A Vapour Pressure Model for Methanol/Gasoline M85 Blends

This paper presents an analytical model for M85 volatility behaviour. It uses a recently developed technique, for gasoline volatility in conjunction with a modified form of Raoult's Law corrected for non-ideality. Vapour pressure, methanol/gasoline distribution in the vapour phase and enthalpy of evaporation are all calculated using explicit equations.

Constants needed for the equations are given for one M85 blend and a simple method of deriving them for any other gasoline/methanol mixture using only measured RVP is outlined.

In addition to describing the new predictive technique, the paper gives the results of an experimental program in which vapour phase composition and vapour pressure behaviour were measured at various temperatures and vapour/liquid ratios. Agreement between predicted and measured results is good.

The new model is used to explain the observation that M85 having the same RVP as gasoline gives poorer cold starting performance.