Fuel system schematic (typical example)|
(1) Fuel supply line
(2) Unit injectors
(3) Fuel gallery (fuel manifold)
(4) Differential pressure switch (fuel)
(5) Fuel pressure sensor
(6) Fuel temperature sensor
(7) Fuel return line
(8) Pressure regulating valve
(9) Secondary fuel filter
(10) Fuel filter base
(11) Fuel transfer pump
(12) Electric fuel priming pump
(13) Pressure relief valve
(14) Primary fuel filter
(15) Fuel tank
(16) Fuel return line to tank
The fuel supply circuit is a conventional design for unit injector diesel engines. The system consists of the following major components that are used to deliver low pressure fuel to the unit injectors:
Fuel tank - The fuel tank is used to store the fuel.
Fuel priming pump - The fuel priming pump is used to evacuate the air from the fuel system. As the air is removed the system fills with fuel.
Fuel filter - The fuel filter is used to remove abrasive material and contamination from the fuel system.
Supply lines and return lines - Supply lines and return lines are used to deliver the fuel to the different components.
The purpose of the low pressure fuel supply circuit is to supply fuel that has been filtered to the fuel injectors at a rate that is constant and a pressure that is constant. The fuel system is also utilized to cool components such as the ECM and the fuel injectors.
Once the injectors receive the low pressure fuel, the fuel is pressurized again before the fuel is injected into the cylinder.
The unit injector uses mechanical energy that is provided by the camshaft to achieve pressures that can be in excess of
Control of the fuel delivery is managed by the engine's ECM. Data from several of the engine systems is collected by the ECM and processed in order to manage these aspects of fuel injection control:
- Injection timing
- Fuel injection timing advance
- Injection duration
- Engine cold mode status
The mechanical electronic fuel system relies on a large amount of data from the other engine systems. The data that is collected by the ECM will be used in order to provide optimum performance of the engine.
The flow of fuel through the system begins at fuel tank (15). Fuel is pulled from the tank by fuel transfer pump (11). The fuel is drawn through electric fuel priming pump (12). The electric fuel priming pump is used to remove air from the fuel system. While the air is removed, the system is filled with fuel. This assembly incorporates primary fuel filter (14) that removes large debris and water from the fuel. The primary fuel filter is a ten micron filter.
The fuel transfer pump incorporates a check valve that will allow fuel to flow around the gears of the pump during priming of the fuel system. The fuel transfer pump also incorporates a pressure relief valve (13). The pressure relief valve is used in order to protect the fuel system from extreme pressure. The fuel transfer pump is engineered in order to produce an excess fuel flow throughout the fuel system. The excess fuel flow is used by the system to cool the fuel system components. The excess fuel flow also purges any air from the fuel system during operation. Air that can become trapped in the fuel system can cause cavitation that may damage the components of the unit injector.
The fuel travels from the fuel transfer pump to fuel filter base (10). Fuel temperature sensor (6) is installed in the fuel filter base. This sensor detects the fuel temperature for the Electronic Control Module (ECM). An electrical signal that represents the fuel temperature is sent to the ECM for processing. Fuel pressure sensor (5) is also installed in the filter base. This sensor detects the fuel pressure for the ECM. An electrical signal that represents the fuel pressure is also sent to the ECM for processing. Differential pressure switch (4) is also installed in the fuel filter base. The differential pressure switch is used to alert the operator in case secondary fuel filter (9) becomes plugged. The secondary fuel filter is a two micron filter that removes abrasive material from the fuel. The fuel flows through a two micron fuel filter (6). The filtered fuel then flows out of the fuel filter base.
Note: If a fuel cooled ECM is installed on the engine, the fuel is pumped into the ECM. The fuel travels through the cored passages of the housing of the ECM in order to cool the control module's electronics.
Note: In some applications, a hand fuel priming pump may be located on the fuel filter base.
The fuel is transfered by fuel supply lines (1) to fuel gallery (3) in the cylinder head. Only a portion of the fuel that is supplied to the fuel injectors is used for engine operation.
The fuel that is unused by the engine is provided for cooling purposes. This unused fuel is discharged into the return passages of the fuel gallery. The fuel is returned to the fuel tank by fuel return lines (7) and (16). A continuous flow of fuel is experienced within the low pressure fuel system.
Pressure regulating valve (8) is located in the fuel filter base. The pressure regulating valve allows the low pressure fuel system to maintain a constant pressure. A flow control orifice is also located in the fuel return. The flow control orifice maintains a system back pressure that is constant. The orifice allows the flow of fuel through the system to be constant. This prevents excessive heating of the fuel.
Note: In some applications, the primary fuel filter and the secondary fuel filter are installed near the fuel supply tank. This location may be far from the engine. The pressure regulating valve will be located in a distribution block that is mounted on the engine.
During engine operation, fuel injectors (2) receive fuel from the low pressure fuel system. The injector pressurizes the fuel to high pressure. The fuel is then injected into the cylinder. The excess fuel is returned to the tank. Refer to Systems Operation, "Unit Injector" for a complete explanation of the injection process.
Fuel heaters help to prevent the plugging of the fuel filters in cold weather. This plugging is called waxing. In cold ambient conditions, the cold engine does not dissipate enough heat into the fuel system in order to prevent waxing. Heaters that are not thermostatically controlled can heat the fuel in excess of
Note: Never use fuel heaters without some type of temperature regulator. Ensure that fuel heaters are turned off during warm weather conditions.
There are two major components of the electronic control system that are necessary in order to provide control of the mechanical electronic unit injectors:
- Personality module (storage for the ECM flash file)
The ECM is the computer that is used to provide control for all aspects of engine operation. The personality module contains the software that defines the characteristics of the engine control. The personality module contains the operating maps. The operating maps define the following characteristics of the engine:
- Torque curves
- Engine speed (rpm)
- Other characteristics
The ECM, the personality module, the engine sensors, and the unit injectors work together in order to control the engine. Neither of the four can control the engine alone.
The ECM maintains the desired engine speed by sensing the actual engine speed. The ECM calculates the amount of fuel that needs to be injected in order to achieve the desired engine speed.
The ECM controls the amount of fuel that is injected by varying the signal to each of the unit injectors. The unit injectors will inject fuel only while the unit injector solenoid is energized. The ECM sends a 105 volt signal to the solenoid in order to energize the injector solenoid. By controlling the timing of the 105 volt signal, the ECM controls injection timing. By controlling the duration of the 105 volt signal, the ECM controls the amount of fuel that is injected.
The ECM sets certain limits on the amount of fuel that can be injected. The FRC fuel position is a limit that is based on boost pressure in order to control the fuel air mixture for the emission control. When the ECM senses an increase in the boost pressure, the ECM increases the FRC fuel position. The rated fuel position is a limit that is based on the horsepower rating of the engine. The rated fuel position is similar to the rack stops and the torque spring on a mechanically governed engine. The rated fuel position provides the horsepower and the torque curves for a specific engine family. The rated fuel position provides the horsepower and the torque curves for a specific horsepower rating. The limits are programmed by the factory into the personality module. The limits are not programmable in the field.
The injection timing relies on the following engine parameters: engine speed, engine load and other engine data. The ECM senses the top center position of number one cylinder from the signal that is provided by the engine speed/timing sensors. The ECM decides when the injection should occur relative to this top center position. The ECM provides the signal to the unit injector at the desired time.
Typical unit injector mechanism|
(1) Unit injector
(2) Adjusting nut
(3) Rocker arm assembly
The unit injector mechanism provides the downward force that is required to pressurize the fuel in the unit injector. When a signal is received from the ECM, the unit injector (1) injects the pressurized fuel into the combustion chamber. The camshaft gear is driven by an idler gear which is driven through the front gear train by the crankshaft gear. The gears of the front gear train that are timed must be aligned in order to provide the correct relationship between the piston and valve movement. During assembly of the front gear train, care must be taken in order to correctly align the timing marks of the gears. The camshaft has three camshaft lobes for each cylinder. Two lobes operate the inlet and exhaust valves, and one operates the unit injector mechanism. Force is transferred from the unit injector lobe on camshaft (4) through rocker arm assembly (3) to the top of the unit injector. The adjusting nut (2) allows setting of the unit injector adjustment. Refer to the Testing and Adjusting, "Electronic Unit Injector - Adjust" for the proper adjustment of the unit injector.
(5) Nozzle assembly
The operation of the Electronic Unit Injector (EUI) consists of the following four stages: pre-injection, injection, end of injection and fill. Unit injectors use a plunger and barrel to pump high pressure fuel into the combustion chamber. Components of the injector include the tappet, the plunger, the barrel and nozzle assembly. Components of the nozzle assembly include the spring, the nozzle check, and a nozzle tip. The cartridge valve is made up of the following components: solenoid, armature, poppet valve and poppet spring.
The injector is mounted in an injector bore in the cylinder head which has an integral fuel supply passage. The injector sleeve separates the injector from the engine coolant in the water jacket. Some engines use a stainless steel sleeve. The stainless steel sleeve fits into the cylinder head with a light press fit.
(A) Fuel supply pressure
(B) Injection pressure
(C) Moving parts
(D) Mechanical movement
(E) Fuel movement.
Pre-injection metering starts with the injector plunger and the injector tappet at the top of the fuel injection stroke. When the plunger cavity is full of fuel, the poppet valve is in the open position and the nozzle check is in the closed position. Fuel leaves the plunger cavity when the rocker arm pushes down on the tappet and the plunger. Fuel flow is blocked by the check valve that is in the tip of the nozzle. Fuel flows past the open poppet valve to the fuel supply passage in the cylinder head. If the solenoid is energized, the poppet valve remains open and the fuel from the plunger cavity continues flowing into the fuel supply passage.
(A) Fuel supply pressure.
(B) Injection pressure
(C) Moving parts
(D) Mechanical movement
(E) Fuel movement.
To start injection, the ECM sends a current to the solenoid on the cartridge valve. The solenoid creates a magnetic field which attracts the armature. When the solenoid is energized, the armature assembly will lift the poppet valve so the poppet valve contacts the poppet seat. This is the closed position. Once the poppet valve closes, the flow path for the fuel that is leaving the plunger cavity is blocked. The plunger continues to push fuel from the plunger cavity and the fuel pressure builds up. When the fuel pressure reaches approximately
End of injection|
(A) Fuel supply pressure
(C) Moving parts
Injection is continuous while the injector plunger moves in a downward motion and the energized solenoid holds the poppet valve closed. When injection pressure is no longer required, the ECM stops current flow to the solenoid. When the current flow to the solenoid stops, the poppet valve opens. The poppet valve is opened by the fuel injector spring and the fuel pressure. High pressure fuel can now flow around the open poppet valve and into the fuel supply passage. This results in a rapid drop in injection pressure. When the injection pressure drops to approximately
(A) Moving parts
(B) Mechanical movement
(C) Fuel movement.
When the plunger reaches the bottom of the barrel, fuel is no longer forced from the plunger cavity. The plunger is pulled up by the tappet and the tappet spring. The upward movement of the plunger causes the pressure in the plunger cavity to drop below fuel supply pressure. Fuel flows from the fuel supply passage around the open poppet and into the plunger cavity as the plunger travels upward. When the plunger reaches the top of the stroke, the plunger cavity is full of fuel and fuel flow into the plunger cavity stops. This is the beginning of pre-injection.