Injectors and Fuel Lines - Overview

03/05/13 Injectors and Fuel Lines - Overview

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006-999 Injectors and Fuel Lines - Overview

Theory of Operation - PT Fuel System

The following is a short description of thetheory of operation of the Cummins PTfuel system.

The PT, or Pressure-Time, fuel systemderives its name from the two mainvariables that affect the amount of fuelmetered per cycle in the Cummins fuelsystem. P stands for pressure at the inletof the injectors. It is controlled by the fuelpump. T refers to the time that isavailable for fuel to flow into the injectors. Itis controlled by engine speed through thecamshaft and injection train.

In the PT system, the amount of fuel burnedby the engine is the amount that is meteredinto the injector cup and injected. Thequantity metered is dependent on the fuelpressure at the injector, the flow area of theinjector and the time the fuel is allowed toflow into the metering chamber of the cup.



The fuel pressure at the injector iscontrolled by the PT fuel pump. Thepressure supplied by the pump variesaccording to different operating conditions.

The rail pressure supplied by the fuel pumpis dependent on the engine speed. It canalso be varied by the operator by changingthe position of the throttle.

The main functions of the fuel pump are toprovide:

Transfer of fuel from tank to engineRail pressure to injectorsIdle speed governingMaximum speed governingOperator control of power outputbelow governed speed (throttle)Control of exhaust smoke duringacceleration (AFC Air FuelControl)Shutdown of the engine.

The flow area of the injectors is determinedby the calibration of a complete set ofinjectors. The injector calibration isdetermined primarily by the parts whichmake up the injector. The Control Parts



List (CPL) manual lists the injectorassembly and flow plus the combinationof other basic engine parts which arenecessary to produce a given level ofengine performance.

The calibration flow rate of the injectors isdetermined by the size of an adjustableorifice. Each injector in the engine iscalibrated to the same flow rate. The flowrate adjustment must be done by aCummins Authorized Fuel SystemsLocation.

The time available for fuel to flow into thecup is determined by the time which themetering orifice in the barrel is uncoveredby the plunger. The metering orifice isuncovered while the cam follower roller ison the inner base circle of the cam. Theplunger is in the retracted position duringthis time. The amount of time the meteringorifice is uncovered is dependent onengine speed.

Injection of the fuel in the cup occurs whenthe cam follower roller travels up theinjection ramp of the cam forcing theinjector plunger downward. The end ofinjection occurs when the plunger bottomsin the cup.



The metering orifice is covered while thecam follower roller is on the outer basecircle of the camshaft. The plunger is in theseated position during this time. Fuelsupplied to the injector during this timebypasses the plunger through the drainport and is returned to the fuel tank.

The engine will produce more horsepowerat rated speed; however, the engine willproduce the most torque at the torquepeak speed. The rail pressure is lower attorque peak speed, but the metering timeis greater so more fuel is metered percycle.

Since more fuel is metered per cycle, morefuel is injected per cycle and more torqueis produced.

Theory of Operation - STC Fuel System

Step Timing Control, commonly referred toas STC, controls the engine timing in aneffort to minimize white smoke at coldengine start-up.



Refer to Step Timing Control Familiarization , Bulletin 3387380, foraddition information on STC.

STC has two stages of injection timing.The engine operates in the ADVANCEDmode of injection timing during starting andlight engine load conditions and atNORMAL timing during medium to highengine load conditions.

STC offers many advantages. DuringADVANCED injection timing, it:

Improves cold weather idlingcharacteristicsReduces cold weather white smokeImproves light load fuel economyReduces injector carboning.

During NORMAL injection timing, STC:

Controls cylinder pressuresReduces nitrogen oxide emissions.



The STC system consists primarily of:

STC injectorsSTC oil control valveSTC plumbing and check valve.

These components control injection timingbased on fuel pump rail pressure (engineload).

In the injector, injection timing is controlledby the STC hydraulic tappet. The tappethas an inner piston (plunger) and an outerpiston (sleeve). These tappet componentswork together with the injector plunger tocontrol injection timing.

In NORMAL timing, no oil is in the tappet.As the cam follower starts up the camshaftinjection ramp, the injector rocker leverbegins to force the inner piston downward.Because no oil is in the tappet, the innerpiston must make direct contact with theouter piston before the injector plunger canbegin its downward travel.

In ADVANCED timing, the tappet is filledwith oil and the injector is metering fuel. Asthe cam follower starts up the camshaftinjection ramp, the injector lever begins toforce the inner piston downward. Since theoil between the pistons forms a solid link,the downward pressure is immediatelytransmitted to the outer piston and theinjector plunger begins its downward travelearlier than it does in NORMAL timing.This causes the fuel to be injected earlier.



Engine oil flows from the STC oil controlvalve through the oil manifold to thetappets. Whenever the oil pressure in theoil manifold exceeds 70 kPa [10 psi], itmoves the tappet inlet check ball from itsseat and fills the cavity between the innerand outer pistons.

During the injection cycle, the oil is heldinside the tappet by the inlet check ball andthe load cell check ball. When the rockerlever forces the inner piston downward, thesolid link of oil causes the injector plungerto contact the fuel earlier; therefore, theinjection timing is in the ADVANCEDmode. At the end of the injection cycle,injection force increased the oil pressure inthe tappet and holds the injector plungerfirmly in the cup.

This increased pressure moves the loadcell check ball from its seat. The oil drainspast the load cell check ball and throughthe drain holes in the injector adapter andreturns to the oil pan through drainpassages in the cylinder head and block.Meanwhile, with continued cam lift, theinner piston makes mechanical contactwith the outer piston and maintains injectorplunger seating force.

The STC control valve uses fuel pressureand spring force to control the position ofan AFC style plunger. The position of theplunger dictates whether the oil passage tothe hydraulic tappets is open or closed.Fuel pressure acts on the piston end of theplunger.



During ADVANCED timing (low fuelpressure), the spring opposes the fuelpressure and holds the plunger in the openposition. Pressurized lube oil flows to thetappets and initiates ADVANCED enginetiming. As fuel pressure increases, thespring holds the plunger in the openposition until the fuel pressure rises abovethe certified switching pressure.

At this certified level, the higher fuelpressure overcomes the spring. Thisaction shifts the plunger and closes the oilpassage. The oil supply to the tappets isinterrupted and the engine begins tooperate in the NORMAL timing mode.

The control valve supplies oil to the STCrocker housing connection through theSTC valve oil outlet line (1).

A check valve in the elbow fitting (2)prevents the oil from draining back into theengine when it is shut off. This prevents anydelay of oil to the tappets during coldstarts.

An internal oil manifold connects the oilsupply to each STC injector in the rockerhousing.



Fuel pressure to the STC valve is providedby a hose (1) between the fuel inletpassages in the cylinder head and theSTC valve. The internal spring cavity of thevalve is vented to the engine crankcase bythe crankcase vent line (2) in order to allowthe plunger to cycle freely.

The oil control valve is calibrated to aspecific flow and pressure using a fuelpump test stand. Tampering with the valveor plumbing will result in the loss of bothfuel economy and engine durability.Correct valve operation is necessary tomaintain acceptable cylinder pressuresand white smoke levels and to assureoptimum fuel economy.

Theory of Operation - CELECT and CELECT Plus Fuel System

The following is a short description of thetheory of operation for the Cummins CELECT and CELECT Plus fuelsystem.



In the CELECT and CELECT Plus fuel system, the amount of fuel burned bythe engine is the amount of fuel injectedinto the injector metering chamber andinjected into the cylinder. The quantitymetered is dependent only on the time thatfuel is allowed to flow into the meteringchamber.

The fuel pump provides a constant fuel flowat a relatively constant pressure to theinjectors. The ECM, controls the time offuel metering.

The main functions of the injector are toprovide:

Metered fuel for injectionInjection timing controlInject fuel into the cylinder.

The main functions of the ECM are toprovide:

Idle speed governingMaximum speed governingOperator control of power outputbelow governed speed (throttle,PTO, Cruise)Control of exhaust smoke duringacceleration (AFC-Air Fuel Control)Control fuel metering timeDetermine injection timing.



Hydromechanical Subsystem

The fuel pump is located in the samelocation as a PT fuel pump.

The fuel pump is a gear type pump. Theassembly includes, a pressure regulator,pulsation damper and a solenoid valve.

1. Pressure regulator2. Pulsation damper3. Solenoid valve.

The ECM mounts on a cooling plate.During engine operation, fuel circulatesthrough the cooling plate to absorb heatgenerated by the ECM.

The injector assembly includes a solenoidvalve, which controls the end of the fuelmetering and the beginning of injection.The solenoid valve is normally open. Anelectronic signal from the ECM closes thevalve as required.



As in the PT fuel system, the CELECT and CELECT Plus fuel systems use thecamshaft to create adequate pressure forinjection.

Here is a cutaway view of the electroniccontrolled injector with the internalcomponents identified.

Injection Cycle

At the start of metering, the meteringplunger and the timing plunger are at thelower limits of their travel. The injectorcontrol valve is closed.



As the camshaft rotates, the timing plungerreturn spring forces the timing plungerupward.

Fuel flows past the metering check ball andinto the metering chamber. This flowcontinues as long as the timing plunger ismoving upward , and the injector controlvalve is closed.

Supply pressure, acting on the bottom ofthe metering piston, forces it to maintaincontact with the timing plunger.

The ECM determines the end of themetering by signaling the injector controlvalve to open .

Fuel at supply pressure then flows into thetiming chamber, thereby stopping meteringpiston travel.

During this time, the bias spring makessure the metering plunger remainsstationary, that it does not drift upward as

the timing plunger moves upward. Thissame force against the metering plungerresults in enough fuel pressure below thepiston to keep the metering check ballseated.

A precisely metered quantity of fuel is nowtrapped in the metering chamber. Thisdetermines the quantity of fuel that will beinjected.



The timing plunger continues to moveupward, and the timing chamber fills withfuel.

Now the timing plunger begins itsdownward travel. Initially, the injectorcontrol valve remains open, allowing fuel toflow from the timing chamber, through theinjector control valve, to the fuel supplypassage.

At the appropriate time, as determined bythe ECM, the injector control valve closes,trapping fuel in the timing chamber. Thistrapped fuel creates a solid, hydraulic linkbetween the timing plunger and meteringplunger.

As a result, the metering plunger is forcedto move downward with the timing plunger.

Because the fuel is trapped, the downwardforce on the timing plunger is transferred tothe metering plunger, thereby increasingpressure in the metering chamber.



When this pressure reaches approximately5000 psi, the needle valve begins to beforced upward.

Continued downward movement of thetiming plunger and metering plunger resultsin steadily increasing fuel pressure. Theresult is that fuel is forced past the needlevalve, through the spray holes, and into thecombustion chamber.

Injection continues until the spill passage ofthe metering plunger passes the meteringspill port.

Metering chamber pressure drops rapidly,allowing the needle valve to close abruptly.This action results in a positive end ofinjection. The positive end of the injectionprevents dribble, and results in cleanerburning.

It is also at this point that the pressure reliefvalve "pops off", thereby reducing theeffects of the high pressure "spike" thatoccurs at the time of metering spill.

Immediately after the metering spill port isopened, the upper edge of the meteringplunger passes the timing spill port.

This action allows the fuel in the timingchamber to be spilled back to the fuel drainas the timing plunger completes itsdownward movement.

This completes the injection cycle.



General Information

Injector Drive Train - CELECT AND CELECT Plus

1. Injector



2. Injector link3. Rocker lever4. Push rod5. Cam follower6. Camshaft.

Injector Drive Train - PT (Type D) STC

1. Injector2. Injector link3. Rocker lever4. Push rod5. Cam follower6. Camshaft.

Last Modified: 08-Sep-2011

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Injectors and Fuel Lines - Overview