Rajan Wilcox
Diesel engines have a return line to send excess fuel back to the tank. This is controlled by a pressure regulation device, which prevents the pump from pushing needlessly and stops it from overheating. Older diesel fuel systems have the diesel pump synchronized with the engine timing, but this can cause issues with timing if it is controlled by computers. Modern engines have the diesel pump make high-pressure fuel, and if the pressure is too high, it is released back to the tank.
| Characteristics | Values |
|---|---|
| Purpose of fuel return lines | To send excess fuel back to the tank |
| Why there is excess fuel | Diesel engines need high-pressure fuel, higher than the compression generated by the pistons |
| Why is it important to send back excess fuel | To prevent the pump from "dead heading" and pushing needlessly, and to prevent overheating |
| How is the return line controlled | By a pressure regulation device |
| Why do older diesel systems not need a return line | They have the diesel pump synchronized with the timing of the engine so that the fuel from the pump gets injected into the cylinders at the exact same time |
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What You'll Learn
Fuel return lines prevent the pump from deadheading
Deadheading is when a pump continues to operate without any fluid flowing through it. This can be caused by a closed discharge valve, line blockage, or a check valve remaining in a seated position. Fuel return lines prevent deadheading by providing a path for excess fuel to flow back to the fuel tank.
In a diesel engine, the fuel return line is a hose that returns excess fuel from the engine to the fuel tank, where a fuel pressure regulator is used to maintain the correct fuel pressure in the system. If the fuel return line becomes clogged or pinched, the excess fuel cannot return to the fuel tank, leading to a build-up of pressure in the system. This can cause the fuel pump to operate in a deadheaded state, as the fuel has nowhere to go and the pump continues to circulate the same fluid volume repeatedly.
The consequences of deadheading can be severe. As the pump circulates the same fluid, the temperature of the fluid will increase due to friction. Eventually, the fluid will reach its flash point and begin to turn into vapour. This vapour can affect the cooling flow circulating through the pump's packing and bearings, leading to excessive wear and heat. The vapour can also heat up any bushings or mechanical seals in the pump, causing them to shatter, crack, or score.
To prevent deadheading, it is essential to ensure that the fuel return line is not clogged or pinched. Regular maintenance and inspections of the fuel return line can help identify any potential blockages or leaks and prevent them from causing larger issues. Additionally, providing a recirculation line from the pump discharge line can protect the pump from operating in a deadheaded state. This recirculation line should be adequately sized to allow enough fluid to flow through the pump and prevent overheating and damage.
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They help prevent overheating
Diesel engines have a return line to send excess fuel back to the tank. This is controlled by a pressure regulation device, which prevents the pump from pushing needlessly and "dead heading". The constant flow offered by the return line helps to prevent the engine from overheating, as the in-tank pumps are cooled by the fuel they are pumping. This is in contrast to return-less systems, which may have a different style of pump or a controller that varies the speed of the in-tank pump.
Diesel engines require high-pressure fuel, higher than the compression generated by the pistons. This means that they need a high-pressure fuel pump, which can lead to challenges in pump design. The return line helps to maintain the high pressure needed for diesel engines by releasing excess fuel back to the tank when the pressure is too high.
In older diesel engines with mechanical injectors, the moving parts in the injector are lubricated by diesel that then returns to the tank via the return line. This design is not commonly found in modern cars.
The return line, therefore, plays a crucial role in preventing overheating by maintaining the necessary fuel pressure and ensuring the proper lubrication of engine components.
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They ensure high-pressure fuel is always available for the injectors
Diesel engines require high-pressure fuel to function, at a pressure higher than the compression generated by the pistons. This pressure is created by a high-pressure fuel pump. If there is no escape for the fuel, it will need to find a way out. Therefore, modern diesel engines are designed to have the diesel pump produce as much high-pressure fuel as possible. If the pressure becomes too high, it is released back into the fuel tank via a return line. This ensures that there is always high-pressure fuel available for the injectors, not just when it is needed.
In older diesel engines with mechanical injectors, the moving parts in the injector are lubricated by diesel. This diesel then flows back into the tank through the return line. This type of design is rarely found in modern cars.
The return line also helps to prevent the pump from "deadheading" and pushing needlessly, as well as preventing the pump from overheating. This is because many in-tank pumps are cooled by the fuel they are pumping, and the constant flow offered by the return line helps to maintain an optimal temperature.
Overall, the fuel return line plays a crucial role in ensuring the availability of high-pressure fuel for the injectors, maintaining the efficiency and performance of diesel engines.
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They lubricate the moving parts in the injector
Lubrication is essential for the efficient operation of a diesel engine. The moving parts in the injector are lubricated by the diesel fuel as it passes through them. This is because the sliding surfaces of these parts are lap-finished, resulting in a very small clearance between them.
The injection pump plungers, delivery valves, injection nozzles, and feed pump piston are among the components lubricated by diesel fuel. This type of lubrication is necessary for high-speed operation under high temperatures and pressures.
It is worth noting that the lubricity of diesel fuel has been a topic of concern with the introduction of new low-sulfur diesel. Sulfur acts as a lubricant in diesel fuel, and its removal during the refining process can impact the fuel's ability to lubricate injector components. Reputable refineries may add lubricity agents to address this issue, but some vehicle owners also choose to add their own additives to enhance lubrication and improve engine performance.
Specialty lubricants, such as biodiesel or two-stroke oil, are often used to reduce wear and tear on the sliding/moving parts of injectors, thereby prolonging their lifespan. These additives help maintain the efficiency of the diesel engine by ensuring that the injectors and injector pumps remain properly lubricated, even with the changes in fuel composition over time.
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They are fed by gravity
Diesel engines, an internal combustion engine invention of Rudolf Diesel, are fed by gravity through fuel return lines. This system is designed to deliver fuel to the engine by utilising the force of gravity, ensuring a consistent and efficient supply.
The basic principle behind this mechanism is straightforward. The fuel tank, positioned higher than the engine, allows for the downward flow of fuel due to the force of gravity. This simple yet effective method eliminates the need for complex pumping systems, as gravity naturally pulls the fuel downwards, facilitating a smooth and uninterrupted flow.
The fuel, typically a liquid such as diesel or gasoline, is stored in the tank, which acts as a reservoir. By strategically placing the tank at a higher elevation relative to the engine, the fuel is naturally drawn downwards when needed. This arrangement ensures that the engine can easily access the required amount of fuel without any additional mechanical assistance.
The fuel return lines play a crucial role in this process. These lines are designed to create a pathway for the fuel to flow from the tank to the engine. They are carefully routed to enable a steady descent, allowing gravity to act upon the fuel and facilitate its movement. The return lines are engineered to maintain an optimal flow rate, preventing any excessive fuel delivery that could lead to waste or inefficient combustion.
This gravity-fed system offers several advantages. Firstly, it is a reliable and straightforward approach that reduces potential mechanical failures associated with more intricate systems. Secondly, it ensures a consistent fuel supply, as the force of gravity remains constant, providing stable delivery. Additionally, this method helps minimise fuel wastage and promotes efficient combustion, contributing to the engine's overall efficiency.
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Frequently asked questions
Diesel engines have fuel return lines to send excess fuel back to the tank. This prevents the pump from pushing needlessly and helps prevent overheating.
Many in-tank pumps are cooled by the fuel they are pumping, and the constant flow offered by the return line maintains this cooling process.
Without a return line, the pump may "dead head", leading to unnecessary pushing and potential overheating.
In older diesel engines with mechanical injectors, the moving parts in the injector are lubricated by diesel that then returns to the tank via the fuel return line.
It is preferable for the return line to be higher than the tank to allow gravity to feed the fuel back. This setup helps to prevent issues like air in the fuel system.
