A New Correlation for Predicting Average Vehicle Evaporative Emissions as a Function of Fuel Volatility

In estimating emissions inventories, it is important to be able to predict the evaporative emissions of the vehicle fleet, and to know the impact of fuel volatility changes. A new relationship has been developed for predicting vehicle evaporative emissions as a function of fuel volatility. A new quantity, vapor generation number N_vp, was defined to indicate the vapor generation potential of a fuel: N_vp = EXP(0.26 RVP) + 0.23 V₇₀, where RVP is the Reid Vapor Pressure of the fuel and V₇₀ is the amount of fuel distilled at 70°C. The first term accounts for the vapor generation from the vehicle fuel tank; the second term accounts for the hot-soak vapor generation from the carburetor and it is equal to zero for fuel-injected cars. The constants in this equation were determined using both mathematical models and experimental data. The average evaporative emissions with a wide variety of fuels were calculated using information from EPA, API, and CRC reports. An excellent correlation (R=0.95) was found between the average evaporative emissions and N_vp.