Heat of Vaporization Measurements for Ethanol Blends Up To 50 Volume Percent in Several Hydrocarbon Blendstocks and Implications for Knock in SI Engines

The objective of this work was to measure knock resistance metrics for ethanol-hydrocarbon blends with a primary focus on development of methods to measure the heat of vaporization (HOV). Blends of ethanol at 10 to 50 volume percent were prepared with three gasoline blendstocks and a natural gasoline. Performance properties and composition of the blendstocks and blends were measured, including research octane number (RON), motor octane number (MON), net heating value, density, distillation curve, and vapor pressure. RON increases upon blending ethanol but with diminishing returns above about 30 vol%. Above 30% to 40% ethanol the curves flatten and converge at a RON of about 103 to 105, even for the much lower RON NG blendstock. Octane sensitivity (S = RON - MON) also increases upon ethanol blending. Gasoline blendstocks with nearly identical S can show significantly different sensitivities when blended with ethanol. HOV was estimated from a detailed hydrocarbon analysis (DHA) as well as using a differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) method. The DHA method allows relatively straight-forward estimation of fuel composition and temperature effects, and errors are estimated at less than 10%. The DSC/TGA produces results in good agreement with DHA, and can provide HOV as a function of fraction evaporated. A striking feature of the HOV results was the insensitivity of HOV to the hydrocarbon blendstock for temperatures up to 150°C - all four hydrocarbon blendstocks had essentially the same HOV in kJ/kg and exhibited the same HOV response to blending with ethanol. HOV is much less variable than RON or S. HOV at 20% evaporated from the DSC/TGA experiment appeared to be unaffected by ethanol content while HOV at 50% evaporated increased monotonically with ethanol content.