Description
Evaporative Emissions
OPERATION
Dual Path Purge System
Atmospheric pressure at the spider trap on ROW vehicles or DMTL intake filter on NAS vehicles is higher than the inlet manifold pressure under all throttle conditions. The pressure differential (vacuum) causes fresh air to be drawn into the spider trap or DMTL intake filter. The fresh air passes through the charcoal canister removing fuel vapor from the charcoal and passing it via the purge hose to the engine, where it is burnt during the combustion process. The purge process is controlled by the ECM (engine control module) which uses PWM (pulse width modulation) signal pulses to open and close the purge valve, regulating the amount of the fuel vapor entering the intake manifold.
Vapor from the charcoal canister passes to the purge valve and cannot enter the air intake when the purge valve is closed (de-energized). When the purge valve is activated by the ECM (engine control module), fuel vapor passes through the purge valve to the double check valve. If the pressure in the intake manifold is lower than the pressure in the venturi, (for example under low load, low turbocharger boost conditions) the vapor is passed through the double check valve and enters the intake manifold via the purge 3 hose to the port after the electric throttle.
Under high load, high turbocharger boost conditions, the pressure in the intake manifold will exceed the purge system pressure, preventing purge gases being drawn into the intake manifold. Under these conditions, purge flow is diverted via the double check valve to the venturi. The pressure differential between the clean air purge 2 hose connection port on the connector port below the electric throttle and purge 2 hose connection on the clean air low pressure duct generates a bypass flow through the venturi (under all conditions) and hence a depression within the venturi, enabling purge gasses to be drawn in and passed through the turbocharger compressor to the engine via the clean air low pressure duct.
The ECM (engine control module) will only operate the purge valve once the engine coolant temperature is more than ?? °C (?? °F). Under these conditions the engine is running smoothly and efficiently with no warm-up enrichment. The purge valve PWM (pulse width modulation) duty cycle (vapor flow) is initially gradually increased as the vapor concentration in the charcoal canister is unknown. A sudden increase in fuel vapor may cause the engine to become over rich with fuel and cause flooding. The fuel concentration is measured by the ECM (engine control module) using adjustments made to achieve the target Air Fuel Ratio (AFR). Once the fuel concentration is determined, the vapor flow through the purge valve can be increased and the ECM (engine control module) can adjust the amount of injected fuel to compensate for the purge vapor and maintain the target AFR.
The purge system does not include a pressure test point. Pressure testing of the system is performed by disconnection of the purge valve hose and connecting special tools to allow the purge system to be pressure tested. The test will confirm the integrity of the purge hose and connections.
DMTL Operation
To check the fuel tank and EVAP (evaporative emission) system for leaks the ECM (engine control module) operates the DMTL air pump and monitors the current draw. Initially, the ECM (engine control module) establishes a reference current by pumping air through a reference orifice and back to atmosphere. Once the reference current is determined, the ECM (engine control module) closes the normally open valve which seals the EVAP (evaporative emission) system. The purge valve remains de-energized and therefore closed. The output from the DMTL air pump is diverted from the reference orifice and into the EVAP (evaporative emission) system.
When the ECM (engine control module) activates the DMTL system, only the air pump is operated. This pumps air through a 0.5 mm reference orifice which causes the air pump motor to draw a specific amperage value. The value equates to the size of the reference orifice and provides the datum for further measurements.
When the normally open valve solenoid is activated the valve is closed, sealing the EVAP (evaporative emission) from atmosphere. Providing there are no leaks, the DMTL pump will begin to pressurize the EVAP (evaporative emission) system and the load and current draw on the air pump motor increases. By monitoring and 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.
Leaks are classified as follows:
- Minor - equivalent to a hole of 0.5 to 1.0 mm (0.02 to 0.04 in)
- Major - equivalent to a 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 pressure altitude (70 kPa (10.15 lbf/in2) derived from engine load calculations) is below 3047m (10,000 ft)
- The ambient air temperature is between 0 and 40°C (32 and 104°F)
- The charcoal canister load value factor is 2 or less (where the load factor is a measure between -1 and +30 of the fuel vapor stored in the charcoal canister. Where -1 is 0% fuel vapor, 0 is the stoichiometric fuel vapor level and +30 is 100% saturated with fuel vapor.
- The fuel tank value is valid and is 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) system components, the ambient air temperature and the fuel level.
NOTE:
A leak test can also be performed using a Land Rover approved diagnostic system.
During normal vehicle operation the ECM (engine control module) energizes the heating element in the air pump to prevent condensation formation and possible incorrect readings.
The ECM (engine control module) performs a leak test for minor leaks after every 2nd major leak check or after refuelling is detected.
When the leak test is completed, the ECM (engine control module) stops the DMTL pump and opens (de-energizes) the normally open valve.
If the fuel cap is opened or refuelling is detected during a leak test, by a sudden drop in the motor draw current or a rise in fuel level, the ECM (engine control module) aborts the leak test.
If a leak is detected during a test, the ECM (engine control module) stores an appropriate DTC (diagnostic trouble code) in its memory. If a leak is detected on two consecutive tests, the ECM (engine control module) illuminates the MIL (malfunction indicator lamp) in the instrument cluster on the next drive cycle.
The duration of the leak test can be between 60 and 600 seconds depending on the test results (developed tank pressure amperage within a specific time period) and the fuel tank level.
DESCRIPTION
Dual Path Purge System Components
Charcoal Canister
Charcoal Canister ROW
Charcoal Canister NAS
The charcoal canister is located in the LH (left-hand) rear wheel arch, above the wheel arch splash shield. A rubber grommet on the bracket is located in a corresponding bracket on the vehicle body. The rear of the canister bracket is secured to a second bracket on the vehicle body with a bolt and spring nut.
The canister has 3 ports which allow for the attachment of the atmospheric vent hose, the fuel vapor purge hose and the tank vent hose. On NAS vehicles the atmospheric hose connection is connected directly to the DMTL air intake filter.
The canister contains a bed of activated charcoal. The charcoal is produced with a special manufacturing technique 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 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 LEV 2 emission regulations.
DMTL Pump NAS
The Diagnostic Monitoring Tank Leakage (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 NAS DMTL system comprises the components previously described with the addition of the DMTL pump and atmospheric air intake filter.
The DMTL pump and filter are located on the same bracket that mounts the charcoal canister. The pump is connected by a hose to atmospheric vent of the charcoal canister and incorporates a PTC (positive temperature coefficient) heating element, a normally open valve and a reference orifice.
The DMTL pump is only operated when the ignition is switched off and is controlled by the ECM (engine control module). The ECM (engine control module) also monitors the electric air pump operation and the normally open valve for faults.
The DMTL atmospheric air intake filter protects the pump from dust being drawn into the system when the pump is operated. The filter is connected to the pump via a short hose.
Purge Valve
The purge valve is located on the LH (left-hand) side of the engine, forward of the intake manifold. The valve is attached to a bracket which is secured to the FEAD (front end accessory drive) idler pulley bracket.
The vapor purge hose from the charcoal canister connects into the upper connection on the valve. The low connection provides for the attachment of the vapor outlet hose to the double check valve. An electrical connector on the top of the valve connects with the vehicle engine wiring harness.
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 correct to allow vapor purging of the charcoal canister.
The purge valve is controlled by PWM (pulse width modulation) by the ECM (engine control module). At this frequency the pulses of purged fuel vapor flows into the intake manifold in an almost continuous flow. The valve is operated at between 7% and 100% duty ratio, with the percentage representing the opening time.
Venturi
The purge valve is located on the LH (left-hand) side of the engine, forward of the intake manifold. The venturi is attached to the same bracket as the purge valve and is attached to the bracket with a plastic clip.
The venturi has three connections; low pressure atmospheric air from the upper air intake duct, vapor purge from the double check valve and high pressure boost air connection to the intake manifold connector pipe.