Sylvie Willis
Diesel engines have two fuel lines, a supply line and a return line, to ensure the engine receives a constant flow of fuel. The supply line delivers fuel from the tank to the engine, while the return line sends back any unused fuel. This system also helps to regulate fuel pressure and prevent leaks, ensuring the engine's optimal performance. Bleeding the fuel lines, or removing air bubbles, is crucial for diesel engines to function properly.
| Characteristics | Values |
|---|---|
| Number of Fuel Pumps in Newer Diesels | At least one electric fuel pump |
| Fuel Line Size | One larger line and one smaller line |
| Fuel Line Function | One line sends fuel, and one returns it |
| Fuel Line Maintenance | Fuel lines may need to be bled to remove air |
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What You'll Learn
One line sends fuel, the other returns it
Diesel engines have two fuel lines because one line sends fuel, and the other returns it. This is known as a return fuel system. In this system, the fuel is pumped through the pump to the engine, dispensed in the rail, and then there is a line that runs back to the fuel tank to dump the excess fuel back in. The larger line is the sender, and the smaller line is the return.
Return fuel systems are designed to return unused fuel from the engine to the tank. The fuel pump is continuously pumping gas through the fuel system at a much higher rate than is actually needed. This keeps the gasoline in the fuel lines cool and flushes out any vapour bubbles that might form, preventing vapour lock in the fuel system. The fuel cools the injectors and head, and sometimes the engine computer. The heat is returned through the fuel to the filter and tank, driving out moisture and reducing algae growth.
Diesel fuel must be filtered several times in most systems. There are three types of diesel fuel lines: heavyweight lines for high pressures between the injection pump and injectors; medium-weight lines for light or medium fuel pressures between the fuel tank and injection pump; and lightweight lines where there is little or no pressure.
Diesel fuel is derived from the heavier residues of crude oil that are left over after the more volatile fuels such as gasoline are removed during the refining process. The most common grade of diesel fuel is 2-D, or ultra-low sulfur diesel (ULSD). A common enemy of diesel fuel systems is water, which can rapidly oxidize ferrous metal components.
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Fuel is used to lubricate the high-pressure pump
Diesel engines have two fuel lines: a high-pressure fuel line and a return line. The high-pressure fuel line delivers fuel to the injectors, and the return line sends excess fuel back to the fuel tank. The pump pressurises the fuel and sends it through the high-pressure fuel line to the injectors.
The injection pump may be a rotary plunger-type unit with an individual piston for each cylinder of the engine, or it may be a single-chamber pump that meters fuel to each cylinder. In either case, the pump sends a small amount of fuel through the high-pressure fuel line to each injector in the appropriate sequence.
The fuel charge is accurately metered and pressurised by the fuel injection pump and passed to the high-pressure injectors via pipes before being sprayed into the combustion chamber, where it ignites. The combustion gasses then drive the piston down to produce the power stroke.
In the past, some consumers have added engine oil to diesel fuel in the belief that this would enhance the lubricity of the fuel. However, this practice can be detrimental, leading to deposits forming in fuel injection pumps, "black sticky gel" plugging fuel filters, and deposits forming on injector tips. Instead, commercially available lubricity additives can be added to diesel fuel after undergoing rigorous no-harm testing. These tests examine the lubricity agent's effect on the performance of other additives in the diesel fuel and the impact of mixing lubricity additives with engine oils.
To test the lubricity of diesel fuel, the BOTD test method can be used. In this test, a ceramic ball rotates against three steel disks that are immersed in the test diesel fuel. The average wear scars created on the disks are measured, with high lubricity diesel fuels generating average wear scars of less than 400 microns and low lubricity fuels yielding average wear scars of over 500 microns.
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Filters may not catch everything on the first pass
This is where the second fuel line and the secondary filter come into play. The secondary filter is designed to catch any remaining contaminants and water that the primary filter might have missed. By having this backup filtration system, the diesel fuel can be further purified, ensuring that only clean fuel reaches the engine. The secondary filter provides an additional layer of protection, reducing the risk of engine damage and performance issues caused by contaminated fuel.
The two-step filtration process is particularly important for diesel engines as they are known to be sensitive to fuel contamination. Water, for instance, can cause significant issues in a diesel engine, leading to corrosion, reduced engine performance, and even engine failure. By having two fuel lines with dedicated filters, the diesel fuel system can effectively mitigate the risk of water contamination and ensure the engine receives clean, dry fuel.
It is worth noting that even with this dual-filtration system, regular maintenance and filter replacements are crucial. Over time, filters can become clogged with captured contaminants, reducing their effectiveness. Therefore, it is essential to adhere to the recommended maintenance schedule and replace the filters as necessary to maintain optimal engine performance and prolong the life of the diesel fuel system.
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Return lines can rupture
There are a few potential causes of return line rupture. One possible cause is excessive fuel pressure. If the fuel pressure regulator, fuel return line to the tank, or vent tube are not functioning properly, it can lead to increased pressure that may cause the return line to rupture. It is important to have these components checked and serviced if there are any signs of fuel pressure issues.
Another potential cause of return line rupture is a restriction in the fuel system. A restriction can create a buildup of pressure, leading to a rupture. It is recommended to have a qualified mechanic test the injector pump, injectors, and rail pressure to check for any restrictions or blockages in the fuel system.
In some cases, the rupture may be due to a manufacturing defect or improper installation. If the return line is not installed correctly, it can be subjected to unnecessary stress, leading to a rupture. It is important to ensure that the return line is installed according to the manufacturer's specifications and that all retaining clips and fittings are secure.
To prevent further issues, it is crucial to address the root cause of the rupture. This may involve testing and servicing various components of the fuel system, as mentioned earlier. Additionally, regular maintenance and inspections can help identify any potential issues before they lead to a return line rupture or other diesel engine problems.
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Air in the fuel lines can cause issues
One of the first signs of air in the diesel fuel system is engine stalling or misfiring. Other symptoms include difficulty starting the engine, reduced engine power after starting, and, in severe cases, the inability to start the engine. Air in the fuel system can also cause cavitation, inconsistent fuel delivery, and poor engine performance.
To prevent issues caused by air in the fuel system, it is recommended to upgrade to a Fuel Air Separation System (FASS). This aftermarket fuel system is designed to improve the performance and longevity of diesel engines by removing air, vapour, water, and debris from the fuel, ensuring a steady and clean supply to the engine. The FASS system includes industry-leading fuel filters that remove contaminants such as dirt, debris, and water, tripling the life of injection systems and reducing issues like rough idling, stalling, and misfires.
Another way to address air-related issues in the diesel fuel system is to locate and fix any leaks or faulty components. This involves checking the fuel lines, connections, and seals for any signs of wear or damage and repairing or replacing the affected parts. Additionally, regular inspection and maintenance of the fuel lines, filters, and fuel pumps can help identify and rectify any issues that could lead to air ingress.
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Frequently asked questions
Diesel engines have two fuel lines, a supply line and a return line, to ensure the engine receives a constant flow of fuel. The supply line delivers fuel to the engine, while the return line sends back any unused fuel to the tank.
The return fuel line ensures that any excess fuel that has not been used by the engine is sent back to the tank to prevent overflow or leakage.
Air in the fuel lines can prevent fuel from reaching the engine, causing issues with starting the vehicle. Bleeding the fuel lines is necessary to remove air from the system.
If your diesel vehicle is having trouble starting, it could be an indication that air has entered the fuel lines. You will need to activate the pump using a diagnostic tool or a 12V battery to bleed the lines and remove the air.
