Evaporative Emissions System: Description and Operation
Evaporative Emissions
COMPONENT LOCATION
OVERVIEW
The evaporative emission (EVAP) control system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank.The system comprises a charcoal canister, purge valve and interconnecting vent pipes and hoses.
Fuel vapor is generated by the fuel in the tank and the amount of vapor produced increases as the fuel heats up.Fuel vapor can flow freely to the charcoal canister via the tank venting system.The venting system consists of roll over valves and a liquid vapor separator mounted internally in the tank and then externally via a breather line.The breather line allows the fuel vapor to flow to the charcoal canister.
On NAS vehicles the vapor generated in the fuel tank during refueling flows without restriction to the charcoal canister.
On all vehicles except NAS, the vapor is restricted in its path to the charcoal canister but can flow freely during the refueling operation to atmosphere, via the filler opening.
The vapor passes into the charcoal canister where it is absorbed and stored by the charcoal.Because there is a limit to the amount of vapor the canister can contain, the fuel vapor is purged from the canister when the engine is running and burned in the engine.
PRINCIPLES OF OPERATION
Charcoal Canister
The charcoal canister contains a bed of activated charcoal or carbon.The charcoal is produced using special manufacturing techniques to treat the charcoal with oxygen.The oxygen treatment opens up millions of pores between the carbon atoms resulting in a highly porous charcoal with a very large effective surface area which is capable of absorbing large quantities of fuel vapor.Once treated the charcoal is known as 'activated' carbon or charcoal.The charcoal canister on NAS vehicles uses a higher grade charcoal to meet the requirements of the emission regulations.
On-board Refueling Vapor Recovery
To meet On-board Refueling Vapor Recovery (ORVR) requirements, the fuel tank and associated components are designed to minimize fuel vapor loss during refueling.This is achieved by preventing fuel vapor from the fuel tank venting directly to the atmosphere.Instead fuel vapor is directed into the Evaporative Emission (EVAP) canister where it is stored before being purged at intervals into the engine's intake manifold.
During vehicle refueling the narrow fuel-filler-tube below the fuel-dispenser nozzle region, provides a liquid seal against the escape of vapor.A check valve also located in the filler-tube opens to incoming fuel to prevent splash back.
As the fuel tank fills, fuel vapor is routed to the EVAP canister's charcoal filter, where it is absorbed by the charcoal's activated granules.The hydrocarbons from the fuel vapor are retained within the canister, allowing only purified air to be released to atmosphere through the canister's vent pipe.The hydrocarbons stored in the canister are purged at intervals into the engine for combustion.
The EVAP canister purge valve is located in the engine compartment.The Engine Control Module (ECM) operates the valve to purge fuel vapor from the EVAP canister into the engine for combustion.Purge rates (the extent that the purge valve opens) are determined by the engine operating conditions.The purge rates are adjusted to maintain vehicle driving characteristics and exhaust emissions at optimum levels.Engine operating conditions that affect the purge rate are:
^ speed and load,
^ engine coolant temperature,
^ time from engine start-up,
^ closed loop fueling
Owing to the limited storage-volume of the EVAP canister:
^ NAS derivative - 2.5 liter
^ ROW derivative -0.7 liter
the charcoal filter is continually regenerated.
Regeneration is achieved when the engine is running, by drawing air through the EVAP canister, via the vent pipe, into the engine for combustion.
Fuel Tank Leakage Monitoring
Fuel tank leakage monitoring is a legislative requirement for NAS vehicles.The monitoring system periodically checks the EVAP system and fuel tank for leaks when the ignition is off.
The monitoring system comprises the previously described components of the EVAP system with the following additional components:
^ air pump, and
^ dust filter
The pump is connected to the vent pipe of the EVAP canister and incorporates:
^ a Positive Temperature Coefficient (PTC) heating element,
^ change over valve, and
^ a reference orifice
The pump is only operated when the ignition is 'off' and as controlled by the ECM.The ECM also monitors the pump operation and the change over valve for faults.The dust filter protects the pump from the ingress of moisture and foreign particles.
The fuel tank leakage monitoring test is performed if the following factors are present:
^ the engine has been operating for at least 10 minutes and is now at rest,
^ the fuel in the tank is between 15 and 85 percent volume,
^ the ambient temperature is above 0 degrees C (32 degrees F) and less than 40 degrees C (104 degrees F),
^ the engine has not been started for at least 3 hours prior to its last operation
The pump is operated to pressurize the fuel tank and the electrical current used to operate the pump is measured with the change-over valve in different states.A comparison of the current draw in each state indicates the degree of any fuel vapor leak; if appropriate the ECM records the appropriate Diagnostic Trouble Code (DTC).