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| Illustration 1 | g00547001 |
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HEUI fuel system (typical example) (1) Unit injector hydraulic pump (2) Oil flow to the engine (3) Oil filter (4) Engine oil pump (5) Injectors (6) Oil cooler (7) IAP control valve (8) IAP sensor (9) Secondary fuel filter (10) Fuel transfer pump (11) Fluid manifolds (12) Fuel tank (13) Fuel pressure regulator (14) Speed-timing wheel (15) Primary fuel filter (16) Water separator (17) Engine speed/timing sensors (18) Oil temperature sensor (19) Engine boost pressure sensor (20) Coolant temperature sensor (21) Coolant level sensor (22) Oil pressure sensor (23) Fuel pressure sensor (24) Fuel temperature sensor (25) Atmospheric pressure sensor (26) Throttle position sensor (27) Data link (28) Alarm warning lamp (29) Diagnostic lamp (30) Synchronization switch (31) Trolling mode switch (32) Slow vessel mode switch (33) Maintenance clear switch (34) Electronic Control Module (ECM) (35) Batteries | |
The operation of the Hydraulic Electronic Unit Injector (HEUI) fuel system utilizes the concepts of hydraulics and the multiplication of force to deliver fuel to the engine. The HEUI fuel system is completely free of adjustment. Adjustments cannot be made to the mechanical components of the system. Changes in performance are made by installing different software in Electronic Control Module (ECM) (34).
This fuel system consists of six basic components:
- Hydraulic Electronic Unit Injector (HEUI) (5)
- Electronic Control Module (ECM) (34)
- Unit injector hydraulic pump (1)
- Injection actuation pressure control valve (7)
- Fuel transfer pump (10)
- Injection actuation pressure sensor (8)
Note: The components of the HEUI fuel system are not serviceable components. These fuel system components must not be disassembled. Disassembly will damage the components. If the components have been disassembled, Caterpillar may not allow a warranty claim or Caterpillar may reduce the warranty claim.
Hydraulic Electronic Unit Injector
The HEUI fuel system utilizes a hydraulically actuated electronically controlled unit injector (5).
The precise delivery of the fuel controls the engine's performance. All fuel systems for diesel engines use a plunger and barrel in order to pump high pressure fuel into the combustion chamber. A fuel injection pump camshaft lobe is typically used to provide a mechanical force to the plunger. The plunger then pumps the precise amount of fuel into the combustion chamber. The HEUI fuel system uses engine oil that has been pressurized by the system's hydraulic pump in order to apply force to the plunger. Control for the exact timing of the fuel delivery is provided electronically by the engine's ECM. Due to the differences in the HEUI fuel system, a technician must use different troubleshooting methods in order to diagnose fuel system problems.
The HEUI fuel system's hydraulic pump pressurizes the engine lubrication oil from
Engine oil is used by the unit injector hydraulic pump in order to supply hydraulic pressure to the injectors. This hydraulic pressure is called injector actuation pressure. The actuation pressure of the oil generates the high injection pressures that are delivered by the unit injector. This injection pressure is greater than actuation pressure by approximately six times. The pressure in the system is multiplied by the intensifier piston that is located in the injector.
Low actuation pressure results in low injection pressures. During conditions of low engine speed such as idle and start, the low injection pressure is due to the low actuation pressure that is being produced by the unit injector hydraulic pump.
High actuation pressure results in high injection pressures. During conditions of high speed such as high idle and acceleration, high injection pressures can be produced because of the high actuation pressures that are produced by the hydraulic pump.
There are many other operating conditions when the injection pressure fluctuates between the minimum and the maximum. Regardless of the speed of the engine, the HEUI fuel system provides infinite control in order to provide the optimum fuel injection pressure.
Electronic Control Module (ECM)
The Electronic Control Module (ECM) (34) is mounted directly on the engine. The ECM is a powerful computer that provides total electronic control of engine performance. The ECM gathers performance data from the engine through a series of engine sensors. This data is used by the ECM in order to modify the engine's fuel delivery, injection pressure, and injection timing. The ECM also contains performance maps in the form of software that define engine's horsepower, torque curves, and rpm.
Most of today's engines are equipped with an ECM that can be reprogrammed in the field. There are electronic service tools that can be used to program the ECM. These electronic service tools use flash programming in order to load new software into the ECM.
The ECM is also used to record engine faults that may occur. These faults are usually triggered when one of the engine sensors detect a parameter that is operating out of the normal range of operation. An electronic service tool can be used in conjunction with the engine ECM to run several diagnostic tests on engine's electrical systems or electronic systems.
The unit injector hydraulic pump (1) is a high pressure hydraulic pump that is located at the front of the engine. The unit injector hydraulic pump is a variable displacement axial piston pump that is driven by the front gear train of the engine. The unit injector hydraulic pump uses a portion of the engine lubrication oil to supply the HEUI fuel system. The unit injector hydraulic pump pressurizes the engine lubrication oil to the correct injection actuation pressure in order to power the HEUI injectors.
Injection Actuation Pressure Control Valve (IAP Control Valve)
The Injection Actuation Pressure Control Valve (IAP Control Valve) (7) is located on the side of unit injector hydraulic pump (1). The pressure control valve assembly controls the outlet flow of the hydraulic pump. The pressure control valve assembly also controls the hydraulic pump pressure.
There are three components of the pressure control valve assembly.
- Injection actuation pressure control valve
- Compensator valve assembly
- Valve base
The compensator valve assembly contains three major parts:
- Load sensing spool
- Pressure limiter spool
- Check valve
The load sensing spool controls the oil flow to the control piston. The control piston controls the swashplate angle. The swashplate angle determines the pressure that is produced by the pump.
In the event of a malfunction of the pump, the pressure limiter spool acts as an emergency relief valve. A malfunction of the pump would cause the pressure to rise above the relief setting. The pressure limiter spool is a simple spring loaded relief valve. The valve opens at a preset pressure. When the valve opens, high pressure oil is sent to the control piston. This will destroke the pump and the oil flow that is being produced by the pump will be reduced.
The check valve works in conjunction with the pressure limiter spool. The valve allows high pressure oil to flow to the control piston when the pressure limiter spool has opened. The check valve remains closed at all other times.
The IAP control valve is an electrically controlled solenoid valve. The IAP control valve works with the load sensing spool in order to control the pump outlet pressure. The IAP control valve is actually an electrically operated hydraulic pressure relief valve. The IAP control valve converts an electrical signal from the ECM to the mechanical control of the spool valve in order to control the pump's outlet pressure.
Under most conditions, the pump is producing an excess oil flow. The IAP control valve instructs the load sensing spool to discharge excess pump flow to the control piston in order to control injection actuation pressure at the desired level. The IAP control valve is a solenoid valve of high precision. The IAP control valve is used to control the actuation pressure that provides hydraulic pressure to the injectors. The performance maps that are programmed into the ECM contain a desired actuation pressure for every engine operating condition. The ECM uses a control current in order to control the IAP control valve. This control current is used to vary the action of the solenoid in order to maintain an actual actuation pressure that is very near to the desired actuation pressure that has been determined by the ECM.
Fuel transfer pump (10) is mounted on the back of unit injector hydraulic pump (1). The fuel transfer pump must first draw fuel from fuel tank (12). Then, the fuel transfer pump must be capable of providing enough flow to the low pressure fuel system in order to maintain a continuous system pressure. A normal system pressure for the low pressure fuel system is usually between
The fuel transfer pump is a fixed displacement gear pump. The fuel transfer pump contains an integral pressure relief valve. This relief valve opens at approximately
Injection Actuation Pressure Sensor (IAP)
IAP sensor (8) monitors the actual injection actuation pressure. The oil manifold supplies the injectors with a continuous flow of actuation oil. This oil is used to power the injectors. The IAP sensor is installed in this high pressure oil manifold. The IAP sensor monitors the oil pressure in the manifold. The ECM is continuously monitoring the IAP sensor for pressure changes. The ECM interprets this signal in order to provide control for the engine's fuel system.
Operation of the HEUI Fuel System
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| Illustration 2 | g00547001 |
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Low pressure fuel system (typical example) (5) Injectors (9) Secondary fuel filter (10) Fuel transfer pump (11) Fluid manifolds (12) Fuel tank (13) Fuel pressure regulator (15) Primary fuel filter (16) Water separator | |
The low pressure fuel system serves four basic functions. The system supplies the injectors (5) with fuel for combustion. Supplies extra fuel flow for cooling of the injectors. This extra fuel flow removes air from the system. The system also supplies the fuel that is used to cool the ECM.
The low pressure fuel system consists of seven basic components:
- Fuel tank (12)
- Primary fuel filter (15)
- Water separator (16)
- Secondary fuel filter (9)
- Fuel transfer pump (10)
- Fuel pressure regulator (13)
- Fluid manifolds (11)
Fuel is drawn from fuel tank (12) and flows through the water separator (16). The water separator is typically a 15 to 30 micron filter. The water separator will filter large debris from the fuel. The water separator also has the capacity that will filter large amounts of water from the fuel. If equipped, the fuel may flow to the primary fuel filter (16). The primary fuel filter is used to filter the fuel before entering the fuel transfer pump.
Fuel flows from the primary fuel filter to the inlet side of fuel transfer pump (10). The fuel transfer pump is mounted on the back of unit injector hydraulic pump. Fuel is drawn into the inlet port of the pump. An inlet check valve in the inlet port of the fuel transfer pump prevents fuel from flowing back into the fuel tank while the engine is not running. The fuel flow is increased by a simple gear pump and the fuel is then discharged through the outlet port of the pump. The outlet port also incorporates a check valve that is used to prevent pressurized fuel leakage back through the pump.
The fuel transfer pump is used in order to pressurize the fuel that supplies the low pressure fuel system. The maximum pressure that is generated by the fuel transfer pump is limited to
Fuel flows from the outlet port of the fuel transfer pump to the secondary fuel filter (9). The secondary fuel filter is a two micron fuel filter. The two micron fuel filter removes very small abrasive contaminants in the fuel. Fuel then flows from the secondary fuel filter to the fuel supply passages that are drilled into fluid manifolds (11).
The fluid manifolds are mounted on top of the cylinder heads. A fuel supply passage runs for the length of the fluid manifold. This passage connects with each unit injector bore in order to supply fuel to the unit injectors. Pressurized fuel flows through the fluid manifold to all of the unit injectors. Excess fuel flows out of the fluid manifold, into the fuel return line, and then to the fuel pressure regulator (13).
The fuel pressure regulator consists of an orifice and a spring loaded check valve. The orifice is a flow restriction that provides a back pressure to the supply fuel. The spring loaded check valve opens at
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| Illustration 3 | g00990201 |
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Actuation Oil Flow (typical example) (1) Unit injector hydraulic pump (3) Oil filter (4) Engine oil pump (6) Oil cooler (7) IAP control valve (8) IAP sensor | |
The injection actuation system serves two functions. The injection actuation system supplies high pressure oil in order to power the HEUI injectors. Also, the injection actuation system utilizes control of the actuation pressure of the oil in order to control the injection pressure of the fuel that is produced by the unit injectors.
The injection actuation system consists of six basic components:
- Hydraulic pump (1)
- Engine oil filter (3)
- Engine oil pump (4)
- Oil cooler (6)
- The Injection Actuation Pressure control valve (IAP control valve) (7)
- The Injection Actuation Pressure sensor (IAP sensor) (8)
Oil from engine oil pump (4) supplies engine oil to unit injector hydraulic pump (1). The capacity of the engine oil pump has been increased in order to meet the additional flow that is required to supply the hydraulic pump.
Oil that is drawn from the engine oil pan is pressurized to the lubrication system oil pressure by the engine oil pump. Oil flows from the engine oil pump through engine oil cooler (6), through engine oil filter (3), and then to the main oil gallery. A separate circuit from the main oil gallery directs a portion of the lubrication oil in order to supply the unit injector hydraulic pump. A steel tube on the left side of the engine connects the main oil gallery with the inlet port of the unit injector hydraulic pump.
Oil flows into the inlet port of the unit injector hydraulic pump and the pump reservoir is filled with engine oil. The pump reservoir provides oil to the unit injector hydraulic pump during engine start-up. Also, the pump reservoir provides oil to the unit injector hydraulic pump until the engine oil pump can increase pressure enough to provide the pump with a steady flow of oil.
The pump reservoir also provides makeup oil to the high pressure oil passage in the cylinder head. When the engine is off and the engine cools down, the oil shrinks. A check valve in the pump allows oil to be drawn from the pump reservoir in order to keep the high pressure oil passage full, even during engine shutdown.
Oil from the pump reservoir is pressurized in the unit injector hydraulic pump and flows out of two outlet ports of the pump under high pressure. The high pressure oil flows from the outlet ports of the unit injector hydraulic pump then flows through a one-way check valve. The oil then flows to the high pressure oil passage that is within the fluid manifold. The check valve is used to prevent high pressure pulses, that are generated by the injectors, from returning to the pump. High pressure pulses would cause the IAP control valve (7) to operate erratically. This would cause the actuation pressure to become unstable and unpredictable.
The high pressure oil passage connects with each unit injector bore in order to supply high pressure actuation oil to the unit injectors. High pressure actuation oil flows from the unit injector hydraulic pump and travels through the fluid manifold to all of the injectors. The high pressure oil is held in the high pressure oil passage until the oil is used by the unit injectors. Oil that has been exhausted by the unit injectors is expelled under the valve covers. This oil returns to the crankcase through oil drain holes in the cylinder head.
Actuation Oil Pressure Control
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| Illustration 4 | g00547161 |
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Injection actuation pressure control system (typical example) (1) Unit injector hydraulic pump (2) Oil flow to the engine (3) Oil filter (4) Engine oil pump (5) Injectors (6) Oil cooler (7) IAP control valve (8) IAP sensor (34) Engine control module (ECM) | |
Unit injector hydraulic pump (1) is a variable displacement axial piston pump. The flow of this pump can be varied from the minimum to the maximum at any engine speed.
The rotating group of the pump changes the rotary motion of the pump shaft to hydraulic oil flow. The rotating group has three components:
- Barrel and pistons
- Swashplate
- Pump shaft
The pump supplies the flow of oil to the injectors. The amount of oil flow controls the system pressure. Pump flow is increased or decreased within the pump by changing the angle of the swashplate.
The swashplate is moved toward maximum flow by a control spring. The maximum angle produces maximum piston stroke and maximum pump flow. The control piston is used to counter the control spring. The control piston is in a retracted state when the swashplate is at the maximum angle. The control spring will be in an expanded state.
Pump flow is reduced by an increased oil flow to the control piston. As the pressure to the control piston increases, the piston pushes the swashplate toward the minimum angle. The swashplate angle will be reduced and the pistons produce minimum stroke at this minimum angle. Minimum output will be produced.
Destroking the pump - This term is used to describe a decrease in the angle of the swashplate in order to decrease the output of the pump. Oil flow is being applied to the control piston.
Stroking the pump - This term is used to describe an increase in the angle of the swashplate in order to increase the output of the pump. Oil flow is being removed from the control piston.
The pump housing contains the following components:
- Rotating group
- Internal oil reservoir
The reservoir provides oil to the unit injector hydraulic pump while the engine is being cranked. The reservoir provides oil to the injection system until oil flow from engine oil pump (4) is established.
Supply oil from the engine lube system flows through the reservoir to the inlet port of the rotating group. The high pressure actuation oil flows from the outlet port of the pump and flows through steel tubing in order to feed the high pressure fluid manifolds that are on each cylinder head.
While the engine is not running, the swashplate control spring in the unit injector hydraulic pump pushes the swashplate to the maximum angle. The maximum pump displacement is achieved. During cranking of the engine, the pump produces maximum flow. This builds actuation pressure rapidly until the desired actuation pressure is reached.
Once the actuation pressure matches the desired pressure, oil is sent from the IAP control valve to the control piston. This will destroke the pump. At idle conditions, a minimum swashplate angle is required to maintain the desired actuation pressure. HEUI injectors (5) use very little actuation oil at either no load conditions or low idle conditions.
When a load is applied to the engine, the desired fuel rate increases. Also, the demand for actuation oil flow and pressure rapidly increase. The Electronic Control Module (ECM) (34) detects the decrease in engine speed that is caused by the increase in load. The ECM then increases the control current to IAP control valve (7). This allows oil to drain from the control piston. This forces the swashplate angle and the pump flow to quickly increase. The swashplate angle will increase until actual pressure equals desired pressure at the flow rate that is required by the injectors.
If the load on the engine is decreased, the actuation oil flow is decreased in order to match the engine requirements. The ECM detects the increase in engine speed and the current that is being sent to the IAP control valve is reduced. Oil is directed to the control piston. This will decrease the swashplate angle. Pump output flow and actuation pressure decrease until actual pressure equals desired pressure.
There are two types of actuation pressure:
- Desired actuation pressure
- Actual actuation pressure
Desired actuation pressure is the injection actuation pressure that is required by the system for optimum engine performance. The desired actuation pressure is established by the performance maps in the ECM and information from the engine sensors. This information is used in order to calculate the optimum pressure to use for the best engine performance. The desired actuation pressure is constantly changing based on various sensor inputs, changing engine speed and load. The following sensors supply signals to the ECM:
- Throttle position sensor (26)
- Engine boost pressure sensor (19)
- Speed/timing sensors (17)
- Coolant temperature sensor (18)
These signals are used by the ECM in order to calculate the desired actuation pressure. The desired actuation pressure is only constant under steady state conditions (steady engine speed and load). The desired actuation pressure is continuously adjusted by the ECM.
Actual actuation pressure is the actual system pressure of the actuation oil that is used to power the injectors (5). The IAP control valve (7) is constantly adjusting the amount of pump flow that is discharged to the drain. The pump flow is discharged to the drain in order to match the actual actuation pressure to the desired actuation pressure.
Three components operate together in order to control injection actuation pressure:
- ECM (34)
- IAP control valve (7)
- IAP sensor (8)
The ECM (34) calculates the desired actuation pressure by sampling sensor inputs and referencing performance maps. The ECM sends a control current to the IAP control valve (7) in order to change the actual actuation pressure. The IAP control valve reacts to the electrical current from the ECM in order to change the actual actuation pressure. The actual actuation pressure is changed when the IAP control valve discharges control pressure oil to the drain. The IAP control valve acts as an electrically controlled relief valve. The IAP sensor (8) monitors the actual actuation pressure in the high pressure oil passage. The IAP sensor reports the actual actuation pressure by sending a signal voltage to the ECM.
The injection actuation pressure control system operates in a cycle. The ECM (34) calculates the desired actuation pressure. After the correct signal has been calculated, the ECM sends an electrical current to the IAP control valve (7) in order to adjust the actuation pressure. The IAP control valve reacts to the electrical current from the ECM by changing the pressure relief setting for the control piston, which changes the actual actuation pressure. The IAP sensor (8) monitors the actual actuation pressure and the IAP sensor sends a signal voltage back to the ECM. The ECM interprets the signal voltage from the IAP sensor in order to calculate the actual actuation pressure. Then, the ECM compares the actual actuation pressure to the desired actuation pressure in order to adjust the electrical current to the IAP control valve. The IAP control valve responds to the change in electrical current by changing the actual actuation pressure. This process is repeated 60 times per second. This cycle of constant repetition is called a closed loop control system.
Increasing current to the IAP control valve (7) causes the actuator solenoid that controls the poppet valve in the IAP control valve to be excited. As the poppet valve closes the drain port, the oil flow from the load sensing spool decreases and the spool allows oil from the control piston to be vented to the case drain. As the control piston retracts, the swashplate angle is increased. There is an increased flow from the pump outlet.
Reducing the current to the IAP control valve causes the following actions to occur. The actuator solenoid that controls the poppet valve in the IAP control valve is relaxed. The poppet valve opens the drain port and a proportional amount of oil is allowed to flow from the load sensing spool. As the load sensing spool reacts, oil is sent to the control piston and the angle of the swashplate is reduced. There is a decreased flow from the pump outlet.
If the IAP control valve fails to receive the control current during engine operation, the only force that will act on the load sensing spool will be the mechanical force of the spool's spring. The pressure that is produced from this spring force is approximately
A margin pressure is necessary for this system in order to establish the engine with a limp home mode in the event of system failure. The spool spring also improves the accuracy of the IAP control valve.
The limp home mode will allow the engine to keep running at a very low actuation pressure. This could happen if the IAP control valve fails or the circuit experiences an open circuit condition.
This spring pressure also improves the ability of the IAP control valve to accurately control lower actuation pressures.
Margin pressure is not a critical adjustment. Margin pressure does not affect normal engine performance. Margin pressure must be set high enough to keep the engine running in the event of an open circuit or a control valve failure. Margin pressure should not be set too high. An excessively high margin pressure will cause overfueling and hard starting of the engine. This will occur when the engine is cold and the oil is thick. Margin pressure is preset at the factory. The pressure should not be adjusted in the field. Increasing or decreasing margin pressure from the factory setting will not increase engine horsepower or engine performance.
The combined force of the spool spring and the oil flow that is controlled by the IAP control valve (7) work together in order to position the load sensing spool. If the margin pressure is changed, the ECM (34) compensates by adjusting the current to the IAP control valve in order to obtain the desired actuation pressure that has been calculated.
The unit injector hydraulic pump contains a pressure limiter spool. The pressure limiter spool is located just above the load sensing spool. The pressure limiter spool will only work when an extreme pressure exists in the system. If an extreme pressure is allowed to exist, the system pressure could exceed the maximum safe operating pressure.
The pressure limiter spool is held in the closed position by a spring. If a malfunction occurs, the pump outlet pressure may exceed the safe limit of the pump. In this case, the pressure would overcome the spring force and the relief spool would vent the excess pressure. This will allow the pump outlet pressure to flow to the control piston. The extra flow to the control piston would destroke the pump. The pump will continue to destroke until the outlet pressure becomes less than relief pressure and the relief valve closes.
This pressure control system also incorporates a one-way check valve that allows the outlet pressure to flow from the relief valve to the control piston. The check valve will not allow oil from the control piston to flow in the opposite direction when the relief valve is closed.
The relief valve is set at the factory. The relief valve should not be adjusted. A low relief setting will cause the relief valve to open below normal operating pressure. This will result in low engine power.
A high relief valve setting will not affect normal operation. A high relief valve setting could rupture the pump housing in the event of a malfunction. Adjusting the relief valve setting will not increase the actuation pressure, engine horsepower, or engine performance.
Most of the high pressure oil flow from the unit injector hydraulic pump is used in order to power the unit injectors. Excess flow is the amount of pump flow that is not required in order to meet the desired actuation pressure. The excess flow is returned to the case drain through the load sensing spool. The excess flow travels through a drilled passage to the front of the pump. Drain oil flows out of the front of the pump over the pump drive gear and flows down the engine front gear train to the engine oil sump.
Operation of the Injection Actuation Pressure Control Valve (IAP Control Valve)
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| Illustration 5 | g00990340 |
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Injection actuation pressure control valve (1) Spring retainer (2) Edge filter (3) Seat assembly (4) Drain port (5) Armature (6) Valve body (7) Adapter (8) Poppet (9) Push pin (10) Control solenoid | |
The IAP control valve is an electrically controlled pilot operated pressure control valve. The IAP control valve is used in order to adjust the actuation pressure. The actual actuation pressure must be constantly adjusted in order to achieve the desired actuation pressure and this pressure must be controlled regardless of engine speed, pump flow, and variable oil demand of the unit injectors. The IAP control valve consists of six basic components:
- Seat Assembly (3)
- Armature (5)
- Poppet (8)
- Push pin (9)
- Control solenoid (10)
The IAP control valve operates by using the variable electrical current from the ECM in order to create a magnetic field in control solenoid (10). This magnetic field acts on armature (5) and the magnetic field generates a mechanical force. This mechanical force is used to adjust the position of the armature. The adjustment on the armature affects the position of push pin (9) and poppet (8).
When the poppet is in the closed position, the poppet is also opposed by the oil pressure that is inside valve body (6). The oil pressure inside the valve body is trying to open the poppet. As the oil pressure from the load sensing spool valve increases, the force on the poppet from the oil pressure also increases. As this force overcomes the mechanical force of the solenoid, the poppet opens. The open poppet allows a flow path to drain port (4) for the oil pressure. Discharging part of the oil pressure to drain lowers the hydraulic pressure that is inside the valve body. When the hydraulic pressure of oil decreases below the magnetic force on the poppet, the poppet closes again.
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| Illustration 6 | g00990427 |
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Operation of the injection actuation pressure control valve (engine off) (1) Oil pressure from load sensing spool (2) Current from ECM (3) Drain port (4) Poppet | |
When the engine is off, there is no oil pressure from load sensing spool (1) and there is no current from ECM (2). The poppet is in the open position.
Valve Operation (Engine Cranking)
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| Illustration 7 | g00990461 |
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Operation of the injection actuation pressure control valve (engine cranking) (1) Oil pressure from load sensing spool (2) Current from ECM (3) Drain port (4) Poppet | |
During engine start-up, approximately
In order for the engine to start quickly, the injection actuation pressure must rise quickly. Because the hydraulic pump is being turned at engine cranking speed, pump flow is very low. The ECM sends a strong current (2) to the IAP control valve in order to keep poppet (4) closed. With the poppet in the closed position, all of the flow through drain port (3) is blocked. Oil flow through the drain port remains blocked until an actual actuation pressure of
Note: If the engine is already warm, the pressure that is required to start the engine may be higher than
Once the unit injectors begin to operate, the ECM begins to control the current to the IAP control valve. The ECM signals the IAP control valve to maintain the actual actuation pressure at
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| Illustration 8 | g00990473 |
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(1) Oil pressure from load sensing spool
(2) Current from ECM (3) Drain port (4) Poppet | |
As the engine cranks, oil pressure from load sensing spool (1) enters the end of the valve body. The oil pressure begins to act against the poppet (4). The hydraulic force that is applied by the oil pressure from load sensing spool attempts to push against the poppet in order to open the drain port. The current from ECM (2) causes the solenoid to generate a magnetic field which forces the poppet against the drain port of the spool chamber. This closes the drain port. The drain port is the only path to the drain for the oil in the valve body. The pump outlet pressure flows to the load sensing spool valve. The load sensing spool valve dumps the oil directly to the case drain. As the pump outlet pressure increases, the pressure in the valve body will also increase. While the pump outlet pressure does not overcome the force on the poppet, this path to the drain will remain blocked. The load sensing spool will continue to dump the oil pressure to the case drain and the angle of the swashplate will remain at the maximum.
The combination of the force from the current from the ECM and the low oil pressure in the valve body will hold the poppet in the closed position. The drain port will remain closed while the poppet is in the closed position. This will continue until the actual actuation pressure reaches
Valve Operation (Running Engine)
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| Illustration 9 | g00990519 |
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Operation of the injection actuation pressure control valve (running engine) (1) Oil pressure from load sensing spool (2) Current from ECM (3) Drain port (4) Poppet | |
Once the engine starts, the current from ECM (2) controls the IAP control valve in order to maintain the desired actuation pressure. The IAP sensor monitors the actual actuation pressure that is in the high pressure oil passage in the fluid manifold. The ECM compares the actual actuation pressure to the desired actuation pressure 60 times per second. If the pressures do not match, the ECM adjusts the current level that is being sent to the IAP control valve. This will bring the actual injection actuation pressure closer to the desired injection actuation pressure.
The amount of current that is sent to the solenoid regulates the amount of magnetic force that is being used to hold poppet (4) closed. The solenoid, the armature, and the push pin simulate a variable spring that is electronically controlled. Increased current results in increased force on the poppet. Decreased current results in a decrease of force that is acting on the poppet.
The magnetic force that is controlled by the ECM is used to hold the poppet closed. When the poppet is closed, the pressure in the valve body increases. When the pressure in the valve body exceeds the force that is holding the poppet closed, the poppet will begin to open. When the poppet opens, the volume of oil that is in the valve body begins to escape to the drain. This causes the pressure in the valve body to drop. When the pressure in the valve body drops, the poppet closes again. As the poppet closes, the pressure begins to increase and the cycle is repeated. This process provides control to the position of the load sensing spool. The position of the load sensing spool controls the oil flow to the control piston.
The IAP control valve uses oil flow to control the position of the load sensing spool. The force of the oil pressure in the valve body provides resistance against the force of the oil pressure from load sensing spool (1). Controlling this pressure helps to control the position of the valve spool. When the IAP control valve allows oil to pass to drain port (3), the load sensing spool is allowed to shift in the bore of the valve body. An oil port that leads to the control piston is opened and the swashplate angle is decreased. This effectively reduces the actual actuation pressure in the fluid rails.
As the pump pressure decreases, the IAP control valve closes the drain port through the poppet. This reduces the flow of oil that is coming from the load sensing spool. The spool repositions in the bore of the valve body and the oil port for the control piston is blocked. An increase in pump outlet pressure will follow.
The amount of control that is provided for the load sensing spool is controlled by the ECM. The electrical current from the ECM is used to control the position of the poppet valve. By opening and closing the poppet valve, the flow of oil from load sensing spool can be regulated. When the poppet is opened the flow of oil from the load sensing spool is increased. The position of the spool changes so that the flow of oil to the control piston of the swashplate increases. When the electrical current from the ECM closes the poppet, the flow of oil from the load sensing spool is decreased. This will reposition the spool in the bore of the valve body so that the flow to the control piston is reduced.
Most of the time, the poppet and the load sensing spool operate in a partially open position. The poppet and the spool are completely open or completely closed only during the following conditions:
- Acceleration
- Deceleration
- Rapidly changing engine loads
When oil flow from load sensing spool (1) enters the end of the valve body, a small amount of oil flows into the chamber of the valve body through the edge filter. The pressure in the valve body is controlled by adjusting the force on poppet (4). Adjusting the force on the poppet allows the poppet to drain off some of the oil in the valve body. The force on the poppet is controlled by the strength of the magnetic field that is produced from electrical current from ECM (2). The poppet also responds to pressure changes in the valve body. The position of the poppet dictates the amount of oil flow that is allowed to reach drain port (3).
The amount of oil that is allowed to pass through the poppet controls the position of the load sensing spool. The position of the load sensing spool determines the amount of oil that is directed to the swashplate's control piston. The process of responding to pressure changes on either side of the load sensing spool occurs so rapidly that the spool is held in a partially open position. This allows the outlet pressure of the injection pump to be closely controlled. The IAP control valve allows infinitely variable control of pump outlet pressure between
Components of the HEUI Injector
The HEUI injector serves four functions. The HEUI injector pressurizes supply fuel from
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| Illustration 10 | g00988690 |
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Components of the HEUI injector (1) Solenoid (2) Poppet valve (3) Intensifier piston (4) Plunger (5) Plunger cavity (6) Barrel (7) Nozzle assembly | |
The HEUI injector consists of five basic components:
- Solenoid (1)
- Poppet valve (2)
- Intensifier piston (3)
- Plunger (4)
- Barrel (6)
- Nozzle assembly (7)
The solenoid (1) is an electromagnet. When the solenoid is energized, the solenoid creates a very strong magnetic field. This magnetic field attracts the armature which is connected to the poppet valve (2) by an armature screw. When the armature moves toward the solenoid, the armature lifts the poppet valve off the poppet valve's lower seat. Energizing the solenoid and lifting the poppet valve off the poppet valve's lower seat is the beginning of the fuel injection process.
The poppet valve (2) has two positions which are opened and closed. In the closed position, the poppet is held on the lower poppet seat by a spring. The closed lower poppet seat prevents high pressure actuation oil from entering the unit injector. The open upper poppet seat vents oil in the cavity that is above the intensifier piston (3) to the drain port. The oil is vented to the drain port through the upper portion of the unit injector. In the open position, the solenoid (1) is energized and the poppet valve is lifted off the poppet valve's lower seat. When the poppet valve is lifted off the poppet valve's lower seat, the lower poppet seat opens allowing high pressure actuation oil to enter the unit injector. When the high pressure actuation oil enters the unit injector, the high pressure actuation oil pushes on the top of the intensifier piston. The poppet is closed against the upper seat of the poppet valve and this blocks the path to the drain port. Blocking the path to the drain prevents the leakage of high pressure actuation oil from the unit injector.
The surface area of intensifier piston (3) is six times larger than the surface area of plunger (4). This larger surface area provides a multiplication of force. This multiplication of force allows
The barrel (6) is the cylinder that holds plunger (4). The plunger moves inside the barrel. The plunger and barrel together act as a pump. Both the plunger and the barrel are precision components that have a working clearance of only
Note: A small amount of controlled leakage is required in order to lubricate the plunger which prevents wear.
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| Illustration 11 | g00547599 |
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Nozzle Assembly (1) Inlet fill check ball (2) Case (3) Check (4) Tip (5) Tip orifice holes | |
The nozzle assembly is similar to all other unit injector's nozzle assemblies. Fuel that has been pressurized to the injection pressure flows from the plunger cavity through a passage in the nozzle to the nozzle tip (4). Fuel flow out of the tip is stopped by check (3), which covers the tip orifice holes (5) in the end of the tip. The force of a spring holds the check down in the closed position. This prevents the leakage of fuel out of the tip and this prevents the leakage of combustion gas into the unit injector when the cylinder fires.
When the injection pressure increases to approximately
Note: VOP and VCP will vary among applications and horsepower ratings in order to meet exhaust emission standards. The above values were used as illustrations only.
The inlet fill check ball (1) unseats during upward travel of the plunger in order to allow the plunger cavity to refill. The inlet fill check ball seals during the downward stroke of the plunger in order to prevent fuel injection pressure leakage into the fuel supply.
Operation of the HEUI Injector
There are three stages of injection with the HEUI injector:
- Pre-injection
- Injection
- End of injection
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| Illustration 12 | g00988773 |
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Pre-injection cycle (1) Drain port (2) High pressure oil inlet port (A) Low pressure oil (B) Fuel supply pressure (C) Actuation oil pressure (D) Mechanical movement of internal components | |
During the pre-injection cycle, all internal components have returned to the spring loaded position. The solenoid is not energized and the lower poppet seat is closed. The lower poppet seat blocks high pressure oil inlet port (2). Actuation oil pressure is blocked from entering the unit injector. The plunger and the intensifier piston are at the top of the bore and the plunger cavity is full of fuel. Fuel pressure in the plunger cavity is equal to the fuel supply pressure. The fuel supply pressure is approximately
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| Illustration 13 | g00988788 |
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Injection cycle (1) Drain port (2) High pressure oil inlet port (A) Low pressure oil (B) Fuel supply pressure (C) Actuation oil pressure (D) Mechanical movement of internal components (E) Fuel flow (F) Injection pressure | |
While the solenoid is energized, the poppet valve remains open. While the poppet valve is open, high pressure oil continues to flow into the injector. The flow of the high pressure oil pushes downward on the intensifier piston and the plunger. The injection pressure fluctuates from
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| Illustration 14 | g00988810 |
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End of injection (1) Drain port (2) High pressure oil inlet port (A) Low pressure oil (B) Fuel supply pressure (C) Actuation oil pressure (D) Mechanical movement of internal components (E) Fuel flow | |
The end of the injection cycle begins when the ECM stops the current to the unit injector solenoid. The magnetic field of the solenoid breaks down and the magnetic field is unable to overcome the spring force of the poppet. The poppet returns to the lower poppet seat which closes high pressure oil inlet port (2). When the poppet valve closes, high pressure oil is stopped from entering the unit injector. As the lower poppet seat closes, the upper poppet seat opens to drain port (1). When the upper poppet seat opens to the drain, the actuation pressure of the oil drops off.
Fuel injection pressure under the plunger exerts an upward force on the plunger and the intensifier piston. As the pressure of the actuation oil above the intensifier piston drops off, the downward force on the intensifier piston drops off. The upward force of the fuel injection pressure under the plunger suddenly becomes greater than the downward force on the intensifier piston. The downward motion of the intensifier piston and the plunger stops.
The exhaust oil on top of the intensifier piston can flow to the drain port through the open upper poppet seat. Then, the oil flows through a vent hole to the rocker arm compartment under the valve cover.
When the downward travel of the plunger stops, fuel flow also stops. While the check is still open, the remaining fuel pressure pushes a small amount of fuel out of the orifice holes. This causes a large pressure drop which lowers injection pressure below VCP. Spring tension on the check now reseats the check into the tip and injection stops.
When the check closes, injection stops. When injection stops, the fill cycle starts. The area above the intensifier piston cavity is open to atmospheric pressure through the drain port. Pressure drops very rapidly in the cavity above the intensifier piston to near zero. The return spring of the plunger pushes up on the plunger and the intensifier piston. As the plunger and the intensifier piston move upward, oil is forced out of the drain port.
As the plunger rises, pressure in the plunger cavity also drops to near zero. The fuel supply pressure is
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| Illustration 15 | g00547740 |
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(1) Fuel inlet
(2) Water separator (3) Drain valve (4) Fuel return | |
Some engines may have a water separator. Water that has been separated from the fuel can be drained from the unit by opening the drain valve (3).