Fuel evaporation
Every fuel tank has a pressure equalization line. This ensures that no negative pressure can be generated by running empty and the tank does not contract. Air flows into the tank via the pressure equalization line.
When the fuel heats up and expands, the pressure equalization prevents the tank from bursting. The gasoline vapors escaping during equalization are fed via a line to the activated carbon filter and caught and temporarily stored there when the vehicle is stationary.
Statistical surveys
According to calculations by the Federal Environment Agency in Berlin, 6–8 liters of petrol per vehicle can evaporate annually under unfavorable conditions (extreme sunlight). With an activated charcoal filter, around 80–90% of these evaporation emissions can be captured, which are then sucked out again with the engine running and sent to combustion.
Legal requirements
Before a new vehicle type is approved, it must be ensured that it complies with the relevant legal regulations. The type test (Type IV) is used to determine evaporative hydrocarbon emissions from vehicles with positive-ignition engines (Otto engines) and consists of two parts:
- The determination of the evaporation losses during hot storage
- The determination of the tank breathing losses
A gas-tight chamber SHED ( sealed housing for evaporative determination ) is used for checking .
During the first part of the test, the vehicle is parked in this chamber at operating temperature.
In the second part, the vehicle is subjected to a precisely prescribed temperature cycle in this chamber.
A total of 2 grams (!) Of hydrocarbons are permitted for these tests .
The vapors come not only from the tank - an estimated 60% share - but also, and especially with new vehicles, from the many plasticizers made from rubber and plastic parts, underbody protection, paints, etc.
A test vehicle which has been driven in 3000 km and in which the activated carbon filter was normally used during this period is required for the type test.
In the past, small holes were punched in the tank cap, whereby the negative pressure was avoided but the fuel could evaporate unfiltered.
literature
- Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3528238763
- Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4