Report no. 13/13
Ethanol, at low concentrations in motor gasoline, is known to impact both the fuelconsumption and emissions from vehicles. Because ethanol has a lower energycontent per litre compared to conventional hydrocarbon gasoline, a vehicle¡¦svolumetric fuel consumption generally increases when running on ethanol/gasolineblends. In principle, factors such as the higher octane number and high latent heatof vaporisation for ethanol could allow better engine efficiency which could mitigatethis effect to some extent. The degree to which modern vehicles can compensatefor the lower energy content of ethanol compared to conventional gasoline is notreliably known, however. This is an important question because it impacts theinterpretation of Well-to-Wheels results for biofuel blends used in conventionalvehicles. For this reason, an assessment of published literature up to 2006 wascompleted in order to evaluate the impact of ethanol content on fuel consumption.The scope of this literature assessment was on the use of low-level ethanol/gasolineblends, specifically 5% (E5) and 10% (E10) v/v ethanol in gasoline. These blendsare the most common ethanol levels in Europe today and have been formalised inthe CEN EN 228 standard for motor gasoline. This literature review did not evaluatethe impact of other oxygenate types that are also allowed in the EN 228 standard.Although many publications were evaluated, the number of studies containingrelevant data was limited primarily because the experimental variability in fuelconsumption results was relatively large. From this analysis, the followingconclusions could be made from the evaluated publications:„h There is a relatively high incidence of incorrectly derived fuel consumptiondata, usually resulting in an underestimate of the increase in fuel consumptionfrom ethanol-containing gasolines.„h For some studies on gasoline blends containing up to E20, the increase inmass fuel consumption (FC) was typically only about 50% of the expectedvalue, based on simply the loss in calorific value from ethanol addition. In themost extreme case, fuels with almost identical energy contents showed a fuelconsumption difference of more than 4%, although for the majority of vehicles,the difference was generally less than 2%. For the largest data set (7 vehiclesusing up to 10 test fuels), the overall trend showed a 3.97% increase in fuelconsumption with a 3.4% reduction in fuel energy content.„h It is not clear that this variation in FC results was related to variations in fuelparameters other than the ethanol content. Rather, it is assumed that thevariability in FC was primarily a consequence of experimental variation andpoorly controlled test procedures. The variability found in these publishedstudies limits the firm conclusions that can be drawn about the influence of lowlevels of ethanol on vehicle fuel consumption.These conclusions suggested that a more definitive vehicle study was warranted todetermine whether modern vehicles can or cannot compensate for the lower energycontent of ethanol-containing gasoline through better engine efficiency. Such a studyhas now been completed by the JEC1 Consortium and is reported elsewhere .