Now that the issue of permeation has seemingly been vaporized with the drafting of the SPI Protocol, plastic fuel tank suppliers are getting ready for an explosion (probably not the best word) of new business, expected to reach around 80% of the market by the turn of the century.

Solvay Automotive Inc. President Norman W. Johnston says: "We see a very nice growth pattern." That chart, he says, goes from 4 million units in 1995, just under 30% of the fuel tank market, to 60% in 1998 and 75-80% by 2004.

Competing Walbro Automotive Inc. is somewhat more optimistic about 1995 and more conservative about the long-term. Walbro Vice President Hans Schwochert says 4.5 million to 5 million plastic fuel tanks will be delivered this year for a 30% to 35% share. He says to add another million in 1996 and expect 6.5 million to 7 million units -- or 55% -- by the end of the century.

"The alternative to plastic that is able to accommodate today's aggressive, alcohol-blend fuels, is coated steel or stainless, which are expensive, heavier and have less volume than plastic," says Mr. Schwochert. "These factors are driving the momentum of plastic tanks in the auto industry."

Although the design flexibility and safety benefits of plastic tanks have been widely known for many years, their widespread proliferation was stymied when the Clean Air Act stipulated that a fuel system could only allow 2 grams of permeation each day. That sent fuel-tank designers back to the drawing board.

Between 1992 and 1994, Mr. Johnston chaired an industry task force that included representatives from the Big Three, plastic fuel tank manufacturers and their suppliers.

The result of the numerous meetings was the SPI Protocol -- a series of real-world tests to prove the integrity of plastic fuel tanks. The protocol is endorsed by automakers and suppliers and has been sent to the Society of Automotive Engineers for its blessing.

"It's a damned torture test," says Mr. Johnston. "There are no metal gas tanks out there that would survive these tests." The test cycle, which costs $50,000 and takes 44 1/2 weeks, includes:

* A 20-week soak with fuel modified with 5% isobutane.

* 10,000 pressure/vacuum cycles.

* 1 million slosh cycles (side to side for cars and front to back for trucks).

* Low-temperature cycles from -20 [degrees Fahrenheit] to 20 [degrees Fahrenheit] to -20 [degrees Fahrenheit] (-29 [degrees Centigrade] to -7 [degrees Centigrade] to -29 [degrees Centigrade]) and from -40 [derees Fahrenheit] to 20 [degrees Fahrenheit] to -40 [degrees Fahrenheit] (-40 [degrees Centigrade] to -7 [degrees Centigrade] to -40 [degrees Centigrade]).

Plastic fuel tank technologies capable of meeting 1998 requirements include the preferred co-extrusion multi-layer structure and single-layer tanks with surface treatments of advanced sulfonate.

Walbro's Mr. Schwochert says the multi-layer tank design is 600 times better than a non-treated plastic fuel tank and 10 times better than a treated tank.

"We know that nothing can beat the multi-layer," says Mr. Schwochert, whose company was focusing on multi-layer designs prior to the findings of the SPI Protocol committee.

Since the task force released its results, companies like Solvay (which had been banking on single-layer technology) have had to invest in machines capable of producing the multi-layer tanks. Solvay says it will have seven presses in North America for molding co-extruded, multi-layer fuel tanks by early this year.

In addition to the economic and weight benefits (30% to 50% compared to metal), plastic fuel tanks offer reduced hydrocarbon emissions (less than 0.1 grams per day), no corrosion and design flexibility.

"The flexibility of high-density polyethylene permits the tank to be designed to maximize available underbody space resulting in increased capacity," says Mr. Johnston. The 16-gallon fuel tank Solvay supplies for the Chrysler Cirrus/Dodge Stratus platform is one example of how a plastic fuel tank offers more volume because of the use of underbody space.

"Now that permeation is a non-issue," Mr. Johnston says, "there are a lot of (plastic fuel tanks) out there, and there will be more out there."