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The above diagram shows a very basic layout of a typical common rail diesel fuel system.
Diesel fuel is drawn from the fuel tank by a low pressure transfer pump, this may be an electric pump located in the fuel tank or mechanical pump which forms part of the main high pressure pump. On its journey to the high pressure pump the fuel will pass through at least one filter assembly.
Fuel at a constant transfer pressure is then passed to the intake of the mechanical high pressure pump which acts to increase fuel pressure to around 200 bar during cranking, to around 1600 bar operating pressure whilst the engine is running.
The pressure at the high pressure pump is regulated by controlling the quantity of fuel sent to the pumping elements within the high pressure pump.
Lubrication of the pump is provided by fuel circulation.
The highly pressurised fuel is pumped to the common rail where it is accumulated . The common rail fuel is delivered to each engine cylinder by a fuel injector.
Each injector is electronically controlled allowing precise operation to deliver the required amount of fuel at the correct time.
Safety Note
Common rail diesel fuel systems operate at very high pressure and can cause severe injury. Fuel pressures of up to 2000bar may be present. Please refer to manufacturers health,safety and training provisions before attempting service or repair of any common rail or pressurised fuel system.
Advantages of the Common Rail Diesel System
The electronically controlled common rail system has many advantages:
Higher Performance through increased torque at low engine speed Lower fuel consumption
Lower emissions Reduced Noise Closed loop control
Programmable characteristics enabling effective use of biofuels
Electronic diagnostics
Fuel Injector Calibration (Coding)
The amount of fuel injected is proportional to the injector duration (opening time) and fuel rail pressure.
A target value for injector duration under specific conditions is programmed into the diesel control unit (DCU).
The fuel injectors are machined to very fine tolerances but because of individual characteristics such as friction,pressure drop,magnetic force ,etc very slight variations in flow can occur.
Slight difference between target flow and actual flow can occur due to high operating pressures.
When some brands of injectors are manufacured they are calibrated on an electrical test rig and given a data code.
The data code is programmed into the engine control unit identifying to which cylinder the injector is fitted.
This data code allows the engine control module to correct the pulse duration applied to each individual injector to maintain optimum performance. Fitment of replacement injectors will require the new injectors to be coded to the engine control unit. We now have facilities for coding of Delphi diesel injectors on Ford vehicles (TDCi).
Common Rail Fuel System Faults
The common rail diesel fuel system is an electro-mechanical system for control of fluid (fuel) pressure and flow.
The most usual faults with the common rail fuel system are due to loss of pressure.
Pressure loss can often be related to faulty fuel injectors causing excessive fuel back leakage (excess of fuel in to fuel return lines).
Fuel pump supply problems such as flow restrictions in the fuel line or worn fuel pump will also cause system pressure loss.
Frequently common rail fuel systems use a fuel accumulator rail mounted pressure sensor which will provide a feedback signal to the engine managemant controller.
A loss of rail pressure may be recognised as a fuel system leak by the engine management controller in which case total system shut down may occur as a safety feature.
Typical example of a vehicle suffering from engine running fault
The vehicle in question was a late model luxury car fitted with a 6 cylinder common rail diesel engine which had covered around 90,000 miles.
The customer reported an intermittent fault which caused sluggish acceleration and rough engine running was sometimes present.
Unusually,the dashboard engine management light , in this particular instance was not illuminated.
The customer had tried to locate the fault by unplugging and reconnecting wiring to various engine components.
A visual check of the engine bay was done, then using the diagnostic scantool we interrogated the engine control module for stored fault codes. Several fault codes relating to different fuel injectors, engine stability, and P3505 pre heat device faults were present.
Fault codes were recorded ,then cleared from engine control unit memory.
The vehicle was then road tested by accompanying the customer.
On road test the vehicle hesitated breifly during acceleration.
After road test the control unit memory was once again read using the diagnostic scanner.
A P0204 cylinder fault code relating to
an electrical fault within the circuit to cylinder 2 injector valve was present.
Moving the wiring to cylinder 2 injector located this fault to a bad connection at the injector electrical connector plug , noted by the audible change in engine note.
The bad connection was rectified and fault codes once again cleared.
A back leakage test was carried out of all six injectors with the engine running. Five of the injectors had back leakage values within 10% of each other. However the remaining fuel injector had almost twice the back leakage fuel flow in comparison to the others indicating a badly worn fuel injector.
Injectors were removed from the engine for test bench assessment of spray pattern ,recondition and calibration.
The serviced injectors were refitted together with new high pressure pipes as recommended and road test carried out resulting in a much smoother drive.
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