Natural Vacuum Leak Detection (NVLD)

The tank leak diagnosis takes place in the rest state after the vehicle is switched off. The temperature difference, e. g. between day and night, causes the fuel in the fuel tank to cool. This causes a vacuum in the fuel tank. If no leak is present, the vacuum remains constant. The pressure switch closes. The fuel evaporation system is identified as leaktight.

The tank leak diagnosis starts after every driving cycle (after switching off the engine) when the starting conditions are met. Due to the diagnosis conditions, the evaluation normally takes place overnight.

Starting conditions (diagnosis conditions): 

The tank-leak diagnosis is started under the following conditions:

• Cold start
• Ambient temperature greater than 4.5 °C
• Cooling of more than 8 °C over longer than 1 hour
• Less than 2500 meters above sea level
• Tank vent valve closed
• Terminal status after-run or vehicle gone to sleep
• Battery voltage between 11 and 16 volts
• Pressure difference for ambient pressure between engine OFF and engine start less than 6 hPa (6 mbar)

The check is always for leaks larger than 0.51 mm. No distinction is made between a minor leak or a micro-leak.

Due to the design of the NVLD, there are 5 mechanical states:

• Rest state 
  Fig 1: NVLD Operation Diagram (Rest State)

  Courtesy of BMW OF NORTH AMERICA, INC.

  Index	Explanation
  1	Partial vacuum
  2	Atmospheric pressure
  3	Excess pressure

  In the rest state, atmospheric pressure exerts its force on the top side of the diaphragm and poppet valve. The inner pressure is not defined here. The pressure switch is opened. The poppet valve forms a perfect seal. There is no active air circulation between the fuel tank and surrounding area.

• Pressure switch closes 
  Fig 2: NVLD Operation Diagram (Pressure Switch Closes)

  Courtesy of BMW OF NORTH AMERICA, INC.

  Index	Explanation
  1	Partial vacuum
  2	Atmospheric pressure
  3	Excess pressure

  A vacuum in the tank ventilation system raises the top rubber diaphragm against the pressure switch. The pressure switch closes. There is no active air circulation between the fuel tank and surrounding area. The bottom poppet valve is pressed against the seal by the spring. The tank ventilation system is not leaking.

  Vacuum approx. 2.5 hPa (2.5 mbar)

• Scavenging air mode 
  Fig 3: NVLD Operation Diagram (Scavenging Air Mode)

  Courtesy of BMW OF NORTH AMERICA, INC.

  Index	Explanation
  1	Partial vacuum
  2	Atmospheric pressure
  3	Excess pressure

  An increased vacuum (scavenging air mode) in the tank ventilation system causes the poppet valve to open slightly. Scavenging air flows from the fresh air filter into the carbon canister. The vacuum present continues to lift the diaphragm against the pressure switch. The pressure switch remains closed.

  Vacuum greater than 8 hPa (8 mbar)

• Excess pressure 
  Fig 4: NVLD Operation Diagram (Excess Pressure)

  Courtesy of BMW OF NORTH AMERICA, INC.

  Index	Explanation
  1	Partial vacuum
  2	Atmospheric pressure
  3	Excess pressure

  If the pressure in the tank ventilation system is somewhat greater than the atmospheric pressure, the diaphragm presses down on the poppet valve. The poppet valve opens. Gas flows out of the tank ventilation system through the fresh air filter and into the open. The pressure switch is opened.

  Pressure greater than 2 hPa (2 mbar)

• Refuelling 
  Fig 5: NVLD Operation Diagram (Refueling)

  Courtesy of BMW OF NORTH AMERICA, INC.

  Index	Explanation
  1	Partial vacuum
  2	Atmospheric pressure
  3	Excess pressure

  The system pressure increases dramatically while refuelling. This causes the top diaphragm to lower. The top diaphragm simultaneously presses down strongly on the poppet valve underneath. Gas flows out of the tank ventilation system through the fresh air filter and into the open. The pressure switch is opened.