1.4.2.1 Monitoring Function - Leak Detection

The EVAP system is monitored for EVAP system leaks with a diameter greater than or equal to 0.02 inches.

The BMW hybrid cars' leakage diagnostic is executed in two ways: the first method is the conventional method which is executed via the Diagnostic Module Tank Leakage (DMTL). This is generally performed when certain conditions are fulfilled. The second method is executed by the DMTL and a separate tank control unit, i.e. Tank Functions Electronics (TFE).

1. DMTL, an electrical actuated pump is located at the atmospheric connection of the EVAP canister and an EVAP system pressure test is performed in the following sequence:

   While the reference leak is being measured, the electrical actuated pump delivers through the reference restriction. The engine management system measures the pump's electrical current consumption in this section.

   Fig 1: Reference Measurement: Valve In Venting Position, Pump ON

   Courtesy of BMW OF NORTH AMERICA, INC.

   During the leak measurement process, the electrically actuated pump delivers through the EVAP canister into the fuel-tank system. The pressure in the EVAP system may reach up to 2.5 kPa, depending on the fuel level in the tank. The engine management system measures the pump's electrical current consumption.

   Fig 2: Leak Detection Diagnosis - System Diagram (Valve In Diagnostic Position, Pump ON)

   Courtesy of BMW OF NORTH AMERICA, INC.

   The first step of the diagnosis is reference measurement. The result of the pump reference current is stored. After the solenoid switches, the venting system is pressurized. When the DMTL current reaches the reference current within the time limit, the system is tight, i.e. the leak is smaller than 0.02 inches. Otherwise, a leak is detected.

2. The TFE is located on the exterior, near the fuel tank. The fuel tank is hermetically isolated from the canister purge (line) by an additional shut-off valve (SOV), i.e. the Fuel Tank Isolation Valve (FTIV). At ignition "off", the Δp in the fuel tank is analyzed for a defined period of time. Based on this analysis, the TFE determines whether a tank leak of 0.015748 inches, a tank leak of 0.02 inches, or no leak is present.

Simultaneously, the DMTL monitors the pressure build-up in the piping from the EVAP CPV to the FTIV.

The TFE only monitors for leaks in the tank when the ECU sends the command.

TFE alone cannot make the decision to execute a leak test.

Fig 3: System Overview Hybrid

Courtesy of BMW OF NORTH AMERICA, INC.

When the EVAP leak detection diagnosis is enabled, the ECU determines whether a tank leak monitor should be executed by DMTL  or by DMTL and TFE  . The following needs to be analyzed before this determination can be made: the pressure in the tank when a delay time period has elapsed after key-off and whether a pending fault code is present from the previous driving cycle.