Onboard refueling vapor recovery

Onboard Refueling Vapor Recovery (ORVR) is a vehicle emission control system that captures fuel vapors from the vehicle gas tank during refueling. The gas tank and fill pipe are designed so that when refueling the vehicle, fuel vapors in the gas tank travel to an activated carbon packed canister, which adsorbs the vapor. When the engine is in operation, it draws the gasoline vapors into the engine intake manifold to be used as fuel. ORVR has been mandated on all passenger cars in the United States since 2000 by the United States Environmental Protection Agency‎. The use of onboard vapor recovery is intended to make vapor recovery at gas stations obsolete.

From http://en.wikipedia.org/