Evaporative Emissions System: Description and Operation
Evaporative Emissions
COMPONENT LOCATION - ALL EXCEPT NAS
COMPONENT LOCATION - NAS
INTRODUCTION
The EVAP (evaporative emission) 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. The vent pipes are connected to the system components using quick release connectors.
Fuel vapor is generated by the fuel in the tank and the amount of vapor produced increases as the fuel heats up. Fuel vapor flows to the charcoal canister through the tank vent pipes, via a liquid/vapor separator.
The vapor from the liquid/vapor separator is absorbed and stored by the charcoal canister. 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 during the combustion cycle.
PURGE VALVE AND PIPES
The purge valve is installed on a bracket attached to the LH (left-hand) cylinder head cover. The pipe to the engine from the purge valve is connected to the intake manifold (naturally aspirated vehicles), or SC (supercharger) front cover (SC (supercharger) vehicles), with a quick release connector. The pipe to the charcoal canister from the purge valve is installed between the LH (left-hand) cylinder head cover and ignition coil cover. From the rear of the LH (left-hand) cylinder head, the pipe then goes across the back of the engine, along the RH (right-hand) side of the transmission, along the fuel tank and rearwards of the tank to the charcoal canister.
The purge valve is a solenoid operated valve, which is closed when de-energized. The valve is controlled by the ECM (engine control module) and is operated when engine operating conditions are suitable for purging of the charcoal canister.
The purge valve is controlled by a PWM (pulse width modulation) signal at 10 Hz from the ECM (engine control module). At this frequency, the pulses of purge gas flow into the engine in an almost continuous flow. The valve operates between 0% and 99% duty or mark space ratio (% open time).
The atmospheric pressure at the air intake vent of the system is higher than the inlet manifold pressure under all throttled engine running conditions. It is this pressure differential across the system that causes the air to flow through the air intake of the purge system and in to the engine. The operation of the supercharger does not affect the purging process.
The ECM (engine control module) waits until the engine is running with a coolant temperature of 55 °C (131 °F) or above and closed loop fuel operational before the purging process is activated. Under these conditions the engine should be running smoothly with no warm up enrichment. The purge valve duty (and flow) is initially ramped slowly because the vapor concentration is unknown (a sudden increase in purge could cause the engine to stall or loss of AFR (air fuel ratio) control to occur). The concentration is then determined from the amount of adjustment that the closed loop fueling is required to make to achieve the target AFR. Once the concentration has been determined, the purge flow can be increased and the injected fuel can be proactively adjusted to compensate for the known purge vapor and the target AFR control is maintained.
When the purging process is active, fresh air is drawn into the charcoal canister via the atmospheric vent filter and, on NAS vehicles, the DMTL pump.
CHARCOAL CANISTER
Charcoal Canister - All Except NAS
Charcoal Canister - NAS
The charcoal canister is located in a central position, forward of the spare wheel. It is attached at the rear with two bolts which screw into the spare wheel carrier. At the front, the canister has two lugs which locate in the EPB (electronic parking brake) module support bracket.
The canister on all except NAS vehicles has a capacity of 1400 cc (85.4 in3).
The canister on NAS vehicles has a capacity of 3000 cc (183 in3).
The canister has three connections for attachment of the pipes from the atmospheric vent, the purge valve and the tank vent. On NAS vehicles, the DMTL pump is installed between the atmospheric vent connection and the atmospheric vent pipe.
The 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 stricter emissions regulations.
A filter on the atmospheric vent prevents dust being drawn into the system. The filter is located by the fuel filler cap.
DIAGNOSTIC MODULE TANK LEAKAGE - NAS ONLY
The DMTL system is a legislative requirement for NAS vehicles. The DMTL system periodically checks the EVAP (evaporative emission) system and the fuel tank for leaks when the ignition is switched off.
The DMTL system comprises the previously described components of the EVAP (evaporative emission) system and a DMTL pump.
DMTL Pump
The DMTL pump is connected to the atmospheric vent of the charcoal canister and incorporates an electric air pump, a PTC (positive temperature coefficient) heating element, a normally open change-over valve and a reference orifice. The DMTL pump is only operated when the ignition is off and is controlled by the ECM (engine control module). The ECM (engine control module) also monitors the electric air pump operation and the change-over valve for faults.
DMTL Operation
To check the fuel tank and the EVAP (evaporative emission) system for leaks, the ECM (engine control module) operates the DMTL pump and monitors the current draw. Initially, the ECM (engine control module) establishes a reference current by pumping air through the reference orifice and back to atmosphere. Once the reference current is determined, the ECM (engine control module) closes the change-over valve, which seals the EVAP (evaporative emission) system. The purge valve remains de-energized and is therefore closed. The output from the air pump is diverted from the reference orifice and into the EVAP (evaporative emission) system.
DMTL System Inactive
In its inactive state, the DMTL pump motor and the change-over valve solenoid are not energized. When the ECM (engine control module) energizes the purge valve, filtered fresh air enters the evaporative system through the open change-over valve of the DMTL pump. The filtered air enters the system compensating for engine vacuum drawing on the hydrocarbon vapors stored in the charcoal canister.
DMTL System Active
Phase 1 - Reference Measurement
When the ECM (engine control module) activates the DMTL system, it first activates only the DMTL pump motor. This pumps air through a 0.5 mm (0.02 in) reference orifice, which causes the electric motor to draw a specific amperage value. This value equates to the size of the reference orifice.
Phase 2 - Leak Detection
When the change-over valve solenoid is energized, the valve closes, sealing the EVAP (evaporative emission) system from atmosphere. Providing there are no leaks, the air pump will begin to pressurize the EVAP (evaporative emission) system and the load and current draw on the pump increases. By monitoring the rate and level of the current increase, the ECM (engine control module) can determine if there is a leak in the EVAP (evaporative emission) system.
During normal vehicle operation, the ECM (engine control module) energizes the heating element in the pump to prevent condensation formation and possible incorrect current readings.
Leaks are classified as:
- Minor - equivalent to a hole diameter of 0.5 to 1.0 mm (0.02 to 0.04 in)
- Major - equivalent to hole diameter of 1.0 mm (0.04 in) or greater.
The ECM (engine control module) performs a check for major leaks each time the ignition is switched off, providing the following conditions are met:
- The vehicle speed is zero
- The engine speed is zero
- The atmospheric pressure is above 70 kPa (10.15 lbf/in2), i.e. the altitude is less than approximately 3047 m (10000 feet)
- The ambient temperature is between 0 and 40 °C (32 and 104 °F)
- The charcoal canister vapor concentration factor is 5 or less (where 0 is no fuel vapor, 1 is stoichiometric fuel vapor and greater than 1 is rich fuel vapor)
- The fuel tank level is valid and between 15 and 85% of nominal capacity
- The engine running time during the previous cycle was more than 10 minutes
- The battery voltage is between 10 and 15 volts
- The last engine off time was more than 180 minutes
- No errors are detected with the EVAP (evaporative emission) components, the ambient air temperature and the fuel level
- High range is selected on the transfer box.
NOTE:
A leak test can be performed using the Land Rover approved diagnostic equipment. This overrides the above conditions and is useful for checking correct system and component operation.
The ECM (engine control module) performs a check for minor leaks after every 2nd major leak check.
When the leak check is complete, the ECM (engine control module) stops the DMTL pump and opens (de-energizes) the change-over valve.
If the fuel filler cap is opened or refueling is detected during the leak check, by a sudden drop in the current draw or a rise in the fuel level, the ECM (engine control module) aborts the leak check.
If a leak is detected during the check, the ECM (engine control module) stores an appropriate fault code in its memory. If a leak is detected on two consecutive checks, the ECM (engine control module) illuminates the MIL (malfunction indicator lamp) in the instrument cluster on the next drive cycle.
The duration of a leak check can be between 60 and 900 seconds depending on the test results (developed tank pressure amperage within a specific time period) and fuel tank level.
The following chart depicts the logic used to determine fuel system leaks:
Test Results
