Today, the most common type of fuel system used on passenger cars with diesel engines is Common Rail. It is also found in trucks.
Below will be given a brief description of this system, its technical features and nuances of operation of fuel equipment from different manufacturers, and the release of Common Rail is engaged in brands Denso, Delphi, Bosch and a number of others.
Maximum simplicity of Common Rail
Against the background of all previous fuel supply systems, Common Rail has a minimum number of parts, and as a result it is much more reliable and convenient in the process of repair.
Fuel pumping in most cases is carried out by means of an electric pump mounted directly in the tank of the car. Rarer schemes with the location of the element under the bottom of the car or in the underhood. Autos without such a pump are very rare. As an alternative or in addition, the diesel can receive a mechanical pump for swap. Such a scheme is used on systems from Denso or Delphi, and Siemens often integrates the functionality of fuel pumping directly into the fuel injector. For Common Rail from Bosch, the mechanical pump is placed together with the fuel pump, but not inside the housing, but outside. If we talk about the newest models of cars, then here immersed pumps directly in the tank are widespread.
The Common Rail fuel injector is needed only for the formation of the necessary working pressure in the system. It does not participate in the distribution of fuel flows to the injectors, does not control injection and other actions. From the point of view of its involvement in the processes, it is the simplest system responsible only for fuel supply, no “smart” functions are provided. Hence the much lower number of breakdowns.
Fuel injectors of the first generation, depending on the company-manufacturer, could differ in drives and plungers. The most unusual designs were offered by Denso and Delphi. For example, the Denso HP2 model provides a pair of high-pressure sections and a pair of plungers for each. The rotation of the moving elements is provided by an ellipse. For the Delphi HPF, the mobility of the plungers is provided by a rotating cage with a cam profile.
Bosch and Siemens have not created such complex designs, their pumps are more reminiscent of star-shaped aircraft engines. In them, the plungers have a radial arrangement supported by a push-pull coupling. This has an eccentric inside it, which causes the plungers to be accelerated alternately. Later, this type of plunger operation became standard for other manufacturers of Common Rail equipment. Accordingly, on the newest cars, the design of the fuel injector does not have serious differences, no matter which company has not released it. Among the latest engineering improvements, for example, on Denso DFP6/Bosch CP4 pumps eccentrics are replaced by cams. The number of plungers is also reduced, and most often there is a single discharge section in the pump.
Regulating Valves
All Common Rail manufacturers use control valves. Most common is the scheme with two such regulators, one of which is located directly on the fuel injector, and the other on the fuel rail. Such a scheme is used by Bosch for its fuel equipment. They work simultaneously depending on the mode of operation of the power unit. The valve on the ramp is necessary to eliminate excessive pressure by withdrawing part of the fuel into the “return”.
All manufacturers use the valve on the housing of the fuel injector, its main task is to control the volume of fuel in the plungers, as well as partial elimination of excess pressure in the system. As a result, the ECU through the control valves changes the performance of the fuel system. PWM signals are used for control.
Bosch and Denso companies sometimes do not put a regulator on the fuel injector, replacing it with an emergency valve, which has a mechanical control device and relieves excessive pressure in the system. But only the reliability of this valve is minimal, it is enough several failures to require its replacement with a new one.
Delphi company in some cases did not install any overpressure regulators on its fuel racks. In them, pressure equalization is carried out directly through the injectors.
Other sensors
Depending on the specific type of diesel engine, its own control system is used. In fact, it is Common Rail that becomes the central element of the entire system, in connection with it and depending on it are the starter, ECU, gas gas pedal and other elements. As a result, in the process of repair, it will not be possible to combine elements of fuel systems of several manufacturers. For example, Bosch injectors will not work with the pressure regulator from Delphi. Exceptions are cases when initially at the stage of creation of the car developers use in the fuel system elements from several manufacturers, the same Delphi and Bosch.
Fuel injectors
Identical in design for the Common Rail system are injectors, whoever their manufacturer is. It is accumulated in a compressed state in the ramp, from where it is fed into the cylinders. Accordingly, in this system, the injectors do not participate in the compression or pressure regulation processes.
After entering the injectors, the fuel is divided between two channels. The first channel ends with an atomizer, the second with a control chamber. A control piston is located in the space between them. In the closed state of the injector, pressure from both sides acts on it.
The engine ECU gives a command, after which the locking mechanism is opened and the fuel is discharged from the control chamber into the “return”. Under the action of fuel pressure, the atomizer needle is lifted, and the fuel is in the combustion chamber. Injection is stopped by disconnecting the voltage supply to the injector, the locking element returns to its original position under the influence of a mechanical spring. Fuel enters the control chamber and the atomizer is closed.
This principle corresponds to the Common Rail system of passenger cars, regardless of the equipment manufacturer.
The main advantages of the system
Today there is simply no more effective fuel system for passenger cars with diesel than Common Rail. What are its real strengths? Firstly, the electronic control contributes to flexibility, when the injection torque is selected by the system in accordance with the actual load on the power unit. In general, the system is completely independent in performance from the speed at which the diesel engine is running. As a result, maximum injection values can be achieved even at low speeds. Other advantages of the system include fast performance and lower energy consumption. The three-plunger fuel injectors used in the system require nine times less energy than systems that utilize distributor-type fuel injectors.
Number of injections
There are several stages in the fuel injection process. First, pilot injections (up to 2). They are done as early as possible, but this has its purpose. The first portions of fuel are minimal in volume, their ignition occurs before the main ignition, warming up the chamber, and providing optimal pressure for better combustion of the main portion of fuel entering the chamber. Fuel ignition in a cold chamber helps to cool the gases, which will result in slower ignition.
The presence of the necessary pressure also optimizes the combustion processes in terms of smoothness of reaching the working pressure of gases, which in turn optimizes the smooth running of the power unit, reduces its noise.
The main injection provides for a minimum delay, and its duration is determined by the ECU taking into account the actual loads. For Common Rail, the fuel injection time reaches up to 36° RPM after TDC. This is largely responsible for the increased torque characteristic of diesel engines.
In the process of afterburning the residual fuel, the system performs another, smaller injection. It is necessary for afterburning, soot formed during the combustion process. The fuel injection in the lower piston position is designed to burn through the particulate filter. Its combustion heats the exhaust gases entering the filter through the exhaust system.
Disadvantages of Common Rail
For all its advantages, the system is not devoid of certain weaknesses and moments, to which it is necessary to pay increased attention in the process of operation.
Common Rail elements are characterized by a minimum distribution of friction pairs, but at the same time the system itself is very dependent on the quality of diesel fuel. The presence of impurities or insufficient cleaning from sulfur will cause accelerated wear of friction pairs. The injectors suffer from such fuel, as a result of which characteristic grooves appear in the seat of the locking element and the atomizer wears out quickly.
In case of wear Common Rail injectors begin to give out a significant part of fuel into the return. They simply cannot hold it in the control chamber. As a result, fuel flows into the return pipes at all stages of the injector’s operation. With a significant degree of wear of the injectors, the atomizer begins to suffer, in connection with which its holes suffer, it is possible that leaks appear or simply jam.
Disadvantages of the fuel injector
For the fuel injector is also characterized by dependence on the quality of fuel, but in this case accelerated wear comes much slower. The section of the pump responsible for pumping the fuel suffers most.
Example, Delphi DFP1. In this pump, the swap section is complex in design, the number of blades inside is four. The presence of moisture in the fuel or solid contaminants is simply destructive. In the section appears in large quantities of metal chips, spread throughout the system by fuel. It should be noted that it is the Delphi equipment has always had the most stringent requirements for fuel, so the original filters are mandatory.
A similar swap design has a model of the Denso HP2 fuel injector. At the same time, it is a little more resistant to chip formation due to low-quality fuel. A similar problem has been observed with the Denso HP3, which uses a gear swap pump.
Problems with chips were also observed in Siemens, perhaps only fuel pumps from Bosch demonstrated excellent resistance to the consequences of working on poor quality fuel, even though they did not completely exclude the appearance of metal. Unfortunately, this is characteristic only for models CP1 – CP3, in the newer CP4 fuel pump the problem is extremely acute, the air in the fuel leads to a violation of the position of the plunger roller, after which begins active friction of metal with all the ensuing consequences.
It is necessary to take into account the better service maintenance of Common Rail of the last years of production, for example, fuel filters are changed according to the manufacturer’s terms. It is necessary to observe the technology of replacement, for example, to pump fuel with a diagnostic scanner, it is reflected in the instructions, and not by “folk” methods. Otherwise, the repair of the fuel system will cost the owners a serious amount.
When it is necessary to appeal to the experts of the service station?
So how to understand that there are malfunctions in the operation of the fuel system? It is worth seeking the help of fuel system specialists in the following cases:
- engine does not start, but the starter turns;
- on cold starting is long and problematic;
- on hot there are also problems with starting;
- the idle speed is uneven;
- exhaust gases have a blue color with a smell of diesel at low speeds;
- the presence of black soot in the exhaust gases;
- unexpectedly stalls the engine when working with a load;
- appearance of fuel pressure errors.
Possible problems of the fuel system
Immediately it must be said, the inability to start the diesel is often associated with the lack of fuel supply, and it is caused by low pressure in the system (a minimum of 200 bar is required). In turn, the pressure is formed by a full-fledged fuel pumping. In rare cases, car owners may encounter a breakdown of the pressure regulator or the appearance of a ramp cut-through.
Unstable operation of the diesel engine can indicate a weak fuel pressure in the ramp. In most cases, this is due to excessively large volumes of diesel fuel ejected into the return line. This is especially true for Denso injectors if they have significant wear.
The presence of blue smoke indicates worn atomizers, but the smoke of black color on the deficit of air in the mixture, while there are no problems with the compression of the engine. It is possible that it appears as a result of poor quality work of injectors.
Engine shutdown under loads can occur due to excessive pressure, when the power unit goes into emergency mode. Or the opposite situation, the pressure is not enough. Such shutdown is characteristic of motors that are not equipped with emergency pressure equalization valves. The appearance of information about overpressure indicates an error in the regulator. It will have to be replaced.
Rarely for diesel engines with Common Rail you can meet the situation when the electronics does not produce any errors, but at the same time the engine smokes, does not produce power. Most often it happens after reinstallation of injectors and is due to mistakes of masters. The use of refractory washers of greater thickness or several washers leads to the atomizer moving upwards, as a result of which the fuel just misses the combustion chamber. At the same time, such “masters” manage to tighten the pressure plates so much that the high-pressure tube fasteners suffer. In general, the problem is purely “human”, not factory.
Remountability
Much during the operation of fuel equipment depends not only on the degree of its reliability, but also repairability. Here it is necessary to take into account the availability of appropriate technical capabilities of the car service. If we talk about Bosch injectors (electromagnetic), they are well repairable, original spare parts are available, technologies of repair works are worked out.
For piezo injectors of the German manufacturer with original spare parts, but available on the market analogs have quite decent quality. Piezo elements are absent on the market completely, so in case of its failure will require a complete replacement of the nozzle or an attempt to find a second-hand element at the scrap yards. The situation is similar with Delphi injectors, and both types, piezoelectric and electromagnetic.
Sufficient repairability have and injectors from Denso. Original spare parts will cost a penny, but there are good analogues on the market, and one of the features of the company is an open access to repair technologies, so if you want in the service station you can organize repair of injectors according to the manufacturer’s technologies. Of course, there are certain peculiarities and nuances, for example, binding to a particular region, but this does not prevent the proper approach to ensure quality repair.
There are no problems to find parts for Siemens/Continental injectors, and recently they have been offered on the market only in the version with piezo elements. They are fully repairable, you can use original spare parts or their analogs, but no one produces piezo elements for injectors of these manufacturers.
In the process of repair of injectors is provided for their adjustment, bench testing, generation of correction code. In normal workshops, specialists should successfully solve issues with linking injectors to ECU after their repair. At the same time, each case has its own nuances.
In general, for Common Rail systems, only injectors and fuel injectors can be repaired. Regulators can be cleaned (remove chips), but this repair technology does not give a significant effect. The only exception is regulators installed by Denso company on HP3 fuel injectors. It provides for the possibility of disassembly and polishing of the spool to improve the efficiency of the fuel system and the stability of engine operation.
Fuel Correction
One of the procedures often used to optimize diesel engine performance is fuel correction. It provides an assessment of the condition of the engine or fuel system based on data from the least worn of the cylinders. In modern autos, the ECU receives cylinder performance data from the crankshaft position sensor. Each ignition of fuel is accompanied by acceleration transmitted to the crankshaft. But at the same time, the performance figures are also dependent on the condition of the elements, both injectors and cylinders. This is where fuel correction comes in handy.
The procedure is carried out only at idling, in the presence of load on the engine, the correction values tend to zero.
What is fuel correction? It is a procedure for controlling the fuel supply in order to equalize performance between cylinders. In the optimal state, the correction value is 0 when the engine is idling. This statement is true for all Common Rail manufacturers except Siemens/Continental. Depending on the size of the power unit, the correction size can reach 5-6 units. For Siemens equipment, it is common practice to correct values in percentages.
A “-” correction indicates” a decrease in fuel delivery, and a ‘+’ correction indicates the need to increase it. To see the real performance of the cylinders is necessary during the operation to request a printout of data on the cylinders at the service station or (at least) demonstrate the parameters on the screen. ECU correction is carried out, that is, a standard diagnostic scanner is quite sufficient to reflect all the adjustments made by the electronics.
Which system would be better?
Even ten years ago, when considering Common Rail, it was possible to form reliability ratings by manufacturing companies engaged in the production of such equipment. Today such questions have lost their relevance. There are no fundamental differences between manufacturers. Regardless of the company, Common Rail are confidently repaired, and prices in most cases remain quite affordable and similar in size.
In their time, the developers successfully coped with the childhood diseases of the first Common Rail. In recent years, there have been much less problems with sales of low-quality fuel in Russia, and the quality of service has improved. By the way, it is an important element of successful operation of fuel equipment, especially in the context of changing fuel filters.
If any problems arise in diesels with such a system, they are caused by improper operation and ignoring the simplest rules of equipment maintenance and servicing.