Sylvie Willis
A diesel gravity feed fuel system is a type of fuel feed system used in some engines, including diesel engines, to deliver fuel to the engine cylinders. This system uses gravity to feed fuel from the tank to the carburetor float chamber, without the need for a fuel pump. The fuel tank is typically mounted at the highest position, above the engine, allowing fuel to flow downwards towards the carburetor. While this design is simpler and more cost-effective, it has limitations, such as the requirement to place the fuel tank over the carburetor for optimal functioning. The PSI (pounds per square inch) of the system can vary based on factors like altitude and the height of the tank, and adjustments may be needed to manage fuel pressure and prevent flooding.
| Characteristics | Values |
|---|---|
| Application | Smaller vehicles such as entry-level two-wheelers, bikes, quads, and aircraft |
| Design | Simple and cheaper to produce and maintain |
| Placement of fuel tank | At the highest position |
| Fuel flow | Gravity |
| PSI | 2-3.5 PSI |
| Altitude | A general rule of thumb is that for each foot of elevation between the tank and carburetor, 0.5 PSI of fuel pressure is produced |
| Fuel tank | Vented to maintain atmospheric pressure |
| Outlet | Single screened outlet |
| Valve | Fuel shutoff valve or multiposition selector valve |
| Strainer | Main system strainer |
| Pump | Micro-electric fuel pump |
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What You'll Learn
Gravity-fed fuel systems are used in high-wing aircraft
In a gravity feed system, the fuel tanks are typically located in the wings of the aircraft. Each tank has a screened outlet that feeds lines connecting to either a fuel shutoff valve or a multiposition selector valve. The shutoff valve allows for the fuel to be turned on or off, while the selector valve enables the pilot to choose which tank is feeding fuel to the engine.
The selector valve may offer options to draw fuel from the right tank, left tank, or both simultaneously. The tanks are vented to maintain atmospheric pressure as they empty and to ensure equal pressure when both tanks feed the engine. Downstream of the shutoff or selector valve, the fuel passes through a strainer before continuing on its path to the engine.
While gravity-fed fuel systems are commonly used in high-wing aircraft, they are not suitable for low- and mid-wing aircraft. This is because, in low- and mid-wing configurations, the fuel tanks are located below the engine, requiring the use of pumps to move the fuel upwards against gravity. Additionally, some high-performance high-wing aircraft may employ fuel injection systems instead of carburettors, combining gravity flow with the use of fuel pumps.
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Fuel tank positioning is crucial for gravity-fed systems
The gravity system is typically found in smaller vehicles such as two-wheelers, bikes, and quads. The design is simple, making it cheaper to produce and maintain. However, the fuel tank must be placed above the carburetor for the system to function correctly. This restriction highlights the importance of fuel tank positioning in gravity-fed systems.
In some cases, gravity-fed systems have been known to cause issues with on-board vehicle diagnostic systems. The main fuel tank may be kept overfull, resulting in a diagnostic trouble code. Additionally, there are safety concerns regarding fuel spillage and the potential for fuel to continue leaking from a damaged transfer fuel hose until the tank is empty.
Due to these factors, the legality of gravity-fed systems is ambiguous, and they may violate certain regulations. For example, in the United States, there are concerns about compliance with the Department of Transportation (DOT) regulations and the Clean Air Act. Therefore, it is essential to carefully consider fuel tank positioning and adhere to engineering best practices to ensure the safety and legality of gravity-fed fuel systems.
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Gravity feed systems are simple and cheap to maintain
The main advantage of a gravity feed system is its simplicity. It relies on gravity to feed fuel into the carburetor, eliminating the need for complex mechanisms or additional pumps. This simplicity translates into lower production costs and easier maintenance for vehicle owners.
The maintenance of gravity feed systems is straightforward and cost-effective. Unlike pressurized fuel systems, gravity feed systems do not require complex sealing or pumping mechanisms, reducing the number of components that could require repair or replacement. The absence of intricate components also means that repairs can often be performed quickly and without specialized tools or expertise.
While gravity feed systems have advantages in terms of simplicity and maintenance, they also have some limitations. One significant disadvantage is the requirement to place the fuel tank over the carburetor for the system to function correctly. This design constraint may limit the layout and styling of the vehicle. Additionally, gravity feed systems may be prone to fuel gauge issues, particularly when fuel feeds into the tank rapidly.
In some jurisdictions, the use of gravity feed systems is regulated or prohibited, especially in commercial applications. For example, certain US states have regulations against gravity feed systems in diesel vehicles due to safety concerns. Therefore, it is essential to check the local regulations before considering the installation or modification of a gravity feed system.
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Fuel injection systems use injectors to supply fuel to cylinders
A fuel injection system uses injectors to supply fuel to the cylinders. This is done to ensure that the engine runs smoothly and efficiently, with the right quantity of fuel supplied according to its wide range of demands. The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine.
There are two main types of fuel injection systems: continuous injection and intermittent injection. In a continuous injection system, fuel flows at all times from the fuel injectors but at a variable flow rate. The most common automotive continuous injection system is the multi-point Bosch K-Jetronic system, which was introduced in 1974 and used until the mid-1990s. Intermittent injection systems can be sequential, batched, simultaneous, or cylinder-individual. In a sequential system, injection is timed to coincide with each cylinder's intake stroke, while in a batched system, fuel is injected to the cylinders in groups without precise synchronization to any particular cylinder's intake stroke.
Modern petrol injection systems use indirect injection, where a special pump sends the fuel under pressure from the fuel tank to the engine bay. From there, the fuel is distributed individually to each cylinder. Depending on the particular system, the fuel is fired into either the inlet manifold or the inlet port via an injector. This works much like the spray nozzle of a hose, ensuring that the fuel comes out as a fine mist. The fuel mixes with the air passing through the inlet manifold or port, and the fuel/air mixture then enters the combustion chamber.
Direct injection systems inject fuel directly into the combustion chamber of each cylinder, rather than the intake manifold. This precise delivery of fuel enhances fuel atomization, resulting in improved combustion efficiency and power output while reducing emissions. Common-rail injection systems are the most common type of system in modern automotive engines. In these systems, fuel from the fuel tank is supplied to a common header (called the accumulator) and then sent through tubing to the injectors, which inject it into the combustion chambers.
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Diesel fuel feed systems differ from petrol engine systems
The main purpose of a fuel feed system is to control the fuel supply to the engine. Diesel fuel feed systems differ from petrol engine systems in several ways. Firstly, diesel fuel systems are designed to inject fuel directly into the engine cylinders, whereas petrol engine systems use a carburetor to mix fuel and air before delivering it to the cylinders. This is because diesel engines rely on the heat of highly compressed air to ignite the fuel, rather than an electrical spark as in gasoline engines.
Diesel fuel systems also differ in their fuel storage and delivery methods. Diesel fuel must be stored in a closed tank to prevent contamination and is typically derived from the heavier residues of crude oil. The tank has specific openings for filling, discharging, and draining fuel, as well as venting to allow air to enter. Diesel engines often use a fuel transfer pump to supply fuel to the injection system, which can be driven by the engine itself or have a hand primer lever.
The injectors in a diesel fuel system are of utmost importance as they deliver a precise amount of atomized and pressurized fuel into each cylinder. Modern diesel injectors, such as those in common rail fuel systems, use piezoelectricity for extremely precise fuel injection. This results in increased power and fuel economy, decreased engine noise, and smoother operation.
Furthermore, diesel fuel feed systems may vary in design depending on the vehicle type. Smaller vehicles like bikes and quads may use a gravity system, where the fuel tank is mounted above the engine, feeding fuel by gravity. In contrast, larger vehicles may employ a pressure feed system with a sealed fuel tank that utilizes an air pump or engine exhaust to create pressure and force the fuel to the engine.
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Frequently asked questions
A gravity feed fuel system uses gravity to deliver fuel from the tank to the carburetor. This system is typically used in smaller vehicles such as two-wheelers, bikes, and quads, and aircraft.
In a diesel gravity feed fuel system, the fuel tank is mounted at the highest position. The fuel then flows downwards by gravity into the carburetor float chamber.
The gravity feed fuel system has a simple design, making it cheaper to produce and maintain. It is also the simplest aircraft fuel system as it does not require a fuel pump.
One disadvantage of this system is that the fuel tank must be placed over the carburetor for it to work correctly. Additionally, it may not be suitable for all types of aircraft, such as low-wing or high-wing aircraft with fuel-injected engines.
In some cases, a fuel pump may be used in conjunction with a gravity feed fuel system. The fuel pump can be placed anywhere, and its purpose is to pump the fuel into the carburetor float chamber.
