Fuel system process
1. Supply from the fuel tank
2. Fuel filter
3. Drain port of the oil-water separator
4. Hand pump
5. Fuel supply to the fuel pump
6. High-pressure fuel pump
7. Fuel supply to the high-pressure fuel rail
8. Fuel rail
9. High-pressure fuel pipe for supplying fuel to the injectors
10. Injector
11. Return oil of the injector
12. High-pressure relief valve
13. Return oil of the high-pressure relief valve
14. Fuel return pipe
15. Fuel returns to the fuel tank
Fuel return oil circuit
Source of return oil:
--Injector
--High-pressure fuel rail
--Fuel pump
The return oil from all injectors converges into a return oil pipe. Inside this pipe, there is a low-pressure one-way valve to prevent the return oil from other components from flowing back to the injectors through this pipe.
The fuel return oil pipe is equipped with a spring-loaded snap ring quick connector at each injector, while all other connectors use quick connectors.
--When installing these connectors, one should be particularly careful. A clicking sound will be heard when the connection is properly made.
Fuel system process
1. Low-pressure oil circuit:
The fuel is drawn out from the fuel tank, passes through the fuel filter on the suction side, and then goes to the high-pressure fuel pump assembly. After entering the high-pressure fuel pump:
● It passes through the gear pump inside the fuel pump assembly to generate low-pressure pressure.
● Some of the fuel goes through the fuel overflow valve to lubricate the high-pressure fuel pump.
● The ECM calculates the desired oil rail pressure value based on the engine's operating condition, compares it with the actual oil rail pressure value, and controls the fuel pump actuator to allow an appropriate amount of fuel to pass through and enter the high-pressure pump chamber for pressurization and then be sent to the high-pressure fuel rail, keeping the oil rail pressure at an appropriate level. (A very small portion passes through the venting hole and returns to the fuel tank along with the high-pressure fuel pump return oil.)
● The excess fuel goes through the fuel overflow valve and returns to the inlet of the gear pump through the internal pipeline of the fuel pump.
2. High-pressure oil circuit:
The pressurized high-pressure fuel, which has been boosted by the high-pressure fuel pump, is transported through a high-pressure fuel pipe to the high-pressure fuel rail. The fuel in the high-pressure fuel rail is then delivered through high-pressure fuel pipes to the fuel injectors of each cylinder. The ECM controls the electromagnetic valves of the fuel injectors, causing them to inject fuel into the cylinders at the appropriate time for combustion.
3. Oil return line:
High-pressure fuel pump return line
High-pressure fuel rail return line
Injector return line
System Technical Specifications
An oil-water separation filter on the suction side needs to be used.
-- This filter is not on the engine and contains a hand pump.
-- Cummins filter part number
● 3968105 (filter element)
● 3964061 (O-ring)
--Fleetguard® Part Number
● FS19925 (including filter element and O-ring)
Oil rail pressure control value
-- 250 to 1600 bar [3626 to 23,520 PSI] Bosch injectors and fuel pump
Low-pressure fuel pipe
All are equipped with quick connectors.
All use molded tubes.
Reduce leakage caused by threads and alignment issues.
Observe for any debris such as paint chips during the disassembly and installation of these pipelines, and keep them clean.
Low-pressure fuel pipes cannot be repaired.
Why not pre-apply pressure to test the fuel filter?
During normal system operation the fuel sent from the inlet side filter has been filtered, but it does not meet the micron-level requirements of the fuel system.
What exactly will occur during the pre-filling process?
During the pre-filling process, the cleaning side of the filter will be contaminated by any potential pollutants present in the added diesel.
Pollution rapidly emerged. The filter medium on both sides was exposed to unfiltered fuel. Now, the key system components are in danger.
Fuel pump
Bosch pump
The fuel pump actuator is the only repairable (replaceable) component of the high-pressure pump. The blocking and sealing covers on the pump body must not be removed.
No need to adjust the timing
Fuel pump actuator
Normally open device
The pulse width modulation signal (PWM) sent by ECM is driven to the closed position
Allows Limp home mode, controlling the output of the high-pressure fuel pump. The only repairable component in the high-pressure fuel pump assembly
Fuel rail
Acting as a high-pressure fuel accumulator
Stainless steel welded by laser
Equipped with oil rail pressure sensor and high-pressure relief valve
Fuel rail pressure relief valve
Single-stage type
Releases when the oil rail pressure drops to 0 Bar
Tested using INSITE ECM
Opening pressure: 1750 to 1700 Bar
--For all engines, INSITE 7.3FP2 and above versions perform leakage tests at higher pressures
After opening, the oil rail pressure is approximately 850 Bar
Not repairable / not adjustable
--Only the oil rail assembly can be selected for replacement
Fuel injector
The high-pressure fuel connector of the injector includes a streamlined filter, which is used to break down small impurities entering the fuel system. The streamlined filter utilizes pulsating high pressure to crush most of the particles, so that the particle size is small enough to pass through the injector.
-- Note: During maintenance, the streamlined filter is not an alternative for cleaning and covering all fuel system connectors, nor is it a substitute for the required fuel filter.
The working principle of the fuel injector
The electromagnetic valve of the injector is not powered on. The spring of the electromagnetic valve keeps the iron core of the valve in the closed position. The same fuel pressure is applied to the top surface (1) of the plunger and the convex area (2) of the needle valve.
The larger force area on the top surface (1) of the plunger results in a greater downward force, which keeps the injector needle valve in the closed position.
When fuel is required to be injected into the cylinder, the ECM sends a voltage signal to the solenoid valve.
The solenoid valve generates an upward electromagnetic force that is greater than the spring force of the solenoid valve, causing the solenoid valve core to move upward.
When the solenoid valve core rises, it will open an oil discharge channel inside the fuel injector.
Due to the opening of the oil leakage channel, the pressure on the top surface of the plunger decreases, resulting in a pressure greater than that on the top surface (1) of the plunger acting on the needle valve boss (2).
This causes the needle valve to rise from its closed position and the fuel is sprayed into the cylinder through the spray holes in the nozzle head.
When fuel injection is not required, the ECM will de-energize the electromagnetic valve of the injector.
The electromagnetic force disappears. Under the action of the spring of the electromagnetic valve, the iron core of the valve moves to the closed position.
The drain channel is closed.
After the drain channel is closed, the pressure on the top of the plunger rises, causing the plunger/piston valve to reset and terminate the fuel injection.
The same fuel pressure is then sent to the top of the plunger (1) and the cam platform (2) of the piston valve.
The larger force-bearing area of the plunger top (1) generates a greater downward force, keeping the fuel injector needle valve in the closed position until the ECM decides to start the next fuel injection.
For the high-pressure common rail fuel system, the "cleanliness concern" of the fuel system is extremely important. Particulate matter entering the small channels inside the injector will impede the flow of fuel.
If the polluted particulate matter blocks the small channels leading to the control chamber at the top of the injector, it will cause the injector to remain in the open position.
If the injector gets stuck in the open position, the amount of fuel injected into the cylinder will lose control, and this will lead to serious malfunctions of the engine.
