History

• The Environmental Protection Agency was given the power to regulate automobile pollutants with the 1970 Clean Air Act. This authority and subsequent regulation led to the development of automobile evaporative emissions systems.
  
  

Theory

• The fuel in a car's tank continuously evaporates, converting to a combustible vapor. To prevent its escape, the evaporative emissions system stores the vapor in a charcoal canister until the engine is started. Once started, the engine will siphon it from the canister so that it may be burned during normal engine operation.
  
  

Operation

• The purge control valve closes to prevent the vapor from reaching the engine when it is turned off. When the engine is started and is ready to receive the canister's contents, the purge control valve opens to allow the vapor flow.