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A fuel supply system supplies the carburetor of fuel injection system with a constant supply of fuel. This is separate from the fuel delivery system, which delivers the appropriate amount of fuel to the intake, or directly into the combustion chamber, at the appropriate time. This article will describe fuel supply systems for both EFI and carbureted gasoline/ethanol engines, as well as Diesel fuel supply systems.
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Gasoline/Ethanol Fuel Supply Systems
Carbureted Engines - Most carbureted gasoline/ethanol engines use a 1-line fuel supply system with a mechanical fuel pump and either an in-line fuel filter or a filter mounted in the carburetor, and a fuel pickup tube assembly with a fuel level sender inside the tank. The mechanical fuel pump is normally a diaphragm-type pump, mounted to the engine block, and driven by a cam located on the cramshaft. A diaphragm pump normally uses a spring to expand the diaphragm, which creates a suction. This suction pulls a check valve closed on the outlet of the pump, then pulls fuel out of the fuel tank via the fuel line. The diaphram is contracted with the pushrod or lever, creating pressure inside the fuel pump. The pressure pushes a check valve closed on the inlet of the pump, then pushes fuel up to the carburetor. When a surplus of fuel has entered the caburetor fuel bowl, it lifts a float inside and closes off the fuel bowl inlet, preventing more fuel from overfilling the fuel bowl. The fuel inside the fuel bowl is stored at atmospheric pressure.
EFI Engines - Most EFI gasoline/ethanol engines use an electric fuel pump mounted inside the fuel tank, with an in-line fuel filter. Some fuel pumps are part of a single fuel module, which consists of a fuel pickup assembly, fuel pump, and fuel level sender. Some fuel modules have the fuel filter mounted internally. Fuel modules used on 1-line fuel supply systems normally also contain a fuel pressure regulator, while 2-line systems have a fuel pressure regulator at the fuel rail outlet. A 1-line system fuel pump flows fuel up to the fuel rail until it reaches a preset pressure, and the regulator opens, allowing excess fuel to bypass into the fuel tank. A 2-line system works similarly, but flows fuel continuously through the fuel rail when the fuel pressure is great enough to open the fuel regulator, back to the tank through a fuel return line.
Diesel Fuel Supply Systems
Diesel engines normally use either a 1-line or 2-line fuel supply system. The fuel supply system on a diesel engine delivers low-pressure fuel to either the fuel injection pump or to the fuel injectors themselves.
- 1-line systems - A typical 1-line system uses a mechanical fuel lift pump, similar to that used on a carbureted engine and either a primary fuel filter and a fuel water separator, or a single fuel filter/fuel water separator, and a pickup tube assembly with fuel level sender inside the tank. Some diesel engines have a mechanical fuel lift pump incorporated into the fuel injection pump. 1-line systems will normally have a primer pump, as they are unable to pull fuel out of the fuel tank when they lose their prime, such as after fuel filter replacement. The fuel injectors on a diesel engine with a 1-line fuel supply system will have fuel return lines that deliver excess fuel to the tank, however these are considered to be part of the fuel delivery system rather than the supply system.
- 2-line systems - A 2-line system will typically have a frame-mounted fuel lift pump, with a fuel filter/water separator, and a fuel pickup tube assembly with fuel level sender and a fuel pressure regulator at the outlet of the return tube. However, some 2-line systems are equipped with mechanical fuel pumps. On most vehicles, the electric lift pump will run for several seconds up to a few minutes when the ignition key is turned on, to prime the fuel supply system with low-pressure fuel, and at all times when the engine is running. The fuel pump flows fuel up to the fuel injection pump or injectors, then back to the fuel pressure regulator, which allows fuel to exit the return line only when the lift pump has generated a preset amount of pressure in the fuel lines.
Summary
The information in this article does not cover all types of fuel supply systems or all variations of the systems described. It is intended to give a general description of some of the more common types of fuel supply systems. Look for more information on specific fuel systems in future Tech Info articles.
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