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Diesel fuel-injection systems:An overviewFields of application 2Technical requirements 2Injection-pump design 4

Common Rail accumulatorfuel-injection systemSystem overview 6

Injection characteristics 8Exhaust-gas reduction 10Fuel system 12Design and functionof the components 14System control using EDC 30

Electronic diesel control EDCTechnical requirements,System overview 42EDC data processing 43Data transmission to other systems 44

Start-assist systems 48

Diesel accumulatorfuel-injection system

Common RailMobility as it is commonly practicednowadays is not least a question ofenvironmental awareness and eco-nomic efficiency.In future, the techniques which per-mit these considerations concerningpersonal mobility to become apractical proposition will become ofdecisive importance particularly withregard to the diesel engine.The increasing needs for higher fueleconomy, less-toxic exhaust-gasemissions, together with the never-ending demands for reductions indiesel-engine noise can no longer becomplied with using mechanicallygoverned fuel-injection systems. Veryhigh injection pressures, coupled witha precise rate-of-discharge curveand exactly metered injected fuel

quantities are needed to comply withthe above requirements.This Technical Instruction contains allthe worthwhile knowledge concer-ning the Common Rail fuel-injectionsystem, its components, its design,and its method of functioning,together with details of how this newsystem is highly successful in fulfil-ling the above requirements.New developments in the system arethe fuel accumulator, the so-called

“rail”, which is permanently underpressure, a special high-pressurefuel supply system, the injectors, andan electronic control which preciselycopes with the most complicatedcontrol tasks: This system will haveno trouble in complying with themore severe exhaust-gas legislationand stipulations of the future.

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Rail-pressure sensor

AssignmentIn order to output a voltage signal to theECU which corresponds to the appliedpressure, the rail-pressure sensor must

measure the instantaneous pressure inthe rail

 – With adequate accuracy, and – As quickly as possible

Design and constructionThe rail-pressure sensor (Fig. 12) com-prises the following components:

 – An integrated sensor element weldedto the pressure fitting,

 – A printed-circuit board (pcb) with electri-cal evaluation circuit, and

 – A sensor housing with electrical plug-inconnection.

The fuel flows to the rail-pressure sensorthrough an opening in the rail, the endof which is sealed off by the sensordiaphragm. Pressurized fuel reaches thesensor’s diaphragm through a blind hole.The sensor element (semiconductordevice) for converting the pressure toan electric signal is mounted on thisdiaphragm. The signal generated by the

sensor is inputted to an evaluation circuitwhich amplifies the measuring signal andsends it to the ECU.

FunctionThe rail-pressure sensor (Fig. 12) opera-

tes as follows:When the diaphragm’s shape changes,the electrical resistance of the layersattached to the diaphragm also change.The change in shape (approx. 1 mm at1500 bar) which results from the build-up of system pressure, changes theelectrical resistance and causes a voltagechange across the 5 V resistance bridge.

This voltage change is in the range 0…70mV (depending upon the applied pres-

sure) and is amplified by the evaluationcircuit to 0.5…4.5 V.The precise measurement of railpressure is imperative for correct systemfunctioning. This is one of the reasons forthe very tight tolerances which apply tothe rail-pressure sensor during pressuremeasurement. In the main operatingrange, the measuring accuracy is approx.± 2% of full-scale reading. If the rail-pressure sensor should fail, the pres-sure-control valve is triggered “blind”using an emergency (limp-home)function and fixed values.

Common Rail 

22 U

     M     K     1     7     0     4     Y

Fig. 11Common Rail injection system on theengine test bench

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Pressure limiter valve

AssignmentThe pressure limiter valve has the same

 job as an overpressure valve. In case ofexcessive pressure, the pressure limiter

valve limits the rail pressure by openingan escape passage. The pressure limiterpermits a short-time maximum rail pres-sure of 1500 bar.

Design and constructionThe pressure-limiter valve (Fig. 13) isa mechanical device comprising thefollowing components:

  – Housing with external thread forscrewing to the rail,

  – A connection to the fuel-tank return

line, – A movable plunger, and – A spring.

At the connection end to the rail, thehousing is provided with a passagewhich is closed by the cone-shaped endof the plunger coming up against thesealing seat inside the housing. Atnormal operating pressures (up to 1350bar), a spring forces the plunger againstthe seat and the rail remains closed. Assoon as the maximum system pressureis exceeded, the plunger is forced up bythe rail pressure against the force of the

spring. The fuel under high pressure cannow escape, whereby it flows throughpassages into the plunger’s interior fromwhere it is led through a collector lineback to the fuel tank. When the valveopens, fuel leaves the rail so that the rail

pressure drops.

Flow limiter

AssignmentIt is the job of the flow limiter to preventcontinuous injection in the very unlikelycase that one of the injectors remainsopen permanently. To comply with thistask, as soon as the amount of fuelleaving the rail exceeds a defined level,the flow limiter closes the line to the

injector in question.

Design and constructionThe flow limiter (Fig. 14) comprises a me-tal housing with external thread forscrewing onto the rail (high pressure) andan external thread for screwing into theinjector lines. The housing has apassage at each end which provides thehydraulic connection to the rail and to theinjector lines.

There is a plunger inside the flow limiterwhich is forced in the direction of the fuelaccumulator by a spring. This plunger

Design and function 

23 

1

2

3

4

5

5431 2 6 7 83

Rail-pressure sensor (schematic)

1 Electric connections, 2 Evaluation circuit,3 Metal diaphragm with sensor element,4 High-pressure connection, 5 Mounting thread.

Pressure limiter valve (schematic)

1 High-pressure connection, 2 Valve,3 Flow passages, 4 Plunger, 5 Spring, 6 Stop,7 Valve body, 8 Fuel return.

Fig. 12 Fig. 13

     U     M     K     1     5     7     7     Y

     U     M     K     1     5     7     6     Y