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. 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, and later that same year, Mercedes-Benz introduced it in their W202 model.
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There are a number of approaches to control the pressure in the common rail. One way is to supply more fuel than is needed to the common rail and use a high pressure regulator—commonly referred to as a pressure control valve—in the high-pressure circuit to spill the excess fuel back to the fuel tank. In this approach, the pressure control valve position is the control system input. While this approach was used exclusively in some early fuel injection systems such as those with Bosch CP1 pumps, poor efficiency and an excessively high fuel return temperatures can result.
Common rail injection: advanced technology for diesel engines
Bosch launched the first common rail system in 1997. The system is named after the shared high-pressure reservoir (common rail) that supplies all the cylinders with fuel. With conventional diesel injection systems, the fuel pressure has to be generated individually for each injection. With the common rail system, however, pressure generation and injection are separate, meaning that the fuel is constantly available at the required pressure for injection.
Diesel engines can start even faster and quieter with Bosch pre-heat systems. In combination with an optimized injection strategy, pre-heat systems can improve the performance of cold engines and reduce emissions. They can also assist heating of the particle filter during regeneration.
The electronic engine control can enable the delivery of the required quantity of fuel from the tank to the high-pressure pump at a pressure of 5 to 6 bar in line with requirements.
In order to contribute to an efficient combustion while conserving resources, the amount of injected fuel must always be adapted accurately to the air mass that enters the cylinder. The air mass is measured accurately by a hot film air-mass sensor.
Electrification and downsizing
Together with the variable injection start and the option of multiple pre- and post-injections as well as short intervals between injections, the system is optimally designed for downsizing concepts with turbocharging as well as for combining with hybrid technology to electrify the powertrain.