Fuel System
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| Illustration 1 | g00459512 |
Fuel System Schematic (1) High pressure oil manifold. (2) High pressure oil line. (3) Hydraulic electronic unit injector. (4) Wiring harness for the unit injector. (5) Electronic Control Module (ECM). (6) Fuel pressure regulator. (7) Fuel filter. (8) Fuel transfer pump. (9) Fuel return line. (10) Injection actuation pressure control valve (IAPCV). (11) Electrical signal. (12) Engine oil cooler. (13) Engine oil filter. (14) Line for low pressure oil. (15) High pressure oil pump. (16) Fuel line. (17) Fuel tank. (18) Return line for low pressure oil. (19) Oil sump. (20) Engine oil pump. | |
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| Illustration 2 | g00459556 |
Fuel System Components (1) High pressure oil manifold. (5) ECM. (7) Fuel filter. (8) Fuel transfer pump. (10) Injection actuation pressure control valve. (15) High pressure oil pump. (21) Injection actuation pressure sensor. | |
The 3126 HEUI fuel system uses hydraulic electronic unit injectors (3) . Hydraulic electronic unit injectors use high pressure oil to pressurize the unit injector.
The fuel supply circuit is a conventional design for injected engines. The circuit uses a transfer pump (8) with a single piston in order to circulate fuel at low pressure. The fuel flows at low pressure through fuel filter (7) into the cylinder head fuel passages. Fuel is delivered at high pressure into the combustion chamber through hydraulic electronic unit injectors (3) . The unit injectors are located near the center of the combustion chambers in the cylinder head between the rocker arms.
Transfer pump (8) draws fuel from fuel tank (17) . The fuel is typically filtered by a primary filter of approximately thirty micron rating, that is installed by the OEM. Some OEM'S may install a primary filter/water separator. The fuel transfer pump incorporates a check valve that permits fuel flow for hand priming. The fuel transfer pump also has a pressure regulating valve in order to protect the fuel system from extreme pressure.
From the fuel transfer pump (8) , fuel passes through a secondary fuel filter (7) of two micron rating. The fuel flows through the fuel filter into the fitting for the fuel manifold, that is located at the front of the engine. Fuel flows continuously around each unit injector (3) through an internal passage that runs the length of the head. Excess fuel exits at the rear of the engine through a pressure regulator in order to return to fuel tank (17) . Excess fuel that is not being used for injection aids in cooling the unit injectors and aids in purging air from the system. During normal under load operating conditions, the fuel pressure is maintained in the range of 400 kPa (58 psi) to 525 kPa (76 psi) by an orificed pressure regulating valve. The pressure regulating valve is located at the rear of the cylinder head. The pressure regulating valve also contains a check valve that serves as a siphon break. Fuel pressure at low idle should be a minimum of 500 kPa (73 psi) .
High pressure oil manifold (1) sends high pressure oil to unit injector (3) . When ECM (5) activates each unit injector the high pressure oil acts on a piston in the upper part of the unit injector. The piston has a surface area that is six times the surface area of the unit injector plunger. This larger surface area provides a multiplication of force. This multiplication of force enables the injectors to inject fuel into the cylinder at pressures higher than 140 MPa (20,000 psi) .
ECM (5) is located at the rear on the left side of the engine. The system provides total electronic control of the start and duration of the fuel injection. The ECM uses data from several sensors in order to adjust the fuel delivery. The adjustments are based on the performance map that is used in the software for engine control.
High pressure oil pump (15) is located at the left front corner of the engine. The high pressure oil pump is a gear-driven axial piston pump. Engine oil at normal pressure enters the high pressure oil pump. The high pressure oil pump increases the oil pressure to the actuation pressure that is required by unit injectors (3) .
Injection actuation pressure control valve (10) is located in high pressure oil pump (15) at the top left of the engine. Operational maps that are stored in the memory identify the optimum oil pressure for the best performance.
Electronic Control System
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| Illustration 3 | g00456376 |
Electronic Controls
The electronic control system consists of two main components:
- Electronic Control Module (ECM)
- Personality Module
The ECM is the computer and the personality module is the software for the computer. The personality module stores the operating maps that define horsepower, torque curves, rpm, etc.. The software in the ECM uses data from the sensors in order to control the unit injectors.
The Throttle Position Sensor (TPS) is connected to the throttle. When the throttle is operated to accelerate the engine, the TPS generates a signal. The signal from the TPS is sent to the interlock system of the Power Take-Off (PTO). If the remote PTO is being operated, the interlock prevents the operator from overriding the throttle signal from the remote PTO. This protects devices that are powered by the PTO from being driven into an overspeed condition. The throttle signal then goes through the logic circuit and through the control circuit for the throttle, the control circuit for the cruise, and the control circuit for the PTO. The signal is then compared to the following inputs:
- Vehicle speed
- Engine rpm
- Customer parameters
The signal then goes into the logic circuit that controls the engine. Data from the coolant temperature sensor and the inlet air temperature sensor are sampled. If the data indicates the need, the ECM will place the engine in cold mode operation. The ECM will output a signal for the desired rpm. The signal is sent to the circuits for the electronic governor.
The PTO application may be operated from a dedicated set of PTO switches in the vehicle cab or from PTO switches in a remote location. The PTO application may also be operated by a remote PTO on/off switch with a separate throttle position sensor for variable engine speed. Parameters are available for applications for the PTO such as cement mixers and bulk haulers.
Fuel Injection
The ECM samples signals from the boost pressure sensor, the inlet air temperature sensor, and the engine speed/timing sensors. The ECM uses the signals from the sensors and the maps of the Fuel Ratio Control in order to establish the FRC fuel position. The FRC fuel position limits fueling during acceleration until boost pressure rises. This prevents overfueling which causes black smoke. Once sufficient boost pressure and sufficient airflow is attained, the FRC limit on the fuel position is normally higher than the fuel position that is desired. Therefore, the FRC limit does not affect engine performance.
The software in the personality module contains torque maps that establish the maximum fuel rates. The torque maps also set the rated fuel position. The personality module cannot be physically removed and replaced. Rerates and updated software are installed by flash programming via an electronic service tool. The engine rpm and the parameters for the PTO are compared to the torque maps. These parameters may limit torque during PTO operation as a protection for the driven device.
The rated fuel position, the FRC fuel position, and the desired fuel position are sent to the electronic governor. The governor sends a signal for the fuel position to the control circuit for fuel injection. The other inputs to the control circuit for fuel injection are the position of Top Center for No. 1 cylinder, and the engine rpm. Both inputs come from the logic circuit of the engine speed/timing signal.
The engine speed/timing sensors detect the Top Center position for the No. 1 cylinder. The engine speed/timing sensors are passive magnetic speed sensors. The sensors are mounted on the upper left side of the engine behind the air compressor. The sensors detect the magnetic pulses from the 25 teeth on the back of the camshaft gear. 24 of the 25 teeth are evenly spaced. The 24 teeth are spaced 15 degrees from the center of one tooth to center of the next tooth. Tooth 25 is placed 22 1/2 degrees after the Top Center of No. 1 cylinder. This allows the ECM to establish the position of No. 1 cylinder. This allows the ECM to fire each unit injector at the correct time and in the correct firing order. There are two engine speed/timing sensors. This permits the engine to continue running without downtime if either one fails. However, an active fault code will be present, and service should be obtained as soon as possible. The sensors must be replaced as a set. The sensors are not identical electrically.
The output of the control circuit for fuel injection is a 110 volt pulse. This DC voltage energizes the unit injector solenoid. The solenoid operates quickly. When the solenoid energizes, the high pressure oil enters the unit injector. The high pressure oil presses against the intensifier piston which multiplies the hydraulic force to several times the input oil pressure. The movement of the intensifier piston pressurizes the fuel and forces the nozzle valve to open. The length of the electrical pulse determines the length of time that the fuel will flow. The hydraulic pressure determines the rate of flow of the fuel. The HEUI fuel system provides more versatility of fuel delivery than fuel systems that are actuated mechanically.
Hydraulic Oil Supply For Unit Injectors
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| Illustration 4 | g00456445 |
Hydraulic Oil Supply For Unit Injectors (1) Jumper tube. (2) Hydraulic electronic unit injector. (3) High pressure oil manifold. | |
The HEUI fuel system uses a high pressure oil system. The oil for this system is drawn from the engine lubrication system on the left side of the block and raised to high pressure by a high pressure oil pump. The high pressure oil pump is a gear-driven axial piston pump. The pressure is regulated by the ECM to the optimum pressure for a given set of conditions. The optimum pressure is derived from maps which are stored in the memory of the ECM. Pressure from approximately 5 MPa (725 psi) to 23 MPa (3335 psi) is available. This is a closed loop system. The ECM senses hydraulic pressure from the injection actuation pressure sensor which is located on high pressure oil manifold (3) . The ECM regulates pressure by sending a control signal to the injection actuation pressure control valve that is mounted on the hydraulic pump. The high pressure oil pump increases the oil pressure to the actuation pressure that is required by unit injectors (2) . The high pressure oil flows from the high pressure oil pump through the high pressure oil line and into the high pressure oil manifold. The high pressure oil manifold is located near the unit injectors. The manifold stores the oil at the actuation pressure. Jumper tubes (1) carry the high pressure oil from the manifold to the unit injectors. After pressurizing a unit injector, the oil is discharged from the unit injectors onto the cylinder head. The oil drains back to the oil pan through the oil drain holes in the cylinder head.
Hydraulic Electronic Unit Injector
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| Illustration 5 | g00456306 |
The HEUI Injector (1) Poppet valve. (2) Lower poppet seat. (3) High pressure oil (fill port). (4) Solenoid return spring. (5) Solenoid. (6) Upper poppet seat. (7) Plunger. (8) Intensifier piston. (9) Poppet chamber. (10) Check valve. (11) Barrel. (12) Piston cavity. (13) Check valve for the fuel inlet. (14) Reverse flow check valve. (15) Fill port for the fuel. (16) Nozzle assembly. (17) Nozzle valve. | |
The injection pump, the fuel lines and the fuel nozzles that are used in traditional Caterpillar Diesel Engines have been replaced with a hydraulic electronic unit injector for each cylinder. Each unit injector has a solenoid (5) . The solenoid controls the amount of fuel that is injected into the cylinder. An Electronic Control Module (ECM) sends a signal in order to control the solenoid.
Fuel flows from the fuel supply manifold through passages in the cylinder head into fill port (15) .
The ECM sends an electrical signal to solenoid (5) that is in the unit injector. The signal energizes the solenoid. The duration of this electrical signal determines the length of time that fuel is allowed to flow through nozzle valve (17) . The high pressure oil that is pressing against intensifier piston (8) determines the injection pressure. The duration is determined from inputs to the personality module of the ECM.
Before the injection cycle begins, solenoid (5) is not energized and poppet valve (1) is held on lower seat (2) by solenoid return spring (4) . The high pressure oil is blocked by the poppet valve and the poppet chamber (9) is opened to the drain. Intensifier piston (8) and plunger (7) are held at the top of the stroke by the spring. Piston cavity (12) is filled with fuel.
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| Illustration 6 | g00456451 |
Oil Flow in the Unit Injector (2) Lower seat. (7) Upper seat. | |
When a pulse energizes solenoid (5) , poppet valve (1) moves off lower seat (2) and onto upper seat (6) . This blocks the path for oil flow to the drain. High pressure oil enters the unit injector through fill port (3) and presses against the top of intensifier piston (8) . The pressure on the top of the piston increases which pushes plunger (7) down. The downward movement of the plunger pressurizes the fuel in piston cavity (12) . Fuel flows past reverse flow check (14) into the fuel passage around nozzle valve (17) . When the valve opening pressure (VOP) is reached, the nozzle valve opens and injection begins.
Intensifier piston (8) continues to move downward until solenoid (5) is de-energized. The de-energized solenoid permits poppet valve (1) , intensifier piston (8) , and plunger (7) to return to the position in order to start the next injection cycle. As the plunger moves upward, fuel is drawn into piston cavity (12) through fill port (15) . The unit injector is now ready to repeat the cycle.
Nozzle assembly (16) is similar to the nozzle assemblies on other unit injectors. The check valve for the fill port unseats during the upward stroke of plunger (7) in order to allow piston cavity (12) to refill with fuel. The check valve for the fill port seals during the downward stroke of the plunger in order to prevent fuel injection pressure from leakage into the fuel supply. Reverse flow check valve (14) opens in order to allow fuel to enter the nozzle assembly. The valve closes in order to prevent reverse flow.
Fuel Heater And Water Separator (If Equipped)
Some engines may have a fuel heater and water separator. The fuel heater is controlled by a thermostat that is located in the base of the unit. The thermostat is preset. When the fuel temperature is below 4°C (40°F), the thermostat will turn on the heater. When the fuel temperature is 15°C (60°F), the thermostat will turn off the heater.
Water that has been separated from the fuel can be drained from the unit by lifting up on drain valve.