Injection System - Fuel Circuit

Injection System - Fuel circuit


The injection system is complex, composed by several components that work together in order to bring the fuel, diesel or gasoline, to the combustion chamber of the engine.


Considering a current Diesel system, contrary to what is usually assumed, the injection system is not only composed only by the injection pump, common rail and common rail injectors. The injection system is much more complex and can be divided into two secondary circuits, a low pressure circuit and a high pressure circuit.


Fuel circuit

In Figure 1, a schematic representation of a complete fuel circuit is shown, in this case an heavy duty system from Bosch.

Picture 1 - Codular common-rail system CRSN25


As previously mentioned, the fuel system can be divided into two sub-circuits.


In Image 1, represented by the blue lines and with an “L”, is identified the low pressure circuit. This includes all components from the fuel tank to the supply pump and the return lines of the high pressure pump, of the common rail and of the injectors.


Represented in red and with an “H”, is the high pressure circuit. It includes the high pressure pump, the common rail and the injectors.


Briefly, the function of each of the components is described below.

Common rail - Distribute fuel evenly and at a stable pressure to the injectors, attenuating pressure spikes.

Pressure control valve - Limits the pressure inside of the high pressure rail;

Injector - Spray the fuel into the combustion chamber, atomizing it;

High pressure pump - Increase the fuel pressure;

Common rail pressure sensor - Reads the pressure inside of the common rail;

Suply pump - Supplies fuel to the high pressure system;

Pre-filter and Fuel filter - Filters the fuel for the suply pump and high pressure circuit;

Fuel tank - Stores fuel;

Engine control unit - Receives data from the sensors and actuators in order to control the engine and its components.


Maintenance and prevention

The automotive sector has undergone numerous changes over the years, technological advances have been enormous and in a few years vehicle systems have been completely transformed. As far as the Diesel injection system is concerned, the maximum working pressures have increased from 200 bar in axial piston rotary injection pumps to around 3000 bar in common rail pumps, an increase in maximum operating pressure of almost 15 times.

The highest operating pressures used are only possible by placing the materials used under much higher forces and using extremely tight geometric tolerances.

In simplified form, the pressure in a system is calculated by the following mathematical expression:



Therefore, maintaining the same contact area within the pressure generating mechanism (high pressure pump), to move from 200 bar to 3000 bar, the pistons / elements that compress the fuel must now withstand a force 15 times greater.

With this in mind, it is inherent that car maintenance is increasingly important. As far as the fuel system is concerned, the points to be aware of are simple.

  • LD Auto recommends that the maintenance plans recommended by the manufacturers are always followed;
  • Use good quality fuel filters;
  • The fuel is responsible for lubricating the moving parts of the system, so it is important to use quality fuel that is free of contaminants such as water or particles.

In case of contamination, to avoid serious problems and repairs that require the replacement of system components, if diagnosed in a timely manner, the components of the fuel system can be repaired. It is then important to eliminate the source of the problems and clean the entire fuel circuit, including tank and piping.


Most frequent breakdowns - Internal components

So far the focus has been on the high pressure pump because it is the element that raises the pressure in the circuit and without it, none of the other components can perform its function.

The following are real failures of some components of a Bosch injector with a magnetic valve, guide, nozzle and needle. Finally, one of the causes that can cause the appearance of leaks in a common rail injection pump of a light vehicle is presented.


Valve assembly - Guide

The guide is one of the components responsible for the operation and return of the injector. Pay attention to the contact area between the two parts of the component, marked in blue, but pay special attention to the condition of the upper part of the guide, marked with the red arrow in Figure 2.

Image 2 - New and damaged valve set


In Image 2, in the first set of 3 photographs, a guide in very bad condition is presented. In the photo on the left it appears that the guide is heavily contaminated with waste and the material is oxidized. The central photograph shows that the guide already has a high wear on its entire surface, being particularly accentuated in the concave area where the sealing is made by the sphere. It is normal for a used guide to have greater wear in the work area of ​​the ball, however it is not tolerable to be as sharp or irregular as it happens in this situation. It is also visible the presence of cracks that cross the fence area, marked in red in the third photo. A bad guide can give rise to several symptoms, the most common being the excess return in the injectors.

In the last set of photographs in Image 2, a guide in good condition is shown. The component's various zones are regular and the critical zone, marked with a red arrow, indicates the seat where the sphere rests. This zone is critical, so wear must be minimal and irregularities are not permitted.


Injector nozzle

The injector nozzle is one of the components responsible for the amount of fuel injected into the combustion chamber, but also for the geometry of the jet that is sprayed. So it has an influence not only on the vehicle's consumption, power but also on the vehicle's emissions.

Image 3 - Injector nozzle


In Image 3, injector nozzles with different problems are visible. On the left side are two deformed nozzles, depending on the level of deformation (external and internal), the defect can be only aesthetic or also functional, affecting the functioning of the component. On the right side there is a damaged nozzle and with the holes deformed, the symptoms of the vehicle can be varied, from making too much smoke due to excessive flow or deficient spray and weak fuel atomization, noisy injection, in the opposite scenario it may lack power , or even prevent the vehicle from starting or operating.


The injector nozzle has a second component inside it, the needle, shown in Image 4.

Image 4 - Injector Needle


In the photo below in Image 4, the appearance of a needle with severe signs of seizure due to poor lubrication and contaminated fuel is visible.

As far as injection pumps are concerned, a problem encountered with some regularity is the lack of tightness of the component.

Image 5 - High Pressure Pump CP1


Image 5 shows a Bosch common rail pump with this problem. With the operation of the pump, especially at high temperatures, the O’ring that seals the fuel line degrades as seen in the photo on the right side causing leaks. Later generations have a fuel shut-off valve for one of the pump elements, which makes it possible to reduce vehicle consumption in situations of lower load, while also lowering the vehicle's operating temperature.


Suggestions - Diagnosis of the low pressure system


The analysis of Image 1 shows that the low pressure circuit has a large number of connections and components, however its analysis in diagnosis is often neglected. However, since the low pressure system is upstream of the high pressure system, its proper functioning is directly related to the correct functioning of the high pressure circuit and motor. In the vast majority of failure situations, the engine control unit does not associate the errors recorded with the low pressure system components, almost always pointing to high pressure circuit components, even when their malfunction is due to insufficient power or other problems upstream.


In the event of malfunctions in the high pressure system, LD Auto recommends that the low pressure circuit function is checked first. In order to verify that the high pressure pump is being properly supplied, the following checks must be carried out:

  • Fuel filter condition, look for residues, swarf, water;
  • Ensure that the entrance to the tank deposit, known as “sucker”, is not obstructed;
  • Check that there is no air in the system;
  • Measure the circuit pressure before the high pressure pump, with some exceptions, the pressure must be between 2.0 bar and 3.0 bar.


Suggestions - Diagnosis of the high pressure system


Ensuring the operation of the low pressure fuel circuit, if the problem persists, the components of the high pressure circuit should be analyzed. In this stage, it is extremely important to have a good knowledge of the system, to know characteristic values ​​and the influence that each parameter has on the others.


The following are some common symptoms and checks that must be carried out in order to diagnose the source of the problem. However, we must bear in mind that each vehicle is a particular case and that the diagnosis must be adapted to each situation.


In addition to the good knowledge of the system, in more complex situations, the knowledge and advanced use of the diagnostic machine is essential, as well as the reading and interpretation of the real values ​​of the system.


After intervention of a component, it has to be tested on a test bench. This is the only way to guarantee that the operating values ​​are correct in each operating condition and that there are no imperceptible problems to be identified only with a visual analysis.