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Common Rail Diesel Injection (CRDI Basics)

Common Rail Diesel Injection ( CRDI Basics )

Diesel injection systems in history

Common Rail Diesel Injection (CRDI Basics)


The system common-rail(CR) is now the fuel system of the fuel mostly used in compression ignition engines.

Major advantages over predecessors:

  • Pressure Injection independent of the rotation regime of the engine
  • Flexible control of injection pressure( pressure range300 2200bar)
  • Flexible and accurate control of the injection quantity
  • Possibility of several injections per cycle depending on the operating condition  
  • Flexibility in the timing of the injections of TDC engine
Alfa Romeo 156 1.9JTD the first car equipped with the common rail system(1997)
Alfa Romeo 156 1.9JTD the first car equipped with the common rail system(1997)

The first passenger car to use the common rail system was the 1997 model Alfa Romeo 156 with a 2.4-L JTD engine, and later that same year, Mercedes-Benz introduced it in their W202 model.

Hydraulic layout

Main components of the hydraulic layout:

Main components of the hydraulic layout
  • Low-pressure pump (usually installed inside the tank)
  • Fuel-filter
  • Pompahigh pressure
  • Rail
  • Pressure sensor installed on the rail
  • Flow regulator (FMV) or pressure regulator (PCV) 
  • Injector

Electrical layout

Electrical layout

The engine control unit (ECU) has as its main function is to manage the system injection:

  • Monitors the pressure inside the rail (injection pressure)
  • Controls the flow regulator or the pressure regulator
  • Controls the injectors managing the duration, the number, and timing of the injections

High-pressure pump

High-pressure pump

The high-pressure pump receives the fuel from the low-pressure circuit and sends it to the rail at pressures are in the range 300 to 2200 bar depending on the operating condition of the engine and on which the system is installed.

It is generally composed of as haft with eccentric (1) realizes that by rotating the suction stroke and the delivery of a number of pumping elements(3).

The pump is lubricated by the fuel itself and on it, you can be installed of devices for adjusting the delivery pressure.


CR 1° generation 

A pressure regulating valve electronics (PCV) is installed on the high-pressure side of the pump.

It is controlled by the control unit to adjust the injection pressure by downloading or less fuel from the high-pressure circuit.

CR 2° generation 

A flow control valve electronics (FMV) is installed on the low-pressure side of the pump.

It is controlled by the control unit to regulate the flow of fuel to the pump in relation to the injection pressure required.


The rail is an accumulator fuel to which are connected the high-pressure pump and the injectors.

On it is installed a pressure sensor that the control unit uses to monitor the injection pressure(PRAIL).

Itmayalsobepresenta a pressure regulator (PCV), if it is not already present on the pump. 

Is there any configuration of the system where the regulator on rail is coupled to the flow control (FMV) installed on the pump.

Pressure control(hints)

How realize the conditions of the mapping for a given operating condition?

The control unit must be able to adjust the pressure inside the rail to obtain the value required by mapping (PRAIL).

Pressure control(hints)
  1. Reading of the pressure inside the rail by means of the integrated pressure sensor.
  2. Control the pressure in the rail by adjusting the flow rate sucked by the high-pressure pump.
  3. Control the pressure in the rail by recirculating or less part of the fuel in the tank.
  4. Comparison of the detected pressure with the required value in the corresponding map for a given operating condition. 

READ: Troubleshooting Common Rail (CR) System


The injectors are classified according to the type of drive according to the type of hydraulic layout. 

  • In the solenoid-operated injector, an electromagnetic force, generated in a solenoid crossed by current, is exploited to actuate the injector.
  • In the piezo injectors utilizes the inverse piezoelectric effect, that is the capacity of the piezoelectric material to deform when subjected to an electric voltage.
  • The electric drive is not directly connected to the pin, there is interposed a pilot valve.
  • The electric drive is directly connected to the needle through a mechanical linkage.



Solenoid injectors

Injectors are indirectly actuated, the solenoid controls a pilot valve the opening of which causes an imbalance of the forces acting on the needle such as to allow the lifting.

Solenoid injectors

Because of the complex internal hydraulic circuit and the presence of mechanical parts with not negligible inertia, the injection is not synchronized with the electrical command supplied to the injector from the engine control.

Piezo injectors ( indirect drive )

The piezoelectric actuator, the extension of which is amplified hydraulically, acts on a pilot valve that opens to download the environment upstream of the needle, which then will rise due to the effect of the pressure of the fuel in the delivery chamber.

The layout of the valve is such that the leaking fuel is much lower than those of the injectors solenoid seen previously.

Piezo injectors ( indirect drive )

Piezo injectors (direct drive) The piezoelectric actuator is directly connected to the pin by means of a mechanical linkage, then NOT is no pilot valve.


1) The dynamics of the injector is improved;

2) It is not necessary to recirculate fuel to actuate the injector ( efficiency ).


1)The piezoelectric actuator must be larger to generate the forces necessary to raise the needle;

2) It is necessary to control the injectors with very high voltages (≈160 volts ) that may require a special shielding of the electric circuit of the injection system

READ: Common Rail Diesel Engine: Working Principle, Pros and Cons


ECU -injection strategy(hints)

  • The basic strategy of injection include:
  • Readingthrottleposition (load);
  • Readingofthe speed of rotation of the engine (rpm);
  • Readingmapsofcontrol:

1. Depending on the load and the speed of rotation is chosen, the related map, the amount of fuel to be injected into the cycle (mb), and the injection pressure(PRAIL);

2. Based on the amount to be injected (sc) and the injection pressure(PRAIL) is chosen, from another map, the duration of the electrical command to be supplied to the injectors (ET).

ECU -injection strategy(hints)

After choosing injection pressure and duration of the electrical control to the injectors, the control unit must choose a number of other parameters with the aim to maintain the engine in top possible conditions, minimizing (to the extent possible ) the emission of pollutants and fuel consumption.

In addition, the unit constantly monitors the conversion efficiency of the devices of after-treatment of the exhaust gas ( oxidizing catalyst, DPF, DeNOx)using, when appropriate, strategies regeneration.

ECU -injection strategy(hints)


Troubleshooting Common Rail (CR) System


Common Rail Diesel Engine: Working Principle Pros and Cons

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