Emissions legislation, weight, packaging, safety and cost are the main factors driving fuel tank innovation. Matthew Beecham talked with Peter Mould, programme manager, Strategic Alliance for Steel Fuel Tanks (SASFT) and Eric Neuwirth, Spectra Premium. SASFT is an international alliance of companies specialising in the various areas of fuel tank production, possessing a common interest in the development, optimisation and application of steel fuel tanks for vehicles.

To what extent could plug-in hybrids benefit from lightweight sealed steel fuel tanks? Is SASFT working on this?

With increased driving distance while running in the electric mode comes fewer opportunities for the engine to purge the carbon canister used for fuel evaporative emission control. Consequently, in current generation designs for Advance Hybrid Electric Vehicles (AHEV) and Extended Range Electric Vehicles (EREV), the fuel tanks are sealed, which raises the maximum internal pressure of the tank. The severe internal operating pressure and vacuum levels of sealed systems represent a technological challenge for plastic fuel tanks, and hence the rigidity of steel offers an advantage over plastic for this type of system, sometimes referred to as a non-integrated system.  However, to date, relatively thick walls (1.4 mm to 2.0 mm) have been used for non-integrated systems, resulting in high mass.

But recently completed studies by SASFT, in conjunction with the United States Advanced Materials Partnership (USAMP) and with funding from the US Department of Energy (DOE), have shown that mass reductions of up to 41% can be achieved, compared with benchmark current design steel tanks. The mass reductions are achieved by a combination of thinner high strength steels for the tank walls and structural enhancing techniques. In contrast, plastic tanks have difficulty in meeting the cyclic fatigue requirements, and any structural improvements to plastic tanks (to meet these requirements) increase the mass of the plastic tanks and reduce the fuel capacity. Thus, steel tanks have an advantage in this vehicle segment.

What other challenges and benefits do the LEV III requirements present manufacturers of steel fuel tanks, lines and vapour management systems?

Because of the inherent impermeability of steel, steel tanks offer an advantage over plastic tanks for achieving the extremely low emission requirements of LEV III. Plastic tank manufacturers must take additional product design steps to achieve LEV III compliance of the system. These additional steps increase mass and cost for plastic tanks. No special product design changes are required for LEV III compatible steel tanks. As a result, steel tanks comfortably meet the low evaporative emissions for LEV III tanks of all vehicle architectures.  In terms of evaporative emissions, plastic tanks are especially challenged for large, high surface area tanks – not so for steel.

During the past two decades, we have seen the gradual increasing fitment of plastic fuel tanks in place of steel. Is that situation changing at all? And are there greater opportunities for steel tanks in emerging markets?

SASFT believes that the decline in steel’s fitment rate over the last two decades has bottomed, for several reasons:

  • Steel fuel tanks have an advantage in highly rigid applications such as plug-in hybrids and conventional vehicles where rigidity and minimum clearance is required such as in RWD and AWD vehicles.
  • Steel fuel tanks have a cost advantage as increasingly severe emission requirements drive up cost and mass for plastic fuel tanks.
  • Opportunities exist for steel manufacturing and assembly facilities close to the end user where the high cost of shipping ‘air-rich’ tanks is minimized and just-in-time supply efficiencies can be realized.
  • Steel commodity prices are much more stable than oil dependent feed stocks for plastic tanks.
  • The full recycling of steel tanks provides environmental and economic benefits in mature markets.  In contrast, no commercially viable process for the recycling of EOL plastic fuel tanks is in place anywhere in the world.
  • The global industry trend toward smaller vehicles favours steel fuel tanks due to the fact that the gauge selection for steel tanks is affected by, among other factors, the fuel volume.  For example, a relatively small fuel tank can be produced using 0.7 mm or 0.8 mm material steel, making it both mass competitive and cost competitive with plastic.

We believe that SASFT has been exploring the effect of spot welds on the corrosion durability of fuel tank steels in alternative fuels. What did you discover?

SASFT has conducted extensive corrosion durability of steel fuel tank systems since 2002 as follows:

  • Part 1 (2002-2004) - Corrosion durability of steels in external (road salts) and internal (CE10A) environments.
  • Part 2 (2009) - Corrosion durability of steels in aggressive ethanol fuels (CE10A, CE22A and CE85A).
  • Part 3 (2010) - Corrosion durability of steels in bio-diesel fuels (B10 RME/SME blend, B20SME, B20AFME and B90SME).

All of these studies showed that today’s new steels are resistant to external salt-rich road environments and to ethanol and bio-diesel fuels. Additionally, the studies demonstrated that steels are available to meet long-life requirements such as 15 years (or 150,000 miles) as stipulated by California’s Air Resources Board.

More recently (2011) SASFT completed studies of the effect of spot welds on the corrosion durability of steels in select alternative fuels (CE85A, B20SME and B90SME). The results showed no failure (perforation) of any of the 10 steels after 12 weeks exposure at 60°C (CE85A) and 90°C (B20SME and B90SME). Most of the steels showed either no corrosion or light corrosion at the cut edges of the welded steel tabs. Some steels showed moderate corrosion but only in the most severe fuel B90 SME. Thus the corrosion life of most fuel system steels is not expected to be negatively impacted by the presence of spot welds.


SASFT

The Strategic Alliance for Steel Fuel Tanks (SASFT) is an international alliance of companies specialising in the various areas of fuel tank production, possessing a common interest in the development, optimisation and application of steel fuel tanks for vehicles.

SASFT was organised by the American Iron and Steel Institute (AISI) in 2000 to bring together the diverse business disciplines involved in designing, manufacturing and supplying steel fuel tanks in the automotive markets.
SASFT’s global reach recognises that different business and technological issues impact the type of fuel tanks produced in different regions of the world. By understanding these issues, rational, optimised approaches can be fully deployed for steel fuel tanks.

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