Pic. 8.12. Scheme of the diesel engine power system: 1 - fuel filter; 2 - high pressure fuel pump; 3 - calibrated hole; 4 - return pipeline; 5 - nozzle; 6 - fuel return pipeline; 7 - fuel tank; 8 - fuel supply pipeline
From the fuel tank diesel fuel pipeline 8 (pic. 8.12) is fed into the fuel filter 1. Then the purified diesel fuel is fed into the high pressure fuel pump (injection pump) 2. Excess fuel through the calibrated hole 3 through the return line 4 is returned to the fuel tank. The high-pressure fuel pump compressed to high pressure is fed through pipeline 6 to injector 5. A small amount of fuel remaining near the injector nozzle needle is also returned to the fuel tank.
Returning excess fuel to the fuel tank
When the diesel engine is running, a significant amount of fuel is injected, the excess of which is returned to the fuel tank through the return pipeline. The injectors do not inject the fuel supplied to them into the swirl chambers: excess fuel is used to lubricate the moving parts of the high pressure fuel pump and injectors. To return excess fuel, the nozzles have special return pipes and are connected by hoses to each other. Fuel returns to the tank from the nozzle of the first cylinder.
Fuel path from fuel tank to injector
Pic. 8.13. High pressure fuel pump (injection pump) in the context: 1 - drive shaft; 2 - fuel supply; 3 - pressure reducing valve; 4 - the handle of the thrust of manual control of the accelerator; 5 – centrifugal regulator; 6 - fuel return hole; 7 - solenoid valve to cut off the fuel supply; 8 – fuel lines to injectors; 9 - plunger of the distribution injection pump; 10 - cam washer; 11 - clutch advancing the moment of fuel injection (rotated 90°here for better representation); 12 - roller; 13 - booster vane pump (also rotated 90°)
The fuel priming vane pump is used to supply fuel from the tank and, together with the pressure control valve, creates pressure in the cavity of the high pressure fuel pump (injection pump), which increases in direct proportion to the frequency of rotation of the crankshaft (pic. 8.13). The suction fuel pump is located in the high pressure fuel pump housing.
Distribution injection pump plunger
The distributor type pump includes only one plunger-sleeve set to supply all cylinders. The plunger is located in the cavity of the high pressure fuel pump. In principle, it acts as an ignition distributor in gasoline engines. The design feature of the injection pump distributor plunger is a plurality of channels and grooves. The special location of the distributor injection pump plunger ensures that it performs two functions. It opens the path of fuel into the cavity and sprays fuel through the nozzles into the swirl chambers of the cylinder head.
The distributor injection pump plunger not only creates the required fuel pressure during its working stroke, but at the same time, while rotating, distributes it over individual outlets. The distributor injection pump plunger rotates at a frequency half that of the engine crankshaft. During one revolution of the drive shaft, the plunger performs a number of strokes equal to the number of engine cylinders. The drive shaft rotates the cam and the plunger to which it is connected. The protrusions on the cam provide axial movement of the plunger and its rotation, i.e. distribution and supply of fuel. The pump continues to deliver fuel during the power stroke as long as the plunger outlet remains closed. Fuel rushes in the direction of the cylinder that is on the compression stroke. The forward movement of the distributor injection pump plunger simultaneously reduces the volume in front of the piston: now the compressed fuel raises the nozzle needle and the fuel is sprayed in the swirl chamber. The fuel pump stops supplying fuel as soon as the outlet is aligned with the hole in the control sleeve. Further, the distributor injection pump plunger, falling into the recess of the cam washer, moves back, and a new portion of fuel enters its cavity.
Functioning of the distributor injection pump plunger
Pic. 8.14. Scheme of functioning of the distributor injection pump plunger: 1 - solenoid valve for stopping the fuel supply; 2 – distributor injection pump plunger; 3 - hole; 4 - channel; 5 - pipeline for returning fuel to the fuel tank; 6 – high pressure chamber; 7 - nozzle
When turning the plunger 2 (pic. 8.14) distribution injection pump, the filling hole 3 through the groove in the plunger is aligned with channel 4. Now the fuel under pressure created by the booster vane pump enters the high pressure chamber 6 in front of the distribution injection pump plunger and fills the entire space. Thus, the filling of the cavity before fuel compression is completed.
Pic. 8.15. Scheme of functioning of the distribution injection pump plunger: 1 - distribution injection pump plunger; 2 - solenoid valve to cut off the fuel supply; 3 - distribution channel; 4 - outlet channel; 5 - pipeline for returning fuel to the fuel tank; 6 – high pressure chamber; 7 - nozzle
After filling plunger 1 (pic. 8.15) distribution injection pump continues to rotate and the filling hole is closed by a plunger. Further, in the rear part of the high-pressure fuel pump, the protrusions of the cam run on the rollers and the distributor injection pump plunger, which is fixedly connected to the cam, moves forward. The pressure in the high pressure chamber rises to the required value, and at this time channel 3 is combined with the outlet channel 4. Then the compressed fuel is supplied through the high pressure pipeline to the nozzle 7. A small amount of the remaining fuel through the return pipeline 5 returns to the fuel tank. The fuel cut off solenoid valve must be open, otherwise fuel cannot enter the high pressure chamber.
Fuel cut off solenoid valve
Pic. 8.16. Connector location (2) solenoid valve (1) engine stop
Before the fuel enters the filling channel of the injection pump distributor plunger, it passes through the open fuel cutoff solenoid valve. As long as the valve is energized, fuel can flow through it unhindered. After power failure, solenoid valve 1 (pic. 8.16) closes and shuts off the fuel supply to the distributor injection pump plunger. If, when checking the valve timing, it is necessary to turn the engine crankshaft with a starter, to prevent the engine from starting, disconnect connector 2 from the engine stop solenoid valve.
Centrifugal Engine Speed Controller
To ensure the normal operation of the diesel engine in various modes and the supply of the required portion of fuel at a strictly defined moment, distribution injection pumps are equipped with a centrifugal regulator. Its weights move outward under the action of centrifugal force and act on the adjusting spool, which opens a hole in the distributor injection pump plunger and thus always adds fuel from the pump cavity.
If the engine speed rises relative to the position of the throttle pedal, the regulator closes the hole and the engine speed decreases again. When starting the engine, the port is fully open and the engine receives the maximum amount of fuel until the idle speed is reached. When the engine is running in all modes, the centrifugal governor always compares the crankshaft speed with the position of the fuel control pedal. Similarly, the centrifugal governor limits the maximum speed, which for the diesel engine of the Fiesta is 5 350+50 min-1.
Clutch for adjusting the fuel injection timing
In order to ensure the normal combustion process and reserve the time required for the preparation of the air-fuel mixture, the start of fuel injection must be carried out before the piston reaches top dead center. This is done by the fuel injection advance clutch, which shifts the injection to an earlier start and achieves an effect similar to the operation of the ignition advance regulator in internal combustion engines. The fuel injection advance clutch is located in the high pressure fuel pump housing.
A device that facilitates the start of a cold engine
A cold starter in a diesel engine performs the same role as an automatic starter in internal combustion engines, but that's where the similarities end. The use of a cold start aid for a diesel engine depends on the air temperature. Without reducing the flow of air into the engine cylinders, it shifts the moment of fuel injection in the direction of an earlier injection. Thus, the atomized fuel has more time to ignite in compressed air and in the swirl chambers heated by glow plugs, while the engine starts more reliably and softly. In addition, a device that facilitates starting a cold engine slightly increases the idling speed: the engine crankshaft rotates at a frequency of 1100 min-1.
Nozzles
Pic. 8.17. Nozzle mounted on the engine: 1 - injecting discharge pipeline; 2 - plug on the fuel return pipe of the 4th cylinder; 3 – fuel return pipeline; 4 - nozzle body
nozzles (pic. 8.17) are the final element of the diesel fuel injection system. They spray fuel at high pressure into the swirl chambers of the individual cylinders. If the pressure in the system exceeds 143 bar, the spring-loaded atomizer needle and nozzle are damaged, causing fuel to flow out of the injection system. To prevent the backflow of burning gases when the nozzle is still open, the pressure in the pressure chamber of the nozzle must be higher than the pressure in the combustion chamber. This is especially important at the end of the injection, when the injection pressure decreases, accompanied by an excessive increase in the pressure of the combustion gases. It can only be ensured by carefully coordinating the operation of the fuel pump, spray nozzle and needle. The fuel circulating in the fuel system also lubricates and cools all moving parts. Since the total mass of fuel is never injected, excess fuel is returned to the fuel tank. The time between the start of injection and ignition is 0.002 s. Therefore, even a slight malfunction in the fuel injection system leads to a violation of the normal operation of the engine. Black smoke from the exhaust pipe and a noisy diesel engine indicate that the fuel injection system needs to be checked and adjusted.
Determining a bad injector
You can identify a broken injector as follows. Start the engine and leave it to idle. Loosen the union nuts that secure the high pressure lines to the injectors one by one. When loosening the nut on a working nozzle, the engine speed should decrease noticeably. If the engine speed does not change when the nut is loosened, then the injector being tested is faulty.
You can also identify faulty injectors by the following symptoms:
- constantly burning tip of the glow plug;
- constant black smoke from the exhaust pipe;
- increased fuel consumption;
- frequent overheating of the engine;
- hard noises in the process of fuel combustion;
- power reduction.
If any of the following symptoms of a Fiesta diesel engine malfunction are present, visit a workshop for an expert opinion on the presence of a malfunction and measures to eliminate it.
Nozzle repair
Standard nozzle bodies include atomizer nozzle and stop needle. The nozzle needle moves freely within the nozzle guide channel and at the same time provides a seal under high injection pressure conditions. There is a conical seal at the bottom of the needle. The nozzle spring presses the needle against the appropriately shaped surface of the atomizer when the nozzle is in the closed position. The conical surfaces of the nebulizer and needle provide high specific pressure contact and effective sealing. The injector opens when the fuel pressure force on the sealing surface exceeds the injector spring force. It will remain open until the system pressure drops below the opening pressure.
Pic. 8.18. Nozzle details: 1 - body; 2 – atomizer housing; 3 - spray needle; 4 - plug-in block of the nozzle body; 5 - thrust bolt; 6 - pressure spring of the nozzle; 7 - installation disk; 8 - the upper part of the body
Without special equipment, it is only possible to visually determine a worn nozzle by external local damage or severe contamination. However, the nozzles usually wear out in the inside of the atomizer, the atomizer needle and the compression spring. To adjust the measurement and injection pressure, i.e. pressure spring force change, special equipment must be used. In most all cases, it is better to replace the faulty injector. If you decide to disassemble the nozzle, do not keep the parts open on the workbench for longer than necessary, as the precision-machined surfaces of the spray needle and body are extremely susceptible to dust or rust. To disassemble the injector, unscrew its upper part from the body and remove all internal parts of the injector (pic. 8.18). The tightening torque for fastening the upper part of the nozzle to the body is 80 Н·м.
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