Ameer Randolph
The fuel return line is a hose that returns excess fuel from the engine to the fuel tank. It is present in carbureted vehicles, and some fuel injection systems. Mechanical diesel fuel systems with an injector pump always have a return line. This line helps to regulate fuel pressure and prevent vapour lock, which occurs when fuel, under the suction of a front-mounted fuel pump, turns into vapour when exposed to a vacuum, usually due to heat from the engine.
| Characteristics | Values |
|---|---|
| Purpose of fuel return lines | To prevent vapour lock by keeping fuel in the lines cool and flushing out vapour bubbles |
| Fuel return line components | A hose that returns excess fuel from the engine to the fuel tank |
| Fuel return line types | Return and returnless systems |
| Returnless system types | Mechanical and electrical |
| Return line use cases | Fuel injection systems, carburetor vehicles, mechanical diesel fuel systems |
| Factors affecting return line effectiveness | Fuel pump type, regulator type, fuel type, emissions, engine temperature, fuel line routing, fuel cell size |
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What You'll Learn
Preventing vapour lock
Vapour lock is a problem that has become more common in recent decades due to the lower boiling point of modern fuel blends. It occurs when the temperature of the fuel gets high enough for it to transform from a liquid to a vapour, which can happen when the heat of a vehicle's engine and exhaust is transferred to the fuel lines. This can cause the fuel pump to fail, as it is designed to pump liquid, not air.
One way to prevent vapour lock is to install a return line, which returns excess fuel back to the fuel tank to cool down. This keeps the fuel lines cool and flushes out any vapour bubbles that might form. This method is favoured by Prestige Motorsports, which has designed its turnkey fuel supply system around this approach.
Another way to prevent vapour lock is to use a thermal-barrier sleeve on the fuel lines, such as a Thermaflect Sleeve from Heatshield Products, which has a built-in, high-temp hook-and-loop fastener system. This solution is particularly useful for protecting the underside of a vehicle where the fuel lines run next to frame rails or floor pans.
Other ways to minimise vapour lock include avoiding steel lines, which offer no insulation from heat, and using a black nylon braided fuel hose instead. It is also important to route the hose away from all heat sources and to use ethanol-free fuel blends, as ethanol blends have a lower boiling point and are a major cause of vapour lock problems.
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Regulating fuel pressure
The FPR works by bleeding off a portion of the fuel flow to the injectors from the fuel pump to control the fuel pressure. The FPR is usually mounted after the fuel rail, ensuring that the fuel rail has priority in fuel flow. The valve in the FPR controls the amount of fuel that is bled from the fuel rail by opening an outlet port, allowing fuel to flow back into the fuel tank. All injectors need a pressure difference between the inlet and the outlet of the injector to spray fuel into the combustion chamber. This is called the base pressure. The base pressure is adjusted on all FPRs via the adjustment screw to suit the injectors and the fuel pump system being used.
A larger FPR can handle more flow and higher pressure while maintaining the ideal 1:1 ratio between the pressure from the fuel rail and the vacuum/boost pressure from the inlet tract. More expensive FPRs can withstand more types of alcohol fuels, such as ethanol and methanol, while cheaper regulators exposed to these fuels may suffer a broken diaphragm, which could cause serious engine damage.
In older cars, there was often no return line. There was simply a fuel pump and a simple float valve in the carburetor that let fuel flow as it was needed. However, engines get hot, and when gasoline gets hot, it turns into gas. Gasoline vapour in the fuel lines can lead to vapour lock. In fuel systems with a return line, the fuel pump continuously pumps gas through the fuel system at a much higher rate than is needed, keeping the gasoline in the fuel lines cool and flushing out any vapour bubbles that might form. This prevents vapour lock.
There are two types of returnless fuel systems: mechanical and electrical. Mechanical returnless systems relocate the pressure-regulating valve into the fuel tank, directly after the pump. Electrical returnless systems regulate the fuel pump speed to control the flow and pressure.
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Returning unused fuel
Fuel return lines are a part of carbureted vehicles. They are hoses that return unused or excess fuel from the engine to the fuel tank. This is done to prevent vapour lock, which occurs when fuel, under the suction of a front-mounted fuel pump, turns into vapour when exposed to a vacuum, usually due to heat from the engine or exhaust. This can cause performance problems such as misfires, poor acceleration, and reduced fuel efficiency.
In fuel systems with a return line, the fuel pump continuously pumps gas at a much higher rate than is needed, keeping the gasoline in the fuel lines cool and flushing out any vapour bubbles that may form. This is particularly important for older vehicles, which did not have a return line and were therefore more susceptible to vapour lock.
Return lines are also used to maintain pressure in the fuel system. The valve in the return line is set to maintain a certain pressure, and if the pressure is too high, it will allow more flow to escape, decreasing the pressure. This excess fuel is then returned to the tank.
The placement of the return line can be important, with some suggesting that returning fuel to the bottom of the tank can create back pressure and burn out the fuel pump. However, others argue that from a physics standpoint, it should take more pressure to drop fuel from the top of the tank than to push it in from the bottom.
Overall, while not all fuel systems have a return line, they can be beneficial in preventing vapour lock and maintaining pressure, especially in carbureted engines.
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Flushing out vapour bubbles
Fuel return lines are used to prevent vapour lock, a phenomenon that occurs when fuel, under the suction of a front-mounted fuel pump, flashes into vapour when exposed to a vacuum, usually facilitated by heat from an exhaust or warm weather.
Vapour lock can also occur when the fuel pump is located above the exhaust manifold or head, causing the fuel in the line to vaporize despite the fuel pump discharge pressure. This can be prevented by insulating the fuel line or relocating the fuel pump to the gas tank, as the pressure in the line prevents vapourisation at high temperatures.
In fuel systems with a return line, the fuel pump continuously pumps 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.
The return line ensures that unused fuel is returned to the tank, as fuel sitting in the line between the pump and the carburettor will increase in pressure if heated. This is especially important when the mechanical pump regulates its own pressure, as the stock fuel line can absorb heat as it passes between the water pump and timing cover.
The presence of vapour bubbles in the fuel line can cause issues such as hard starting, rough idle, and stalling. To diagnose a vapour bubble issue, one can use clear fuel lines to inspect for bubbles or perform a vacuum test.
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Maintaining fuel temperature
Fuel return lines play a vital role in maintaining fuel temperature by facilitating the return of unused fuel to the tank. This prevents fuel from stagnating in the lines, which can lead to increased pressure and temperature. By returning unused fuel, the system avoids issues caused by heated fuel, such as vapor lock. Vapor lock occurs when the fuel temperature rises to the point that it turns into vapour instead of remaining a liquid. As a result, the fuel pump, which is designed to pump liquid fuel, cannot draw the vapourised fuel, leading to engine problems or even failure.
The presence of a fuel return line helps regulate fuel pressure and temperature. When fuel passes through the engine, it absorbs heat, and without a return line, this heated fuel can raise the overall fuel temperature in the tank. This, in turn, can affect the fuel's combustibility and the vehicle's overall efficiency. By returning the unused fuel, the system can maintain optimal fuel temperature and pressure, ensuring the engine receives fuel at the correct temperature and pressure.
In addition to preventing vapor lock and maintaining fuel temperature, fuel return lines also contribute to the longevity of the fuel pump. By reducing the pressure and heat in the fuel lines, the pump experiences less stress and is less prone to failure. This, in turn, enhances the reliability and durability of the fuel system as a whole.
It is worth noting that while fuel return lines offer these benefits, modern returnless fuel delivery systems have been designed to mitigate the drawbacks of not having a return line. Returnless systems utilise in-tank regulators and fuel "returns" to manage pressure and temperature without the need for a dedicated return line. However, even in these systems, maintaining fuel temperature remains a critical aspect of ensuring the safe and efficient operation of the vehicle.
In conclusion, maintaining fuel temperature is essential for the proper functioning of a vehicle's fuel system. Fuel return lines play a crucial role in achieving this by facilitating the return of unused fuel, preventing vapor lock, regulating pressure and temperature, and prolonging the life of the fuel pump. While returnless systems have been developed, the principle of managing fuel temperature remains a key consideration in the design of safe and efficient automotive fuel systems.
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Frequently asked questions
Fuel pumps have return lines to prevent vapour lock, which occurs when fuel, under the suction of a front-mounted fuel pump, turns into vapour when exposed to a vacuum, usually due to heat from the exhaust or warm weather.
Vapour lock happens when fuel turns into gas after it heats up due to close exposure to the engine. Vapour lock is typically accompanied by performance problems such as misfires, poor acceleration, and reduced fuel efficiency.
In fuel systems with a return line, the fuel pump continuously pumps 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.
There are two types of returnless systems: mechanical and electrical. Mechanical returnless systems are less efficient but much easier to implement, while electrical returnless systems require knowledge of both the current pump flow rate and the required engine flow rate.
