STUTTGART - Common rail is the latest development in direct-injection diesel technology. Its developer, Robert Bosch GmbH, says it will allow automakers to meet tough emissions standards proposed by the EU.
In conventional direct-injection diesel engines the engine's speed regulates the pressure at which fuel is delivered each injection cycle.
In a common-rail engine the fuel-injection pressure is independent of the engine's speed. It is generated by a high-pressure pump. The pressure is constant, which has several advantages. First, it improves engine performance, therefore reducing fuel consumption. Second, it cuts emissions and noise.
A third advantage, says Bosch, is that the system can be fitted to existing engines without a costly redesign of the engine block. 'A diesel engine does not require major modifications for the application of a common rail system,' says Wilhelm Polach, director of Bosch's Automotive Product Division's engine test laboratory.
'A high-pressure pump replaces the injection pump, and an injector replaces the nozzle-holder assembly in the cylinder head,' he says.
Inside the common rail and the high-pressure fuel lines to the injector nozzles, the pressure is nearly constant. It reaches a maximum of 1600 bar.
In the past, says Polach, fuel economy was the No.1 priority. Now other factors rank with it: low emissions, low noise, high power reserves and good driveability.
'The injection system plays an important role in meeting these requirements,' he says. 'It must inject fuel during each work cycle with great precision, at the right moment, in the right amount, and at optimal pressure.
'High injection pressures - ranging from 1000-2000 bar - are indispensible if the goal is to achieve low fuel consumption coupled with low particulate emissions and high torque.'
The sensors and electronic control unit are similar to those used in conventional time-controlled pump systems. But the constant pressure allows very precise control over the injection rate, the quantity of fuel used and the atomization of the fuel.