Sylvie Willis
Dual fuel tank systems are often used in trucks to provide adequate on-board fuel storage capacity. In some cases, a mid-mounted fuel tank is combined with a smaller rear-mounted tank to meet the vehicle owner's fuel storage demands. The dual fuel tank system allows fluid to flow between the two tanks. The system draws fuel from one tank until a switch is flipped, activating a module that signals which fuel pump to turn on. The sender assembly handles the switching of the fuel through internal valves.
| Characteristics | Values |
|---|---|
| Purpose | To provide adequate on-board fuel storage capacity |
| Functionality | Draws fuel from one tank until a switch is flipped, activating a module that signals which fuel pump to turn on |
| Components | Fuel Delivery Module (FDM) or low-pressure pump and sending unit in each tank with a single high-pressure pump on the frame rail |
| Fuel Transfer | Fluid can flow between the two tanks through a fill limiter tee |
| Fuel Switching | Sender assembly handles switching of fuel through internal valves |
| Fuel Reservoirs | Prevent fuel flow interruptions during extreme vehicle maneuvers with low tank fill levels |
| Fuel Filters | Four filtering/screening devices: fuel intake sock, filter screen on fuel rail port of the injector, filter screen on fuel inlet side of pressure regulator, and fuel pump assembly |
| Issues | May experience problems with fuel gauge accuracy and fuel pump functionality |
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What You'll Learn
Dual Function Reservoir (DFR)
A dual-tank fuel system typically consists of two independent tanks, each with its own low-pressure pump and sending unit, and a single high-pressure pump located on the frame rail. In between the two tanks lies a mechanically operated Dual Function Reservoir (DFR), which is controlled by the low-pressure pumps.
The DFR plays a crucial role in the dual-tank fuel system by facilitating the selection of a specific tank from which fuel will be drawn. When a tank is selected, the fuel pressure actuates a valve within the DFR, allowing fuel to flow from the chosen tank while simultaneously blocking the ports connected to the other tank. This mechanism ensures that fuel is only drawn from the desired tank and prevents any unintended crossflow between the tanks.
The DFR is usually situated on the low-pressure side of the circuit, operating within a pressure range of approximately 5-7 PSI. It is important to note that the DFR may not function properly if the fuel system incorporates high-pressure pumps. Ford, for instance, recommends against using air pressure exceeding 5 PSI when testing the DFR.
While some individuals have expressed dissatisfaction with the DFR system, citing concerns about its functionality, others have shared positive experiences, highlighting its reliability. Nevertheless, it is essential to be aware of potential issues, such as a stuck DFR valve, which can restrict access to one of the tanks. Regular maintenance and troubleshooting can help identify and rectify any problems that may arise with the DFR or other components of the dual-tank fuel system.
The DFR is an integral component of the dual-tank fuel system, enabling the seamless selection and utilisation of fuel from either tank. Its functionality is dependent on the proper operation of the low-pressure pumps and valves within the system. By understanding the workings of the DFR, users can effectively manage their fuel supply and address any potential challenges that may arise.
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Fuel Delivery Module (FDM)
A Fuel Delivery Module (FDM) is a critical component of a vehicle. During the driving process, the fuel supply must be timely, sufficient, and stable, and it should provide an accurate amount of remaining fuel to remind the driver when to fill the tank. The FDM includes three parts: a gas supply pump, a gas pump controller, and liquid level sensors for the gas tank. Fuel pumps deliver fuel to pipes, sending gas to the engine, and the fuel tank liquid surface sensor determines the gas volume by floats that monitor height change and send signals to the engine ECU.
The FDM is also known as a sender assembly, which handles the switching of fuel through internal valves. The system contains four filtering or screening devices. Firstly, the fuel intake sock is a screen located on the intake side of the fuel pump and is part of the fuel pump assembly. Secondly, the filter screen on the fuel rail port of the injector is part of the fuel injector assembly and cannot be serviced separately. Thirdly, the filter screen on the fuel inlet side of the pressure regulator is part of the fuel pressure regulator assembly and cannot be serviced separately. Finally, the fuel pump and fuel tank liquid level sensor are suitable for various kinds of fuel, as fuel contains many different chemical substances that may lead to erosion. Therefore, the proper selection of materials and processes is necessary to overcome these issues.
The FDM is also responsible for fuel volume measurement, providing a real-time accurate amount of remaining fuel volume to the driver. This information helps drivers avoid accidents and safety hazards by ensuring they do not run out of fuel. These sensors can help reserve a safety volume of fuel to prevent such occurrences.
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Fuel reservoirs
The primary function of the fuel reservoir is to prevent fuel flow interruptions, ensuring a consistent supply of fuel to the engine regardless of the vehicle's movements. This is particularly crucial during extreme manoeuvres, such as abrupt turns, acceleration, or braking, which can cause fuel to shift or slosh within the tank.
In a dual-tank fuel system, the reservoirs are typically part of the Fuel Delivery Module (FDM) or Dual Function Reservoir (DFR). This module is present in each tank and includes a fuel pump, gauge sender, and check valve. The DFR, situated between the two tanks, is operated by the low-pressure pump, controlling the flow of fuel by activating specific valves.
The design of the fuel reservoir varies depending on the vehicle and tank configuration. In some cases, the reservoir may be moulded or welded into the tank, creating a seamless integration. Alternatively, it may be incorporated as part of the Fuel Delivery Assembly, ensuring its functionality within the fuel system.
The fuel reservoir works in conjunction with other components, such as the sender assembly, to manage fuel flow. The sender assembly uses internal valves to direct the flow of fuel, ensuring it reaches the engine without interruptions. Additionally, the system incorporates multiple filtering and screening devices to maintain fuel quality and prevent contaminants from entering the engine.
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Fuel intake sock
A fuel intake sock, also known as a fuel sock or a fuel filter sock, is a crucial component of a vehicle's fuel system. It is a screen located on the intake side of the fuel pump and is part of the fuel pump assembly.
The fuel sock serves multiple important functions:
- Strain out large solids: The sock acts as a filter, straining out large solid particles and impurities from the fuel before it reaches the fuel pump. This helps protect the fuel pump and other vital components of the fuel system, such as the fuel rails and fuel injectors, from damage and clogging.
- Prevent water entry: The sock also functions as a water filter or barrier, preventing water from entering the fuel system. This is essential as water in the fuel can cause significant issues, including fuel starvation and damage to the fuel pump and injectors.
- Act as a wick: In some vehicles, the fuel pickup tube doesn't reach the bottom of the fuel tank. The fuel sock acts as a wick, allowing fuel to be drawn out of the tank even when the fuel level is below the tube. This ensures that the vehicle can utilise all the fuel in the tank, including the last few drops.
- Facilitate fuel flow: The sock can also facilitate fuel flow to the fuel heater by including a bypass valve. When the filter is covered with wax, the valve opens, allowing fuel to flow through the sock to the heater.
It is important to maintain and replace the fuel intake sock periodically. If the sock becomes damaged, clogged, or dislodged, it can lead to fuel pump issues and even leave a vehicle stranded. Additionally, the sock should be checked regularly to ensure it is securely attached and functioning correctly.
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Fuel transfer lines
In a dual-tank fuel system, the tanks are independent, and each has a low-pressure pump and sending unit. There is a single high-pressure pump on the frame rail. Between the two tanks is a mechanically operated Dual Function Reservoir (DFR) that is operated by the low-pressure pump. When a tank is selected, the fuel pressure moves a valve to allow fuel to flow from that tank and block the ports to and from the other tank.
The sender assembly handles the switching of the fuel through internal valves. The system contains four filtering or screening devices. The fuel intake sock is a screen located on the intake side of the fuel pump and is part of the fuel pump assembly. There is also a filter screen on the fuel rail port of the injector, which is part of the fuel injector assembly and cannot be serviced separately. Additionally, the filter screen on the fuel inlet side of the pressure regulator is part of the fuel pressure regulator assembly and cannot be serviced separately.
The safety regulations for fuel transfer lines are stringent, especially when it comes to pipes passing through ballast tanks, due to the risk of pollution. These pipes must be short, have increased wall thickness, and be equipped with stronger flanges. Additionally, the Safety of Life at Sea Convention (SOLAS) mandates the use of special double-skinned pipes for delivering fuel to engine combustion chambers. These pipes, made of low carbon steel alloys, can withstand high pressures ranging from 600 to 900 bar. The double-skinned design is a safety measure to prevent fuel spray in the event of pipe fracture.
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Frequently asked questions
The dual tank fuel system in a Ford F-150 allows the vehicle to draw fuel from either the front or rear tank. When the switch is flipped, a module is activated that tells the fuel pump to turn on, and fuel is drawn from the selected tank.
The system uses a sender assembly to handle the switching of fuel through internal valves. This assembly can be replaced if needed. The system also includes a fuel gauge sender, which consists of a variable screened resistor controlled by a float in the fuel tank. The resistance changes with the fuel level, allowing the system to monitor the amount of fuel in each tank.
One issue is that the fuel gauge may become inaccurate or maxed out, regardless of the selected tank. This could be due to a faulty sending unit, in which case, one of the tanks should display an accurate reading. Another issue is that the fuel pump may fail, or the check valve may leak, resulting in fuel returning to both tanks.
