Fuel System 3408E and 3412E Part 2 - Caterpillar Electronic Engine
Fuel System - 3408E and 3412E Industrial Engines
Actuation Oil Pressure Control
Injection actuation pressure control system (typical example)
(1) Unit injector hydraulic pump
(3) Oil filter
(4) Engine oil pump
(5) Injectors
(6) Oil cooler
(7) IAP control valve
(8) IAP sensor
(30) 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) (30) 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
-Engine boost pressure sensor
-Speed/timing sensors
-Coolant temperature sensor
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. The IAP control valve 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 (30)
-IAP control valve (7)
-IAP sensor (8)
The ECM calculates the desired actuation pressure by sampling sensor inputs and referencing performance maps. The ECM sends a control current to the IAP control valve 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 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 calculates the desired actuation pressure. After the correct signal has been calculated, the ECM sends an electrical current to the IAP control valve 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 samples 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 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.
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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 5 to 6 MPa (725 to 870 psi). This pressure is called margin pressure.
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 work together in order to position the load sensing spool. If the margin pressure is changed, the ECM 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)
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.
Valve Operation (Engine Off)
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)
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 6.2 MPa (900 psi) of injection actuation pressure is required in order to activate the unit injector. This low injection actuation pressure will generate a fuel injection pressure of about 35 MPa (5000 psi). Actuation pressure will continue to increase until the desired actuation pressure is reached. The desired actuation pressure during engine start-up is approximately 7 MPa (1000 psi).
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 6.2 MPa (870 psi) is achieved. The ECM does not send a signal to the unit injectors until this minimum actual actuation pressure is reached.
Note: If the engine is already warm, the pressure that is required to start the engine may be higher than 6.2 MPa (900 psi). The values for the desired actuation pressures are stored in the performance maps of the ECM. These values for desired actuation pressures vary with engine temperature.
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 7 MPa (1000 psi) until the engine starts. The ECM monitors the actual actuation pressure through the IAP sensor. The ECM uses the signal from the IAP sensor, signals from other engine sensors, and the performance maps in order to calculate the desired actuation pressure. Once the desired actuation pressure has been calculated, the ECM compares the desired actuation pressure to the actual actuation pressure in the high pressure oil passage. The ECM adjusts the current levels to the IAP control valve in order to reach the desired actuation pressure.
Oil Flow (Engine Cranking)
(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 6.2 MPa (900 psi).
Valve Operation (Running Engine)
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
Oil Flow (Running Engine)
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 6 MPa (900 psi) and 24 MPa (3500 psi).
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Actuation Oil Pressure Control
Injection actuation pressure control system (typical example)
(1) Unit injector hydraulic pump
(3) Oil filter
(4) Engine oil pump
(5) Injectors
(6) Oil cooler
(7) IAP control valve
(8) IAP sensor
(30) 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) (30) 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
-Engine boost pressure sensor
-Speed/timing sensors
-Coolant temperature sensor
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. The IAP control valve 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 (30)
-IAP control valve (7)
-IAP sensor (8)
The ECM calculates the desired actuation pressure by sampling sensor inputs and referencing performance maps. The ECM sends a control current to the IAP control valve 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 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 calculates the desired actuation pressure. After the correct signal has been calculated, the ECM sends an electrical current to the IAP control valve 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 samples 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 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.
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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 5 to 6 MPa (725 to 870 psi). This pressure is called margin pressure.
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 work together in order to position the load sensing spool. If the margin pressure is changed, the ECM 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)
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.
Valve Operation (Engine Off)
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)
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 6.2 MPa (900 psi) of injection actuation pressure is required in order to activate the unit injector. This low injection actuation pressure will generate a fuel injection pressure of about 35 MPa (5000 psi). Actuation pressure will continue to increase until the desired actuation pressure is reached. The desired actuation pressure during engine start-up is approximately 7 MPa (1000 psi).
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 6.2 MPa (870 psi) is achieved. The ECM does not send a signal to the unit injectors until this minimum actual actuation pressure is reached.
Note: If the engine is already warm, the pressure that is required to start the engine may be higher than 6.2 MPa (900 psi). The values for the desired actuation pressures are stored in the performance maps of the ECM. These values for desired actuation pressures vary with engine temperature.
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 7 MPa (1000 psi) until the engine starts. The ECM monitors the actual actuation pressure through the IAP sensor. The ECM uses the signal from the IAP sensor, signals from other engine sensors, and the performance maps in order to calculate the desired actuation pressure. Once the desired actuation pressure has been calculated, the ECM compares the desired actuation pressure to the actual actuation pressure in the high pressure oil passage. The ECM adjusts the current levels to the IAP control valve in order to reach the desired actuation pressure.
Oil Flow (Engine Cranking)
(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 6.2 MPa (900 psi).
Valve Operation (Running Engine)
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
Oil Flow (Running Engine)
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 6 MPa (900 psi) and 24 MPa (3500 psi).
Read More:
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