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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.

New regulations require small gasoline engine evaporative diurnal emission control. In the past, it was not possible to design a proper evaporative emission control system (it was mostly trial and error design). In this study, a mathematical model was developed for estimating activated carbon canister size and nomographs were developed for estimating canister purge air requirements and diurnal bleed emissions. The model and the nomographs can be used to design an evaporative emission control system for any SI engine fuel system.

Mathematical models were developed for estimating refueling vapor generation as a function of tank fuel and dispensed fuel Reid vapor pressures and temperatures, and fill type (bottom fill vs. top fill). Since refueling vapor generation is a strong function of fill type (bottom fill vs. top fill), simple illustrative diagrams are presented to explain the mechanism of higher vapor generation from bottom fill type fuel tanks. The models are useful for estimating refueling vapor generation in any real world ambient conditions and with any fuel. The models have been verified with published experimental data.