Tank vent valve
The tank ventilation system captures volatile hydrocarbons in the carbon canister. These hydrocarbons are fed to the intake air for combustion. The tank ventilation system prevents hydrocarbons from escaping into the environment.
The creation of volatile hydrocarbons in the fuel tank depends on:
- Fuel temperature and ambient temperature
- Air pressure
- Fill level in the fuel tank
- Time.
If the pressure and temperature remain the same, evaporation decreases over time, as only the volatile hydrocarbons evaporate.
The vacuum in the fuel tank resulting from withdrawal of the fuel is balanced out by the ventilation and venting.
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 depending on the loading of the activated carbon. The purge air enriched with hydrocarbons is then fed to the N55 engine for combustion.
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 amount of fuel vapor drawn in by the combustion engine from the activated carbon canister must be adapted to each operating condition of the engine. To achieve this, the tank-ventilation valve is activated by the Digital Engine Electronics (DME) by means of a pulse-width modulated signal with fixed or variable frequency.
With the engine off and after the engine start, the fuel tank ventilation valve remains closed as long as the coolant temperature is still below 40 °C. However, the fuel tank is still ventilated via the activated carbon filter to the ambient air. The fuel vapors that escape here are collected by the carbon canister.
The 1st tank ventilation phase starts as of a coolant temperature of 40 °C. In the 1st tank ventilation phase, the fuel tank ventilation valve is opened for approx. 170 seconds.
In the 1st adaptation phase, the fuel tank ventilation valve is closed. For the 3 load ranges (idle speed, lower load range and medium load range), a fuel mixture adaptation as deviation to Lambda value = 1 is made at certain engine speeds and load ranges.
In the 2nd tank ventilation phase, the fuel tank ventilation valve may be opened for the entire phase (270 seconds), provided no other requests are pending (for example, overrun fuel cutoff or diagnosis).
In the 2nd adaptation phase, the fuel tank ventilation valve is closed again. For the 3 load ranges (idle speed, lower load range and medium load range), a fuel mixture adaptation as deviation to Lambda value = 1 is made at certain situations for load and engine speed. All other adaptations run in exactly the same way.
| Index | Explanation |
|---|---|
| 1 | Tank-ventilation valve open |
| 2 | Tank vent valve closed |
| 3 | 1st tank ventilation phase |
| 4 | 1st adaptation phase |
| 5 | 2nd tank ventilation phase |
| 6 | 2nd adaptation phase |
| 7 | Time |
The fuel vapors are cached in a carbon canister and fed to combustion.
The following graphic shows the tank ventilation system on the F10.
| Index | Explanation |
|---|---|
| 1 | Connection to tank ventilation line from carbon canister |
| 2 | Connection upstream of throttle valve |
| 3 | Tank vent valve |
| 4 | Connection downstream of throttle valve |
| 5 | Connection upstream of twin-scroll exhaust turbocharger |