Common Rail Diesel Injection

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On diesel engines, The first common rail system systems featured a high-pressure (over 1,000 bar or 100 MPa or 15,000 psi) fuel rail feeding individual solenoid valves, as opposed to earlier low-pressure fuel pump feeding unit injectors (or pump nozzles).

Third-generation common rail diesels now feature piezoelectric injectors for increased precision, with fuel pressures up to 3,000 bar (300 MPa; 44,000 psi)

The following information should give you a basic insight into how the common rail diesel systems are constructed and how the system works. Most chip tuners ignore the facts of the system and decide to make changes anyway. By the time you read have this article you should be able to understand why messing around with injection protocols can turn things bad. See Also Chip Tuning - The Reality, The Myths and Everything Inbetween


Heres a list of Acronyms manufacturers use to Brand their common rail systems

  • Ashok Leyland: CRS (used in U Truck and E4 Busses)
  • BharatBenz: 4d34i (used in 914R and 1214R)
  • BMW: D (also used in the Land Rover Freelander as TD4)
  • Chevrolet: VCDi (licensed from VM Motori)
  • Cummins and Scania: XPI (developed under joint venture)
  • Cummins: CCR (Cummins pump with Bosch injectors)
  • Daimler: CDI (and on Chrysler's Jeep vehicles simply as CRD)
  • Fiat Group (Fiat, Alfa Romeo and Lancia): JTD (also branded as MultiJet, JTDm, and by supplied manufacturers as CDTi, TiD, TTiD, DDiS and Quadra-Jet)
  • Ford Motor Company: TDCi (Duratorq and Powerstroke)
  • Honda: i-CTDi and i-DTEC
  • Hyundai and Kia: CRDi
  • Isuzu: iTEQ
  • Jeep: CRD
  • Komatsu: Tier3, Tier4, 4D95 and higher HPCR-series
  • Mahindra: CRDe, DiCR, m2DiCR
  • Mazda: MZR-CD and Skyactiv-D (are manufactured by the Ford and PSA Peugeot Citroen joint venture) and earlier DiTD
  • Mitsubishi: DI-D (mainly on the recently developed 4N1 engine family)
  • Nissan: dCi (Infiniti uses dCi engines, but not branded as dCi)
  • Opel: CDTI
  • Proton: SCDi
  • PSA Peugeot Citroën: HDI or HDi (developed under joint venture with Ford) – See PSA HDi engine
  • Renault: dCi (joint venture with Nissan)
  • SsangYong: XDi (most of these engines are manufactured by Daimler AG)
  • Subaru: TD or D (as of Jan 2008)
  • Tata: DICOR and CR4
  • Toyota: D-4D and D-Cat
  • Volkswagen Group (Volkswagen, Audi, Seat and Skoda): TDI (more recent models use common rail, as opposed to the earlier unit injector engines)
  • Volvo: D and D5 engines (some are manufactured by Ford and PSA Peugeot Citroen), Volvo Penta D-series engines

Basic System Principles

Diagram of the common rail system

Solenoid or piezoelectric valves make possible fine electronic control over the fuel injection time and quantity, and the higher pressure that the common rail technology makes available provides better fuel atomisation. To lower engine noise, the engine's electronic control unit can inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing its explosiveness and vibration, as well as optimising injection timing and quantity for variations in fuel quality, cold starting and so on. Some advanced common rail fuel systems perform as many as five injections per stroke.[9]

Common rail engines require a very short (< 10 seconds) to no heating-up time, depending on ambient temperature, and produce lower engine noise and emissions than older systems.

Diesel engines have historically used various forms of fuel injection. Two common types include the unit injection system and the distributor/inline pump systems. While these older systems provided accurate fuel quantity and injection timing control, they were limited by several factors:

  • They were cam driven, and injection pressure was proportional to engine speed. This typically meant that the highest injection pressure could only be achieved at the highest engine speed and the maximum achievable injection pressure decreased as engine speed decreased. This relationship is true with all pumps, even those used on common rail systems. With unit or distributor systems, the injection pressure is tied to the instantaneous pressure of a single pumping event with no accumulator, and thus the relationship is more prominent and troublesome.
  • They were limited in the number and timing of injection events that could be commanded during a single combustion event. While multiple injection events are possible with these older systems, it is much more difficult and costly to achieve.
  • For the typical distributor/inline system, the start of injection occurred at a pre-determined pressure and ended at a pre-determined pressure. This characteristic resulted from "dummy" injectors in the cylinder head which opened and closed at pressures determined by the spring preload applied to the plunger in the injector. Once the pressure in the injector reached a pre-determined level, the plunger would lift and injection would start.

In common rail systems, a high-pressure pump stores a reservoir of fuel at high pressure — up to and above 2,000 bars (200 MPa; 29,000 psi). The term "common rail" refers to the fact that all of the fuel injectors are supplied by a common fuel rail which is nothing more than a pressure accumulator where the fuel is stored at high pressure. This accumulator supplies multiple fuel injectors with high-pressure fuel. This simplifies the purpose of the high-pressure pump in that it only needs to maintain a commanded pressure at a target (either mechanically or electronically controlled). The fuel injectors are typically ECU-controlled. When the fuel injectors are electrically activated, a hydraulic valve (consisting of a nozzle and plunger) is mechanically or hydraulically opened and fuel is sprayed into the cylinders at the desired pressure. Since the fuel pressure energy is stored remotely and the injectors are electrically actuated, the injection pressure at the start and end of injection is very near the pressure in the accumulator (rail), thus producing a square injection rate. If the accumulator, pump and plumbing are sized properly, the injection pressure and rate will be the same for each of the multiple injection events.

Overview of a common EDC16-C39 system (opel/vauxhall/saab/fiat/alfa)

The EDC-16C39 Common Rail system is a high pressure electronic injection circuit for fast, direct injection diesel engines.

Its main features are:

  • availability of high injection pressure (up to 1400 bar);
  • possibility of modulating pressures between 150 bar up to a maximum service pressure of 1400 bar, regardless of engine speed and load;
  • ability to work at high engine rpms (up to 5000 rpm in full load conditions);
  • injection control precision (injection advance and duration);
  • reduction in fuel consumption;
  • reduction in emissions.

Main system functions are essentially as follows:

Breakdown of system components

File:Edc16 overview.png
overview of edc16c39 components

1. Auxiliary fuel pump

2. Fuel filter

3. Fuel return manifold

4. Pressure pump

5. Pressure regulator on pump

6. Supercharging sensor

7. Pressure regulator on rail

8. Pressure sensor

9. Rail

10. Throttle body

11. E.G.R. solenoid valve

12. Oil level sensor

13. Timing sensor

14. Heater plug

15. Turbo vane actuator control solenoid valve

16. Flow meter

17. Rpm sensor

18. Water temperature sensor

19. Minimum oil pressure switch

20. Lambda sensor downstream of the catalytic converter

21. Lambda sensor downstream of the filter (DPF)

22. Differential pressure sensor

23. Filter (DPF)

24. Pedal assembly

25. Diagnostic connector

26 Injection control unit

27. Vehicle wiring

28. Engine wiring

29. Swirl throttle control actuator

System Components


cross section of a bosch Common Rail Injector cross section

The injectors are fitted on the cylinder head and controlled by the injection control unit.

The injector can be divided into two sections:

  • actuator/spray consisting of pressure rod (1), pin (2) and nozzle (3);
  • control solenoid consisting of coil (4) and pilot valve (5).

  • 1. Pressure rod
  • 2. Pin
  • 3. Nozzle
  • 4. Coil
  • 5. Pilot valve
  • 6. Ball plunger
  • 7. Control area
  • 8. Feed volume
  • 9. Control volume
  • 10. Fuel return - low pressure
  • 11. Control duct
  • 12. Feed duct
  • 13. Electrical connection
  • 14. Fuel return connector - high pressure
  • 15. Spring
  • 16. IMA code

Injector operation is divided into three stages.

  • REST POSTION - the coil (4) is de-energized and the plunger (6) is in the closed position to prevent fuel from being introduced into the cylinder Fc > Fa (Fc: due to line pressure acting on control area 7 of rod 1; Fa: due to line pressure acting on feed volume 8).
  • START OF INJECTION - the coil (4) is energised and causes the plunger (6) to rise.

The control volume fuel (9) flows out towards the return manifold (10) bringing about a pressure drop in the control area (7). Simultaneously, the line pressure flowing through the intake duct (12) exerts a force Fa > Fc in the supply volume (8) causing the pin (2) to lift and allowing fuel into the cylinders.

  • END OF INJECTION - the coil (4) is de-energised and causes the plunger (6) to return to the closed position. This rebalances the forces so that the pin (2) returns to the closed position and the injection ends.

IMA classification

During the test stage the injectors are checked with the specifications verified in different pressure/flow rate conditions. All the injectors that do not meet a certain standard are rejected; the remaining ones are classified using a 9 character alphanumerical code, known as an IMA code, imprinted by laser on the top part of the magnet.

During fitting on the vehicle the control unit should memorise the individual code and, if one or more injectors is replaced in a service situation, the alteration to the code should be made during the diagnosis using the appropriate equipment.

Reconditioned Injectors

  • Be wary of reconditioned/repaired injectors. When sending in old injectors to a diesel center for reconditioning, make a note of the old IMA NUMBERS, We still dont know to this day how you can recondition an injector and leave the IMA the same. As above, reconditioned injectors are cheaper than new ones, but also the fuel pressure/flow rates will be incorrect. This can lead to a number of problems, as knocking, smoking, incorrect cylinder balance etc etc.

High Pressure Fuel Pump

This is typical of a Bosch EDC16 system

The pump is radialjet type with three radial pistons and is driven by a toothed belt without the need to adjust timing

typical example of a bosch high pressure fuel pump


Because the pressure pump must be fed at a pressure of at least 3.5 bar, the fuel system is equipped with an auxiliary fuel pump submerged in the tank.

The maximum delivery pressure reaches 1600 bar.

The pressure pump is lubricated and cooled(1) by the fuel via special channels

  • (1) Take note of lubrication and cooling especially with cheap grade fuel, getting more common nowadays is, premature wear of the high pressure pump and also blocking of channels. Cheap/lower grade diesel seems to have less lubricating properties than quality fuel, First signs of premature wear of the pump would be presence of brass particles usually found in the fuel filter or housing. This is the first signs of the pump eating away at itself due to lack of lubrication and over heat. this can also cause problems with the fuel pump pressure regulator.
cross section of typical bosch high pressure fuel pump

The movement of the pistons is determined by the rotation of a triangular shaped cam on the pump shaft. This cam causes the movement of the three pistons in succession through the movement of a mechanical interface (tappet) between the cam and the foot of the piston. The contact between the cam and each individual tappet is ensured by means of a spring.

Each pumping unit is equipped with an intake valve and a supply ball valve. All the three supplies for the pumping elements are joined inside the pump and send the fuel to the shared manifold by means of a single duct.

There is a low pressure regulator solenoid valve on the pump to adjust the supply pressure at the pump intake in order to only compress the diesel fuel needed to reach the pressure mapped in the control unit

Pressure Regulator


This is fitted on the pressure pump and controlled by the injection control unit. It adjusts fuel feed pressure to the injectors.

The pressure regulator consists mainly of the following components:

  • a spherical shutter (1);
  • a valve (3) controlling pin (2);
  • a preloading spring (4);
  • a coil (5).
cross section of a bosch high fuel pressure regulator
  • 1. Spherical plunger
  • 2. Pin
  • 3. Valve
  • 4. Preload spring
  • 5. Coil
  • 6. Case
  • 7. Anchor

Fuel pressure sensor

It is fitted at the end of the of the single fuel manifold rail and its function is to provide the injection control unit with a feedback signal to enable it to:

  • modulate injection pressure;
  • regulate fuel injection duration.

As you can see just from this single sensor alone the dramatic effect that it has on fuelling each injection stroke, And why adjusting fuel pressure on top of injection duration etc, causes poor calibration tunes to smoke.

Lambda Probes

This sensor is used to measure the oxygen content and the Lambda value (ratio between quantity of intake air and theoretical quantity of air required for complete combustion of fuel injected) in automotive engine exhaust gases.


When the mass of oxygen present coincides with the amount required to burn all the fuel, the mixture is known as stoichiometric and the ratio is 14.6.In this case the Lambda is at the unit value.

The Lambda can be expressed as the ratio between the quantity of intake air and the quantity of air required for the combustion of the fuel injected.

The Lambda values range from 0.65 (rich mixture, shortage of oxygen) to infinite (percentage of oxygen equal to percentage present in the air 20.95% approx).

The value measured by the Lambda sensor is compared with the Lambda value calculated by the engine management control unit and, if necessary, modifications are made to the map for the injectors and the air flow rate meter in order to maintain the operation of the engine and the harmful emissions within legal limits; its operation is therefore to monitor the engine over a period of time.

The sensor works by comparing the concentration of oxygen in the reference cell, housed inside the sensor, with the combustion gas flowing inside the comparison cell next to the reference cell. Depending on the resulting imbalance, the engine management control unit regulates a current signal (lp) that rebalances the oxygen content in the comparison cell through an electrochemical action. The value of lp is proportional to the Lambda value measured according to the graph illustrated below.


Sensor structure

The sensitive element is the planar type, the sensor consists of three layers of zirconium oxide and its particular shape allows the sensor to be very compact with low intensity pumping currents which further decreases the power absorption of the sensor.


The sensitive part is protected from the flow of exhaust gases with a cover with two layers with access openings perpendicular to one another.

The diagram below shows an exploded view of a sensor illustrating the various components.


1. Insulated cable

2. Rubber cap

3. Metal cap

4. Glass

5. Metal duct

6. o-ring

7. Sintered products

8. Hexagon

9. Ceramic heater

10. Sensitive element

11. Dual protection pipe


  • Heater nominal supply voltage: 7.5 V
  • Dissipated power at 7.5 V: 7.5 W
  • Lambda measuring range: from 0.8 to infinity
  • System tolerance: ΔO2/O2 = +/- 4 %
  • Exhaust temperature: ≤ 980 °C
  • Exhaust pressure: ≤ 2.5 bar
  • Supply: ≥ 10.8 V
  • Activation time: ≤ 10 s
  • Potentiometer range: from 30 to 300 Ohm

The Lambda sensor connector is illustrated in the diagram below.


1 - Pumping current (lp)

2 - Virtual mass

3 - Heater (-)

4 - Heater (+)

5 - Calibration current

6 - Nerst voltage