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CATEGORIES (articles) > Engine, Gearbox > Technical > Common rail diesel explained

Common rail diesel explained

Common rail direct fuel injection is a modern variant of direct injection system for Diesel engines. It features a high-pressure (1000+ bar) fuel rail feeding individual solenoid valves, as opposed to low-pressure fuel pump feeding pump nozzles or high-pressure fuel line to mechanical valves controlled by cams on the camshaft. Third generation common rail diesels now feature piezo injectors for even greater accuracy, with fuel pressures up to 1700 bar.


The common rail system prototype was developed in the late 1960s by Mr. Hiber of Switzerland. After that, Ganser of the Swiss Federal Institute of Technology developed the common rail technology further. In the mid-nineties, Dr. Shohei Itoh and Masahiko Miyaki, of the Denso Corporation, a Japanese automotive parts manufacturer, developed the Common Rail Fuel System for Heavy Duty Vehicles and finally turned into its first practical use on their ECD-U2 Common Rail system, which was mounted on the Hino Raising Ranger truck and sold for general use in 1995.The modern common rail system was extensively prototyped in the 1990's, with collaboration between Magneti Marelli, Centro Ricerche Fiat and Elasis. After research and development by the Fiat Group, the design was aquired by the German company Robert Bosch GmbH for completion of development and making suitable for mass-production, whom later in 1997 extended its use for passenger cars.

Common rail engines had been used in marine and locomotive applications for some time. The Cooper-Bessemer GN-8 (circa 1942) is an example of a hydraulically operated common rail diesel engine, also know as a modified common rail.The engines are suitable for all types of road car, ranging from city cars such as the Fiat Nuova Panda to large family cars like the Alfa Romeo 159.

Common rail today

Today the common rail system has brought about a revolution in diesel engine technology. Delphi Automotive Systems make common rail systems. Different car makers refer to their common rail engines by different names:

  • DaimlerChrysler's CDI (and on Jeep vehicles simply as CRD)
  • Fiat Group's (Fiat, Alfa Romeo and Lancia) JTD
  • Hyundai's CRDI
  • Ford Motor Company's TDCi,
  • Renault's DCi
  • General Motors'/Opel's CDTi (most of these engines are manufactured by Fiat, the others by Bosch)
  • Mitsubishi's DI-D
  • PSA Peugeot Citroën's HDI or HDi
  • Toyota's D-4D
and so on.


Solenoid or piezoelectric valves make possible fine electronic control over the injection time and amount, and the higher pressure that the common rail technology makes available provides better fuel atomisation. In order to lower engine noise, a small "pilot" amount of fuel can be injected just before the main load, effectively reducing its explosiveness; some advanced common rail fuel systems perform as many as five injections per stroke.

Common rail engines require no heating up time, and produce lower engine noise and lower emissions than older systems.

In older diesel engines, a distributor-type injection pump, regulated by the engine, supplies bursts of fuel to injectors which are simply nozzles through which the diesel is sprayed into the engine's combustion chamber. As the fuel is at low pressure and there cannot be precise control of fuel delivery, the spray is relatively coarse and the combustion process is relatively crude and inefficient.

In common rail systems, the distributor injection pump is eliminated. Instead an extremely high pressure pump stores a reservoir of fuel at high pressure—up to 1,800 bar (180 MPa)—in a "common rail", basically a tube which in turn branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid. Driven by a computer (which also controls the amount of fuel to the pump), the valves, rather than pump timing, control the precise moment when the fuel injection into the cylinder occurs and also allow the pressure at which the fuel is injected into the cylinders to be increased. As a result, the fuel that is injected atomises easily and burns cleanly, reducing exhaust emissions and increasing efficiency.

In addition, the engine's electronic control unit can inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing noise and vibration, as well as optimising injection timing and quantity for variations in fuel quality, cold starting, and so on.

Most European automakers have common rail diesels in their model lineups, even for commercial vehicles. Some Japanese manufacturers, such as Toyota, Nissan and recently Honda, have also developed common rail diesel engines. Some Indian companies have also successfully implemented this technology, notably Mahindra & Mahindra for their 'Scorpio' lineup of SUV's. They call it the CRDe technology.

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