The function of the diesel fuel system is to inject a precise amount of atomized and pressurized fuel into each engine cylinder at the proper time. Combustion in a diesel engine occurs when this rush of fuel is mixed with hot compressed air. (No electrical spark is used as in a gasoline engine.)

The fuel system consists of the following components. 


There are many different types and shapes of fuel tanks. Each size and shape is designed for a specific purpose. The fuel tank must be capable of storing enough fuel to operate the engine for a reasonable length of time. The tank must be closed to prevent contamination by foreign objects. It must also be vented to allow air to enter, replacing any fuel demanded by the engine. Three other tank openings are required--one to fill, one to discharge, and one to drain.


There are three types of diesel fuel lines. These include heavyweight lines for the high pressures found between the injection pump and the injectors, medium weight lines for the light or medium fuel pressures found between the fuel tank and injection pump, and lightweight lines where there is little or no pressure.


Diesel fuel must be filtered not once, but several times in most systems. A typical system might have three stages of progressive filters--a filter screen at the tank or transfer pump, a primary fuel filter, and a secondary fuel filter. In series filters, all the fuel goes through one filter and then through the other. In parallel filters, part of the fuel goes through each filter.

For more information on fuel filters, see Diesel Fuel Filter Basics.


Simple fuel systems use gravity or air pressure to get fuel from the tank to the injection pump. On modern high speed diesel engines, a fuel transfer pump is normally used. This pump, driven by the engine, supplies fuel automatically to the diesel injection system. The pump often has a hand primer lever for bleeding air from the system. Modern injection pumps are almost all jerk pumps which use the plunger and cam method of fuel injection.


There are four primary systems for injecting fuel:

1.      Individual pump and injector for each cylinder

2.      Combined pump and injector for each cylinder (unit injector type)

3.      One pump serving injectors for several cylinders (distributor type)

4.      Pumps in a common housing with injectors for each cylinder (common rail system)

The common rail system is rapidly gaining popularity for on-road applications. The in-line and distributor types are used on off-road vehicles and industrial machines.

High Pressure Common Rail Fuel System Diagram



Diesel fuel injectors are arguably the most important fuel system component. The job of the injectors is to deliver a precise amount of atomized and pressurized fuel into each cylinder. Highly atomized, pressurized fuel distributed evenly throughout the cylinder results in increased power and fuel economy, decreased engine noise, and smoother operation.  

Modern diesel fuel injectors, such as those found in common rail fuel systems, use piezoelectricity. Piezoelectric injectors are extremely precise and can handle the very high pressures found in common rail applications.  


The fuel used in modern high speed diesel engines 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, more commonly known as ultra low sulfur diesel (ULSD). 

For more information on diesel fuel, see Ultra Low Sulfur Diesel Basics.


A common enemy of diesel fuel systems is water. Unfortunately, water is more common in diesel fuel than most people realize. Should water find its way into an injection system, it will rapidly oxidize ferrous metal (steel) components. Some of the most common failures attributed to water include:

  • Injection component seizure
  • Sticky metering components in both the pump and injector
  • Governor/metering component failure


A diesel fuel system is a critical component of any diesel engine and its optimum operation is essential for peak performance. E-ZOIL manufactures several additives formulated to address common issues encountered by the diesel fuel system. DIESEL AID, DIESEL AID + CETANE, ARTIC POWER, ARTIC FLO and CLEAN & LUBE increase fuel system lubricity preventing premature failure of fuel pumps and injectors.  Additionally, DIESEL AID and DIESEL AID + CETANE totally disperse water, which also causes premature failure of fuel system components.