Evaporative Purge System Monitoring
General DescriptionEvap. Families XBMXR0160E38, XBMXR0160E39
The evaporative system monitoring permits the detection of leaks in the evaporative system with a diameter of 1 mm and up.
By means of a Leak Detection Pump (LDP), a vacuum actuated pump located at the atmospheric connection of the charcoal canister, a pressure test of the evaporative system is performed in the following order;
- During the Fast Pulse Phase the evaporative system is set under a defined pressure. The pressure in the evaporative system after the Fast Pulse Phase may be higher or lower than the defined pressure depending on the fuel level in the tank.
- The Natural Frequency Phase that follows the Fast Pulse Phase is therefore needed to adjust the pressure to its correct value.
- During the final Measurement Phase the time between the pump strokes of the LDP is evaluated. In case of a time between pump strokes below a preset threshold a leak is assumed to be present.
During the pressure test the purge valve needs to be shut. After the test canister purge is resumed and consequently the remaining pressure in the evaporative system is bled off.
To have the maximum capacity (or minimal loss of purge air due to leak detection) the test normally runs after engine cold start. However under certain circumstances the test may be preliminary aborted and started again later during engine run if all conditions for the restart of the test are met.
Monitoring Structure
Please refer to the following information.
2.1 Cold start conditions
2.2 Conditions for preliminary abortion
2.3 Conditions for repetition after preliminary abortion
2.4 Conditions for final abortion of monitoring sequence
2.5 Monitoring sequence
Conditions for preliminary/final abortion are checked every 100 mS
Appendix A
Use of ambient temperature signal provided by the vehicle's CAN bus.
The LDP component is specified for defined temperature range. To prevent the component from being operated under unspecified conditions the ambient temperature is used as a condition for starting the monitoring sequence after cold start.
The ambient temperature signal is checked for circuit continuity later in the driving cycle by means of the intake air temperature monitor (TAL_REF Monitor):
- At the time of engine start the value of TAL_REF is initialized to the maximum possible value.
- If a specified load and a specified vehicle speed is exceeded over a specific time the value of TAL_REF is updated with the value provided by the intake air temperature sensor. This however is only done if the intake air temperature value is lower than the value that is already represented by TAL_REF. Thus TAL_REF always represents the minimum value of the intake air temperature sensor found during the conditions mentioned above.
Using this technique the following sequences are possible:
- The ambient temperature is within specified range and no leak is detected during the monitor sequence:
In this case the system is considered to be without a leak and no fault code is stored.
- The ambient temperature is within the specified range and a leak is detected during the monitoring sequence:
After the conditions for updating TAL_REF are met and TAL_REF is within specified range a fault code is stored.
In case TAL_REF is out of specified range no fault code is stored.
- The ambient temperature is out of specified range at the time of engine start:
This leads to a preliminary abortion of the monitoring sequence, the monitoring sequence is carried out as soon as all conditions for repetition after a preliminary abortion
Appendix B
Downhill run detection.
The LDP measures the difference between the internal tank pressure and the ambient pressure. In case of a vehicle moving downhill the internal tank pressure remains constant whereas the ambient pressure increases. Consequently the pressure difference measured by the LDP decreases which may cause the detection of a leak that is not present, i. e. a false alarm.
To prevent this, a downhill run detection that compares the engine torque with a torque that is needed for driving the car on level terrain at a given speed, is implemented.
Once a downhill run is detected the monitoring sequence is preliminary aborted
Appendix C
Typical pressure over time (1 mm leak, fuel level approx 40% of nominal capacity)
Appendix D
System schematics
EVAPORATIVE PURGE SYSTEM FLOW CHECK
The purge flow from the charcoal canister through the purge valve is monitored after fuel system adaption is completed and the lambda controller is at closed loop condition. The diagnosis is started during regular purging.
Monitoring Structure
Step 1 - For rich or lean mixture
Flow through the purge valve is assumed as soon as the lambda controller is compensating for a rich or a lean shift.
After this procedure the diagnosis is completed and the evaporative purge system resumes working normally.
Step 2 - For a stoichiometric mixture
In this case the lambda controller does not need to compensate for a deviation. Therefore after finishing the regular purging the purge valve is opened and closed abruptly several times. The effect of additional cylinder charge triggers a variation of the engine idle speed.
A predetermined value is reached if the system functions properly and the diagnosis procedure is completed.
To start the diagnosis function (step 2) several conditions have to be satisfied.
- vehicle speed = 0
- engine at idle speed
- closed loop of lambda controller
- coolant temperature > fixed limit
Furthermore if the diagnosis has already been started and one of the conditions has not been satisfied continuously, the process will be interrupted and started again later.
- engine idle speed variation < fixed limit