System Check, Fuel Tank
System Check, Fuel Tank
System check, fuel tank
The tank-ventilation system captures evaporated components of the fuel and feeds these to the combustion. The tank-ventilation system prevents hydrocarbons from escaping into the environment. On one hand, the fuel expands. If the pressure and temperature remain the same, however, evaporation decreases over time, as only the light hydrocarbons evaporate. On the other hand, if there were no tank-ventilation system, a partial vacuum would be created in the tank when fuel is withdrawn. This is why there is ventilation at the back of the tank.
NOTICE: National-market versions US and Korea.
Emission regulations mean that the tank ventilation system is supplemented with the 'tank leak' diagnosis module.
Brief component description
The following components for the fuel tank system check are described:
Tank vent valve
The tank vent valve regenerates the activated carbon canister by means of scavenging air. The scavenging air which is drawn through the activated carbon canister is then enriched with hydrocarbons and fed to the combustion engine.
The tank vent valve is closed when in a de-energized state. This prevents the ingress of fuel vapor from the activated carbon canister into the intake pipe when the engine is at a standstill.
The tank-ventilation valve is supplied with 12 Volts of vehicle voltage via the DME. The DME activates the tank ventilation valve on the earth side.
The tank vent valve is closed when in a de-energized state. This prevents the ingress of fuel vapor from the activated carbon canister into the intake pipe when the engine is at a standstill.
Diagnostic module for tank leaks
In the following countries, a leak diagnosis system is compulsory: USA, Canada, Korea.
The tank leak diagnosis allows the leak-tightness of the tank ventilation system to be determined. Tank leak diagnosis allows a difference to be made between:
- Minor leak greater than 1 mm
- Superfine leak 0.5 mm to 1 mm
The diagnostic module for tank leaks is located on the activated carbon filter. The leak test of the tank ventilation system is carried out after a completed drive cycle after stopping the engine. Here, various processes run in the after-run period of the engine control system.
The diagnostic module for tank leaks consists of an electrically driven overpressure pump (leak diagnosis pump) as well as an electromagnetic changeover valve and a reference bore hole. There is also a heating element integrated in the diagnosis module to remove any condensate by heating it up.
System functions
The following system functions for the fuel tank system check are described:
Tank ventilation
The tank vent valve controls the regeneration of the activated carbon canister by means of scavenging air. Scavenging air drawn through the activated carbon canister is enriched with hydrocarbons (HC) depending on the loading of the activated carbon. The scavenging air is subsequently fed to the engine for combustion.
The creation of hydrocarbons in the fuel tank depends on:
- Fuel temperature and ambient temperature
- Air pressure
- Fill level in the fuel tank
The tank vent valve is closed when in a de-energized state. This prevents the ingress of fuel vapor from the activated carbon canister into the intake pipe when the engine is at a standstill.
Diagnostic module for tank leaks (US, Canada, Korea)
The leak test of the fuel system is run regularly after stopping the engine. The following processes run in the after running period of the DME:
- Initial situation
During normal engine operation, the changeover valve in the diagnosis module is in the position "Regeneration". Fuel vapors are stored in the carbon canister and fed into the engine depending on the actuation of the tank vent valve (please refer to "Fuel tank ventilation").
- Checking start conditions
The tank-leak diagnosis is started under the following conditions:
- Engine off: engine management system in after-run (approx. 25 seconds waiting period)
- Cold start: coolant temperature less than ambient temperature plus 6.8 °C
- Duration of the current journey: greater than 10 minutes (totalled over 40 km/h)
- Fuel tank level: between 7.5 liters and 42.5 liters
- Ambient temperature: between 1.5 and 38.25 °C
- Ambient pressure: greater than 730 mbar
- Filling of activated carbon filter: less than limit value (virtually full)
- Battery voltage: between 10.9 and 16 Volts
With a positive result, the tank leak diagnosis is started with a comparison measurement.
- Comparison measurement The tank vent valve is always closed after the engine is switched off. The changeover valve of the diagnosis module remains in the position "Regeneration". The electric leak diagnosis pump impels fresh air from the surrounding area through a defined leak of 0.5 mm diameter. The power consumption needed for this is stored as a value. The actual tank leak diagnosis then follows.
- Tank leak diagnosis The tank vent valve remains closed. The changeover valve of the diagnosis module switches to the position "Diagnosis". The leak diagnosis pump pumps fresh air from the environment into the fuel tank, slowly raising the internal pressure. At the start of the tank leak diagnosis, the internal pressure corresponds to the ambient pressure. This is why power consumption is low. As the pressure inside the tank increases, the power consumption also increases. The DME evaluates the power consumption of the leak diagnosis pump.
- Evaluation of the pump current
The DME evaluates the rise in the power consumption within a certain time. If the power consumption exceeds the value stored within this time, the fuel system is considered to be OK. Tank leak diagnosis is ended. If the power consumption does not reach the value stored, the fuel system is considered to be defective. Tank leak diagnosis allows a difference to be made between:
- Major leak, e.g. fuel filler cap missing
- Minor leak
- Micro-leak
The relevant fault is entered in the DME fault memory. Tank leak diagnosis is then terminated.
- End of the tank-leak diagnosis
The changeover valve is switched back to the position "Regeneration". The after-running period of the DME is available for other functions.
Notes for Service department
General notes
The tank leak diagnosis can also be started using the diagnostic system. In this case, the processes take place as described above.
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