For some time, fuel tank manufacturers have come under increasing pressure from legislation, particularly in the US, to design and build tanks that can withstand higher impacts without leaking. Matthew Beecham reports.

“Legislative mandates and rulings led by environmental concerns are the main technology and design drivers in the fuel system world,” said Manouchehr Kambakhsh, vice president, global advanced technology, TI Automotive.  “Lower targets for fuel tank evaporative emission are driving more creative solutions in a number of areas, including process technology, barrier material technology and fuel tank architecture. Lower fuel tank weight also helps with the overall vehicle fuel economy and improves CO2 exhaust emission. The transition to smaller fuel efficient vehicles is driving “flat fuel tank” geometry which helps the OEMs to offer larger interiors and cargo space in overall smaller vehicle packages.”

In addition to continued demand for fuel tank range and fuel economy, we are seeing more complex shapes and geometries, particularly driven by the all/rear wheel drive increase in smaller car segments.  This creates a unique packaging environment that influences the design selection and requirement for those programmes.

Material matters

“For some time, the automotive industry has been moving toward the increasing fitment of plastic tanks,” said Dr Paul Wouters, vice president, research, Inergy Automotive Systems.  “The main forces driving this trend include light weight, corrosion resistance and crash resistance benefits of the material. These are the main drivers.”

While the angular shape of modern plastic fuel tanks may appear a little strange, their blow-moulded design allows maximum fuel holding capacity in a minimum amount of space.  Plastic tanks are also considered more crashworthy because of their seamless design and ability to deform.  And depending on the volumes produced, plastic fuel tanks can beat steel in terms of cost.

Given the legislation in Europe and California, can steel tanks really make a comeback? “I don’t think that the legislation is giving steel fuel tanks any advantages,” said Albert Boecker of TI Automotive.  “We have proved that PZEV [partial zero emission vehicle] application can be fulfilled using a plastic fuel tank. That has prompted several OEMs to go back to plastic fuel tanks even if they initially used a steel tank for their first PZEV application.”

Ray Sheffield, director of fuel tanks for US-based Martinrea International added: “Steel fuel tanks have lost market share since the 1980s, yet the market share has stabilised and signs of growth are emerging due to an expected increase in the number of PZEV and hybrid gasoline / electric vehicles, as well as cost competitiveness. … Plastics technology has developed to the point of meeting PZEV requirements. However, we believe steel will be the preferred material for PZEV tanks due to lower cost and concern about higher evaporative emissions with plastic tanks when a new test fuel — containing ethanol similar to typical market fuels — is specified by CARB [California Air Resources Board].”

Fueling the bio-diesel debate

Bio-diesel is already popular in Europe and is reported to be spreading to the North American market. However, there have been concerns about using plastic fuel tanks to store bio-diesel.  “The main issue with bio-fuels is not in the plastic tank itself but all the metal parts incorporated into the reservoir which can have a corrosion effect,” said Wouters.”So we have to consider the metallic elements that we use in the fuel system.”

Harald Lehmann of Kautex added: “Any plastic parts that are assembled and in contact with fuel are, to our knowledge, not impacted by biofuels.”

Kambakhsh told us: “For plastic tanks the issue is polymer compatibility as bio-diesel fuel ages and eventually breaks down into chemically more reactive constituents.  The solution lays in adjustments in polymerisation process of the HDPE (high density polyethylene) and complementing it with stabilisers.  TI Automotive’s Advanced Materials team over the past years has worked closely with the supply base in the polymer industry to ensure continued compatibility of HDPE with bio-diesel blends.  Steel corrosion in bio-diesel is a well known phenomenon to TI Automotive Advanced Materials team.  As the global leader in steel and plastic fuel line and brake tube manufacturer TI Automotive has studied the effects of bio-diesel, at different concentration levels, on various grades of steel.  In summary, for steel tanks to withstand the aggressive nature of bio diesel it is necessary to move up to higher grades of stainless steel or use surface coating.  Both options will add cost to the product making it less competitive and you still have the weight problem.”

Although bio-fuels bring a number of benefits, they also present significant engineering challenges. For bio-ethanol the biggest challenge is the aggressive nature of the fuel in the vehicle tank. Sheffield told us:  “Flex fuels (up to E85) are becoming more available in the US and Canada and carmakers have produced substantial quantities of flex-fuel vehicles. We are aware that at intermediate levels of ethanol (5 – 20%), the permeation rate through HDPE tanks increases and is exacerbated by the presence of water (a common contaminant). This is likely to make it more difficult for plastic tanks to meet PZEV evapourative emission requirements in real-world conditions.  SASFT recently completed durability tests of ten different steel systems in various ethanol fuels. Only slight to moderate corrosion was observed in two of the steels at E85 – no weight changes, no pitting, nor any visible effects on the fuels were observed. This is consistent with field results in North America, where millions of vehicles have been certified for use with E85 fuels over the past 15 years or so. Ford in particular has used steel tanks in E85 vehicles such as the Taurus without any significant performance issues.” In an attempt to get automakers to reconsider steel fuel tanks, the American Iron and Steel Institute formed SASFT (Strategic Alliance for Steel Fuel Tanks), an international alliance of companies specialising in the various areas of fuel tank production.

Fueling innovations

A number of interesting fuel tank innovations have also recently emerged. For example, BMW is now fitting its incorrect fuelling protection system (IFPS) to all diesel-powered models. Ford has also introduced a similar system on its models in Europe. IFPS works by preventing the fuel filler nozzle of a petrol pump from entering the fuel tank neck of a diesel-powered car. The larger diameter diesel nozzle is able to touch two opposing contact patches that then release the locking system, allowing the nozzle to engage. A smaller petrol pump nozzle cannot make the same connection and this access is blocked. The system was initially introduced on some models last March, but is now also fitted to 6 and 7 Series and X5 models. Its introduction is expected to prevent most of the 1,000 cases a year of mis-fuelling that occur on BMW cars in the UK which can each cost owners thousands of pounds to rectify. BMW reckons that 150,000 vehicles are mis-fuelled in the UK alone every year.

Wouters said:  “I have seen some statistics about the number of motorists that have made mis-fueled their vehicles. It sometimes occurs when a family has two cars, a diesel-engined one and a gasoline-engined one. So mistakes can happen at the filling station. So I think that kind of feature is going to be more and more popular, especially with the diesel expanding in the US.”

Boecker added: “Also, with the new technologies such as common rail then the motorist is no longer able to immediately sense if he is driving a diesel or a petrol engine. In the past, each type of engine delivered a very different driving performance. Nowadays, that difference is hardly noticeable.”

Meanwhile, Ford has already eliminated the petrol filler cap on the 2008 Ford Explorer, 2009 F-150 pick-up truck and 2009 Lincoln MKS. The Ford transition to capless fuel systems comes from NASCAR where a spring-loaded flap inside the filler neck pulls back as the fuel nozzle is inserted.  When the nozzle is removed, the flap closes tightly.

“While it is a convenience for the end user, there is also a cost associated with that,” said Wouters.  “So it is very dependent with the OEM’s philosophy and the value that they place on that technology feature. But we are seeing an increasing use of that feature on a global stage.”

Matthew ‘Beechy’ Beecham

See also: Global market review of fuel tanks – forecasts to 2016 (download)