Trapped gas

Automotive Design & Production, March, 2003 by Christopher A. Sawyer

Even though it hasn't been given a snappy name yet by marketing, Kautex-Textron has a new multi-layer, blow-molded plastic fuel tank system heading for production. Designed to help automakers meet California's strict Partial Zero Emission Vehicle CPZEv) emission standards, the unit seals all of the sending units, lines and other equipment within the tank, eliminating as many leak paths as possible, and sealing the rest with specially designed fluro-elastomeric seals.

"Years ago, we postulated that all of the components of the fuel system should be put in the fuel tank when possible to meet tighter evaporative emission standards," says Dr. Ulrich Karsch, director, Research and Materials, Kautex-Textron GmbH (Bonn, Germany). "The idea behind this concept," he says, "being to avoid cutting holes in a tight shell." At first, the California Air Resources Board's 15-year anticorrosion requirement for PZEV vehicles, combined with the need for an effective vapor barrier, had the industry concentrating on sealed stainless steel tanks. However, these proved to be both costly and heavy. Yet no one was certain that a blow-molded plastic fuel tank--which promised to be appreciably lighter and less expensive than stainless steel--would be capable of stopping fuel vapors from leaking out. Plastic fuel tanks had been around only since the mid-1990s (Kautex-Textron supplied a multi-layer blow molded design to Chrysler for the 1994 jeep Grand Cherokee), and no one knew whether or not the m aterial's permeability would increase over time. "The Jeep gave us the confidence to move forward with this concept," says Karsch, "because our testing showed that there was no appreciable degradation of the material, even in worst-case scenarios." And while this proved the durability of the materials, it still left the problem of vapor permeation unanswered.

The answer almost literally was found in the grocery store. EVOH (Ethylene Vinyl Alcohol) film is commonly used in packaging food, and provides an excellent barrier against the elements. Adding this film to the tank's composition prevents fuel vapors from reaching the atmosphere. The inner and outer layers of the Kautex-Textron PZEV fuel tanks are made of polyethylene. Two adhesive layers imbed the EVOH barrier, and the sixth is a regrind layer that uses production scrap for feedstock. "For the sealing surfaces," says Karsch, "you need fluro-elastomeric seals and improved designs." The level sending units, rollover valves, pressure sending unit, filters and vent lines are placed on a carrier within the mold, and the six-layers formed around them. "The main thing we've had to improve during development has been the thickness of the EVOH layer," says Karsch, "especially in high fuel flow areas where smoothness is important." Complex shapes have, so far, not proven to be a problem, and Karsch postulates there would be no problem making a tank with relatively sharp radii for a mid-size PZEV vehicle. "We know the limits, as well as how to adapt the process to a radius that performs adequately," he says.

The Kautex-Textron fuel system eliminates the weight and expense of stainless steel, can be blow-molded to give the maximum fuel capacity for a given-- often oddly shaped--area, and still meets the tough California PZEV permeation requirements. With 80% of European cars currently using a plastic fuel tank, and a similar percentage of North American-built vehicles expected to reach this level in 2005 (Asian automakers will have this technology on 36% of their vehicles by then), it will be a relatively painless move for automakers to switch to this technology for their PZEV-compliant fleet.

RELATED ARTICLE: The PZEV Rutes

California sets the stage For many things in the United States, including emission standards. Automakers still Face ZEV Izero Emission Vehicle] requirements in the state, though the regulations have been relaxed to allow them to use PZEVs [Partial Zero Emission Vehicles] to meet at least part oF the obligation until hydrogenFueled or Fully electric vehicles are ready For prime time. PZEV baseline requirements include certiFying that the vehicle meets: (11 SULEV [Super Ultra Low Emission Vehiclel emission standards For 150,000 miles, [2] the state's zero evaporative emission standards, and (3] 06011 requirements For SULEVs. In addition, manuFacturers must extend their perFormance and deFect warranties to iS years/i 50,000 miles From the current I 0 year/i 00,000 mile standard For non-PZEV vehicles. Until plastic Fuel tanks had sufficient real-world testing and showed little to no degradation, it was unclear whether it would be possible to meet the legislation with this technology.