Common rail

Diesel fuel injector as installed in a MAN V8 Diesel engine

Common-rail direct fuel injection is a direct fuel injection system for diesel engines.

On diesel engines, it features a high-pressure (over 100 bar or 10 MPa or 1,500 psi) fuel rail feeding solenoid valves, as opposed to a 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 2,500 bar (250 MPa; 36,000 psi).[1]

High pressure injection delivers power and fuel consumption benefits over earlier lower pressure fuel injection, by injecting fuel as a larger number of smaller droplets, giving a much higher ratio of surface area to volume. This provides improved vaporization from the surface of the fuel droplets, and so more efficient combining of atmospheric oxygen with vaporized fuel delivering more complete combustion.

In petrol engines, it is used in gasoline direct injection engine technology.


Common-rail fuel system on a Volvo truck engine

The common-rail system prototype was developed in the late 1960s by Robert Huber of Switzerland, and the technology was further developed by Dr. Marco Ganser at the Swiss Federal Institute of Technology in Zurich, later of Ganser-Hydromag AG (est.1995) in Oberägeri.

The first successful use in a production vehicle began in Japan by the mid-1990s. 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 turned it into practical use on their ECD-U2 common-rail system mounted on the Hino Ranger truck and sold for general use in 1995.[2] Denso claims the first commercial high-pressure common-rail system in 1995.[3]

Modern common-rail systems, although working on the same principle, are governed by an engine control unit, which opens each injector electrically 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 refinement for mass production. In hindsight, the sale appeared to be a strategic error for Fiat, as the new technology proved to be highly profitable. The company had little choice but to sell Bosch a licence, as it was in a poor financial state at the time and lacked the resources to complete development on its own.[4] In 1997, they extended its use for passenger cars. The first passenger car to use the common-rail system was the 1997 model Alfa Romeo 156 2.4-L JTD,[5] and later that same year, Mercedes-Benz introduced it in their W202 model.

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.

Vickers pioneered the use of common-rail injection in submarine engines. Vickers engines with the common-rail fuel system were first used in 1916 in the G-class submarines. It used four plunger pumps to deliver a pressure up to 3,000 pounds per square inch (210 bar; 21 MPa) every 90° of rotation to keep the fuel pressure adequately constant in the rail. Fuel delivery to individual cylinders could be shut off by valves in the injector lines.[6] Doxford Engines used a common-rail system in their opposed-piston marine engines from 1921 to 1980, where a multicylinder reciprocating fuel pump generated a pressure around 600 bars (60 MPa; 8,700 psi), with the fuel being stored in accumulator bottles.[7] Pressure control was achieved by an adjustable pump discharge stroke and a "spill valve". Camshaft-operated mechanical timing valves were used to supply the spring-loaded Brice/CAV/Lucas injectors, which injected through the side of the cylinder into the chamber formed between the pistons. Early engines had a pair of timing cams, one for ahead running and one for astern. Later engines had two injectors per cylinder, and the final series of constant-pressure turbocharged engines was fitted with four injectors per cylinder. This system was used for the injection of both diesel and heavy fuel oil (600cSt heated to a temperature near 130°C).

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