Seeking neutral: controlling charge in auto fuel systems

Plastics Engineering, March, 2005 by Dwight Smith, Stan Weitz, Bernhard Forschler

The flow of fuel in automotive fuel systems creates a static charge. While this is not a problem if the charge dissipates as it forms, it can become a serious problem if the electrostatic potential builds to the point where arcing occurs. Repeated arcing can pierce a hole through a plastic fuel-system component, which will cause fuel or vapors to escape. During refueling, even a single electrostatic discharge can cause a thermal event.

Several trends in automobile fuel systems have heightened the potential for charge buildup. One is the growing use of plastics, which has generally been a boon to the industry. As insulators, however, plastics heighten electrostatic issues. Another is the shift to fuel injection, which prompted designers to move the fuel pump from near the engine to the fuel tank. This change increased the volume of fuel through the fuel lines, increasing charge accumulation. Charge accumulation may increase in the future as fuel pressure is boosted to aid fuel injection. In addition, some oxygenates blended into fuels can increase static charge as they flow through the system.

All plastic elements that carry fuel--filler necks, fuel modules, filter, lines, fuel rails, onboard vapor recovery refueling systems and more (Table 1)--receive close scrutiny regarding electrostatic charge. The industry's fuel-system ESD standard, Society of Automotive Engineers (SAE) J1645, "Fuel System--Electrostatic Charge," includes test methods for measuring the electrostatic characteristics of materials, components, and assemblies to ensure they will not cause electrostatic hazards in vehicles.

Fuel systems are governed by many standards and regulations. For instance, U.S. Dept. of Transportation FMVSS 301 says fuel systems must withstand impacts of 45 mph without rupture. Also, the SAE J1681 "Gasoline, Alcohol, and Diesel Fuel Surrogates for Materials Testing" standard defines fuel recipes for evaluating fuel-system materials. It determines the chemical resistance required in fuel-system polymers. In addition, permeability limits in California's Low Emission Vehicle II and Partial Zero Emission Vehicle mandates regulate the amount of fuel vapors allowed to escape through materials like plastics and elastomers.

Plastics That Limit Charge Buildup

Polymers dissipate static charge when they contain conductive additives like carbon powder, carbon or stainless-steel fibers, or selected nanomaterials. Plastic is an insulator until a threshold loading of one of these additives alters a material's resistance to the point where its percolation curve drops to the conductive state (Fig. 1).

[FIGURE 1 OMITTED]

Conductive additives affect the elongation, impact strength, modulus, and other mechanical properties of a resin (Table 2). Carbon fiber grades are stiffer and stronger and elongate less than unfilled polymers. They also usually have improved friction and wear resistance. Carbon powder grades allow moderate elongation and are similar in strength and stiffness to the neat polymer. Grades made with stainless-steel fiber offer the best combination of mechanical performance and static dissipation. These grades have moderately higher stiffness and strength and good elongation versus unfilled grades.

The effects of conductive additives can be seen in acetal copolymer, the most common family of polymers used in fuel-system components. Consider stiffness--one standard, unfilled acetal copolymer (Celcon[R] M90) has a tensile modulus of about 3000 MPa. The modulus of the carbon fiber-filled grade (Celcon[R] EF10), which is based on the standard grade, jumps dramatically to 8400 MPa, while the moduli of the carbon powder and stainless-steel grades are much closer to that of the unfilled resin.

Designers and molders must ensure that a conductive additive is sufficiently dispersed to give parts the best possible electrical and mechanical properties. Uneven dispersion can make conductivity spotty and render a part nonconductive. Designers use uniform wall thicknesses and generous fillets and corner radii to keep material flowing freely and to prevent a filler or fiber from concentrating in part geometries such as comers.

Molders take steps to ensure the integrity of conductive fibers. They use general-purpose screws having non-aggressive profiles that are sized to reduce shear and thus fiber breakage. They also size sprues and bushings to reduce shear, use larger gates than with unfilled resins (typically about 2 mm in diameter), and avoid long barrel-residence times. Molders must predry grades containing conductive fillers that draw moisture into the polymer.

Assembly also has special considerations. For instance, the drop in elongation at break caused by conductive additives can make snap-fits more difficult to use and make it more difficult to join parts by spin, vibration, or hot-plate welding.

Seeking Neutral

General ESD Considerations

ESD fuel-system measurement occurs at three levels: the first certifies polymers for resistance and static dissipation; the second evaluates components to ensure that processing does not significantly degrade the polymer's ability to dissipate charge; and finally, assemblies are tested to ascertain that bonding among components allows the charge to travel freely to ground.