Operation CHARM: Car repair manuals for everyone.

Evaporative Emissions System: Description and Operation





SYSTEM OPERATION

This system is used to prevent fuel vapors from escaping into the atmosphere. This is accomplished through the use of a non-vented fuel tank, a special fuel filler cap, charcoal canister, special air cleaner, two rollover/pressure relief valves, and hoses necessary to connect these components together.

When the pressure in the fuel tank is below 3 psi, the rollover/pressure relief valves open allowing fuel vapors to flow to the evaporative canister where they are absorbed by charcoal granules. This prevents excessive pressure build-up in the fuel tank.

Air Cleaner Venturi:






A venturi in the air cleaner assembly is utilized as a purge line vacuum source.
The effect of the venturi increases the speed of the intake air flowing by the purge inlet slots in the venturi wall. This creates a low pressure area at the inlet slots that draws vapors from the canister into the airstream flowing through the venturi. The vapors then pass through the intake manifold into the combustion chambers where they are consumed during combustion.

CHARCOAL CANISTER

The fuel resistant nylon body of the canister contains activated charcoal granules which absorb and store fuel vapors until they are drawn into the induction system to be burned along with the normal air/fuel mixture. During engine operation outside air is drawn into the canister through a filter pad located at the bottom of the canister causing the stored vapors to be drawn out of the canister into the airstream in the air cleaner snorkel.



Pressure-Vacuum Filler Cap:






PRESSURE-VACUUM FILLER CAP

The fuel filler cap uses a two-way (vacuum and pressure) relief valve that is closed during normal operating conditions. The relief valve is calibrated to open only when pressure in excess of 1.5 psi or vacuum in excess of 1.8 inch Hg develops in the fuel tank. When vacuum or pressure is relieved, the valve returns to its normally closed position.
It is normal to occasionally encounter an air pressure release when removing the fuel filler cap.

Rollover and Pressure Relief Valve:





ROLLOVER AND PRESSURE RELIEF VALVE

These dual function valves relieve fuel tank pressure and prevent fuel flow through the fuel tank vent lines should the vehicle be rolled over. The valve consists of a plunger, spring, and an orifice and guide plate. The valve is normally open. In the closed position, force applied by spring pressure seats the plunger against the guide plate, shutting off the flow of fuel vapor from the fuel tank to the evaporative canister. When fuel does not reach the valve, the spring pressure is overcome and the plunger moves away from the guide plate allowing fuel vapor to flow through the valve orifice.

In vehicle rollover the valve is inverted. In this position the plunger is forced against the guide plate and raw fuel is prevented from flowing through the valve orifice into the fuel tank vent tube.