A petrol fuel tank is a safe container for flammable fluids. Though any storage tank for fuel may be so called, the term is typically applied to part of an engine system in which the fuel is stored and propelled (fuel pump) or released (pressurized gas) into an engine. Fuel tanks range in size and complexity from the small plastic tank of a butane lighter to the multi-chambered cryogenic Space Shuttle external tank.
Typically, a fuel tank must allow or provide the following:
- Storage of fuel: the system must contain a given quantity of fuel and must avoid leakage and limit evaporative emissions
- Filling: the fuel tank must be filled in a secure way, without sparks
- Provide a method for determining level of fuel in tank, gauging (the remaining quantity of fuel in the tank must be measured or evaluated)
- Venting (if over-pressure is not allowed, the fuel vapors must be managed through valves)
- Feeding of the engine (through a pump)
- Anticipate potentials for damage and provide safe survival potential.
Plastic (high-density polyethylene HDPE) as a fuel tank material of construction, while functionally viable in the short term, has a long term potential to become saturated as fuels such as diesel and gasoline permeate the HDPE material.
Considering the inertia and kinetic energy of fuel in a plastic tank being transported by a vehicle, environmental stress cracking is a definite potential. The flammability of fuel makes stress cracking a possible cause of catastrophic failure. Emergencies aside, HDPE plastic is suitable for short term storage of diesel and gasoline. In the U.S., Underwriters Laboratories approved (UL 142) tanks would be a minimum design consideration.
Fuel tank construction
While most tanks are manufactured, some fuel tanks are still fabricated by metal craftsmen or hand-made in the case of bladder-style tanks. These include custom and restoration tanks for automotive,aircraft, motorcycles, and even tractors. Construction of fuel tanks follows a series of specific steps. The craftsman generally creates a mockup to determine the accurate size and shape of the tank, usually out of foam board. Next, design issues that affect the structure of the tank are addressed - such as where the outlet, drain, fluid level indicator, seams, and baffles go. Then the craftsmen must determine the thickness, temper and alloy of the sheet he will use to make the tank. After the sheet is cut to the shapes needed, various pieces are bent to create the basic shell and/or ends and baffles for the tank. Many fuel tanks' baffles (particularly in aircraft and racecars) contain lightening holes. These flanged holes serve two purposes, they reduce the weight of the tank while adding strength to the baffles. Toward the ends of construction openings are added for the filler neck, fuel pickup, drain, and fuel-level sending unit. Sometimes these holes are created on the flat shell, other times they are added at the end of the fabrication process. Baffles and ends can be riveted into place. The heads of the rivets are frequently brazed or soldered to prevent tank leaks. Ends can then be hemmed in and soldered, or flanged and brazed (and/or sealed with an epoxy-type sealant) or the ends can be flanged and then welded. Once the soldering, brazing or welding is complete, the fuel tank is leak-tested.
Automotive Fuel tanks
For each new vehicle a specific fuel system is developed, to optimize the use of available space. Moreover, for one car model, different fuel system architectures are developed, depending on the type of the car, the type of fuel (gasoline or diesel), nozzle models, and region.
Two technologies are used to make fuel tanks for automobiles:
- Plastic high-density polyethylene (HDPE) fuel tanks made by blow molding. This technology is increasingly used as it now shows its capacity to obtain very low emissions of fuel (see Partial zero-emissions vehicle). HDPE can also take complex shapes, allowing the tank to be mounted directly over the rear axle, saving space and improving crash safety. Initially there were concerns over the low fracture toughness of HDPE, when compared to steel or aluminum. Concern for safety and long term ability to function should be considered and monitored.
- Metal (steel or aluminum) fuel tanks welded from stamped sheets. Although this technology is very good in limiting fuel emissions, it tends to be less competitive and thus less on the market.
Modern cars often feature remote opening of the fuel tank fuel filler flap using an electric motor or cable release. For both convenience and security, many modern fuel tanks cannot be opened by hand or otherwise from the outside of the car.
Sometimes called the reserve tank is a secondary fuel tank (in many cars/bikes it contains around 15% of the capacity of the primary tank) these are more commonly found on bikes, older cars (some without fuel gauge) and vehicles designed for long distance or special use. A light on the instrument panel indicates when the fuel level dips below a certain point in the tank. There is no current standard, although some efforts are made to collect this data for all automobiles.
In vehicles modified for endurance the primary tank (the one that comes with the car) is made into a reserve tank and a larger one installed. Some 4x4 vehicles can be fitted with a secondary (or sub-tank) by the dealership.
Racing fuel cell
A racing fuel cell has a flexible inner liner to minimize the potential for punctures in the event of a collision or other mishap resulting in serious damage to the vehicle. It is filled with an open-cell foam core to prevent explosion of vapor in the empty portion of the tank and to minimize sloshing of fuel during competition that may unbalance the vehicle or cause inadequate fuel delivery to the motor (fuel starvation).
Placement and safety
For safety considerations, in modern cars the fuel tank is usually located ahead of the rear axle, out of the crumple zones of the car.
Automobiles such as the Ford Pinto or the models that still use the Ford Panther platform (Ford Crown Victoria, Lincoln Town Car, and Mercury Grand Marquis) are notorious for having the fuel tank behind the rear axle. Since 1980 new Ford models corrected this problem and had the fuel tank in front of the rear axle.
General Motors 1973-1987 C/K pickup trucks have the fuel tank located outside the frame. According to the Center for Auto Safety this creates a fire hazard. In automotive applications, improper placement of the fuel tank has led to increased probability of fire in collisions. Circa 1990, General Motors faced over a hundred product liability lawsuits related to fires allegedly caused by GM's decision to place the fuel tanks in its pickup trucks outside the protection of the vehicle's frame . In 1993, reportage on this matter for NBC News created a scandal over vehicles rigged to catch fire for the television camera.
Ford's Pinto also sparked controversy for putting the fuel tank in a poorly reinforced area which can cause deadly fires and explosions if the car got into a rear end collision, costing Ford US$125 million.
Proper design and construction of a fuel tank play a major role in the safety of the system of which the tank is a part. In most cases intact fuel tanks are very safe, as the tank is full of fuel vapour/air mixture that is well above the flammability limits, and thus cannot burn even if an ignition source were present (which is rare).
Bunded oil tanks are used for safely storing domestic heating oil and other hazardous materials. Bunding is often required by insurance companies, rather than single skinned oil storage tanks.