Evaporative Emissions System: Description and Operation

1988 PRELUDE

Fig. 19 Evaporative emission control system.:

This system, Fig. 19, consists of a charcoal canister, a vapor purge control system, a carburetor vapor control system and a fuel tank vapor control system. The system minimizes the amount of fuel vapor escaping to the atmosphere.



CHARCOAL CANISTER



The charcoal canister is a container for temporary storage of fuel vapor until it can be purged from canister into engine and burned.



VAPOR PURGE CONTROL SYSTEM



The vapor purge control system controls when canister is to be purged. When coolant temperature is above 145°F, and intake air temperature is below 149°F, manifold vacuum is directed to the purge control valve, which allows venturi vacuum to draw fresh air through the bottom of the canister into the engine, purging the fuel vapor from the canister.



CARBURETOR VAPOR CONTROL SYSTEM



The air vent cut-off solenoid valve regulates air flow to the carburetor float bowls. When the engine is not running, the outer air vent passage opens, so that fuel vapor in the float bowls can be vented into the charcoal canister. When engine speed is above 400 RPM, the air-vent cut-off solenoid valve opens the inner air vent passage, so that fuel vapor in the float bowls can be vented into the air cleaner.
When the engine is hot started, with coolant temperature above 176°F, the opening of the inner air vent passage is delayed momentarily to ease engine start up.
There is also an inner vent solenoid valve to control air flow to the carburetor float bowls. When engine speed is above 400 RPM, the inner vent solenoid valve normally opens the sub inner air vent passage, but will only allow fuel vapor to be vented to the air cleaner.



FUEL TANK VAPOR CONTROL SYSTEM



The fuel tank vapor control system consists of a fuel cut-off valve, liquid/vapor separator, a two way valve and fuel filler cap. All fuel vapor inside the fuel tank is directed to the charcoal canister through the fuel cut-off valve and the liquid/vapor separator to prevent build up of vapor in the tank which might otherwise escape when the fuel filler cap is removed.
The two way valve regulates both the vacuum and pressure in tank.
There is also a relief valve in the fuel filler cap in case there is an excessive build up of pressure or vacuum.