Fuel Permeation Rates of Elastomers After Changing Fuel

The fuel permeation rate of elastomers is an important factor to consider in the design of automotive fuel system components. Evaporative hydrocarbon emission losses are now tightly regulated. The new 2 gram in 24 hour limit in a 20-40°C diurnal SHED test must be met not only when the car is built, but also at any time up to 10 years or 100,000 miles of service, or for trucks 11 years or 120,000 miles. The SHED test protocol calls for draining the fuel system on a car that has been in service and refilling with a non-oxygenated test fuel. However, the car may have been using an oxygenated fuel prior to the test. There has been speculation that the more permeable oxygenated fuel will influence the permeation rates through the elastomeric components for some time after the original oxygenated fuel has been drained from the vehicle.

This paper presents data which simulates the condition described above where fuel system elastomers such as NBR and fluoroelastomers are exposed to various fuel-oxygenate blends and tested for permeation. The fuel was then removed and replaced by a non-oxygenated test fuel. Permeation rate was measured again and trends noted. Testing was done by both the Thwing Albert Cup method on diaphragms, and on actual rubber and plastic fuel hose at 40°C.