Evaporative Emissions System: Description and Operation
Evaporative Emissions
COMPONENT LOCATION (ALL EXCEPT NORTH AMERICAN SPECIFICATION)
NOTE:
System on supercharger vehicle shown, system on naturally aspirated vehicle similar.
COMPONENT LOCATION (NORTH AMERICAN SPECIFICATION)
NOTE:
System on naturally aspirated vehicle shown, system on supercharger vehicle similar.
INTRODUCTION
The EVAP (evaporative emission) system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank. The system comprises of an EVAP (evaporative emission) canister, an EVAP (evaporative emission) purge valve and interconnecting vapor pipes. The vapor 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 can flow freely to the EVAP (evaporative emission) canister via the tank venting system.
On NAS vehicles the vapor generated in the fuel tank during refueling flows without restriction to the EVAP (evaporative emission) canister.
On all vehicles except NAS, the vapor is restricted in its path to the EVAP (evaporative emission) canister, but can flow freely during the refueling operation to atmosphere via the fuel filler pipe opening.
The vapor passes into the EVAP (evaporative emission) 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.
DIAGNOSTIC MODULE - TANK LEAKAGE PUMP (NAS ONLY)
The DMTL (diagnostic module - tank leakage) pump 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 with the following additional components; a DMTL pump and a DMTL filter.
The DMTL pump is connected to the atmospheric vent of the EVAP (evaporative emission) 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 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 change-over valve for faults.
The DMTL filter protects the pump from dust being drawn into the system when the pump is being operated. The filter is located on the fuel filler head and is connected to the DMTL pump by a vapor pipe.
The DMTL test is performed after the engine has stopped following a run of 10 minutes or more, providing that the vehicle fuel tank is between 15 and 85% full, the ambient temperature is above 0 °C (32 °F) and less than 40 °C (104 °F) and the vehicle was not started for at least 180 minutes prior to this run.
The DMTL pump is driven to pressurize the fuel tank and the current is measured with the change-over valve in different states.
A comparison of the current draw in each state indicates the degree of any leak, and the ECM (engine control module) then sets the appropriate DTC (diagnostic trouble code).
DMTL Pump 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 EVAP (evaporative emission) canister 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.
When the change-over valve is closed, the load on the air pump falls to zero. Providing there are no leaks, the air pump will begin to pressurize the EVAP (evaporative emission) system and the load and current draw in 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, 15 seconds after the engine has started, the ECM (engine control module) energizes the heating element in the pump to prevent condensation formation and possible incorrect readings. The heater remains energized until either the engine and ignition are off (if no DMTL test is running) or until after the DMTL test is completed."
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 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 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 EVAP (evaporative emission) 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.
NOTE:
A leak test can be performed using a Jaguar recognized diagnostic tool. 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 results and fuel tank level.
EVAP CANNISTER
EVAP Canister (All Except NAS)
EVAP Canister (NAS)
The EVAP (evaporative emission) canister is located immediately behind the fuel tank. Two bolts attach the EVAP (evaporative emission) canister to the underside of the center floor section.
The EVAP (evaporative emission) 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 EVAP (evaporative emission) canister on NAS vehicles uses a higher grade of charcoal to meet the stricter emissions' regulations.
A mounting bracket on the RH (right-hand) end of the EVAP (evaporative emission) canister contains the FPDM (fuel pump driver module). For additional information, refer to Fuel Tank and Lines.
On all except NAS vehicles, the EVAP (evaporative emission) canister has a capacity of 1400 cc (85.4 in.3). Two connectors on the EVAP (evaporative emission) canister allow for the attachment of the vapor pipe from the fuel tank and the vapor pipe to the EVAP (evaporative emission) canister purge valve. An atmospheric vent is located on the underside of the EVAP (evaporative emission).
On NAS vehicles, the EVAP (evaporative emission) canister has a capacity of 3000 cc (183 in.3). Three connectors on the EVAP (evaporative emission) canister allow for the attachment of the vapor pipe from the fuel tank, the vapor pipe to the EVAP (evaporative emission) canister purge valve and the DMTL pump.
EVAP CANISTER PURGE VALVE
The EVAP (evaporative emission) canister purge valve is located on the LH (left-hand) side of the engine, below the ignition coils cover. A vapor pipe is routed from the EVAP (evaporative emission) canister purge valve to the inlet of the intake manifold (naturally aspirated vehicles) or the supercharger (supercharged vehicles). The EVAP (evaporative emission) canister purge valve is controlled by the ECM (engine control module) and is operated when engine operating conditions are correct to allow purging of the EVAP (evaporative emission) canister. The EVAP (evaporative emission) canister purge valve is a solenoid operated valve which is closed when de-energized.
A vapor pipe, which runs parallel with the fuel delivery line under the LH (left-hand) side of the vehicle, connects the EVAP (evaporative emission) canister purge valve to the EVAP (evaporative emission) canister.
The EVAP (evaporative emission) canister purge valve is operated at 10 Hz by a PWM (pulse width modulation) signal from the ECM (engine control module). At this high frequency, the pulses of fuel vapor flow into the intake manifold/supercharger in an almost continuous flow. The valve operates between 7% and 100% duty or mark space ratio (percentage open time).
Atmospheric pressure is higher than the pressure at the inlet of the intake manifold/supercharger under all throttle settings and engine running conditions. It is this pressure differential that causes air to flow through the EVAP (evaporative emission) system to the engine.
The ECM (engine control module) waits until the engine is running above 55 °C (131 °F) coolant temperature with 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 EVAP (evaporative emission) canister purge valve duty (and flow) is initially ramped slowly because the vapor concentration is unknown (a sudden increase in purge could cause unstable engine running or cause it to stall due to an extremely "rich" air/fuel mixture). The concentration is then determined from the amount of adjustment that the closed loop fueling is required to make to achieve the target Air Fuel Ratio (AIR). Once the concentration has been determined, the purge flow can be increased rapidly and the injected fuel can be pro-actively adjusted to compensate for the known purge vapor and the target AIR control is
maintained.
When the purging process is active, fresh air is drawn into the EVAP (evaporative emission) canister via the DMTL filter and pump on NAS vehicles, or via the vent port on the EVAP (evaporative emission) canister of non NAS vehicles.