This phenomenon is illustrated in Figure 2 (Tanaka et al. One notable point regarding the vapor pressure of ethanol blend is its tendency to increase more quickly than that of gasoline with increasing temperature. In some regions, higher vapor pressures are allowed for gasoline-ethanol blends, if they contain ethanol. If strict fuel specifications are to be met, this rules out the possibility of so-called splash-blending of ethanol. Vapor pressure of blends can be adjusted by using base fuel with low vapor pressure. With ethanol content of some 30-50%, vapor pressure is at the same level as for gasoline without oxygenates (Environment Australia 2002, Furey 1985). When ethanol is added into gasoline, vapor pressure increases with blending ratios of 5-10%, but then gradually declines (Figure 1). Vapor pressure of neat ethanol is low at only 16 kPa (Owen and Coley 1995). Blending vapor pressures for alcohols are significantly higher than their nominal vapor pressures. In particular, the vapor pressure and distillation characteristics of ethanol/gasoline blends are non-linear. When adjusting the octane number of ethanol/gasoline blend, octane numbers in the other distillation ranges of gasoline may need consideration.Įthanol forms azeotropes with hydrocarbons of gasoline, which impacts volatility. Ethanol increases octane numbers of the front-end distillation range of gasoline. Kalghatgi 2005 observed that knock sensor equipped cars performed better when using fuels with lower MON for a given RON. However, even special cars for ethanol use (FFVs) are still being a compromise to dedicated ethanol cars. The high octane numbers of ethanol could enable optimization of engine to increase the thermal efficiency. The sensitivity (RON-MON) is typically 8-10 units for gasoline, but 14 units for ethanol. The blending RON of ethanol is about 120-135, and the blending MON 100-106. Alcohols tend to increase the research octane number (RON) more than the motor octane number (MON). Octane numbers of low-molecular mass alcohols are high, and therefore they have been even used as octane boosters. Complete requirements and standards are available from respective organizations. Selected properties and requirements for anhydrous ethanol, including EN 15376 and US ASTM D 4806.
Typical properties of ethanol together with the selected requirements and recommendations are shown in the following Table. Automanufacturers have defined recommendations for fuel ethanol in "WWFC Ethanol Guidelines". In the US ASTM 4806 defines requirements for denatured fuel ethanol used in blending of gasoline. The European Committee for Standardization (CEN) has defined requirements for undenatured fuel ethanol used for low-concentration ethanol/gasoline blends (EN 15376). Gasoline containing up to 5 vol-% of ethanol is available in European markets at least until 2013 for cars, which do not tolerate E10.
In Europe, Fuel Quality Directive 2009/30/EC allows maximum 10 vol-% ethanol in gasoline starting from 1 January 2011. US EPA regulations allow 1 psi higher vapor pressure for blends containing 9 to 10 vol-% ethanol. In the US, ASTM D 4814 is a specification for gasoline containing up to 10% ethanol (E15 waiver accepted for model year 2001 and newer cars). Hydrogen to carbon ratio of ethanol is 3.0, whereas it is 1.85 for gasoline.
Most modern cars equipped with closed-loop control system can compensate leaning effect of gasoline containing low concentration of ethanol. About 10 vol-% ethanol represents 3.7 wt-% oxygen in gasoline. Thus, ethanol may improve gasoline composition by dilution effect. Ethanol is aromatic-, olefin- and sulfur-free compound.
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