Common Rail Fuel Injection Systems

by Andre Lorico, Clemson Automotive Engineering Graduate Student

Basic Description

Diesel vehicles have gained a reputation of being slow, loud, soot-producing engines only to be used in trucks. While this has been true of diesel engines for a long time, recent advances in injection technology have transformed diesel engines into quiet, clean-burning and efficient power plants. The common rail diesel injection system currently used in the vast majority of modern diesel engines is the single most important factor in increasing efficiency and performance of the diesel combustion cycle.

Historically, diesel engines have used the unit injection system and the distributor/inline pump system. These older systems had two main disadvantages: Firstly, they were cam-driven and as a result the maximum pressure for fuel injection was only achieved at high engine speeds. Secondly, they were limited in the number of injections that they could deliver per combustion cycle. The common-rail injection diesel systems build upon direct-injection technology which allows fuel to be injected directly into the combustion chamber. Common-rail injection systems use a rail to store the fuel at pressures in excess of 2,000 bar. This rail is shared between all cylinders, hence the name common-rail. The common rail allows a steady high-pressure supply of fuel independent of engine speed. The transfer pump is electronically activated to maintain the optimal pressure in the rail. Early generations of common-rail injectors used magnetic solenoids to control the injection of fuel into the cylinders. The latest generation of common-rail injectors use piezoelectric inline actuators to control fuel injection. These new actuators use hundreds of piezo-crystal wafers which expand very rapidly when a current is applied. Piezoelectric inline actuators can be integrated very close to the injector jet needle, providing friction-free movement. These injectors allow for much more precise fuel-injection times with shorter response times, as well as improved atomization of the fuel.

Common Rail diesel injection systems have several main advantages over older fuel injection systems. The high pressure fuel supply from the rail allows for the fuel to be injected quickly and at high pressure into the cylinders. This allows the electronic diesel control unit (EDU) to very accurately control the fuel injection. Such accurate control allows for up to 5 fuel injections per combustion stroke ("pilot", "pre", "main", "after", and "post" injections). The "pilot" injection, which occurs before ignition allows for good fuel-air mixture. The subsequent "pre" injection shortens the delay in the main injection, drastically reducing NOx gases, vibrations and other noise. The "after" injection occurring immediately after the "main" injection ensures that all particle matter is burned. Finally, the "post" injection allows for control of exhaust gas temperature, increasing after-treatment efficiency.

The overall result of using advanced common rail diesel injection systems is an engine that operates more efficiently, and is capable of outperforming it's gasoline-powered counterpart. Virtually all previous downsides of operating a diesel engine, such as noise, weak performance, vibrations and particle emissions have been eliminated with common rail direct injection diesel engine technology.

Common Rail Engine Names by Manufacturer:
   Audi: TDI
   BMW: D-engines
   Daimler: CDI
   Fiat Group: JTD
   Ford: TDCi Duratorq and Powerstroke
   GM Opel/Vauxhall: CDTi and DTi
   GM Daewoo/Chevrolet: VCDi
   Honda: i-CDTi
   Hyundai/Kia: CRDi
   Land Rover: TD5
   Mazda: CiTD
   Mitsubishi: DI-D
   Nissan: dCi
   Peugeot: HDI or HDi
   Renault: dCi
   Subaru: TD
   Toyota: D-4D
   Volkswagen: TDI
   Volvo: D and D5

Manufacturers

Bosch, Continental, Delphi, Denso