Only specialty elastomers such as Viton® fluoroelastomer can function in the increasingly hostile automotive fuel system environments driven by tough new legislation. The following article based on a paper given recently at the ISATA Automotive Conference in Florence, Italy by Dr. Steve Bowers of DuPont Performance Elastomers explains why.

Modern automotive fuel systems have a tough job to do. In addition to fuel storage, metering and fuel delivery to the engine, such systems must comply with ever more stringent regulations defining not only emission levels, but often the composition of fuels as well. These factors combine to create exceptionally hostile working conditions for fuel system components.

The USA, in particular the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), has traditionally led in vehicle emission legislation. Recently, the European Program on Emissions, Fuels and Engine technologies (EPEFE) was set up to establish European legislation and the basis for future fuel composition and emission limits (the EURO-2000 regulations) that will put much higher demands on fuel system components.

It is a fact of life that fuel compositions are changing rapidly to meet the emission limits of these new regulations. Traditional anti-knock agents like tetraethyllead and tetramethyllead are being replaced by oxygenated alcohol or ether additives to maintain fuel octane ratings while reducing hydrocarbon and carbon monoxide pollutants. However, any number of compositions may satisfy legislation and the variety of legal fuels is vast.

Many fuel compositions contain additives that cause swelling and property deterioration in elastomers. To simplify the problem of how to decide the suitability of elastomers, various test fuels have been adopted by the industry as a basis for comparison of elastomer performance. In most cases, these fuels ensure worst case testing scenarios. Consequently, the evolution of engine and fuel systems to meet stringent legislation and aggressive new fuels has resulted in a much more hostile working environment for many elastomeric under-hood components like seals, hoses, diaphragms and gaskets. Increasingly, only specialty elastomers such as Viton® fluoroelastomer can function. Test data is available to demonstrate the outstanding performance of the fluoroelastomer in these representative test fuels.

Why specify Viton®?

Viton® fluorohydrocarbon elastomers offer the highest continuous heat resistance of any conventionally processed rubber. They also have outstanding resistance to swell and permeation in contact with aggressive automotive fuels and additives. The fluoroelastomer is specified by automotive engineers for fuel system seals, O-rings and hose because it continues to function in environments that destroy parts made from many other plastic and rubber materials, and offers increased safety margins and long term durability.

Test Details

Various Viton® types were tested for volume swell, percentage of original elongation at break and tensile strength retained, hardness change and permeation after aging for 168 hours at 23°C (73°F) in fuel C / methanol / MTBE / ETBE / bio and low-sulphur diesel.

Volume Change

Volume change or swell is a reaction of an elastomer to fuel that can precede seal failure and leakage, or compromise accuracy of fuel metering systems and reduce engine efficiency. Compounds of Viton® exhibit lower volume swell in fuels than any other class of elastomers except for perfluoroelastomers. Testing also indicates that Viton® is suitable for service in low sulphur and bio-diesel fuels.

Change in Physical Properties

Physical property change usually goes hand in hand with volume swell. Viton® provides excellent long term compression set resistance and retention of sealing characteristics after fuel immersion giving functionality for the life duration of the vehicle.

Chemical Attack

Non-reversible chemical changes may occur in elastomers as a result of exposure to fuel. Viton® is especially resistant to both additives and fuel oxidation by-products in gasoline and traditional diesel fuels, and test results so far indicate its resistance to additives in low sulphur and bio-diesel fuels, although the final composition of such fuels is currently undecided.

Permeability

All elastomers are permeable to fuels, although the resistance of individual classes of elastomer varies widely. Fluoroelastomers like Viton® have the lowest permeability to fuel of any other class of elastomers used commercially in automotive fuel systems.

Applications of Viton® in Fuel Systems

Viton® has evolved over its 40 years of existence and today's new range of high performance Viton® comfortably meets the demands of the automotive industry in many fuel system sealing and hose applications

Viton® Types for Use in Automotive Fuel Systems

Applications Service Requirements Viton® Family In Tank Hose and Tubing Fuel Resistance Compression Set Resistance Low Swell B Fuel Pump Dampeners Fuel Resistance Low Swell Damping Characteristics Compression Set Resistance Stress Relaxation Resistance B Filler Neck Hose Fuel Line Hose F-200 veneers Fuel Resistance Permeation Resistance Compression Set Resistance B F Quick Connect Seals Fuel Resistance Permeation Resistance Compression Set Resistance Stress Relaxation Resistance Low Swell F Quick Connect Seals Low Temperature Sealing GFLT Fuel Injector Seals Fuel Resistance Compression Set Resistance Stress Relaxation Resistance Low Temperature Sealing AL Fuel Injector Seals (Low Temperature) Fuel Resistance Compression Set Resistance Stress Relaxation Resistance Low Temperature Sealing GLT Fuel Pump Seals Fuel Resistance Compression Set Resistance Stress Relaxation Resistance Low Swell B Diaphragms Fuel Resistance B Diaphragms (Low Temperture) Fuel Resistance & Low Temperature Flex Life GLT GFLT Emission Control Devices Heat Resistance B Air Intake Manifold Gaskets Heat Resistance Compression Set Resistance A