Rajan Wilcox
Gravity-fed fuel tanks are a popular topic among vehicle enthusiasts, with many seeking to build their own auxiliary tanks to extend their range. While some opt for purchasing ready-made solutions, others take on the challenge of custom fabrication, employing materials like steel, aluminium, and plastic. The process involves careful plumbing, often utilising a combination of pumps, valves, filters, and hoses to ensure safe and efficient fuel transfer. However, it is important to approach this project with caution, as fuel storage and gravity feeding regulations vary by region, and improper setups can lead to hazardous situations.
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What You'll Learn
Check local laws and regulations
It is important to check local laws and regulations before installing a gravity feed fuel tank in your vehicle. The regulations surrounding these fuel systems are stringent and exist for your safety. Many state and federal agencies regulate fuel systems, including the US Department of Transportation (DOT), the National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (CFR), Federal Motor Vehicle Safety Standards (FMVSS), California Code of Regulations (CCR), and the California Air Resources Board (CARB).
The DOT has been known to issue permits for the manufacture and sale of refueling tanks that can store gas, diesel, ethanol, methanol, kerosene, and aviation fuel. These refueling tanks meet or exceed testing and certification requirements. However, the legality of gravity-fed systems remains unclear due to DOT ambiguity. Transfer Flow, for instance, believes that gravity feed auxiliary fuel systems may violate portions of US DOT regulation 49 CFR 393.67, a requirement of all fuel systems.
There are several requirements that must be met for a fuel tank to be compliant. Each fuel tank must be securely attached to the motor vehicle, and a fuel system must not supply fuel by gravity or siphon feed directly to the carburetor or injector. The fuel tank installation must not extend beyond the widest part of the vehicle, and no part of the fuel tank should be forward of the front axle of a power unit. Fill pipe openings must be located outside the vehicle's passenger and cargo compartments. Fuel lines that are not completely enclosed in a protective housing must not extend more than 2 inches below the fuel tank or its sump. Diesel fuel crossover, return, and withdrawal lines that extend below the bottom of the tank or sump must be protected against damage from impact. Every fuel line must be long enough and flexible enough to accommodate normal movements without incurring damage and must be secured against chafing, kinking, or other causes of mechanical damage.
There are also requirements for the fuel tank itself. Each fuel tank must be equipped with a non-spill air vent, such as a ball check valve, and must be clearly and permanently marked with the manufacturer's name, manufacture date, gallon size, and a statement that it "meets all FMCSA requirements for non-side-mounted fuel tanks" and "not to exceed 95% of capacity." The fuel connections from the auxiliary fuel tank to the primary system, including a selection control valve, shall not render inoperative any of the functions of the primary system.
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Choose the right fittings
Choosing the right fittings is a crucial aspect of constructing a gravity-feed fuel tank. This process involves selecting the appropriate components to ensure the safe and efficient transfer of fuel. Here are some key considerations and guidelines to help you choose the right fittings:
Firstly, it's important to understand the legal requirements for fuel tank fittings. In some regions, gravity or siphon feeding of fuel directly to the engine is prohibited. For example, the regulation "49 CFR CH.III (393.65) 6d" states that "gravity or siphon feeding is prohibited". Additionally, the same regulation specifies that "fuel withdrawal fittings must be located above the normal level of fuel in the tank when the tank is full". This means that fuel must be withdrawn from the top of the tank rather than from a fitting at the bottom.
To comply with these regulations, you may need to incorporate specific fittings and components in your fuel tank design. This includes the use of appropriate valves, such as shut-off, vent, and overflow valves. A ball valve, for instance, can be used as a shut-off valve to control the fuel flow. Additionally, consider incorporating a check valve or electric solenoid valve to prevent fuel from flowing back into the tank and potentially causing an overflow.
The type of fuel you intend to use will also influence your choice of fittings. For example, if you plan to use diesel fuel, ensure that any pumps or valves you select are compatible with diesel. This includes diesel fuel pumps and diesel filters, which can be essential components in your setup.
Another consideration is the size and compatibility of the fittings with your fuel tank and hoses. Ensure that the fittings you choose match the size and thread pattern of your fuel tank's openings. For instance, if you have a 1-inch hose, you'll need a 1-inch fitting. Similarly, if you're using a rubber hose, ensure that the fittings are suitable for use with rubber and won't cause leaks or damage.
Finally, don't underestimate the importance of fuel filters. While they may not be strictly classified as fittings, incorporating fuel filters into your setup is crucial for maintaining fuel quality and preventing debris from entering your engine. Consider using an inline fuel filter between the auxiliary and main tanks to ensure that only clean fuel is transferred.
By carefully considering these factors and complying with legal requirements, you can choose the right fittings for your gravity-feed fuel tank, ensuring a safe and functional system.
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Venting and pressure relief
The venting mechanism in a gravity-feed fuel tank serves as a vital lifeline, ensuring that air can enter and exit the tank freely. It works in harmony with the inlet valve, creating a balance that ensures smooth and safe fuel flow. A blocked or kinked vent line can lead to vapor locking, disrupting fuel flow and causing operational issues.
To address this, it is essential to incorporate a vent that relieves pressure in both outward and inward directions. This bidirectional venting capability ensures that air can escape from the tank when needed and also allows air to enter, maintaining pressure balance.
When designing the venting mechanism, consider factors such as the tank's capacity, the rate of fuel flow, and the potential for pressure fluctuations. Regular inspections of the venting mechanism are also necessary to identify any blockages or unusual wear that could impact its functionality over time.
In addition to the venting mechanism, incorporating a pressure relief valve can provide further protection against excessive pressure build-up. This valve can be set to open at a specified pressure threshold, releasing excess pressure and preventing potential damage to the system.
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Fuel filtration
When designing a fuel filtration system for a gravity-fed fuel tank, it is essential to select an appropriate filter type and micron rating. Fuel filters with fine mesh or small micron ratings can effectively remove contaminants but may restrict fuel flow, especially in gravity-fed systems. It is recommended to use a filter with a micron rating between 40 and 150, depending on the specific application. For example, the Briggs and Stratton red filter with a 150-micron rating is commonly used and suitable for gravity feed systems.
Additionally, consider the fuel type and source. If the fuel is sourced from high-volume truck stops or unknown sources, it may contain rust particles or other contaminants. In such cases, implementing a dual-filter system or using a finer mesh filter in conjunction with a fuel pump can help ensure adequate filtration.
The location of the fuel filter is another important consideration. Inline fuel filters are commonly used and can be placed between the auxiliary tank and the main tank or in the fuel line leading to the engine. Ensure that there is sufficient fuel line length to accommodate the filter and that it is easily accessible for maintenance and replacement.
When installing a fuel filter, it is crucial to consider the direction of fuel flow. Typically, the filter should be installed before the fuel pump in the fuel line. This allows the filter to catch any contaminants before they reach the pump, protecting the pump and ensuring a clean fuel supply to the engine.
Regular maintenance and replacement of fuel filters are essential to maintain optimal performance. The frequency of replacement will depend on the filter type, fuel quality, and operating conditions. Inspect the filter at regular intervals and replace it when necessary to ensure the fuel system's efficiency and longevity.
By following these guidelines and considerations, you can design and implement an effective fuel filtration system for your gravity-fed fuel tank, ensuring the reliable and efficient operation of your engine.
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Tank materials
The materials used in the construction of a fuel tank are important for safety and durability. Nearly all automobiles and aircraft use fuel tanks, and depending on the type of vehicle or aircraft, one type of fuel tank may be more suitable.
Fuel tanks can be made from a variety of materials, including steel, aluminium, plastic, and stainless steel. Steel is one of the least expensive customisable fuel tank materials and is exceptionally strong. It is also susceptible to rust, so steel tanks are often made of thicker plates to account for this. Steel must meet ASTM standards and be a minimum of 0.0747 inch thick.
Aluminium is another popular choice for fuel tanks, especially in marine applications. It is lightweight and easy to cut, but it is also prone to corrosion. To combat this, aluminium tank manufacturers offer to coat their tanks with corrosion-inhibiting epoxies. ABYC guidelines require aluminium tanks to be constructed of 5000 series alloy and be a minimum of 0.090 inches thick, although thicker is usually better.
Other materials used for fuel tanks include black iron, fiberglass, and plastic. When choosing a fuel tank, it is important to consider the material thickness, as this affects the durability of the tank. Thicker materials are generally more desirable, as they provide more protection and can help to prevent leaks. It is also important to consider the environment in which the tank will be used, as some materials may be more susceptible to corrosion or damage in certain conditions.
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Frequently asked questions
You will need a feed or transfer tank, a shut-off valve, a vent, and an overflow. You will also need a pump to get the fuel to flow properly.
Gravity or siphon feeding is prohibited in some areas, so it is important to check your local laws and regulations. Make sure to use a diesel fuel pump and a diesel filter to avoid leaks and spills.
There are a few ways to gravity feed an auxiliary fuel tank. One way is to use a plain gravity feed with a manual shut-off valve. Another way is to use a pump and a ball valve to control the flow of fuel.
