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What Is Common Rail Fuel Injection System?

Common rail fuel injection uses high-pressure fuel rail feeding solenoid valves to inject fuel. This is in contrast to low-pressure fuel pump feeding unit injectors (or pump nozzles) that use lower pressure fuel. High-pressure fuel injection offers fuel consumption and power benefits over lower pressure fuel injection.

It injects fuel in smaller droplets which results in a higher surface area to volume ratio. This allows for better vaporization and more efficient mixing of atmospheric oxygen and vaporised fuel, resulting in more complete combustion.

Common rail injection is a common technique used in diesel engines. Common rail injection is also used in gasoline direct injection systems for petrol engines.


Vickers was the first to use common rail injection in submarine engines. In 1916, Vickers engines using the common rail fuel system were used in the G-class submarines. The four-pumper pumps were used to produce a pressure of up to 3,000 lb/sq in (210 bar; 21 Mpa) for every 90deg rotations. This was to maintain the rail's fuel pressure. Valved in the injector lines could shut down fuel delivery to individual cylinders.

From 1921 to 1980, Doxford Engines used common rail technology in their opposed-piston marine engines. Multicylinder reciprocating fuel pumps generated 600 bar pressure (60 MPa; 8,700 PSI). Fuel was stored in accumulator containers. An adjustable pump discharge stroke and a "spill valve" were used to control pressure. The spring-loaded Brice/CAV/Lucas injectors were supplied by camshaft-operated mechanical time valves.

They were injected through the cylinder into the chamber between the pistons. The timing cams were used for both ahead and back running in early engines. Two injectors were used for ahead running and one for astern. The final series of constant-pressure turbocharged engines featured four injectors per engine. This system was used to inject diesel and heavy fuel oil (600cSt at a temperature of near 130 degrees).

Since the beginning of time, common rail engines have been used for locomotive and marine applications. A hydraulically-operated common rail diesel engine is the Cooper-Bessemer GN-8 ( around 1942).

Robert Huber, a Swiss engineer, developed the common rail system prototype for automobile engines in the late 1960s. Dr Marco Ganser, a Swiss Federal Institute of Technology in Zurich and later Ganser-Hydromag AG (est. 1995 in Oberageri.

The first common-rail-Diesel-engine used in a road vehicle was the MN 106-engine by East German VEB IFA Motorenwerke Nordhausen. It was made into one IFA W50 in 1985. The development was cancelled due to a lack of funds and mass production was not possible.

In Japan, the mid-1990s saw the first mass-produced vehicle use this fuel system. Denso Corporation, a Japanese manufacturer of automotive parts, created the common rail fuel system for heavy-duty vehicles. They mounted it on the Hino Ranger truck and made it practical. Denso claimed the 1995 commercial launch of a high-pressure common rail system.

Although they work on the same principle as common rail, modern common rail systems are controlled by an engine control unit. This opens each injector electronically rather than mechanically. In collaboration with Magneti Marelli and Centro Ricerche Fiat, this was extensively prototyped during the 1990s. The design was developed by the Fiat Group and acquired by Robert Bosch GmbH to complete development and refinement in mass production.

The sale of the design was a strategic mistake for Fiat as the new technology proved highly profitable. Bosch was a license that the company couldn't refuse to sell since it was in poor financial condition at the time and didn't have the resources to develop its technology on its own.

They extended the use of the common rail system to passenger cars in 1997. The 1997 Alfa Romeo 156, a passenger car with a 2.4-L JTD motor, was the first to use the common rail system. Later, Mercedes-Benz introduced it in the W202 model.


Common rail is compatible with all types of diesel-powered road vehicles, including executive cars like the Audi A8 and city cars such as the Fiat Panda. BOSCH, Denso and Delphi are the main suppliers of modern common railway systems. They are now owned by Continental AG.

Acronyms and branding used

The automotive manufacturers refer to their common rail engines by their own brand names:

  • Ashok Leyland: CRS (used in U Truck and E4 Busses)

  • Audi: TDI , BiTDi The "Bi" stands for BiTurbo

  • BMW Group (BMW and Mini): d (also used in the Land Rover Freelander as TD4 and the Rover 75 and MG ZT as CDT and CDTi), D and SD

  • Chevrolet (owned by GM): VCDi (licensed from VM Motori) and Duramax Diesel

  • Chrysler CRD

  • Citroën: HDi , e-HDi and BlueHDi

  • Cummins and Scania: XPI (developed under joint venture)

  • Cummins: CCR (Cummins pump with Bosch injectors)

  • Daimler: CDI

  • Fiat Group (Fiat, Alfa Romeo and Lancia): JTD (also branded as MultiJet, JTDm, and by supplied manufacturers as TDi, CDTi , TCDi, TiD , TTiD, DDiS and QuadraJet )

  • Ford Motor Company: TDCi (Duratorq and Powerstroke) and EcoBlue Diesel

  • Honda: i-CTDI and i-DTEC

  • Hyundai, Kia and Genesis: CRDi


  • Isuzu: iTEQ , Ddi and DI TURBO

  • Jaguar: d

  • Jeep: CRD and EcoDiesel

  • Komatsu: Tier3 , Tier4 , 4D95 and higher HPCR -series

  • Land Rover: TD4 , eD4, SD4, TD6, TDV6, SDV6, TDV8, SDV8

  • Lexus: d (e.g. 450d and 220d)

  • Mahindra: CRDe , m2DiCR , mEagle , mHawk , mFalcon and mPower (Trucks)

  • Maserati: Diesel

  • Mazda: MZR-CD and Skyactiv-D (are manufactured by the Ford and PSA Peugeot Citroën joint venture) and earlier DiTD

  • Mercedes-Benz: CDI and d

  • Mitsubishi: Di-D

  • Nissan: DDTi

  • Opel/Vauxhall: CDTI , BiTurbo CDTI , CRI , Turbo D and BiTurbo D

  • Porsche: Diesel

  • Proton: SCDi

  • Groupe PSA (Peugeot, Citroën and DS): HDi , e-HDi or BlueHDi (developed under joint venture with Ford) – See PSA HDi engine

  • Renault, Dacia and Nissan: dCi and BLUEdCi (Infiniti uses some dCi engines as part of the Renault-Nissan Alliance, branded d )

  • Saab: TiD (The 2.2 turbo diesel engine was also called "TiD", but it didn't have Common rail) and TTiD The double "T" stands for Twin-Turbo

  • SsangYong: XDi , eXDI, XVT or D

  • Subaru: TD , D or BOXER DIESEL (as of Jan 2008)

  • Suzuki: DDiS

  • Tata: 2.2 VTT DiCOR (used in large SUV-class such as Safari), VARICOR (used in large SUV-class such as Safari Storme, Aria and Hexa), 1.05 Revotorq CR3 (used in Tiago and Tigor) 1.5 Revotorq CR05 (used in Nexon and Altroz), 1.4 CR4 (used in Indica, Indigo), 3.0 CR4 (used in Sumo gold ) 1.3 Quadrajet (supplied by Fiat and used in Indica Vista, Indigo Manza and Zest), and 2.0 Kryotec (also supplied by Fiat and used in SUV Harrier and All new Safari), 3.3 L Turbotronn and 5L Turbotronn ( used in M&HCV Trucks).

  • Toyota: D-4D and D-CAT

  • Volkswagen Group (Volkswagen, Audi, SEAT and Škoda): TDI (more recent models use common rail, as opposed to the earlier unit injector engines). Bentley term their Bentayga diesel simply Diesel

  • Volvo: D , D2 , D3 , D4 and D5 engines (some are manufactured by Ford and PSA Peugeot Citroën), Volvo Penta D-series engines


The common rail technology allows for better fuel atomisation thanks to the availability of higher pressures. The engine's electronic controller unit can inject small amounts of diesel right before the main injection event ("pilot") injection to reduce engine noise.

This helps reduce engine vibrations and explosiveness, as well as optimise injection timing and quantity to accommodate variations in fuel quality, cold start, and other factors. Advanced common rail fuel systems can perform up to five injections per stroke.

Common rail engines are very efficient and require little to no heating up, depending on the ambient temperature. They also produce less noise and emissions than older systems.

Different types of fuel injection have been used in diesel engines for a long time. Two common types include the unit-injection system and the distributor/inline pump systems. These older systems can provide precise fuel quantity and timing control but are limited by a few factors.

  • Cam driven. Injection pressure is proportional to engine speed. This means that you can only achieve the highest possible injection pressure at high engine speeds. As engine speed drops, the maximum injection pressure is reduced. This is true for all pumps, including those on common rail systems. Unit or distributor systems tie the injection pressure to the instantaneous pressure from a single pumping event without an accumulator. This relationship can be more problematic and noticeable.

  • These systems are limited in terms of the number and timings of injection events that can be commanded during one combustion event. Although multiple injection events can be achieved with older systems, this is more difficult and expensive.

  • The typical distributor/inline system starts injection at a predetermined temperature (often referred to as pop pressure), and it ends at the same temperature. This is due to "dumb" injectors within the cylinder head that open and close at pressures set by spring preload. The injection begins when the pressure inside the injector is at a predetermined level.

A high-pressure pump is used to store fuel at high pressure in common rail systems. It can hold up to 2,000 bar (200 MPa; 29,000 PSI). Common rail refers to the fact all fuel injectors are connected by a common fuel train. This is nothing but a pressure accumulator that stores the fuel at high pressure.

The accumulator supplies fuel injectors with high-pressure fuel. This makes it easier for the high-pressure pump to serve its purpose. It only needs to maintain the desired pressure (either electronically or mechanically). The engine control unit (ECU) controls the fuel injectors. The engine control unit (ECU) controls the fuel injectors. A hydraulic valve, consisting of a plunger and nozzle, opens mechanically or hydraulically to allow fuel to be sprayed into the cylinders at the desired pressure.

Because the fuel pressure energy and injectors can be stored remotely, the injection pressure at the start and end of the injection is very close to the pressure in the rail. This results in a square injection rate. The injection pressure and rate for multiple injection events will be identical if the pump, accumulator and plumbing are properly sized.

Diesel third-generation common-rail models now have piezoelectric injectors to increase precision and fuel pressures of up to 2,500bar (250 MPa; 36,000 PSI).

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