Common rail injection

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Common rail system on a truck engine
Common rail system on BMW engine N47D20

In the common rail , which also memory injection is called, is injection systems for internal combustion engines , in which a high-pressure pump brings the fuel to a high pressure level. The pressurized fuel fills a pipe system that is constantly under pressure when the engine is running.

Origin of the term

The term common rail comes from English and stands for common distribution pipe . It describes the use of a common high-pressure fuel reservoir, usually in the form of a pipe, to which the injection nozzles (injectors) are connected to supply the cylinders with fuel.

scope of application

Injection in the common rail process

The basic idea is the complete separation of pressure generation from the actual injection process. In this way, an injection controlled exclusively by maps is possible. Injection timing and injection quantity are regulated by an electronic engine control . This electrically actuates a valve per cylinder (individual cylinder), the so-called injector , which replaces the conventional injection nozzles of classic diesel units.

The injection process at the end of the compression stroke is divided into three groups:

  • the pilot injection; up to two pilot injections are possible for smooth engine operation
  • the main injection, which can still be subdivided into the first and second main injection
  • and the post-injection, which can be used to reduce the formation of soot inside the engine, for a low NO x value in SCR catalytic converters or to burn off diesel particulate filters .

Differences to classic injection

Engines with in-line or distributor injection pumps have a separate high-pressure line for each cylinder between the injection pump and the injection nozzle . These high pressure lines are not connected to one another. Injection at the nozzle into a cylinder is triggered directly by an associated pumping process of the injection pump.

With the classic injection pump (in-line pump , multi-piston pump , i.e. one pump element per cylinder), the injection quantity and duration, i.e. the height of the effective piston stroke of the injection pump, does not depend on the crank angle, because the pistons are rotated with the accelerator pedal position and have a different effective stroke obtained by a circumferential sloping edge (= control edge) on the piston when the accelerator pedal is not depressed the high-pressure delivery starts later or ends earlier. This means that less fuel is pumped and injected with little torque requirement. The construction principle of the in-line and distributor pumps allows only one injection per work cycle; The beginning and end of the injection are determined by the control edge geometry and can be moved together by an injection adjuster if necessary.

It is different with common rail technology: Here the injection quantity and duration can be electronically controlled independently of the crank angle and thus pre, main and post injections are also possible; As of 2012, up to eight separate partial injections can be implemented per work cycle of the engine. The pre-injection is primarily used to reduce the combustion noise, the post-injections are used to reduce internal engine particles or to increase the exhaust gas temperatures in the burn-off cycles if the pressure loss of the fine dust filter (soot particle filter) in the exhaust system is too high.

Shortly before the triumphant advance of common rail injection systems, distributor injection pumps (BOSCH VP44 radial piston pump and VP30 and VP37 axial piston pump) were also equipped with high-pressure solenoid valves for volume metering. This technology makes it possible to influence the injection process, which is directly linked to the crank angle, during the compression phase of the fuel through the valve and to cause a pulsation in the fuel column between the pump piston and the injection valve during a piston stroke. This made it possible to implement up to three injection processes per work cycle in distributor pump technology. However, the possible degrees of freedom of a common rail system were not achieved.


In 1937 Prosper L'Orange developed ideas for direct injection in diesel engines . Common-Rail emerged from research at the ETH Zurich between 1976 and 1992, but has not yet been used on a vehicle there. By continuously pumping diesel fuel into a central pressure pipe, a high injection pressure of over 1000 bar is generated. This common distribution pipe (common rail) serves as a reservoir for all injection valves.

In the 1970s, a development department at the Aken injection device factory, WTZ Dieselmotoren Roßlau and SKL Magdeburg worked on an electronically controlled diesel injection system (EDES) for stationary diesel engines. Since the rail pressure hardly changes over the duration of an injection, the term constant pressure injection was initially used. At the Leipzig trade fair in 1981, the SKL presented the full engine 6 VDS 26/20 ALE-2 as a common rail system after successful endurance testing over several thousand hours. The documentation at hand shows that the engine was also operated with light heavy fuel oil (36  cSt ). The fuel consumption was reduced by 9 g / kWh and the exhaust gas opacity to 60%. The electronic control worked with remarkable speed and precision.

The world's first common rail diesel engine for a road vehicle was the MN 106 engine from VEB IFA Motorenwerke Nordhausen, which was installed in a modified IFA W50 truck . On May 16, 1985 the W50 drove for the first time on a public road in Chemnitz. 17,000 km had been covered by 1986. In the GDR the system was called EDES ("Electronic Diesel Injection System"). The development was canceled due to a lack of financial resources. After a restoration, the engine was exhibited in the August Horch Museum until March 2014 . The engine has been on loan at the IFA Museum Nordhausen since 2014.

In the 1980s, based on the results of the ETH, the preparation of the Unijet common rail system began. The system was developed by Magneti Marelli , Centro Ricerche Fiat and Elasis until 1993. Problems with the tolerances of the injectors initially prevented planned series production. Bosch bought the patents at the end of 1993 and developed the common rail system to make it ready for series production. Ten years after the first car with direct injection ( Fiat Croma TD id), the first road vehicle with common rail injection came onto the market in October 1997: the Alfa Romeo 156 JTD . Shortly beforehand, in September 1997, MTU's BR4000 engine series for rail and off-highway applications with a common rail system from L'Orange went into series production.

In 1998 Daimler-Benz followed as the first German manufacturer with the OM 611 . BMW also offered a common rail engine with the BMW M57 . In the same year, Citroën also started development and introduced its own system with the C6 .

The PSA-Peugeot-Citroën Group , in cooperation with Siemens, brought the first piezo injectors onto the market. With the short reaction times of piezo technology, the injection times can be controlled more precisely and more quickly. Up to eight injections are possible per combustion process. This further enhances the combustion process and the acoustic running properties, the engine achieves lower emission values ​​and lower consumption with the same output.

The most important suppliers of common rail systems today are Bosch , L'Orange , Delphi , Denso , Magneti Marelli and Continental .

Derived Systems

The Mitsubishi Carisma GDI (gasoline direct injection) was the first series production car with stratified direct fuel injection to hit the market in 1997.

Among other things, VW (with the supplier Bosch) is one of the representatives of gasoline direct injection in the case of cars with Otto engines . Here, too, a fuel rail (common rail) supplies the electrically operated high-pressure injection valves located under the intake port in the cylinder head. However, at 200 bar (20 MPa ), the fuel pressure is relatively low compared to diesel injection. In recent years, a large part of the VW / Audi engine range has been converted to the technology called FSI or TFSI .

The diesel common rail system

System overview
Common rail high pressure pump

The diesel common rail system is known as accumulator injection. A high-pressure pump constantly maintains the fuel pressure in the distributor pipe. It is usually mechanically coupled to the engine. The performance of the high-pressure pump is designed so that more fuel can be delivered at any time and in any operating state than the engine needs. This means that the pump is oversized for normal operation. A pressure control valve is used to regulate the pressure in the case of unregulated pumps, which relieves the unneeded amount of fuel from the distributor pipe to ambient pressure and returns it to the fuel tank. As a result, the fuel at the pressure control valve can heat up to 140 ° C or more, which can damage or destroy fuel-carrying parts and make it necessary to use a fuel cooler. The main disadvantage of this system with an unregulated pump and pressure control valve, in addition to the fuel cooling that may be required, is the high power requirement of the pump, which always delivers the maximum amount of fuel.

High-pressure pumps with element switch-off represent an improvement. Individual pump elements of the high-pressure pump are switched off as long as the remaining active pump elements can cover the fuel requirements of the engine. In this system, the excess power of the pump is partially reduced, the remaining excess still has to be controlled by a pressure control valve.

Systems with so-called suction throttle control can be operated without a pressure control valve. With this principle, only as much fuel is fed to the high-pressure pump as is necessary to maintain the desired pressure in the rail. The energy requirement for generating high pressure is therefore as low as possible, and the elimination of the pressure control valve and the heat generated there, the use of a fuel cooler can be avoided.

The combination of the suction throttle control with a pressure control valve on the high pressure side enables performance-optimized operation, a rapid pressure reduction in overrun mode and can also use the quantity controlled by the pressure control valve to heat the fuel (e.g. in winter).


Diesel fuel is compressible. The common rail system uses this compressibility to dampen the pressure surges from the stroke of the individual pump pistons. A larger storage volume enables a more uniform pressure with lower pressure peaks. However, this leads to a more sluggish system because the pump needs more time to set a different required pressure. The smoothing of the delivery pressure prevents a pressure wave peak from being active on one injector during its injection phase and thus more fuel being injected than specified by the map, while a pressure wave trough is active on another injector and therefore injects less. A fast system is required in order to inject the optimum amount of fuel even when the load and operating conditions change.

The pressure accumulator volume integrated directly in the high-pressure pump, the rail itself and, with various manufacturers, a pressure accumulator per injector, which is arranged as close as possible to the injector, serve as accumulators.

Achievable pressures

The rail pressure ( i.e. the pressure in the pressure accumulator) of a maximum of 300 MPa (3000 bar ) can be used for very high injection pressures .

Some manufacturers are also working on a pressure-boosted common rail system. The injection pressure is increased with the aid of a lower pressure in the pressure accumulator during the injection phase to pressures of currently up to 250 MPa (2500 bar) at the nozzle. The pressure intensification is carried out by a hydraulic pressure intensifier with control functions integrated in the injector. The principle is also known as the Amplified Pressure Common Rail System (APCRS). The lower pressure load on the high-pressure pump, which only has to provide the lower supply pressure in the rail, is advantageous here. This means that the pressure-dependent leakage losses in the pump and injector are also lower. Disadvantages are the required higher delivery rate of the high pressure pump and additional hydraulic losses due to the pressure intensification, as well as the higher expenditure due to more complex injectors. Such a system from Bosch has been in series production for commercial vehicle engines from Daimler Trucks since 2011.

Purpose and benefits

  • Common rail injection optimizes the combustion process and the engine running properties and reduces particle emissions . Due to the very high pressure, the fuel is very finely atomized. Small fuel droplets have a large surface area in relation to their volume. On the one hand, this favors the speed of the combustion process and, on the other hand, a low particle mass in the emissions. As a disadvantage, the percentage of small particles is greater, which contributes to the fine dust problem.
  • The high-pressure pump, driven by the combustion engine, brings the fuel provided by the pre-delivery system (in current systems in passenger cars usually an electric pre-delivery pump, in trucks usually a mechanical pump) from the tank to the required injection pressure in the pressure accumulator, as specified by the control unit. The injectors (injection nozzles) are connected to the common high-pressure distribution pipe (fuel rail) and inject the fuel directly into the combustion chamber.
  • Common rail injection has structural advantages. On the one hand, there is the decoupling between pressure generation and injection control compared to injection using an injection pump or pump-nozzle system : with a CR system, the injection time can be freely selected. On the other hand, less attention has to be paid to the position of the existing auxiliary drives ( toothed belt , timing chain , etc.) for the pressure generator unit.

Injection performance

In order to permanently maintain the high rail pressure, a certain pumping capacity must be applied by the engine, depending on consumption.

Illustrative example for calculating the hydraulic power of the injection processes

Given is:

  • Average consumption: 5 dm³ per 100 km at 160 km / h, which corresponds to 8 dm³ in one hour.
  • Common Rail pressure: 1800 to 2200 bar
  • The limit value is 1600 to 2500 bar. Higher or lower values ​​can usually be traced back to:
    • Motor cold or too warm
    • Underload or idle
    • Injector very dirty
    • Injection volume not compatible with injection control unit

The required injection capacity results from the injection volume and the pressure increase.

The following applies to the injection performance:

in this:

  • Delivery rate, volume flow per unit of time
  • pressure
  • Drive power of the pump in watts
  • Efficiency, in practice always smaller 1

and used:

p = 2,000 bar = 200,000,000 N / m²
: 1 (simplified)

The average required power of 440 W (corresponds to approx. 0.6 HP) is by far compensated for by the increase in efficiency through direct injection (see there). It must be taken into account that internal leakages and return quantities are not taken into account and that a larger injection quantity and thus more power are required when accelerating. In contrast, no pumping power is required in overrun mode .


  • In some common rail systems, more fuel is pressurized at various operating points or even in the entire usable engine map than is required for injection, control and leakage. The excess amount is controlled by a pressure control valve and fed back into the fuel tank, which creates a high expansion temperature. The overall engine efficiency is reduced by this output; the temperature of the spill volume makes a more temperature-resistant fuel system and, in some cases, fuel cooling necessary. As countermeasures, high-pressure pumps with cylinder deactivation or needs-based high-pressure delivery with the use of a suction throttle solenoid valve are used. As a result, fuel cooling can usually be dispensed with because of the lower return flow into the tank.
  • The permanent high pressure can lead to continuous injection in the event of a malfunction of the injection valve (jamming or contamination of the nozzle or control valve). The valves and the exhaust system can be thermally overloaded as a result, with the risk of major engine damage or even an engine fire. In the case of classic systems or pump-nozzle systems, this risk is considerably lower because the high pressure is only temporarily applied. In the case of large engines, there is a safeguard against this malfunction by means of flow control valves, which prevent continuous injection and thus destruction of the engine and allow the engine to continue operating with the remaining cylinders.

Injection control

The opening of the injectors ("injection nozzles") is not triggered by the fuel pressure as in distributor injection systems or slide valve in-line injection systems, but by electrical control, whereby the fuel pressure provides the essential force for lifting the nozzle needle. The injection process can be influenced by the duration and the current intensity of the injector control and extremely short opening times can be achieved, which enable one or more pre-injections before the main injection or one or more post-injections after the main injection. Pre-injections are also possible as a one-off process with distributor pumps that can be electronically influenced and with the pump-nozzle system. They preheat the combustion chamber, so to speak, and thus lead to an overall smoother combustion process for the following main injection. Furthermore, the nitrogen oxide formation can be reduced with the help of this pre-injection, since the pre-injection u. a. the maximum combustion temperature is reduced. In addition, the temperature rise over time is somewhat smaller, which is gentle on the engine components and reduces noise emissions. A post-injection following the main injection can reduce the particle emissions within the engine. Further downstream injections can be used to regenerate the particle filter.

The injection nozzles are actuated electromagnetically or piezoelectrically, controlled by the electronic engine control unit.

The control unit uses the signals from several temperature sensors (cooling water, charge air and lubricating oil), air mass meter, accelerator pedal position sensor, if necessary lambda probe, speed and phase sensor and rail pressure sensor to calculate the required injection quantity or injection duration and actuates the injectors with the corresponding control pulses for the start and duration of injection. In the most modern systems in particular, several pre-injections are used.

Both the injection times and the respective injection pressure and, in some cases, the time course can be set almost freely. This makes it easier to adapt to the respective operating state of the internal combustion engine.

In the meantime, one or more post-injections are also provided for burning off the soot particle filters in order to temporarily increase the energy content in the exhaust gases for the burning process.

Although the common rail system has made a big leap in terms of exhaust gas behavior and, in particular, the running behavior of diesel engines, this requires a much higher number of components, which results in very high demands on their reliability and increases the system complexity.


Road vehicles

Almost all car manufacturers now use the common rail system. Everyone has their own abbreviations . The Volkswagen Group , which for a long time relied on the competing pump-nozzle (PD) system, has largely switched to common rail. The competitive situation of the pump nozzle (Siemens VDO Automotive competed against Bosch ; since 2008 Continental Automotive Systems ) was expected to result in more lively competition and, above all, attempted the exhaust gas limit values ​​without using the PD system, which was initially technically superior in terms of the injection pressures that could be achieved Achieve particle filter. Pump-nozzle engines have a slight consumption advantage over common-rail engines with an unregulated high-pressure pump because no excess high-pressure quantity is generated.

Since 2007, the accumulator pressure of common rail systems has caught up with that of the PD system, and the increasing use made the system costs of the CR system fall. In addition, a maximum of three closely spaced injections per work cycle are possible with the pump-nozzle element, while the piezo injectors of the common rail system can implement up to eight injections that are further apart per engine cycle. Achieving the Euro 6 emissions standard (valid from September 1, 2014) is therefore impossible with the pump-nozzle system, and further development with this additional constructive and financial expense for a PD unit in the passenger car area was therefore uneconomical.

Pump-line-nozzle systems, however, can still be found in commercial vehicle engines from DAF and on the four-cylinder engines of the Mercedes-Benz Atego . In modern heavy commercial vehicles, common rail is now state of the art and is also used in large numbers in series production (example: MAN Truck & Bus ). Because of the use of passenger car diesel engines in light commercial vehicles, the proportion of common rail systems is also increasing here.

A common rail system can also be used for gasoline direct injection with significantly lower system pressures. The formation of vapor bubbles plays an important role here. Gasoline and diesel injection systems are also constructed differently with regard to the lubricating properties of components moving against one another, such as slide bearings, pump elements and coatings. Therefore, from a technical and economic point of view, standardization of the two systems is only possible with a few subcomponents.

There are several providers on the market. Important quality features of injection systems include speed of pressure build-up, efficiency, injection quantity deviation, controller quality, noise emissions and durability.

Large engines

In addition to its use in motor vehicles (high-speed engines), common rail injection is also used in large diesel engines, i.e. in four-stroke medium-speed engines and two-stroke low-speed engines that are used, for example, as marine diesel . The main application is shipping, where even highly viscous fuels - heavy oil called (Heavy Fuel Oil = HFO with viscosities up to 700 cSt at 50 ° C) - are burned. L'Orange, Bosch and Heinzmann are the only companies that also offer common rail technology for heavy oil engines. L'Orange has been producing for MTU Series 4000 engines since 1996 . Wärtsilä and Caterpillar (for the MaK brand) work together with L'Orange, MAN Diesel in Augsburg is developing together with Bosch.

Aircraft engines

In addition, common rail technology is also used in aircraft engines , for example in the Thielert Centurion 1.7 .


  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 28th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2238-0
  • Jan Drummans: The car and its technology. 1st edition, Motorbuchverlag, Stuttgart, 1992, ISBN 3-613-01288-X

Web links

Commons : Common Rail Injection  - Collection of Images, Videos and Audio Files

Individual evidence

  1. Hans-Jürgen Grönke : On industrial history in the southern Harz , Lukas-Verlag, Berlin, 2016, ISBN 9783867322232 , p. 355 ff.
  2. Press release Bosch
  3. The new heavy-duty engine generation from Mercedes-Benz . In: Mercedes-Benz Passion Blog . March 25, 2011 ( [accessed January 31, 2017]).