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Diurnal evaporative emissions from a vehicle's gasoline (petrol) tank are due to the increase in gasoline vapor pressure with ambient temperature over the course of a 24-hour period. Graphical illustrations are used to explain both the mechanism of these emissions and various control methods. The diurnal emissions can be controlled either by using either an activated carbon canister or a sealed/pressurized fuel tank. Mathematical models were developed for estimating tank vapor generation and activated carbon canister volume for storing the tank vapor. Nomographs were developed for estimating the canister purge air requirements and canister bleed emissions. The models and the nomographs can be used for designing an evaporative emission control system for a given vehicle fuel system. Another mathematical model was developed for estimating the sealed/pressurized fuel tank pressure as a function of fuel RVP (Reid Vapor Pressure) and temperature.

Commingling of fuels containing various concentrations of ethanol can occur in a vehicle fuel tank. It was not possible to estimate the RVPs of commingled ethanol containing fuels because they form complex non-ideal solutions. A nomograph and an equation were developed using the theory and experimental data for estimating the RVP of a commingled ethanol fuel blend from the RVPs of two base fuels containing any amounts of ethanol. The model is also useful for general purpose estimation of vapor pressures of ethanol/gasoline blends such as seasonal blending of E85 fuels with the right vapor pressure gasoline and the right amount of ethanol to meet the seasonal fuel vapor pressure requirement.