Hydrocarbon pollutants also escape into the air through fuel evaporation. With today's efficient exhaust emission controls and today's gasoline formulations, evaporative losses can account for a majority of the total hydrocarbon pollution from current model cars on hot days when ozone levels are highest. Evaporative emissions occur several ways:
DIURNAL: Gasoline evaporation increases as the temperature rises during the day, heating the fuel tank and venting gasoline vapors.
RUNNING LOSSES: The hot engine and exhaust system can vaporize gasoline when the car is running.
HOT SOAK: The engine remains hot for a period of time after the car is turned off, and gasoline evaporation continues when the car is parked.
REFUELING: Gasoline vapors are always present in fuel tanks. These vapors are forced out when the tank is filled with liquid fuel.