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.