An "evaporative emissions control system" (EVAP) is a system in a vehicle's fuel system designed to prevent gasoline
vapors from escaping into the atmosphere by capturing them in a charcoal canister and then burning them off in the
engine during operation, effectively reducing emissions; key components include the fuel tank, gas cap, charcoal
canister, and a purge valve that controls the release of stored vapors back into the engine for combustion.
Function of evaporative emissions control systems
The primary goal of an EVAP system is to trap fuel vapors that naturally evaporate from the fuel tank, preventing
them from escaping into the environment and contributing to air pollution.
Components
- Fuel tank: The sealed fuel tank plays a crucial role in containing fuel vapors.
- Gas cap: A tight-fitting gas cap with a pressure-regulating valve is critical to prevent vapor leaks.
- Charcoal canister: This canister, filled with activated charcoal, absorbs and stores fuel vapors.
- Purge valve: This valve opens at the appropriate time to release the stored vapors from the canister into the engine intake manifold to be burned.
- Fuel tank pressure sensor: Monitors the pressure inside the fuel tank to detect potential leaks.
How it works
- When the engine is off, fuel vapors from the tank are absorbed by the charcoal in the canister.
- When the engine is running, the purge valve opens, allowing the trapped vapors to be drawn into the engine and burned during combustion.
Diagnostics
Modern vehicles have onboard diagnostics (OBD) systems that can detect faults in the EVAP system, often triggering a check engine light if a leak is detected.