Fuel Tank: Description and Operation

Diagnosis Module For Tank Leakage (DM-TL)










PRINCIPLE OF OPERATION
The diagnosis module comprises an electromechanically driven 1-overpressure pump together with a 2-change-over valve with integrated 3-reference restriction. In the change-over valve's setting "a" as shown, the pump delivers through the reference restriction. In the other setting the pump delivers through the 4-canister into the fuel-tank system. The engine-management system measures the pump's electrical current consumption in each setting. A comparison of the currents is a measure for the leakage in the tank. If the electrical current consumption is identical in both settings, this means that there is a tank-system leak which is equal to the reference restriction. In order to make the procedure independent of environmental influences and tolerances in the overpressure pump, the system performs a comparison with a reference restriction. Outside of the leakage-diagnosis mode, the change-over valve remains in the "a" setting. The canister can be regenerated via a large flow cross-section, and only an insignificant pressure loss occurs. This is advantageous above all on vehicles with high canister-regeneration rates.