Leakage Diagnostic
Leakage Diagnostic
Vapor that evaporates from the fuel in the fuel tank is routed to and stored in the EVAP canister from where it is introduced into the combustion process via the Canister Purge (CP) valve.
A leak diagnostic has been introduced in certain markets to ensure that there are no leaks in the fuel tank system. The diagnostic is designed to detect leakage corresponding to a 0.20 inch or larger hole. The fuel tank system consists of fuel tank, fuel filler pipe, EVAP canister, CP valve and all pipes between these components. To be able to diagnose the fuel tank system, it is also equipped with a diagnostic module (DMTL = Diagnostic Module Tank Leakage) including the electrical driven air pump.
The diagnostic is divided into different phases as follow:
- Reference leak measurement, performed every LD
- Rough leak test, performed every DCY
- Small leak test performed every second DCY when enabling conditions are met.
The diagnostic is performed by measuring the motor current and then compares it to a specified reference current. If a fault is detected in any of the phases the diagnostic is interrupted and the diagnostic trouble code (DTC) for the component identified is stored. Diagnosis is carried out in the following stages:
- While fuel level sensors are working correctly and the fuel level is higher than 85% all leakage tests and healing attempts are aborted.
- While the fuel level sensors are not working correctly, the test is aborted if the initial rate of change is higher than a calibrated level due to a combination of high fuel level and high evaporation. In case of healing when the fuel level sensor are not working correctly the attempt is aborted if the initial rate of change is higher than a calibrated level due to a combination of high fuel level and high evaporation. This level is calibrated to approximate 70%.
1. Reference leak measurement phase
For the reference current measurement, the motor pump is switched on. In this mode fresh air is pumped through a 0.02-inch reference orifice, situated internally in the module, and the pump motor current is measured. At some unusual operating conditions the pump current may not stabilize. In this case the leak check is aborted and a new leak check will be performed in the next after run. To prevent a permanent disablement of the leak check due to a DM-TL module problem, the number of subsequent irregular current measurements is counted and a module error is set as soon as the counter exceeds a calibrated value.
2. Rough leak test phase
In this monitoring mode the changeover valve is switched over (the purge control valve remains closed). The motor current drops to a zero load level. Fresh air is now pumped through the canister into the tank. This creates a small overpressure at a tight evaporative system, which leads to a current increase.
The rough leak check (≥0.04-inch) is performed by monitoring the pump motor current gradient. Relative pump motor current is created by using minimum pump motor current and reference pump motor current. Area ratio is created by dividing integrated relative current with ideal area, which is the linear integrated area from minimum pump current to current sample of the current. If the relative current has increased above an upper limit but not exceeded a calibrated area, within a calibrated time, the rough leak check has passed without a fault. If the calibrated area ratio is reached before the relative pump current limit, within the calibrated time, a rough leak fault code is set. The integrated relative pump current area A-int is defined by;
A-int = A1 + A2
and the ideal area A-ideal,
A-ideal = A2.
See figure below.
3. Small leak test phase
If the conditions for a small leak check (≥0.02-inch) are set the pump motor remains on in monitoring mode until an elliptic combination of the relation pump current and area ratio are fulfilled, or a maximum time limit has been reached.The judgment is based on a test value which is a combination of the actual area ratio and gradient of area ratio with respect to relative pump current.
Results from simulation using old measurements and creating the area ratio and relative pump current and plot them versus each other.Blue curves correlates to no leakage, red curves to 0.5 mm leakage and the magenta to 1.0 mm leakage.
Results from simulation using old measurements and plotting the area ratio vs. the ideal area.Blue curves correlates to no leakage, red curves to 0.5 mm leakage and the magenta to 1.0 mm leakage.
If the motor current decreases or increases too much during one of the tests, the test is aborted and a new leak test will be performed in the next after-run.