Elemental Composition Determination and Stoichiometric Air-Fuel Ratios of Gasoline Containing Ethanol 2010-01-2112
Carbon, hydrogen and oxygen are major elements in modern fuels. Varying combinations of these elements in motor fuel alter the stoichiometric air-fuel ratio (A/F). Stoichiometric A/F ratio is an important parameter in engine calibration affecting vehicle performance, emissions and fuel economy.
With increasing use of ethanol in automotive fuels in recent years, since it can be made from renewable feedstocks, oxygen contents in fuel are increasing. Oxygen contents can be around 1.7 mass % in European E5 gasoline or 3.5 mass % in U.S. E10 gasoline and up to 29 mass % in E85 fuel. The increase in oxygen content of fuel has resulted in changes in other physical and chemical properties due to the differences between ethanol and hydrocarbons refined from fossil oil. A previous paper (SAE 2010-01-1517) discussed the change in energy content of automotive fuel and the estimation of net heating values from common fuel properties. This paper will discuss the estimation of elemental composition and stoichiometric A/F ratios of gasoline-ethanol blends from common fuel properties. While the carbon, hydrogen, and nitrogen contents can be measured by combustion of the fuel in a tube and then quantifying the combustion products as described in ASTM D5291 or E191, the test is time-consuming and expensive. The repeatability of the method is marginally satisfactory. It is generally satisfactory and more convenient to estimate the elemental composition from other commonly-measured fuel properties. Several standardized empirical methods have been developed in the past for estimating the hydrogen content of hydrocarbon fuels such as gasoline, diesel fuel, and jet fuel. However, the addition of ethanol to gasoline has become very common in some countries, and most of the estimation methods developed for hydrocarbon fuels are not satisfactory for use with gasoline-ethanol blends, especially blends containing more than about 10 volume-percent ethanol. This paper summarizes compositional data from recently collected retail samples of E10 and E85 gasoline-ethanol blends in the U.S. and E20 blends in Brazil, as well as laboratory blends containing different concentrations of fuel ethanol. The compositional data obtained by different standardized test methods are compared and discussed. New empirical methods and equations are proposed for gasoline containing any concentration of ethanol. Stoichiometric A/F ratios are then computed with standard methods.