Evaporative Emissions System: Description and Operation

PURPOSE
The evaporative control system prevents gasoline vapors from escaping into the atmosphere. Vapors from the fuel tank are routed to the evaporative control canister. Carbon in the canister stores the gasoline vapors until the engine is started. The intake manifold vacuum of the running engine draws the vapors trough the diaphragm and EVAP solenoid valve into the combustion chamber, where the vapors are burned.

Fig. 33 Evaporative Control System:

CONSTRUCTION
The evaporative control system consists of an expansion tank, carbon canister, diaphragm valve, EVAP valve, connecting hoses and the fuel filler cap.

OPERATION
When the engine is at idle, the diaphragm and EVAP solenoid valves are closed. Fuel vapors are absorbed by the carbon in the canister. When the engine is running at normal or high load conditions, the vacuum in the intake manifold will open the diaphragm valve. This enables the vapors to flow through it. When the engine has reached operating temperature and is above idle, the EVAP solenoid valve will open and close in cycles depending on air flow measured by the air flow sensor. A high air flow will keep the valve open for a longer amount of time than a low air flow. The gasoline vapors can now be drawn out of the carbon canister, through the open diaphragm and EVAP solenoid valve into the intake manifold. There they get burned in the combustion chamber.

COMPONENTS:

EXPANSION TANK
The expansion tank is used as a fuel overflow canister. When the fuel tank is full and the outside temperature changes from cold to hot (or other influencing factors), the fuel expands into the expansion tank. As the fuel level drops, the fuel flows back into the tank via a hose connection.

CARBON CANISTER
The canister contains carbon (activated charcoal) which absorbs the gasoline vapors from the fuel tank. The suction created by the intake air system draws the fuel vapors from the carbon out of the canister. The use of alcohol-gasoline fuels will reduce the effectiveness of the carbon in the canister.

DIAPHRAGM VALVE
This valve is controlled by a vacuum line connected to the throttle housing. When no vacuum is applied to the valve, the vapor hose connections are blocked off. With vacuum at the diaphragm valve, the connection is opened.

EVAP SOLENOID VALVE
The solenoid valve is controlled by the ECU via electrical connection. When without voltage, the valve is open. When 12V current is applied to the solenoid connection, the valve closes the vapor in- and outlet ports.

ELECTRONIC CONTROL UNIT (ECU)
The control unit (located under the driver side seat) uses many input sources to determine how long the EVAP solenoid valve should be open. It closes the valve by sending an electrical current in cycles of six times per minute. The "open time" is determined by the duration of each cycle. This "open time" varies depending on the ECU input sources (air flow, engine temperature, etc...).

FUEL FILLER CAP
The filler cap is a non vented type to keep the fuel vapors in the fuel tank system. Never replace non vented type filler cap with vented type cap.