High Pressure Common Rail

Stringent emission levels, improved engine performance, lower fuel consumption and the need for quiet running engines has led to the development of Common Rail fuel injection systems.

This style of system has a number of basic components:

1. A low pressure pump (mechanical or electrical) to supply fuel to the high pressure pump from the fuel tank via the filtration system.
   
2. High pressure pump - this pump is designed to produce fuel under pressure irrespective of engine speed (up to 1850 bar).
3. High pressure accumulator (rail) - this stores the high pressure fuel.
   
4. Injectors - these electronically controlled devices have solenoids that are actuated via the Electronic Control Unit (ECU). An open command lets fuel from the rail into the injector where atomisation occurs in the usual way. A close command causes pressure to collapse and injection to cease. The high speed at which the injector operates can be used to initiate pre and post injection events to improve noise and emissions.
5. Pressure control and monitoring circuit - each common rail system has a pressure sensor, which ultimately monitors the rail pressure. A fast acting solenoid valve is employed in the Bosch system to increase/decrease the fuel pressure from the high pressure pump.

This valve is directly controlled via the ECU to ensure precise control of the rail pressure.(The pressure control valve does vary from system to system - some manufacturers use valves which act on the fuel supply to the pump to cause pressure rise or decay)

Common faults - High Pressure Common Rail

• Fuel contamination - common rail systems are very intolerant to dirt ingress or petrol contamination
• Lack of power - injectors can exhibit problems with excessive back leakage
• Lack of power - some high pressure pumps can have internal problems leading to low efficiencies and therefore incorrect flows of fuel to the rail

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