On the surface, a carbon canister looks like a straightforward leak testing application. The part is made from rigid plastic, it isn’t too big, and the leak rate specification is comparable to other fuel system components.
The purpose of a carbon canister is to block or capture vaporized hydrocarbons in order to prevent them from being released into the atmosphere. The vapours can result from refuelling, “hot soak” which occurs when a hot engine continues to evaporate the fuel that is in the system, temperature changes, and permeation that occurs when the fuel system components become saturated with fuel.
Hydrocarbon molecules are attracted to the non-polar surfaces of activated carbon contained within the canister and they are stored within the pores by physical adsorption. Canister filling occurs when temperature changing events and refuelling.
When a vehicle is in operation the canister is purged back to the fuel delivery system. The carbon filled canister must be tested after it is filled with the activated carbon. During the leak test, the carbon will move and compress absorbing the pressure.
This can make it appear as though the canister is leaking. Since the specification for the leak test is strict because of the limits expected when reviewing a part for potential fuel leakage, even the slightly change in pressure will need to be investigated. When carbon fuel recovery testing is done using air, the following must also be considered:
The carbon can absorb the test air in the same way that it absorbs the hydrocarbons. Therefore, performing repeatability tests, the wait times between each test on the same part should be greater than 5 minutes.
Don’t use standard leak testing calculators for the purposes of calculating cycle times. While the canister measures 2 litres of empty volume, it might look like 4 to 6 litres of volume to the leak tester. This is a result of the carbon’s absorption of air. It does not release air molecules easily.
Because of the geometry of most canisters, the external pressure and air currents have the ability to change the internal pressures somewhat during a test causing repeatability problems or false rejects. To resolve this issue, make sure to keep the part that is being tested protected from fans or air conditioner vents that could blow air on them. You should also avoid setting up the test near doors that open and close.
With over 30 years of experience, Sealtick is the leading supplier of leak tester equipment for the packaging industry. To learn more about the products that we offer, contact us on 03 9540 5100.
