Regardless of the pollutants created by the combustion within the engine, a vehicle will emit considerable amounts of unburned hydrocarbons. These hydrocarbon emissions can stem from leaks in the fuel system, but may also be due to exhaustive storage capacity of the carbon canister (carbon canister becomes permeable).
For this reason, an OBD II requirement concerns the fuel system and the fuel tank ventilation system. The maximum permissible level of escaping fuel fumes has been determined anew. Moreover, leaks larger than 0.5 mm in the fuel and fuel tank breather system must be recognized by the DME.
To this end, the following measures have been implemented:
- Fuel supply to the injection rail via the fuel filter with integrated pressure regulator.
- The fuel controlled by the fuel pressure regulator is routed in the filter via the return line to the fuel tank.
- A modified injection tube counteracts the formation of vapour bubbles and now no longer features a fuel return line to the fuel tank.
- Realization of a fuel tank leak diagnosis facility in the engine control unit with the aid of a motor-driven pressure pump (vane pump) and a changeover valve with integrated reference leak orifice (0.5 mm) > diagnosis module for fuel tank leakage (DMTL)
Fuel tank leakage diagnosis is performed automatically in predefined cycles. During normal operation it is carried out in the control unit afterrunning phase after shutting off the engine. The leakage tightness of the fuel and tank breather system is monitored pneumatically. An overpressure is generated by a motor-driven pump and the current intake serves as a measure for the tank overpressure.
Operating principle of fuel tank leakage diagnosis
During normal operation, the module-internal changeover valve is in the regeneration position, i.e. the fuel tank is connected via the carbon canister to the surroundings so as to ensure regeneration of the carbon canister when the fuel tank vent valve (TEV) is open.
During normal operation diagnosis is carried out in the control unit afterrunning phase after shutting off the engine. The tank vent valve is always closed during this phase. Initially, the reference current is determined by pumping through the reference leak via the changeover valve. The changeover valve is then switched over from the regeneration position to the diagnosis position so that fresh air can be pumped into the fuel tank. The pump current is low as the tank pressure initially correspond to the ambient pressure. The pump current increases as tank pressure rises. If the previously measures reference current is exceeded within a defined period of time, the diagnosis is completed and the fuel tank system is considered to be leak-tight. If the determined reference current is not reached after a defined period of time, the fuel tank system has a leak and diagnosis is ended. A corresponding fault code is entered in the DME fault code memory. Finally, the changeover valve is reset back to the (de-energized) regeneration position and control unit afterrunning is ended.
The DM-TL features complete diagnosis capabilities. A corresponding fault code is entered in the DME fault code memory if a fault occurs during engine operation. For the purpose of checking operation and troubleshooting, the diagnostic program offers the option of activating the system test via the DIS tester / MoDiC / GT-1.