Evaporative Emissions System: Description and Operation

PURPOSE
The evaporative control system prevents gasoline vapors from escaping into the atmosphere.

CONSTRUCTION

Evaporative Control System:

The evaporative control system consists of an expansion tank, roll-over valve, evaporative control canister, thermo vacuum valve, canister purge valve # 1, canister purge valve # 2, connecting hoses and the fuel filler cap.

OPERATION
The fuel tank is sealed and the vapors 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 two diaphragm valves into the combustion chamber, where the vapors are burned.

The vapors from the gas tank are routed to the evaporative control canister. When the engine is off, the fuel vapors collect in the evaporative control canister where they are absorbed by the carbon. Carbon in the canister stores the gasoline vapors until the engine is started.

When the engine is running, the fuel vapors are removed from the evaporative control canister. This is done by opening the connection between the evaporative control canister and the intake manifold, so the vapors can be drawn into the combustion chamber. Canister purge valve # 1 is opened by throttle port vacuum from the throttle valve after it passes through the thermal vacuum valve. Throttle port vacuum is shut off at closed throttle. The thermo vacuum valve opens at engine temperatures above 58° C (136° F). When throttle port vacuum is applied to canister purge valve # 1, the connection between the evaporative control canister and canister purge valve # 2 opens. Canister purge valve # 2 does not open until manifold vacuum is applied, this is to insure that the vapors are not vented to the engine while it is not running. When both canister purge valve # 1 and # 2 are open, the fuel vapors are drawn into the intake manifold to be burned.

COMPONENTS:

Expansion Tank
The expansion tank is used as a gas overflow canister. When the gas 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 gas flows back into the tank via a hose connection.

Roll-Over Valve
A roll-over valve is installed between the expansion tank and the evaporative control canister. It operates through mechanical motion. The valve is a safety device that stops liquid fuel from leaking into the evaporative control canister when the vehicle is at an angle. At a specific angle, for example, roll-over of vehicle, accident, parked on steep hill etc, the valve closes and the liquid fuel can not exit the tank system.

Evaporative Control 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.

Thermal Vacuum Valve
When the engine has reached a temperature of 58° C (136° F), it opens the vacuum connection from the throttle body to canister purge valve #1.

Canister Purge Valve # 1
This valve is controlled by the thermal vacuum valve and the throttle housing vacuum. A vacuum line is routed from canister purge valve #1 through the thermal vacuum valve to the throttle housing. When the thermal vacuum valve is open and no vacuum (closed throttle) is applied to the canister purge valve, the vapor hose connections are blocked off. With vacuum at the canister purge valve, the connection is opened and the fuel vapors can pass through it.

Canister Purge Valve # 2
This valve is controlled manifold vacuum from a port on the throttle housing. When the engine is NOT running there is no vacuum applied to canister purge valve # 2, the vapor hose connections are blocked off. With the engine running there is vacuum at the canister purge valve, the connection is opened and the fuel vapors can pass through it.

Fuel Filler Cap
The filler cap is a non vented type to keep the fuel vapors in the gas tank system. Never replace non vented type filler cap with vented type cap.