Evaporative Purge System Monitoring
Evaporative System Leak MeasurementGeneral Description of Leak Measurement
The evaporative system monitoring permits the detection of leaks in the evaporative system with a diameter of 1,0 mm and up.
By means of a DiagnosticModul TankLeakage (DM_TL), a electrical actuated pump located at the atmospheric connection of the evaporative canister, a pressure test of the evaporative system is performed in the following order:
a) During the Reference Leak Measurement, the electrical actuated pump delivers through the reference restriction. The engine-management system measures the pump's electrical current consumption in this section.
b) During the Leak Measurement, the electrical actuated pump delivers through the charcoal canister into the fuel-tank system. The pressure in the evaporative system may be up to 25 hPa depending on the fuel level in the tank. The engine-management system measures the pump's electrical current consumption. A comparison of the currents of the reference leak measurement and the leak measurement is a measure for the leakage in the tank.
c) 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.
Evaporative system monitoring structure
Diagnosis Frequency and MIL illumination
Diagnosis Frequency and MIL illumination: No refueling detected
Diagnosis Frequency and MIL illumination: After refueling detected and no leak
Description of chart
The upper chart describes the diagnostic frequency after refueling is detected and no leak in the tank. Reason for this chart is to demonstrate the capability of the "check filler cap" diagnostic. Before the driving cycle > 20 mm. starts, the tank was refilled. The first diagnostic is performed while driving to check if the filler cap was replaced. Afterwards, two normal diagnostic cycles are performed.
Diagnosis Frequency and MIL illumination: After refueling is detected, filler cap left open:
Description of chart
The chart above displays the situation: After refueling was detected and the filler cap was left open. But due to the "check filler cap"- message, which will be displayed in the instrumentation panel, the driver has the possibility to recognize his mistake. The emissions are reduced to a minimum compared to systems without this functionality. To give the driver the possibility to replace the filler cap before illuminating the MIL, the first "leak supposed"-flag will be ignored.
Diagnosis Frequency and MIL illumination: After refueling detected, "check filler cap" message ignored:
Description of chart
The chart above describes the case: After refueling was detected and the "check filler cap" message was ignored by the driver. While driving the "check filler cap" message will illuminate in the panel immediately after a leak diagnostic was performed. During soak, a leak diagnostic is started. The flag "leak supposed" is set. After restarting the engine the "check filler cap" message will be displayed immediately in the panel again. The following two cycles will lead to MIL illumination.
It has to be mentioned, that the "check filler cap" functionality is only available for M3 and M5, but not for Z8!
Evaporative Purge System Flow Check
The purge flow from the charcoal canister through the purge valve is monitored after fuel system adaptation is completed and the lambda controller is at closed loop condition. The diagnosis is started during regular purging.
Monitoring Structure of Evaporative Purge System Flow Check
Monitoring Cycle of Evaporative Purge System Flow Check
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