Evaporative Emissions - Evaporative Emissions
Description and Operation
To reduce the emission of fuel vapour, the fuel tank is vented to atmosphere through activated evaporative emission canisters which collect the fuel vapor. The evaporative emission
canister is periodically purged of fuel vapor when the evaporative emission canister purge valve opens the vapor line between the evaporative emission canister and the air intake induction
elbow. This action allows manifold depression to draw air through the evaporative emission canister atmospheric vent, taking up the deposited fuel vapor from the charcoal adsorber inside
the evaporative emission canister and burning the resulting fuel vapor in the engine.
There are two variants of the evaporative emissions system. All systems use the charcoal adsorber storage evaporative emission canisters and purge valve and operate as described
above. The specific features of each system are described below. The evaporative systems are designated as:
Vehicles with on-board refueling vapor recovery
Vehicles without on-board refueling vapor recovery
Evaporative Emissions Canister Purge Valve
The evaporative emission canister purge valve controls the flow rate of fuel vapor drawn into the engine during the canister purge operation. The valve is operated via inputs from the
engine control module (ECM).
With no ECM signal applied to the valve solenoid, the valve remains closed.
Canister Purge Operation
The following pre-conditions are necessary for purging to commence:
after battery disconnection/reconnection, engine management adaptations must be re-instated.
engine has run for at least 8 seconds.
engine coolant temperature is not less than 70°C.
engine not running in the fuel cut off condition (eg overrun).
the adaptive fuel correction function has not registered a rich or lean failure.
the evaporative emission leak test has not failed.
no faults have been diagnosed in the relevant sensor and valve circuits - mass air flow (MAF) sensor, engine coolant temperature (ECT) sensor, evaporative canister purge valve
and evaporative emission canister vent solenoid.
If these conditions have been satisfied, purging is started. If any failures are registered, purging is inhibited.
The canisters are purged during each drive cycle at various rates in accordance with the prevailing engine conditions. The engine management software stores a map of engine speed
(RPM) against engine load (grams of air inducted/rev). For any given engine speed and load, a vapor purge rate is assigned (purge rate increases with engine speed and load).
The preset purge rates are based on the assumption of a vapor concentration of 100%. The actual amount of vapor is measured by the closed loop fueling system: the input of evaporative
fuel into the engine causes the outputs from the upstream oxygen sensors to change, the amount of change providing a measure of the vapor concentration. This feedback causes the
original purge rate to be adjusted and also reduces the amount of fuel input via the injectors to maintain the correct air to fuel ratio.
Engine speed/load mapping and the corresponding purge rates are different for vehicles with on-board refueling vapor recovery and vehicles without on-board refueling vapor recovery .
Vehicles With On-board Refueling Vapor Recovery.