Evaporative Emissions System: Description and Operation
DESCRIPTION AND OPERATION
Evaporative Emissions Control
To reduce the emission of fuel vapor, the fuel tank is vented to atmosphere through activated charcoal adsorption canister(s) which collects the fuel droplets. The charcoal is periodically purged of fuel when the EVAP Canister Purge Valve opens the vapor line between the canister(s) and the air intake induction elbow. This action allows manifold depression to draw air through the canister atmospheric vent, taking up the deposited fuel from the charcoal absorber and burning the resulting fuel vapor in the engine.
The EVAP Canister Purge Valve is controlled by the engine management system ECM. Purging is carried out in accordance with the engine management fueling strategy (see below).
The fuel tank vapor outlet is via a removeable flange assembly on the top surface of the tank. The vapor storage canister or canisters are fitted on the underside of the vehicle below the rear seats.
There are three variants of the evaporative system. All systems use the charcoal adsorbate storage canisters and purge valve and operate as described above. The specific features of each system are described below. The evaporative systems are designated as:
SINGLE CANISTER SYSTEM
RUNNING LOSS SYSTEM
RUNNING LOSS WITH ON-BOARD RE-FUELING VAPOR RECOVERY (ORVR) SYSTEM
EVAP Canister Purge Valve
}<n Evaporative Canister Components:
The EVAP canister purge valve controls the flow rate of fuel vapor drawn into the engine during the canister purge operation. The valve is opened by a vacuum feed from the induction elbow : the vacuum feed is controlled by the integral valve solenoid and is applied when the solenoid is energized. The solenoid is pulsed on (energized) and off by a fixed frequency (100Hz) variable pulse width control signal (pulse width modulation). By varying the pulse on to off time, the ECM controls the duty cycle of the valve (time that the valve is open to time closed) and thus the vapor flow rate to the engine.
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/connection, engine management adaptations must be re-instated.
Engine has run for at least 8 seconds.
Engine coolant temperature is not less than 70°.
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 - Air Flow Meter (AFM), Engine Coolant Temperature sensor,
Evaporative Canister Purge valve and Canister Close Valve (CCV)
If these conditions have been satisfied, purging is started. if any failures are registered,purging is inhibited.
The canister(s) is 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 (grains 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 single canister, running loss and ORVR evaporative systems.
Single Canister System
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This system uses a single charcoal canister with a pressure control valve between the canister and the fuel tank vapor outlet. A vacuum control pipe is connected from engine intake induction elbow to the pressure control valve. The vapor outlet from the fuel tank is taken via a safety rollover valve fitted to the removeable flange at the top of the tank.
With the engine stopped, the pressure control valve is closed, maintaining a slight positive pressure in the tank : any further increase in pressure causes the valve to open and release vapor to the canister.
When the engine is running, manifold depression (via the vacuum control pipe) holds the pressure control valve open. Air is drawn into the tank to maintain atmospheric pressure as fuel is used and vaporized fuel is deposited in the charcoal canister.
Canister purge operation is as described in Evaporative Emissions Control.
Evaporative flange assembly
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The evaporative loss flange assembly is fitted to the top of the tank via a seal and locking ring. The assembly is removeable complete with the fitted components. The vapor vent / rollover valve is a push fit via a sealing grommet. The fuel pump connector is push fitted and crimped into a location tube on the underside of the flange.
Running Loss System
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The running loss system has the following features:
A normally open pressure control valve is fitted at the lefthand side of the tank.
Two charcoal canisters are connected in series to reduce the concentration of fuel in vapor vented to atmosphere.
Tank pressure sensor and canister close valve are fitted to allow the on-board diagnostic facility to test for leaks in the fuel and evaporative system.
The pressure control valve allows continuous venting of vapor to the charcoal canisters during normal running but prevents fuel entering the vent line during refueling.
Fuel vapor from the tank passes through the valve to the lefthand canister and a third pipe connects the valve to the filler neck. When refueling, the difference in pressure between the tank interior and the open filler neck causes the valve to close, shutting off the vapor vent line. When the fuel cap is replaced, the pressures are equalized and the pressure control valve opens the vent line to the canisters.
The canister close valve is a solenoid operated device controlled by the ECM. The valve is normally open and is closed only during the leak test sequence.
The fuel pressure sensor is fitted to the evaporative loss flange and provides a voltage to the ECM which is proportional to tank vapor pressure.
Canister purge operation is as described in Evaporative Emissions Control.
Evaporative Flange Assembly
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The evaporative loss flange assembly is fitted to the top of the tank via a seal and locking ring. The assembly is removeable complete with the fitted components.
The vapor vent / rollover valve and pressure sensor are a push fit via sealing grommets. The fuel pump connector is push fitted and crimped into a location tube on the underside of the flange.
Pressure Control Valve
Component View:
The pressure control valve is fastened by adhesive to the lefthand side of the tank. It is a non-serviceable component.
Canister,CCV and Fittings
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The canisters are fixed to the underside of the vehicle either directly or via semi-enclosed mounting brackets. Two fixing bolts are used at the front of the canister(or bracket) and a single rear bolt supports the canister and the canister close valve (CCV).
The vapor pipes to the canisters, other than the CCV, use multi-tang connectors which are push fitted and pulled out without the use of tools.
The canister close valve has a stub pipe with 'O' ring seal which is a simple push fit into the canister. A mounting bracket on the canister close valve enables it to be secured to the underbody via the canister rear mounting bolt.
A hose connects the CCV to the bracket mounted adaptor into which the vent system air filter is screwed.
Running Loss with On-board Refueling Vapor Recovery (ORVR) System.
p Component View:
The system has the following features:
On-board refueling vapor recovery (ORVR) to reduce the fuel vapor vented directly to atmosphere from the filler nozzle when refueling.
Two charcoal canisters are connected in series to reduce the concentration of fuel in vapor vented to atmosphere.
A tank pressure sensor and canister close valve are fitted to allow the on-board diagnostic facility to test for leaks in the fuel and evaporative system.
The canister close valve is a solenoid operated device controlled by the ECM. The valve is normally open and is closed only during the leak test sequence.
The fuel pressure sensor is fitted to the evaporative loss flange and provides a voltage to the ECM which is proportional to tank vapor pressure.
Operation of ORVR System
The ORVR system enables fuel vapor generated during re-fueling to be collected by the charcoal canisters. During normal running of the vehicle, the vapor is collected and purged in the same way as for non-ORVR systems.
The ORVR system features are:
Narrow fuel filler pipe and tank check valve.
Fuel level vent valve (FLVV) fitted to the evaporative flange and consisting of a two stage shut-off valve with rollover protection and a pressure relief valve.
Grade vent valve with rollover protection, fitted to the evaporative flange and with an outlet pipe connected to the FLVV vapor outlet pipe.
Large bore vapor vent pipes.
The fuel filler pipe has a reduced diameter between the nozzle guide and the tank, providing a liquid seal when re-fueling and preventing the fuel vapor venting directly to atmosphere. There is no breather tube fitted between the tank and the filler nozzle. To prevent spit back when re-fueling, a check valve is fitted at the lower end of the filler pipe inside the tank (see Figure).
During re-fueling, the tank is vented to atmosphere via the fuel level vent valve (FLVV), large bore vapor pipes and the charcoal canisters. The FLVV incorporates a flow valve which is closed by the rising fuel level, creating a back pressure and causing the fuel delivery to stop. In the closed position, the FLVV also sets the fuel level.
With the FLVV closed (tank full), any increase in pressure or overfilling is relieved by a separate rollover protected grade vent valve. The outlet from this valve feeds into the main FLVV vapor outlet pipe, bypassing the closed FLVV.
When the fuel level is below full, the FLVV opens to allow unrestricted venting via the canisters.
A pressure relief valve is incorporated into the FLVV assembly and has an outlet pipe to the filler nozzle,. If a blockage or other restriction (eg, canister close valve failing the closed position) occurs in the vapor vent system, the pressure relief valve opens to allow venting to atmosphere via the filler nozzle guide and fuel filler cap.
Canister purge operation is as described in Evaporative Emissions Control.
Filler Tube and Check Valve
The fuel filler cap is the same as that used on non-ORVR systems.
The check valve assembly is a push fit into the end of the fuel filler tube and is accessible with the evaporative flange removed. The valve has a light spring loading and opens under the inflow of fuel.
OVRR Evaporative Loss Flange Assembly
The evaporative loss flange assembly is fitted to the top of the tank via a seal and locking ring arrangement identical to that used for non-ORVR systems. The assembly removeable complete with the fitted components.
The fuel level vent valve (FLVV) is mounted in the ELF assembly via a bayonet fitting: it is turned approximately 90° clockwise to release. The grade vent valve and pressure sensor are push in fits.via sealing grommets: note that, due to the tight fit, removal of these components may require cutting the grommets. The fuel pump connector is push fitted and crimped into a location tube on the underside of the flange.
Cannister,CCV and Fittings
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The canisters are fixed to the underside of the vehicle either directly or via semi-enclosed mounting brackets. Two fixing bolts are used at the front of the canister(or bracket) and a single rear bolt supports the canister and the canister close valve (CCV).
The vapor pipes to the canisters, other than the CCV, use multi-tang connectors which are push fitted and pulled out without the use of tools.
The canister close valve has a stub pipe with 'O' ring seal which is a simple push fit into the canister. A mounting bracket on the canister close valve enables it to be secured to the underbody via the canister rear mounting bolt.
A hose connects the CCV to the bracket mounted adaptor into which the vent system air filter is screwed.
Circuit Diagram, Evaporative Emissions Systems:
