanol-gasoline and ethanol-gasoline blends can be estimated conveniently and more accurately from their molar composition by linear interpolation between the octane numbers of the base gasoline and the pure alcohol. Table 1. Propertiesa of Gasoline, Methanol, and Ethanol
1. Introduction There is interest in the use of ethanol in blends with gasoline to address energy security and climate change concerns and to provide rural stimulus. Ethanol can be made from domestically grown, renewable, nonpetroleum feedstocks including corn and sugar cane, with natural gas, coal, or residual biomass used for processing energy. Lifecycle analysis of ethanol for transportation fuel suggests it can provide substantial CO2 emissions reductions relative to conventional gasoline, but the amount of reduction is subject to continuing research and debate.1 Most ethanol is used as blends of 10% by volume (E10) or less in conventional gasoline vehicles, with some additional use as E85 (75-85% denatured ethanol by volume or 70-83% ethanol in the U.S.2) in flex-fuel vehicles (FFVs). In fact, mandates requiring E10 are currently in place in several states in the U.S. Ethanol use in the U.S. is expected to increase as a result of the Energy Independence and Security Act of 2007 which established a new Renewable Fuel Standard (RFS2) that requires greater biofuel use through 2022. Absent more widespread use of E85 in FFVs, conventional gasoline will need to exceed the E10 level to meet the RFS2 schedule,3 even though current non-FFV engines were not designed for ethanol content greater than E10. Ethanol and methanol present certain performance issues and benefits when used in engines as blends with gasoline. For
molecular weight (g/mol) density (kg/m3) liquid molar volumec (cm3/mol) heat of vaporization (MJ/kg) higher heating value (MJ/kg) lower heating value (MJ/kg) RON MON
methanol
ethanol
∼110 720-780 147 0.35 47 44 91-99 82-89
32.0 787b 41 1.16 22.7 20.0 109d 89d
46.1 785b 59 0.84 29.7 26.9 109d 90d
a All data from Heywood4 unless noted otherwise. All properties at 1 atm and 25 °C. b Data from Poling et al.5 c For gasoline, a density of 750 kg/m3 and molecular weight of 110 g/mol was assumed. d Data from Hunzwartzen6 and Bauer et al.13
example, the resulting ethanol-gasoline blend has lower energy content than gasoline (e.g., compare ethanol and gasoline in Table 1), potentially higher or lower vapor pressures,7 altered distillation properties,8 and potential for water-induced phase separation.9 However, ethanol and methanol have long been realized to present certain advantages for spark-ignited engines, particularly their high research octane number (RON) and motor octane number (MON) as compared to gasoline.10 These alcohols also have a greater latent heat of vaporization (4) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill, Inc.: New York, 1988. (5) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids, 5th ed.; McGraw-Hill, Inc.: New York, 2001. (6) Hunwartzen, I. SAE Technical Paper 820002, 1982. (7) Andersen, V. F.; Anderson, J. E.; Wallington, T. J.; Mueller, S. A.; Nielsen, O. J. Energy Fuels 2010, 24, 3647–3654. (8) Andersen, V. F.; Anderson, J. E.; Wallington, T. J.; Mueller, S. A.; Nielsen, O. J. Energy Fuels 2010, 24, 2683–2691. (9) Mueller, S. A.; Anderson, J. E.; Wallington, T. J. J. Chem. Educ. 2009, 86, 1045–1048. (10) Wigg, E. E.; Lunt, R. S. SAE Technical Paper 741008, 1974.
*To whom correspondence should be addressed. E-mail: jander63@ ford.com. (1) Hill, J.; Polasky, S.; Nelson, E.; Tilman, D.; Huo, H.; Ludwig, L.; Neumann, J.; Zhenga, H.; Bonta, D. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 2077–2082. (2) ASTM D5798-07, Standard Specification for Fuel Ethanol (Ed75Ed85) for Automotive Spark-Ignition Engines, 2007. (3) Anderson, J. E.; Baker, R. E.; Hardigan, P. J.; Ginder, J. M.; Wallington, T. J. SAE Technical Paper 09FFL-0302, 2009. r 2010 American Chemical Society
gasoline
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: DOI:10.1021/ef101125c
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than gasoline (Table 1), which contributes to their higher RON values and provides additional charge cooling in directinjection (DI) engines.11-13 Octane number (ON) parameters such as RON and MON are critical properties of fuel that reflect their propensity to resist “knock” in spark-ignited engines, which can cause engine damage when severe. Knock results from premature autoignition of the mixture of fuel, air, and combustion end-gas located outside of the advancing flame front in the cylinder.4 Autoignition is caused by the increased temperature and pressure in the cylinder that results from normal combustion of fuel within the flame front. Alcohols and other oxygenates are believed to retard the progress of the initial, low-temperature autoignition reactions through consumption of radical species and production of unsaturated hydrocarbons that retard oxidation reactions.14 As a result of their greater resistance to knock, fuels with higher ON can provide modest improvements in fuel economy in standard compression-ratio gasoline engines with knock-limited spark advance technology15 and enable the use of engines with higher compression ratios having greater thermal efficiency and potential for further engine downsizing to improve vehicle fuel economy.11,12,16 The RON and MON methods were developed and standardized in the 1930s and 1940s.17 RON was intended to be representative of light motoring whereas MON represented more aggressive motoring. Historically, different engines were found to exhibit different knock responses to a given fuel, with performance of some engines correlating better to MON and others to RON. As a result, the “Anti-Knock Index” (AKI) was developed (defined as the arithmetic average value of RON and MON) and is used on labels on pumps at U.S. retail filling stations. Typical regular-grade U.S. unleaded gasoline has an AKI of 87 (e.g., 91 RON and 83 MON), while premium gasoline has an AKI of 91 (e.g., 94 RON and 88 MON). Gasoline outside the U.S. and Canada is typically marketed using RON only. In general, regular-grade gasoline in most E.U. countries has a higher RON than in the U.S.18 The types and concentrations of the various hydrocarbons present in gasoline determine its ON. The ON of hydrocarbons can vary greatly with chemical structure. In general, RON and MON values of pure hydrocarbons decrease in the following order: aromatics, isoalkanes (isoparaffins), alkenes (olefins) and cycloalkanes (naphthenes), and n-alkanes (n-paraffins). Branching, decreasing molecular weight, and presence of unsaturation each tend to increase octane number. RON and MON are method-defined parameters measured on a specially designed, carbureted, single-cylinder, four-cycle
test engine with a variable compression ratio whose specifications and operating conditions are described in ASTM D269919 and D2700,20 respectively. For both RON and MON, the sample fuel is rated based on comparison to primary reference fuel blends of n-heptane and 2,2,4-trimethylpentane (“isooctane”). These reference fuels are used to define the ON ranges, with 100% n-heptane at 0 ON and 100% isooctane at 100 ON. Fuels with ON greater than 100 are compared to reference fuels comprised of isooctane with different amounts of tetraethyl lead, an antiknock compound, using a method-defined empirical relationship. Both methods are specified to be applicable over an ON range of 40-120 but are most precise in the ranges of typical gasoline (i.e., 80-90 MON and 90-100 RON).19,20 Both ON test procedures are carried out by first adjusting the air-fuel ratio to obtain maximum knock intensity when operating the engine with the sample fuel. Next, the compression ratio is adjusted within a range of 4:1 to 16:1 (by changing cylinder height) to obtain a method-defined standard knock intensity. Two reference fuels are then selected that closely bracket the knock intensity at the previously determined compression ratio (but at air-fuel ratios providing maximum knock). The ON result is an interpolated value between the ONs of two reference fuels that matches the knock of the sample fuel. Operational differences between the RON and MON test methods have been discussed elsewhere4,14 and include engine operating speed, inlet air and air-fuel mixture temperatures, and spark advance. RON, using an engine speed of 600 rpm, was intended to be representative of light motoring whereas MON, with an engine speed of 900 rpm, was representative of more aggressive motoring. Spark advance is fixed in the RON test but varies with the compression ratio in the MON test. For fuels containing alcohols, a key difference is that the MON test specifies the temperatures of the inlet air (38 °C/ 100 °F) and the air-fuel mixture (149 °C/300 °F), whereas the RON test specifies a higher temperature for the inlet air (nominally 52 °C/125 °F) and does not specify a temperature for the air-fuel mixture. Alcohols, particularly methanol and ethanol, have a significantly greater heat of vaporization than gasoline (Table 1). As a result, the intake air-fuel mixture temperature decreases with increasing alcohol content in the RON test but is held constant in the MON test. This factor is believed to result in a greater increase in RON than MON when either ethanol or methanol is added to gasoline.21 This means that the inherently beneficial high heats of vaporization of alcohols are a factor in the RON test but not the MON. The additional cooling provided by evaporation of alcohols in the RON test is believed to contribute to their high RON values.21 The accuracy and appropriateness of the ASTM RON and MON methods for fuels with high alcohol content has been questioned due to the lesser heat of combustion and greater heat of vaporization of the alcohols as compared to gasoline and their impact on fuel consumption rate (RON and MON) and air-fuel mixture temperature (MON only). While ASTM D2699 and D2700 note that the methods may not be applicable for samples with high oxygenate content, the applicable
(11) Stein, R. A.; House, C. J.; Leone, T. G. SAE Technical Paper 2009-01-1490, 2009. (12) Milpied, J.; Jeuland, N.; Plassat, G.; Guichaous, S.; Dioc, N.; Marchal, A.; Schmelzle, P. SAE Technical Paper 2009-01-0324, 2009. (13) Bauer, K.; Heilmann, G.; Koepcke, G.; Reders, K.; Erwig, W.; Hunwartzen, I.; Moeller, P.; Starke, K. Abschlussbereicht: Klopffestigkeitsbestimmung (ROZ und MOZ) von Alkoholen und Alkoholmischkraftstoffen in CFR-Pruefmotoren (German). DGMK Forschungsbericht 260-01, 1980. (14) Hamilton, B.; Falkiner, R. J. Motor Gasoline In Fuels and Lubricants Handbook: Technologies, Properties, Performance, and Testing; Totten, G. E., Ed.; ASTM International: West Conshohocken, PA, 2003. (15) Russ, S. SAE Technical Paper 960497, 1996. (16) Nakata, D.; Uchida, A.; Ota, S.; Utsumi, K.; Kawatake, K. SAE Technical Paper 2007-01-2007, 2007. (17) Searle, G. R. Octane Quality and Knock. In Motor Gasoline; Marshall, E. L.; Owen, K., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1995. (18) Snelgrove, D. G. Motor Gasoline Specifications and Stability In Motor Gasoline; Marshall, E. L.; Owen, K., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1995.
(19) ASTM International. Standard Test Method for Research Octane Number of Spark-Ignition Engine Fuel, ASTM D2699-07, ASTM: West Conshohocken, PA, 2007. (20) ASTM International. Standard Test Method for Motor Octane Number of Spark-Ignition Engine Fuel, ASTM D2700-09, West Conshohocken, PA, 2009. (21) Moran, D. P. SAE Technical Paper 941861, 1994.
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concentration of alcohol added to the base gasoline,29 the ON of the base gasoline,30,31 and the hydrocarbon composition of the base gasoline.22,32 In short, the ON of every alcohol-gasoline blend of interest needs to be measured. As a result of this blending nonlinearity, bON values are typically much higher than measured ONs of the pure alcohols. As described by Yates et al.,33 this has led to considerable confusion, with bON values and pure compound ON values being reported interchangeably or without designation. We show that RON and MON values for methanol- and ethanol-gasoline blends are better described using a molar concentration dependence. Such behavior seems reasonable since fuels in the RON and MON tests are vaporized in the carburetor and are present in a gaseous state in the engine cylinder where the autoignition-related chemical reactions occur. Reaction rates are determined by partial pressures of the compounds involved, which according to the ideal gas law will scale proportionately with molar composition. We suggest that the octane number of a methanol-gasoline or ethanolgasoline blend can be estimated from its molar composition by linear interpolation between the octane numbers of the base gasoline and the pure alcohol.
concentration ranges are not indicated. Houben states that these methods should not be used without modifications for fuels containing more than 20% (v/v) oxygenate. The European equivalents of the ASTM RON and MON standards (DIN EN ISO 5164 and 5163)23,24 specify that no accuracy estimate can be provided for gasoline containing more than 4% (m/m) oxygen, corresponding to approximately 11% (v/v) ethanol and 8% (v/v) methanol. Above these concentrations, RON and MON results may be influenced by an inability to supply fuel at a rate sufficient to achieve maximum knock, instead being constrained by the method-defined fuel level limits in the fuel sight glass. The MON result may also be influenced by an inability to maintain the specified air-fuel mixture temperature. Method modifications have been identified to allow more accurate measurement of RON and MON of fuels with high alcohol content and were codified in the German standard DIN 51756-7.25 For both RON and MON, adjustable-orifice fuel jets are used in lieu of fixed-diameter jets to provide the greater fuel flow required and to more precisely maintain both the fuel level in the fuel sight glass and the resulting fuel flow.6,13,25,26 For MON, the additional heat required to vaporize the fuel and heat the fuel-air mixture to the specified temperature is provided by an auxiliary external electric heating element.6,13,25,27 ASTM D2699 and 2700 allow (but do not require) an adjustable-orifice fuel jet for RON and MON but specify a different sight glass fuel level. It has also been reported26 that normal combustion of pure methanol and high methanol-content blends may be erroneously detected as knock by the standard knock meter. Ethanol and methanol have ONs higher than that of typical gasoline (Table 1). The addition of small amounts of alcohol [e.g., 10% (v/v)] results in a disproportionately large and nonlinear increase in ON28 when considered on a volumetric basis, which is industry standard practice. Despite this nonlinearity, a linear calculation approach has commonly been used to describe the ON increase for alcohol-gasoline blends. This approach makes use of a “blending octane number” (bON) in lieu of the measured ON of the pure alcohol. The calculation is shown in eq 1, where ONblend = ON of the alcohol-gasoline blend, ONgasoline = ON of the base gasoline, bONv,alc = blending ON of the alcohol in the base gasoline based on volumetric concentration, and Calc = volumetric alcohol fraction in the alcohol-gasoline blend. ONblend ¼ ð1 - Calc ÞONgasoline þ ðCalc ÞbONv, alc
2. Experimental Section RON and MON data for methanol- and ethanol-gasoline blends and the corresponding base gasolines were obtained from available literature sources. Only unleaded fuels were considered, and these included commercial gasolines, reference or test gasolines, petroleum refinery streams used in gasoline, and reference hydrocarbon mixtures (e.g., mixtures of isooctane and n-heptane). Whenever possible, a copy of the original publication describing each ON measurement was obtained. When base gasoline density and molecular weight were not provided, values of 750 kg/m3 and 110 g/mol were assumed. Base gasoline hydrocarbon composition was also recorded when provided. In cases where volumetric bON values were reported instead of actual ON measurements, the ONs of the blends were calculated using eq 1. Where multiple alcohol concentrations were tested for a single base gasoline, bON values were calculated for each alcohol concentration referencing back to the base gasoline. In certain cases where data were provided only in graphical form, the data were obtained from the author34,35 or extracted from figures in the literature source.29,36 The raw data used in the analysis (RON, MON, alcohol concentration, base gasoline density, molecular weight, hydrocarbon composition, and literature citation) are provided in the Supporting Information.
ð1Þ 3. Results and Discussion
However, this volumetric blending ON approach is of extremely limited value because the resulting bON values depend on the
3.1. Methanol. Considerable RON and MON data for methanol-gasoline blends were obtained from studies completed in the 1970s, a time when there was greater interest in the use of methanol as an alternative fuel. Johnson and Riley26,29 provide a complete set of RON and MON data for a wide range of methanol concentrations added to four different base gasolines having different RON and MON
(22) Houben, M. C. M. Oxygenated Blending Components for Gasoline Alcohols and Ethers. In Motor Gasoline; Marshall, E. L.; Owen, K., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1995. (23) International Organization for Standardization. Determination of knock characteristics of motor fuels - Research method. ISO 5164, 2005. (24) International Organization for Standardization. Determination of knock characteristics of motor and aviation fuels - Motor method. ISO 5163, 2005. (25) Deutsches Institut f€ ur Normung. Determination of knock characteristics (octane number) of alcohols and alcohol/fuel mixtures using the CFR engine. DIN 51756-7, 1986. (26) Johnson, R. T.; Riley, R. K. Evaluation of Methyl Alcohol as a Vehicle Fuel Extender. U.S. Department of Transportation, DOT-TST76-50, 1975. (27) Most, W. J.; Longwell, J. P. SAE Technical Paper 750119, 1975. (28) Szybist, J.; Foster, M.; Moore, W. R.; Confer, K.; Youngquist, A.; Wagner, R. SAE Technical Paper 20100-01-0619, 2010. (29) Johnson, R. T.; Riley, R. K. SAE Technical Paper 760377, 1976.
(30) Brinkman, N. D.; Gallopoulos, N. E.; Jackson, M. W. SAE Technical Paper 750120, 1975. (31) Keller, J. L. Hydrocarbon Proc. 1979, 58, 127–38. (32) Society of Automotive Engineers. Alternative Automotive Fuels, SAE Standard J1297, 2002. (33) Yates, A.; Bell, A.; Swarts, A. Fuel 2010, 89, 83–93. (34) da Silva, R.; Catalu~ na, R.; de Menezes, E. W.; Samios, D.; Piatnicki, C. M. S. Fuel 2005, 84, 951–959. (35) da Silva, R. Personal communication, 2010. (36) Karonis, D.; Anastopoulos, G.; Lois, E.; Stournas, S. SAE Technical Paper 2008-01-2503, 2008.
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Figure 1. RON and MON measurements for methanol-gasoline blends using four base gasolines plotted versus methanol content on a volumetric basis (panel A) and a molar basis (panel B). Data from Johnson and Riley.29
accomplished using eqs 2 and 3, where xalc = molar alcohol fraction in the alcohol-gasoline blend, Calc = volumetric alcohol fraction in the alcohol-gasoline blend, and rmv = ratio of liquid molar volumes of alcohol (valc, cm3/mol) and base gasoline (vgasoline, cm3/mol).
Table 2. Reported RON and MON Values for Methanol and Ethanol methanol a
ethanol a
RON
MON
∼112 109.6 109.5* 106-115 112 108.7* ( 0.4b 107 109
∼90 87.4† 86.0* 88-92 91 88.6*† ( 0.2 89 89
114.4
94.6
a
RON
MONa
ref year
1975 1975 1975,6 106-107.5 89-100 1979 111 92 1979 108.6* ( 0.4 89.7*† ( 0.3 1982 106 92 1988 109 90 1995 108 92.9 2006 111.4 94 2007
ref 37 27 26, 29 31 38 6, 13 4 39 40 41
xalc ¼ Calc =½Calc þ ð1 - Calc Þrmv
ð2Þ
The liquid molar volume ratio, rmv, is determined using the molecular weights (Malc and Mgasoline, g/mol) and densities (Falc and Fgasoline, kg/m3) of the alcohol and base gasoline (Table 1). rmv ¼ valc =vgasoline ¼ ðMalc =Falc Þ=ðMgasoline =Fgasoline Þ ð3Þ
a
RON and MON values shown are measured values for pure compounds, not “blending” octane numbers. Underlined values are known to be determined as part of the referenced study. RON and MON values followed by an asterisk (/) were identified as having been determined using an adjustable-orifice fuel jet. MON values followed by a dagger (†) were identified as having been determined using an auxiliary heater for the intake mixture. In all other cases, there was no indication of the use of such measures. b Value and 95% confidence interval.
The density of gasoline is typically within a few percent of 750 kg/m3.4,32,42 Molecular weight distributions of gasoline are not commonly reported, but available literature values show average molecular weights in the range of 95-115 g/ mol.4,32,42 For the purpose of this calculation, the median value may be more appropriate than the mean and should be based on molar fractions rather than mass or volume fractions. However, the values obtained using these two approaches typically do not greatly differ and the choice has little effect on the result. In the absence of reported data, values of 750 kg/m3 and 110 g/mol were assumed for the density and representative molecular weight of the base gasoline. A value of rmv less than one implies that molecules of the alcohol occupy less volume than gasoline in the liquid state. The implication is that alcohol concentrations expressed on a molar basis can be considerably greater than when expressed on a volumetric basis. Figure 2 shows how molar concentrations relate to the corresponding volumetric concentrations in gasoline for methanol as well as for other alcohols and ethers used or considered for blending into gasoline. The effect of this transformation is most pronounced when the molecular weight of the alcohol is much less than that of gasoline (e.g., methanol). When the RON and MON data of Johnson and Riley29 are instead plotted as a function of molar methanol concentration (Figure 1B), a linear relationship is apparent. This is significant in that it justifies the use of a linear blending ON
values. Adjustable-orifice fuel jets were used for RON and MON tests as well as an adapted knock-detection scheme,26 but no mention was made of an auxiliary heater for MON. As shown in Figure 1A, RON and MON values are highly nonlinear as a function of volumetric methanol concentration. Addition of methanol at the lowest concentrations tested [5-15% (v/v)] results in a disproportionately large increase in both RON and MON. At higher concentrations, the RON and MON increase is progressively less significant. The RON (109.5) and MON (86.0) values measured by Johnson and Riley29 for pure methanol are in good agreement with other literature values (Table 2). As discussed above, molar alcohol concentration may be more appropriate than volumetric concentration to describe the dependence of RON and MON on methanol content. Conversion from volumetric to molar concentrations can be (37) Ingamells, J. C.; Lindquist, R. H. SAE Technical Paper 750123, 1975. (38) Wagner, T. O.; Gray, D. S.; Zarah, B. Y.; Kozinski, A. A. SAE Technical Paper 790429, 1979. (39) Owen, K.; Coley, T. Automotive Fuels Reference Book, 2nd ed.; Society of Automotive Engineers: Warrendale, PA, 1995. (40) Ghosh, P.; Hickey, K. J.; Jaffe, S. B. Ind. Eng. Chem. Res. 2006, 45, 337–345. (41) van Basshuysen, R., Schafer, F., Eds. Modern Engine Technology: from A to Z; SAE International: Warrendale, PA, 2007.
(42) American Petroleum Institute. Alcohols and Ethers: A Technical Assessment of their Application as Fuels and Fuel Components, Publication 4251, 3rd ed.; American Petroleum Institute: Washington, DC, 2001.
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Figure 2. Calculated molar concentrations of oxygenates [methanol, ethanol, isobutanol, methyl tert-butyl ether (MTBE), and ethyl tertbutyl ether (ETBE)] versus volumetric oxygenate concentration when blended into typical gasoline (assumed to have 750 kg/m3 density and 110 g/mol molecular weight).
approach which greatly simplifies the calculation of RON, MON, and AKI for these methanol-gasoline blends. It implies that a single blending ON value (the ON of pure methanol) is sufficient to estimate the ON for any methanol concentration in the four base gasolines. (In Figure 1, blending ONs are the slopes of line segments connecting the ONs of a methanolgasoline blend and its base gasoline.) The ON of an alcoholgasoline blend can be calculated on a molar basis using eq 4, where bONmol,alc = blending ON of alcohol in the base gasoline based on molar alcohol fraction, xalc. ONblend ¼ ð1 - xalc ÞONbase þ ðxalc ÞbONmol, alc
Figure 3. Blending RON and MON values for methanol-gasoline blends using four base gasolines, calculated using a volumetric basis as in eq 1 (panel A) and a molar basis as in eq 4 (panel B). Data from Johnson and Riley.29
ð4Þ
Blending ON values calculated on this molar basis (bONmol,alc) are essentially a constant value equal to the measured ON value of pure methanol, showing little dependence on methanol concentration (Figure 3B) or ON of the four base gasolines. In contrast, volumetric blending values vary considerably with methanol concentration and differ for the four base gasolines (Figure 3A). Assuming molar concentration is appropriate for ON blending calculations, the higher volumetric blending values at low methanol concentrations are a predictable result of eqs 2-4. Volumetric blending values approach the ON of pure methanol as methanol concentrations approach 100%, explained by the fact that volumetric concentrations are equal to molar concentrations at the two extremes: 0% and 100% (Figure 2). To more broadly evaluate the applicability of the molarbased blending ON approach, RON and MON data from a total of 52 methanol-gasoline blends in 28 base gasolines were obtained from 12 literature sources, including Johnson and Riley.29 Volumetric and molar blending ONs were calculated and are plotted versus methanol concentration in panels A and B of Figure 4, respectively. Observations from these data are consistent with those described above. Volumetric blending values increase with decreasing methanol concentration whereas the molar blending values do not. The molar blending values are grouped around the reported ONs for pure methanol, with few exceptions. The greater scatter at low methanol concentrations for both calculation approaches is likely due to uncertainties in the ON measurements, whose
impacts are magnified in eqs 1 and 4. The blending values are also plotted versus the ON of the base gasoline in Figure 4C, D. Little dependence on base gasoline ON is evident in either calculation approach for these methanol-gasoline blends. 3.2. Ethanol. As a result of the recent interest in ethanol, considerably more ON data were available for ethanolgasoline blends. RON and MON data were obtained from a total of 178 ethanol-gasoline blends in 62 base gasolines from 19 literature sources. Blending ON values were again calculated on both a molar and volumetric basis. As with methanol, there is considerably less variation in blending values with the molar approach (Figure 5B,D) than with the volumetric approach (Figure 5A,C) and the molar blending ON values are grouped around the reported ONs for pure ethanol. As such, the ON data appear to be linear with respect to molar ethanol concentrations. Upon close inspection it appears that the molar blending RON and MON values at concentrations less than 50% (v/v) are on average a few ON units higher than reported RON and MON values for pure ethanol. At ethanol concentrations greater than 50% (v/v), however, the blending RON values are a few units lower than the RON of pure ethanol whereas the blending MON values are approximately equal to the MON of pure ethanol. The possible significance of these trends is unclear at this point. As reported previously,30,31 volumetric blending ON values are dependent on base gasoline ON (Figure 5C) with higher blending values for base gasolines with lower 6580
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Figure 4. Blending RON and MON values, calculated using a volumetric basis (panels A,C) and molar basis (panels B,D), plotted versus methanol content (panels A,B) and base gasoline ON (panels C,D) for 52 blends in 28 base gasolines. RON and MON values for pure methanol from Hunzwartzen6 are shown for comparison. See the Supporting Information for tabulated data and references.
ON. Molar blending values are not dependent on the base gasoline ON (Figure 5D). As shown in Figure 6, the highest molar blending RON values in Figure 5 correspond to blends in base gasolines with high [>50% (v/v)] isoparaffin content. [None of these were commercial gasolines, instead, they were mixtures of isooctane and n-heptane or blends containing large fractions of refinery alkylate (isoparaffin).] The above observation is also true for the highest blending MON values. The dependence of ON response on base gasoline composition has been reported previously,22,32 but the linearity provided by this molar blending approach may enable such hydrocarbon composition dependence to be more readily identified. The linear ON blending relationship using a molar basis also enables the identification of unusual data that would otherwise be masked by the nonlinearity inherent in a volumebased approach. For example, data in several reports show an apparent inconsistency or discontinuity between RON measurements for mid- and high-level ethanol-gasoline blends [i.e., 15-85% (v/v) ethanol] and RON measurements for pure ethanol reported by others (Table 2). The American Petroleum Institute recently issued a report containing ON data43 for ethanol blended at 10-33% (v/v) into 21 base gasolines. These data (included in Figures 5 and 6) are consistent with other studies in showing a linear increase in
RON and MON with molar ethanol content. However, several blends with high-octane base gasoline appeared to exhibit a plateau at 102 RON, i.e., RON values increase linearly on a molar basis up to 102 RON [reached at 30-50% (mol/mol) or 15-30% (v/v) ethanol] but remain at 102 RON for higher ethanol concentrations. A similar plateau at 102103 RON was observed in data from two other recent studies,28,44 but the plateau was reached at different ethanol concentrations [20%44 and 50%28 (v/v)]. These unexpectedly low RON values at high ethanol concentrations are evident in Figure 6A. However, another recent study45 showed RON values increasing in a linear fashion to 106 RON at the maximum ethanol concentration tested [30% (v/v)]. In none of these studies was it noted that special measures were necessary or employed to address the potential issues of RON/MON fuel metering or MON fuel-air mixture heating. RON values of up to 106 RON were found for blends containing up to 87% (v/v) ethanol in the study by FVV,46 which followed DIN 51756-725 and used an adjustableorifice carburetor jet for RON/MON and an auxiliary heater for MON. In the other four studies, it is unclear if and how (44) Yucesu, H. S.; Topgul, T.; Cinar, C.; Okur, M. Appl. Thermal Eng. 2006, 26, 2272–2278. (45) AVL. Ethanol effects on gasoline-like HCCI combustion, Report PEI-0341, rev. 3, for Coordinating Research Council Project No. AVFL-13B, 2009. (46) Forschungsvereinigung Verbrennungsmotoren (FVV). Examination and Evaluation of Alternative Fuels for Operation in Modern Gasoline DI Engines, Project No. 942, 2009.
(43) American Petroleum Institute. Determination of the Potential Property Ranges of Mid-Level Ethanol Blends: Final Report, American Petroleum Institute: Washington, DC, 2010.
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Figure 5. Blending RON and MON values, calculated using a volumetric basis (panels A,C) and molar basis (panels B,D), plotted versus ethanol content (panels A,B) and base gasoline ON (panels C,D) for 178 blends in 62 base gasolines. RON and MON values for pure ethanol from Hunzwartzen6 are shown for comparison. See the Supporting Information for tabulated data and references.
the resulting RON and MON data were impacted by the lack of these measures and if this contributed to the low RON values. The corresponding MON data from all five studies showed no indication of a similar plateau, suggesting a possible heat of vaporization and fuel-air mixture temperature influence. It is currently unclear if this RON plateau is a real and reproducible outcome inherent to the RON method, and if its presence or absence in these studies is due to methodological differences and/or fuel differences between studies. Renewed efforts are needed to ensure accurate measurement of RON and MON of pure alcohols and alcohol-gasoline blends with high alcohol content. As noted earlier, it has been reported that the modifications for the ASTM RON and MON methods are required for ethanol concentrations above 11% or 20% (v/v).22-24 Limiting the RON and MON data to these concentrations (in Figure 6A,B), however, does not appear to change the conclusions reached using the entire data set. As discussed earlier, most of the scatter in molar-based blending ON values at low ethanol concentrations is likely the result of magnification of experimental errors or uncertainty in ON measurements. In any event, even for data sets exhibiting a plateau at 102-103 RON, the molar blending approach provides a means to better estimate ONs for low-level ethanol blends [0-10% (v/v)] and midlevel ethanol blends [10-20% (v/v)] which are of immediate interest in the U.S. Furthermore, gasoline and ethanol-gasoline blends available in nearly all retail filling stations (other than E85) have RON values less than 102.
3.3. Estimation of Octane Number of Methanol- and Ethanol-Gasoline Blends Using the Molar Concentration Approach. With the use of the molar concentration-based approach, ONs of each of the methanol- and ethanol-gasoline blends were estimated by linear interpolation between the ONs of the base gasoline from each reference and the ON of the pure alcohols (as reported by Hunwartzen.6) The predicted ONs are compared to the measured ONs in Figure 7. In general, the molar-based method provides an improved means for estimating ONs of the blends over a wide range of base gasoline ONs. As discussed above, some of the scatter in the data for ethanol blends can be explained by hydrocarbon composition (i.e., measured RON and MON values of blends using base gasoline with high isoparaffin content were higher than the predicted values). Composition information was not available for most studies, so it is possible that additional variability could be due to this factor. Data affected by the possible RON measurement plateau seen in three studies also may explain some scatter (i.e., predicted RONs higher than measured RONs of 102-103). Lastly, the molar blending ON values were determined that provide the best fit to the data set using minimization of squared prediction errors. Excluding blends with base gasolines known to have more than 50% (v/v) isoparaffin content, optimal molar blending ON values were as follows: methanol bRON = 108.7, methanol bMON = 87.1, ethanol bRON = 108.9, ethanol bMON = 93.4. The best-fit blending RON values are within 0.3 ON units of RON values for the pure alcohols reported by Hunwartzen.6 The best-fit 6582
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Figure 6. Molar-based blending RON values (panels A,C) and blending MON values (panels B,D) for 178 ethanol-gasoline blends in 62 base gasolines, differentiated by isoparaffin content in the base gasolines, plotted versus ethanol content (panels A,B) and base gasoline ON (panels C,D).
blending MON values for methanol and ethanol are 1.5 ON units less than and 3.7 ON units greater than (respectively) the corresponding MON values reported by Hunwartzen but are within the ranges of published MON values in Table 2. Given that the MON test modifications employed in the study reported by Hunwartzen do not appear to have been used in the many studies included here, it is possible that our data set is dominated by MON measurements that are systematically biased. However, the fact that the methanol and ethanol blending RON values deviate in opposite directions may suggest otherwise. 3.4. Other Compounds. It is plausible that the same molar blending number approach is valid for other oxygenated compounds. Higher molecular weight alcohols (e.g., isomers of propanol and butanol) have higher energy content than methanol and ethanol and are also reported to have ONs high enough to make them desirable as gasoline components.22,41,47 However, there are few literature data concerning the ON behavior of these alcohols in blends. Various ethers [e.g., methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME)] have also been used as gasoline blending components, in part because of their beneficially high ONs. The molecular weights of these alcohols and ethers are greater than methanol and ethanol and closer to gasoline hydrocarbons, making their molar and volumetric concentrations more similar (Figure 2). As a result, there is little difference in blend ON calculations for
these compounds using a volumetric or molar concentration basis. The estimation of ONs for hydrocarbons blended into gasoline has also been described as nonlinear, and the volumetric blending ON approach has been used to describe relative differences between hydrocarbons.39,48 Molar-based blending ONs may provide a better representation, particularly for hydrocarbons with molecular weights much lower than that of gasoline (e.g., butanes, pentanes). An examination of data from API Research Project 45,48 in which individual hydrocarbons were blended into 60:40 (v/v) isooctane/n-heptane, suggests that the use of the molar-based blending calculation approach reduces some, but not all, of the disparity between ONs of the pure compounds and blending values (data not shown). A molar-based approach may also be applicable to cetane number estimation for blends of biodiesel in petroleum diesel fuel, as this property is also derived in an engine setting and involves in-cylinder chemical reactions. However, compression-ignition engines typically involve combustion of some nongaseous fuel in the cylinder. Also, the molecular weight of biodiesel is not greatly different from that of petroleum diesel. Other oxygenated compounds under consideration as diesel fuel additives (e.g., dimethyl ether, ethanol, and 1-butanol) do have molecular weights considerably less than petroleum diesel and might be better described by a molar blending (48) American Society for Testing Materials. Knocking Characteristics of Pure Hydrocarbons, Special Technical Publication No. 225, 1958.
(47) Popuri, S. S. S.; Bata, R. M. SAE Technical Paper 932953, 1993.
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Figure 7. Predicted versus measured RON (panels A,B) and MON (panels C,D) for methanol-gasoline blends (panels A,C) and ethanolgasoline blends (panels B,D) differentiated by isoparaffin content in the base gasoline. Predictions were made by interpolating (molar basis) between the reported base gasoline ON from each reference and alcohol ON from Hunzwartzen.6
approach, particularly since they are more likely to be present in the gas phase at the onset of combustion. Few data are available to test this hypothesis. 3.5. Modeling Octane Numbers for Alcohol-Gasoline Blends. The molar-based blending octane number approach proposed here satisfies Rusin’s49 definition of a “transformation method” that employs “common sense” physical mixing behavior and consists of three steps: (a) transformation of measured component properties according to a predetermined rule, (b) linear blending of these transformed properties, and (c) inverse transformation of the results to yield estimated blend properties. The molar-based blending approach requires only the first two steps of this approach: (a) transformation from volumetric to molar concentrations using eq 2 and (b) linear blending using molar concentrations using eq 4. The model eventually proposed by Rusin et al. recognizes the relevance of gas-phase concentrations of knock-producing agents in the end-gas derived from the fuel components but does not utilize molar concentrations of fuel components themselves.49 The molar-based approach also satisfies the principle suggested by Ghosh et al.40 in that ONs are reconciled for blends and their “pure” components (gasoline and alcohol, representing limiting values of the blends). An implicit assumption in all modeling approaches is that there is no fundamental difference in the octane number scale above 100 and below 100, even though calibration methods differ (i.e., isooctane/n-heptane blends below 100 and isooctane with tetra-ethyl lead above 100).26
Other researchers have attempted to capture the nonlinear ON blending behavior by using models with nonlinear structure, e.g., empirical fit using an exponential dependence on volumetric methanol concentration.29 More advanced approaches are obviously in use in the fuel refining industry for evaluating and optimizing octane blending using various refinery components and/or pure hydrocarbons, alcohols, and ethers.40 These approaches are considered proprietary information but have been reported to involve concepts such as blending bonuses, interaction factors,17 data transformations,29 neural networks, and other techniques.40 4. Conclusions Given the trend toward greater concentrations of oxygenates (primarily ethanol) in gasoline, there is an opportunity to increase the RON of fuels used in gasoline engines. This change would result in a modest increase in the fuel economy of existing late-model vehicles and enable greater fuel economy gains in future vehicles through higher compression ratio engines and downsizing. However, the measurement of RON in gasoline with high oxygenate content is prone to greater error and variability than gasoline with little or no oxygenate. Renewed efforts are needed to ensure the most accurate measurement of RON and MON of pure oxygenates and high concentration oxygenate-gasoline blends. The use of the volumetric basis for describing fuel composition is standard practice, most likely because this is how different refinery streams are blended to produce motor fuels of desired properties. Many properties (e.g., heating values,
(49) Rusin, M. H.; Chung, H. S.; Marshall, J. F. Ind. Eng. Chem. Fundam. 1981, 20, 195–204.
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composition, density) are measured on a volume basis and are estimated for blends with acceptable accuracy using a volumetric-blending approach. However, chemical properties involving gas-phase phenomena (e.g., vapor pressure) often have molar concentration dependence. The fact that ON values of alcohol-gasoline blends are better described using a molar concentration basis is most likely due to the fact that liquid fuels are vaporized in the ON tests. The proposed
method for estimating the octane number of alcohol-gasoline blends based upon their molar composition may be useful for fuel refiners, fuel suppliers, and engine manufacturers (e.g., flexible-fuel or dual-fuel engines). Supporting Information Available: Tabulated literature data and references used in this analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
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