Tank Vent Valve
Tank Vent Valve
Tank vent valve
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On the other hand, if there were no tank-ventilation system, a partial vacuum would be created in the tank when fuel is withdrawn.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.
Functional description
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
- Time
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 vent valve is activated by the engine control by means of a pulse-width modulated signal with fixed or variable frequency.
Structure and inner electrical connection
The tank vent valve is a solenoid valve. In de-energized state, the tank vent valve closes off the fuel vapor line between the carbon canister and the intake pipe.
Activation and setpoint values
When the engine is started after it has been switched off for a certain period of time, the tank vent valve remains closed as long as the engine temperature is still below 40 °C. However, the fuel tank is still ventilated to the ambient air via the carbon canister. The fuel vapors that escape here are collected by the carbon canister. 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 tank vent 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.
Observe the following setpoint values for the tank vent valve:
Size Value
Voltage range 9 to 16 volts
Duty cycle 0 to 100%
Frequency range 0 to 30 Hz
Power consumption 1 A
Temperature range -30 °C to 140 °C
Diagnosis instructions
Failure of the component
If the tank vent valve fails, the following behavior is to be expected:
- Fault entry in the engine control unit
- Emergency operation with substitute value
General notes
If the oxygen sensors before the catalytic converter in cylinder bank 1 and cylinder bank 2 do not supply any plausible values, or a malfunction is diagnosed with both oxygen sensors, the tank vent valve switches to emergency operation. In emergency operation, the flow rate of the scavenging air depends on the engine speed and throttle valve position (lower than during normal operation).
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