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