Cummins N14 STC, Celect, Celect Plus – Service Manual 006-999   Injectors and Fuel Lines – Overview

Theory of Operation – PT Fuel System

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The following is a short description of the theory of operation of the Cummins® PT fuel system.

 
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The PT, or Pressure-Time, fuel system derives its name from the two main variables that affect the amount of fuel metered per cycle in the Cummins® fuel system. “P” stands for pressure at the inlet of the injectors. It is controlled by the fuel pump. “T” refers to the time that is available for fuel to flow into the injectors. It is controlled by engine speed through the camshaft and injection train.

 
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In the PT system, the amount of fuel burned by the engine is the amount that is metered into the injector cup and injected. The quantity metered is dependent on the fuel pressure at the injector, the flow area of the injector and the time the fuel is allowed to flow into the metering chamber of the cup.

 
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The fuel pressure at the injector is controlled by the PT fuel pump. The pressure supplied by the pump varies according to different operating conditions.

 
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The rail pressure supplied by the fuel pump is dependent on the engine speed. It can also be varied by the operator by changing the position of the throttle.

 
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The main functions of the fuel pump are to provide:

  • Transfer of fuel from tank to engine
  • Rail pressure to injectors
  • Idle speed governing
  • Maximum speed governing
  • Operator control of power output below governed speed (throttle)
  • Control of exhaust smoke during acceleration (AFC — Air Fuel Control)
  • Shutdown of the engine.
 
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The flow area of the injectors is determined by the calibration of a complete set of injectors. The injector calibration is determined primarily by the parts which make up the injector. The Control Parts List (CPL) manual lists the injector assembly and flow plus the combination of other basic engine parts which are necessary to produce a given level of engine performance.

 
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The calibration flow rate of the injectors is determined by the size of an adjustable orifice. Each injector in the engine is calibrated to the same flow rate. The flow rate adjustment must be done by a Cummins® Authorized Fuel Systems Location.

 
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The time available for fuel to flow into the cup is determined by the time which the metering orifice in the barrel is uncovered by the plunger. The metering orifice is uncovered while the cam follower roller is on the inner base circle of the cam. The plunger is in the retracted position during this time. The amount of time the metering orifice is uncovered is dependent on engine speed.

 
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Injection of the fuel in the cup occurs when the cam follower roller travels up the injection ramp of the cam forcing the injector plunger downward. The end of injection occurs when the plunger bottoms in the cup.

 
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The metering orifice is covered while the cam follower roller is on the outer base circle of the camshaft. The plunger is in the seated position during this time. Fuel supplied to the injector during this time bypasses the plunger through the drain port and is returned to the fuel tank.

 
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The engine will produce more horsepower at rated speed; however, the engine will produce the most torque at the torque peak speed. The rail pressure is lower at torque peak speed, but the metering time is greater so more fuel is metered per cycle.

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

 
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Theory of Operation – STC Fuel System

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Step Timing Control, commonly referred to as STC, controls the engine timing in an effort to minimize white smoke at cold engine start-up.

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

 
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STC has two stages of injection timing. The engine operates in the ADVANCED mode of injection timing during starting and light engine load conditions and at NORMAL timing during medium to high engine load conditions.

 
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STC offers many advantages. During ADVANCED injection timing, it:

  • Improves cold weather idling characteristics
  • Reduces cold weather white smoke
  • Improves light load fuel economy
  • Reduces injector carboning.
 
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During NORMAL injection timing, STC:

  • Controls cylinder pressures
  • Reduces nitrogen oxide emissions.
 
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The STC system consists primarily of:

  • STC injectors
  • STC oil control valve
  • STC plumbing and check valve.

These components control injection timing based on fuel pump rail pressure (engine load).

 
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In the injector, injection timing is controlled by the STC hydraulic tappet. The tappet has an inner piston (plunger) and an outer piston (sleeve). These tappet components work together with the injector plunger to control injection timing.

 
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In NORMAL timing, no oil is in the tappet. As the cam follower starts up the camshaft injection ramp, the injector rocker lever begins to force the inner piston downward. Because no oil is in the tappet, the inner piston must make direct contact with the outer piston before the injector plunger can begin its downward travel.

 
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In ADVANCED timing, the tappet is filled with oil and the injector is metering fuel. As the cam follower starts up the camshaft injection ramp, the injector lever begins to force the inner piston downward. Since the oil between the pistons forms a solid link, the downward pressure is immediately transmitted to the outer piston and the injector plunger begins its downward travel earlier than it does in NORMAL timing. This causes the fuel to be injected earlier.

 
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Engine oil flows from the STC oil control valve through the oil manifold to the tappets. Whenever the oil pressure in the oil manifold exceeds 70 kPa [10 psi], it moves the tappet inlet check ball from its seat and fills the cavity between the inner and outer pistons.

 
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During the injection cycle, the oil is held inside the tappet by the inlet check ball and the load cell check ball. When the rocker lever forces the inner piston downward, the solid link of oil causes the injector plunger to contact the fuel earlier; therefore, the injection timing is in the ADVANCED mode. At the end of the injection cycle, injection force increased the oil pressure in the tappet and holds the injector plunger firmly in the cup.

 
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This increased pressure moves the load cell check ball from its seat. The oil drains past the load cell check ball and through the drain holes in the injector adapter and returns to the oil pan through drain passages in the cylinder head and block. Meanwhile, with continued cam lift, the inner piston makes mechanical contact with the outer piston and maintains injector plunger seating force.

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

 
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During ADVANCED timing (low fuel pressure), the spring opposes the fuel pressure and holds the plunger in the open position. Pressurized lube oil flows to the tappets and initiates ADVANCED engine timing. As fuel pressure increases, the spring holds the plunger in the open position until the fuel pressure rises above the certified switching pressure.

 
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At this certified level, the higher fuel pressure overcomes the spring. This action shifts the plunger and closes the oil passage. The oil supply to the tappets is interrupted and the engine begins to operate in the NORMAL timing mode.

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

A check valve in the elbow fitting (2) prevents the oil from draining back into the engine when it is shut off. This prevents any delay of oil to the tappets during cold starts.

 
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An internal oil manifold connects the oil supply to each STC injector in the rocker housing.

 
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Fuel pressure to the STC valve is provided by a hose (1) between the fuel inlet passages in the cylinder head and the STC valve. The internal spring cavity of the valve is vented to the engine crankcase by the crankcase vent line (2) in order to allow the plunger to cycle freely.

 
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The oil control valve is calibrated to a specific flow and pressure using a fuel pump test stand. Tampering with the valve or plumbing will result in the loss of both fuel economy and engine durability. Correct valve operation is necessary to maintain acceptable cylinder pressures and white smoke levels and to assure optimum fuel economy.

 
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Theory of Operation – CELECT™ and CELECT™ Plus Fuel System

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The following is a short description of the theory of operation for the Cummins CELECT™ and CELECT Plus™ fuel system.

 
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In the CELECT™ and CELECT Plus™ fuel system, the amount of fuel burned by the engine is the amount of fuel injected into the injector metering chamber and injected into the cylinder. The quantity metered is dependent only on the time that fuel is allowed to flow into the metering chamber.

 
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The fuel pump provides a constant fuel flow at a relatively constant pressure to the injectors. The ECM, controls the time of fuel metering.

 
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The main functions of the injector are to provide:

  • Metered fuel for injection
  • Injection timing control
  • Inject fuel into the cylinder.
 
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The main functions of the ECM are to provide:

  • Idle speed governing
  • Maximum speed governing
  • Operator control of power output below governed speed (throttle, PTO, Cruise)
  • Control of exhaust smoke during acceleration (AFC-Air Fuel Control)
  • Control fuel metering time
  • Determine injection timing.
 
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Hydromechanical Subsystem


The fuel pump is located in the same location as a PT fuel pump.

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

  1. Pressure regulator
  2. Pulsation damper
  3. Solenoid valve.
 
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The ECM mounts on a cooling plate. During engine operation, fuel circulates through the cooling plate to absorb heat generated by the ECM.

 
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The injector assembly includes a solenoid valve, which controls the end of the fuel metering and the beginning of injection. The solenoid valve is normally open. An electronic signal from the ECM closes the valve as required.

 
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As in the PT fuel system, the CELECT™ and CELECT Plus™ fuel systems use the camshaft to create adequate pressure for injection.

 
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Here is a cutaway view of the electronic controlled injector with the internal components identified.

 
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Injection Cycle


At the start of metering, the metering plunger and the timing plunger are at the lower limits of their travel. The injector control valve is closed.

 
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As the camshaft rotates, the timing plunger return spring forces the timing plunger upward.

Fuel flows past the metering check ball and into the metering chamber. This flow continues as long as the timing plunger is moving upward , and the injector control valve is closed.

Supply pressure, acting on the bottom of the metering piston, forces it to maintain contact with the timing plunger.

 
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The ECM determines the end of the metering by signaling the injector control valve to open .

 
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Fuel at supply pressure then flows into the timing chamber, thereby stopping metering piston travel.

During this time, the bias spring makes sure the metering plunger remains stationary, that it does not drift upward as the timing plunger moves upward. This same force against the metering plunger results in enough fuel pressure below the piston to keep the metering check ball seated.

A precisely metered quantity of fuel is now trapped in the metering chamber. This determines the quantity of fuel that will be injected.

 
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The timing plunger continues to move upward, and the timing chamber fills with fuel.

 
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Now the timing plunger begins its downward travel. Initially, the injector control valve remains open, allowing fuel to flow from the timing chamber, through the injector control valve, to the fuel supply passage.

 
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At the appropriate time, as determined by the ECM, the injector control valve closes, trapping fuel in the timing chamber. This trapped fuel creates a solid, hydraulic link between the timing plunger and metering plunger.

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

Because the fuel is trapped, the downward force on the timing plunger is transferred to the metering plunger, thereby increasing pressure in the metering chamber.

 
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When this pressure reaches approximately 5000 psi, the needle valve begins to be forced upward.

Continued downward movement of the timing plunger and metering plunger results in steadily increasing fuel pressure. The result is that fuel is forced past the needle valve, through the spray holes, and into the combustion chamber.

 
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Injection continues until the spill passage of the metering plunger passes the metering spill port.

Metering chamber pressure drops rapidly, allowing the needle valve to close abruptly. This action results in a positive end of injection. The positive end of the injection prevents dribble, and results in cleaner burning.

It is also at this point that the pressure relief valve “pops off”, thereby reducing the effects of the high pressure “spike” that occurs at the time of metering spill.

 
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Immediately after the metering spill port is opened, the upper edge of the metering plunger passes the timing spill port.

 
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This action allows the fuel in the timing chamber to be spilled back to the fuel drain as the timing plunger completes its downward movement.

This completes the injection cycle.

 
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General Information

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Injector Drive Train – CELECT™ AND CELECT Plus™

  1. Injector
  2. Injector link
  3. Rocker lever
  4. Push rod
  5. Cam follower
  6. Camshaft.

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Injector Drive Train – PT (Type D) STC

  1. Injector
  2. Injector link
  3. Rocker lever
  4. Push rod
  5. Cam follower
  6. Camshaft.

Last Modified:  08-Sep-2011