Evaporative Emissions System: Description and Operation

Fuel Tank:

This system consists of the fuel tank, expansion tank, charcoal canister, [1][2]charcoal canister by-pass valve, breather valve, and rollover (gravity) vent valve. It is designed to limit emission of fuel vapors into the atmosphere and prevents raw fuel from escaping the fuel tank. An expansion chamber for the fuel tank and vent lines are part of the fuel tank vent system. These components prevent raw fuel from escaping at extremely high outside temperatures and when the vehicle is driven or parked on an incline or non-level position.

Rollover/Gravity Valve (typical):

Vapors from the fuel tank are passed through a rollover (gravity) valve, and trapped in the charcoal canister which is filled with activated charcoal. This canister is connected to the fuel tank vent system and allows fuel vapors to pass through it and deposit hydrocarbons on its surface.

Charcoal Canister:

The [1][2]charcoal canister by-pass valve regulates the flow of vapors purged from the charcoal canister. When the engine is running, vacuum opens the charcoal canister by-pass valve and fresh air enters the charcoal filter through an opening. This cleans the charcoal and hydrocarbons are routed via the air cleaner, back to the engine where they are burned during combustion.