Evaporative Emissions System: Description and Operation
Natural Vacuum Leak Detection
Natural Vacuum Leak Detection (NVLD)
Natural Vacuum Leak Detection (NVLD) is a passive diagnosis system for the fuel evaporation system.
The NVLD requires a longer engine cutout time in order to identify a leakage. Therefore a short leakage test is not possible. Depending on the ambient conditions, the diagnosis normally requires that the engine is off for between 6 and 12 hours.
The California Air Resources Board (CARB) has issued a regulation. This regulation states that an overall leakage in the fuel evaporation system exceeding 0.51 mm (0.02 inches) must be identified.
The fuel evaporation system must therefore meet the following requirements:
- Sealed against the environment
- Identify and display a leak
NOTICE: USA and Korea national-market versions.
The leak diagnosis on the fuel tank for petrol engines is a legal requirement in these countries.
Brief component description
The following components for the Natural Vacuum Leak Detection (NVDL) are described:
Fuel evaporation system overview (USA, Korea)
Basic version: Natural Vacuum Leak Detection temperature sensor and pressure switch
Two separate components are fitted in the basic version. The temperature sensor and pressure switch belong to the NVLD. The temperature sensor and corresponding electronics are secured near the fuel tank. The engine control communicates with the temperature sensor through a data line connecting the two. The electronics and temperature sensor are supplied from terminal 30F.
The pressure switch is connected to the electronics and temperature sensor. The pressure switch is located on the side of the carbon canister housing.
Integrate version: Natural Vacuum Leak Detection temperature sensor and pressure switch
In the integrated version, the NVLD is just one component. The temperature sensor and pressure switch are integrated. The engine control communicates with the NVLD through a data line connecting the two. The NVLD supplied with terminal 30F. The Natural Vacuum Leak Detection (temperature sensor and pressure switch) are located on the carbon canister housing.
Physical principle and pressure pattern
The physical principle is based on the ideal gas law:
Pressure and temperature have a direct proportional relationship when quantity and volume remain constant. The NVLD uses this physical principle to identify leaks in the fuel evaporation system.
This means that a vacuum forms when the temperature in the fuel tank drops. The NVLD uses a temperature sensor to measure the temperature. The NVLD uses the diaphragm-operated pressure switch to determine the vacuum. This diaphragm switches the pressure switch at a certain vacuum / ambient pressure combination.
The following graphic demonstrates the temperature profile and pressure pattern both when there is a leak in the fuel tank and when there is no leak.
System functions
The following system functions for the fuel tank system check are described:
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 metres 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
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
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
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
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
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.
Tank ventilation
The tank vent valve controls the regeneration of the activated carbon canister by means of scavenging air. The scavenging air sucked in by the carbon canister accumulates hydrocarbons (HC) depending on the carbon filter load. 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.
Notes for Service department
General notes
With the starting conditions (diagnosis conditions) in place, the NVLD identifies the position of the pressure switch (closed or opened). The state identified is requested by the engine control next time the engine started.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.