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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 piezoelectric injectors for even greater accuracy, with fuel pressures up to 180 MPa / 1800 bar, although a new version of Delphi’s proven diesel common rail system will allow compliance with Euro 6 and US Tier 2 Bin 5 without costly next-generation injection technologies.

The common rail system prototype was developed in the late 1960s by Robert Huber 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 ECU controlled common rail system whilst working on the same principle, is controlled by an electronic control unit which opens each injector electronically, rather than mechanically. This was extensively prototyped in the 1990s, with collaboration between Magneti Marelli, Centro Ricerche Fiat and Elasis. After research and development by the Fiat Group, the design was acquired by the German company Robert Bosch GmbH for completion of development and making suitable for mass-production. In 1997 they extended its use for passenger cars. The first passenger car that used the common rail system was the 1997 model Alfa Romeo 156 1.9 JTD and later on that same year Mercedes-Benz E 320 CDI.

Common rail engines have 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 known as a modified common rail.

The engines are suitable for all types of road cars with diesel engines, ranging from city cars such as the Fiat Nuova Panda to large family cars like the Alfa Romeo 159.

Today the common rail system has brought about a revolution in diesel engine technology. Robert Bosch GmbH, Delphi Automotive Systems, Denso Corporation and Siemens VDO are the main suppliers of modern common rail systems. Different car makers refer to their common rail engines by different names:

 AUDI TDI ENGINES 

• BMW D-engines
• Daimler's CDI (and on Chrysler's Jeep vehicles simply as CRD)
• Fiat Group's (Fiat, Alfa Romeo and Lancia) JTD (also branded as MultiJet, JTDm, Ecotec CDTi, TiD, TTiD , DDiS)
• Ford Motor Company's TDCi Duratorq and Powerstroke
• General Motors'/Opel's Vauxhall's CDTi (manufactured by Fiat and GM Daewoo) and DTi (Isuzu)
• GM Daewoo's/Chevrolet's VCDi (licensed from VM Motori; also branded as Ecotec CDTi)
• Honda's i-CTDi
• Hyundai-Kia's CRDi
• Mahindra's CRDe
• Maruti Udyog's DDiS (Manufactured under license from Fiat)
• Mazda's CiTD
• Mitsubishi's DI-D
• Nissan's NEO-Di
• PSA Peugeot Citroën's HDI or HDi (Volvo S40/V50 uses engines from PSA 1,6D & 2,0D.)
• Renault's dCi
• SsangYong's XDi (most of these engines are manufactured by DaimlerChrysler)
• Tata's DICOR
• Toyota's D-4D
• Volkswagen Group: The 4.2 TDI (V8) and the latest 2.7 and 3.0 TDI (V6) engines featured on current Audi models use common rail. The 2.0 TDI in the future Audi A4 and the future Volkswagen Tiguan will also use common rail.
• Volvo D5-engines are called common rail

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, 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.

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 2,000 bar (200 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.

Most European automakers have common rail diesels in their model lineups, even for commercial vehicles. Some Japanese manufacturers, such as Isuzu, 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-CRDe' and Tata Motors for their 'Safari-DICOR'.

• Turbo Direct Injection
• Gasoline Direct Injection

• BSG GmbH: common rail system codeveloped by Daimler-Benz, Fiat, Elasis and Bosch
• Common rail 101
• Common rail injection animation
• Video about Common Rail Injection on Sprinter 311 CDI (Portuguese)