|
The
common rail diesel injection system was developed by Bosch . Common rail
diesel engines can be found in many modern vehicles including cars, vans, buses, trucks, and earthmovers.
The common rail diesel fuel system is an
electro-mechanical system for control of fluid
(fuel) pressure and flow. Diesel fuel
must be delivered at the correct moment
and atomised in a fine spray to form an air
fuel mixture inside the engine combustion
chambers to allow efficient burning .
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 soot emissions - use of diesel particulate filter ( dpf )
in exhaust system. Reduced Noise Closed loop control
Programmable characteristics enabling effective use of biofuels
Electronic diagnosticsDiesel Lambda Control using exhaust gas recirculation to reduce NOx emission
The above diagram shows a very basic layout of a typical common rail diesel fuel system.
Electronic sensors and actuators not shown .
General
System Overview
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
fuel 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 220 bar during cranking, to around 1600 bar
(approx 23000psi) operating pressure whilst the engine is running at full
load. Newer systems are capable of operation at higher fuel pressure.
The
pressure at the high pressure pump is regulated by controlling the quantity of fuel
entering the pumping elements within the high pressure pump or by control of
the amount of pressurised fuel returning from the fuel rail to the tank.
Lubrication of the pump is provided by fuel circulation.
The highly pressurised fuel is pumped to the common rail . The fuel
accumulated in the common rail is delivered to each engine cylinder by an
electronically controlled fuel injector. A fuel rail pressure
sensor is used to provide a fuel pressure feedback signal to the
engine controller.
Each injector is electronically controlled allowing precise operation to deliver the required amount of fuel at the correct time.
Common rail diesel fuel systems with electromagnetic solenoid valve type
injectors allow a degree of fuel to return to the tank by internal leakage
through the injector body, referred to as injector back leakage.
Excessive back
leakage of fuel at the fuel injectors due to wear can cause a drop in common
rail fuel pressure and in some circumstances may prevent the vehicle engine
from being started.Regular
servicing is essential to prevent fuel flow restriction, contamination of
precision components and consequent engine running problems. Fuel quality
is of paramount importance due to close tolerance of machined precision
system components.
Safety Note
Common rail diesel fuel systems operate at very high pressure and can cause severe injury. Fuel pressures of up to 2200bar 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.
We only recommend servicing by qualified personnel.
Fuel Injector Calibration (Coding)
The amount of fuel injected is proportional to
the injector duration (opening time) , fuel
rail pressure and fuel temperature &
fluid viscosity.
A target value for injector duration under specific conditions is programmed into the diesel control unit (DCU)
engine control map.
The fuel injectors are machined to very fine tolerances but because of individual characteristics such as friction, pressure drop, magnetic force ,
wear during service 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 have facilities for
injector coding of several makes of vehicle.
Common Rail Fuel System Faults
Faults with the common rail fuel system often result in loss of fuel pressure.
Worn fuel injectors causing excessive back leakage and loss of fuel
pressure.
Fuel Quality resulting in contamination of filters causing flow
restriction.
Incorrect Fuel used causing damage to system components - particularly
when a driver has accidentally refuelled with petrol instead of diesel.
Electrical wiring , Sensor and Actuator faults
Most common rail fuel systems use a fuel accumulator rail mounted pressure sensor which will provide an
electrical feedback signal to the engine management controller.
A loss of fuel pressure may be interpreted as a fuel system leak by the engine management controller in which case total system shut down may occur as a safety feature .
Certain electronic management systems can lock out and prevent engine start
until faults have been repaired.Faults caused by Exhaust gas recirculation resulting in excessive carbon build up
and blockage of intake components.
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 already 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
and new fuel filters as recommended and road test carried out resulting in a much smoother drive.
|
