Common rail refers to a small accumulation tank called Rail where the pressure of the fuel remains almost constant and always available in order to supply the electronic injectors and therefore for an optimum injection. The protection of the environment, the need to reduce the consumption of fuel and to make the diesel engines more silent and better performing are the key factors that determined the study and development of the Unijet common rail system. Born as a project from Marelli in 1987, it was afterwards acquired by Fiat’s research centre in Bari who set it up and tested it on a vehicle in 1992. The project was transferred to Bosch Group for the final industrialization process in 1994. The first vehicles with Unijet Common Rail installation were introduced into the market in 1997.
CRDi FUNCTION
A pump inhales the fuel from the tank (ELECTRONIC PUMP) and continuously sends the quantity of requested fuel towards a second pump (HIGH PRESSURE PUMP) by making it pass first through the fuel filter that purifies it from any impurity which would cause a premature wear of its components. The high-pressure pump compresses the fuel at a pressure of 1350 bar and transfers it through a connection pipe to the high-pressure accumulation duct (Rail). This tank develops the function of mitigating the pressure oscillations caused by the opening and closing of the injectors and by the continuous discharges of the pump. The fuel is then transferred from the Rail through some connection pipes to the electronic injectors, which - instructed by an electromagnetic valve –inject the correct amount of fuel directly into the combustion chamber of the engine.
The fuel in excess, required for the opening of the nozzles, is sent back to the tank along with the leakages of fuel coming from the pressure regulation valve and from the high pressure pump itself. In this type of system, the quantity of injection is being established by the driver through the accelerator pedal while the initial stage and the pressure of injection are calculated and controlled by the electronic control unit (EDC). Through the accelerator pedal’s sensor, the electronic control unit registers the driver’s intention while, thanks to other sensors, it registers the exercising conditions of the engine and vehicle and – according to the information acquired – it carries out the intervention for the engine regulation
The power supply of the fuel is divided into low-pressure circuit and high-pressure circuit.
The low-pressure circuit is made of:
- auxiliary immersed electronic pump
- diesel filter;
- leakage manifold
The high-pressure circuit is made of:
-pressure pump;
- repair collector
 Different car makers refer to their common rail engines by different names: * BMW's D-engines (also used in the Land Rover Freelander TD4 * 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, Quadra-Jet) * Ford Motor Company's TDCi Duratorq and Powerstroke *General Motors Opel/Vauxhall CDTi (manufactured by Fiat and GM Daewoo) and DTi (Isuzu) *General Motors Daewoo/Chevrolet VCDi (licensed from VM Motori; also branded as Ecotec CDTi) *Honda's i-CTDi *Hyundai-Kia's CRDi *Mahindra's CRDe *Maruti Suzuki's DDiS (manufactured under license from Fiat) *Mazda's CiTD *Mitsubishi's DI-D *Nissan's NEO-Di *PSA Peugeot Citron'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) *Subaru's Legacy TD (as of Jan 2008) *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, as opposed to the earlier Pumpe-Dse engines. The 2.0 TDI in the VW Tiguan SUV uses common rail, as does the 2008 model Audi A4. *Volvo D5-engines are called common rail | 
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