Sustainable stainless steel fuel tanks for hybrid and ICE vehicles

The majority of internal combustion engine (ICE) road vehicles currently use plastic fuel tanks. However, the drive for sustainability together with growth in hybrid electric vehicles (HEVs) as well as the development of new carbon-neutral synthetic fuels, is creating new interest in stainless steel designs. This article explains the potential benefits of making the switch. They include pressure resistance, weight saving, crashworthiness, ease of assembly, corrosion resistance and 100% recyclability.

Performing under pressure

The trend towards electric vehicles (EVs) is growing fast. According to the International Energy Association (IEA), the global EV stock across all transport modes is set to expand from over 11 million in 2020 to almost 145 million vehicles by 2030, an annual average growth rate of nearly 30%. Some 47 million of these electric vehicles will be HEVs that combine an electric motor and battery with an internal combustion engine. Together with the ICE vehicles that will remain in production for some time, it is clear that many vehicles will still require safe, durable and lightweight fuel tanks for conventional and synthetic fuels.

However, HEVs in particular bring new challenges for fuel tank manufacturers. This is because when a vehicle is running in purely electric mode, the engine shuts off, causing fuel vapor to build pressure in the tank. Therefore, a pressure-resistant fuel tank is required to maintain the in-tank pressure. This is just one area where a stainless steel design offers a particular advantage over plastic due to its inherent higher pressure resistance.

Light, strong and crashworthy

Development programs carried out with major manufacturers have shown that it is possible to create stainless steel tanks with a wall thickness of just 1 mm. This enables a potential weight saving of around 3 kg over a plastic design, which can be crucial when the focus is on shaving every gram from a vehicle’s total weight. Practical experience suggests that the space saved could allow a typical passenger vehicle to carry around 3 liters of extra fuel, further extending its range.

There is an added benefit that reducing the weight could mean less CO₂ is used in production of the raw material. At this stage it is difficult to provide a rigorous comparison between the carbon footprint of a single layer stainless steel fuel tank and one constructed of multi-layer plastic. However, figures published by the International Stainless Steel Forum (ISSF) in 2015 indicate that the production of one tonne of stainless steel generates 2.9 tonne of CO₂. Various sources suggest that typical fossil plastics will release 1.7 to 3.5 tonne of CO₂ for every tonne produced. Since the two materials are broadly similar in this respect, the weight saving potential of stainless steel would be reflected in terms of a reduced carbon footprint.

The strength and energy absorption of stainless steel is another important factor for enhanced crashworthiness. It is also formable by relatively straightforward processes to create a half-shell that can easily be assembled by welding to create the finished fuel tank. These half-shells are easily stackable, meaning that they can be transported very efficiently to local manufacturers and completed using simple and well-established production processes. In contrast, the manufacture of a plastic tank requires the use of complex, high-value molding and forming machinery that is unsuitable for local manufacturers. Furthermore, the finished product takes up a lot of space during transportation as effectively tanks of air are being moved from site to site.

Corrosion resistant

A major advantage of stainless steel is its exceptional, inherent corrosion resistance. That means a long-lasting, maintenance-free fuel tank can be manufactured with no need for further surface treatments or coating. 

A sustainable solution

Stainless steel is a superb material for sustainable transport as it is efficient, long lasting and 100% recyclable at the end of the vehicle’s life. Furthermore, Outokumpu is a leader in sustainability as our stainless steel has the lowest carbon footprint in the industry, this is thanks to a recycled content of more than 90%. We also have a commitment to be carbon-neutral by 2050. Stainless steel is 100% recyclable and it can be recycled time after time, with no adverse effect on its properties. In contrast, while significant advances are being made in recycling of plastics, the properties of recycled plastic tend to be impaired compared with virgin material. The result is plastic recycled from a plastic fuel tank would necessarily have to be “downcycled” for use in a less demanding application.

Better for baffling

Some HEV designers aiming to reduce noise levels to virtually “silent” levels require tanks with integral baffling to reduce the effect of fuel sloshing. Incorporating baffles is an expensive process for plastic fuel tanks, but easily accomplished with stainless steel. Prototype tests have shown that an anti-slosh stainless steel tank is only 8% heavier than the plastic equivalent, but achieves superior noise reduction, of approximately 4-5 dB (62 -> 57,8 dB).

Tailored solutions based on a family of grades

The majority of Outokumpu’s fuel tank development programs are based on our Core 304/4301 grade, which is a classic 18% chromium, 8% nickel austenitic stainless steel. It’s an all-purpose product with good corrosion resistance and is suitable for a wide variety of applications that require good formability and weldability.

Depending on the application, a variety of other austenitic grades such as Core 301/4310 or Core 304L/4307 might also be specified. Another alternative which offers interesting possibilities is the Forta H-Series. This new fully austenitic, nickel-free grade offers a unique combination of high yield strength and high elongation to fracture made possible by its TWIP (Twinning Induced Plasticity) strengthening mechanism.  The harder it’s pushed, the stronger Forta H-Series stainless steel becomes. Furthermore, Forta H-Series also offers stable pricing as it eliminates the volatility associated with nickel. Currently also seen as very promising for fuel tanks is 1.4376, a nickel-reduced grade that combines the advantages of price stability and good corrosion resistance with elevated strength. 

The possibilities do not end there. If a tank design requires even greater strength, then a broad range of grade options are available. These include duplex grades that combine the advantages of austenitic and ferritic stainless steels.

Progress is accelerating

Stainless steel has often been seen as something of a niche material for constructing fuel tanks for specialist, high-end vehicles. Now, with over a dozen development projects currently ongoing with major manufacturers, stainless steel is fast emerging as the ideal choice for fuel tanks on the new generation of vehicles, both hybrid and ICE.