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Evaporative emission canister - North American Specification (NAS) derivative
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Fuel tank leakage monitoring pump
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Fuel tank leakage monitoring pump filter
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Fuel delivery pipe to engine
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Evaporative emission canister purge valve
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,