As a professional in Cummins engine maintenance, the correct installation and adjustment of injectors are crucial steps to ensure that the engine meets performance, fuel consumption, and emission standards. This article will provide a detailed introduction to the installation and adjustment procedures for various series of Cummins injectors, aiming to help you improve the quality of maintenance.
Overview of the Cummins PT Fuel System
The Cummins PT fuel system is a patent of the American Cummins Engine Company. It differs significantly from the fuel systems of general diesel engines in terms of composition, structure, and working principles. The PT fuel system controls the cyclic fuel supply by adjusting the pressure (Pressure) of the fuel pump and the time (Time) of the injector, thereby meeting the requirements of the engine under various operating conditions.
Adjustment Methods for PT Injectors
There are mainly two methods for adjusting Cummins PT injectors: the torque method and the lift method. Both methods must be carried out strictly in accordance with technical specifications.
1. Torque Adjustment Method
The torque adjustment method involves tightening the injector adjusting screw to a specified torque to ensure the seating pressure of the plunger on the barrel head. The specific steps are as follows:
(1) Calibrate the timing marks. Put the cylinder in the decompression state. Rotate the timing pulley in the direction of crankshaft rotation to align the timing mark on the pulley with the mark on the gear chamber cover.
The "TC" on the pulley indicates that the piston is at the top dead center position, and "VS" indicates that the piston is at a 90° position after the top dead center. The numbers in front of the TC and VS marks represent the corresponding cylinder numbers. After aligning the marks, determine the cylinder in the compression stroke based on the states of the intake and exhaust valves. Note that only one cylinder can be adjusted for each alignment of the marks.
(2) Adjustment operation. When the engine is cold, loosen the lock nut on the injector rocker arm. Screw in the adjusting screw to lower the plunger. After the lower end of the plunger contacts the conical seat of the barrel head, screw in the screw an additional 15° to squeeze out the remaining fuel in the barrel head.
Loosen the adjusting screw by one full turn, and then use a torque wrench to tighten it to the specified torque. Use a screwdriver to hold the adjusting screw in place, and tighten the lock nut to the specified torque.
(3) Recheck. After all injectors have been adjusted, start the diesel engine. When the oil temperature reaches 60°C, recheck the adjustment.
2. Lift Adjustment Method
The lift adjustment method uses a dial indicator and a rocker arm pressure rod to adjust the injector plunger to a specified value. The specific steps are as follows:
Fix the dial indicator bracket on the cylinder head, with the measuring rod of the indicator vertically pressing on the flange surface of the injector plunger.
When the plunger is at the lowest position in contact with the barrel head seat surface, set the dial indicator reading to zero.
Rotate the crankshaft in the direction of diesel engine rotation to raise the plunger to the highest position. At this time, the reading on the dial indicator represents the stroke of the plunger.
If the stroke does not meet the specifications, adjust the adjusting screw on the rocker arm until it meets the requirements.
Inspection and Adjustment of Injection Timing
The inspection and adjustment of injection timing are carried out using an injection timing instrument based on the relationship between the piston position and the injector push rod position.
The specific steps are as follows:
(1) Install the injection timing instrument. First, remove the rocker arm cover, rocker arm assembly, and injector of the first cylinder. Install the injection timing instrument. The timing instrument has two dial indicators: one measuring rod contacts the piston (piston stroke dial indicator), and the other measuring rod presses on the push rod ball seat (push rod stroke dial indicator).
Note that the installation position of the injection timing instrument must be parallel to the cylinder centerline.
(2) Inspection and adjustment. Rotate the crankshaft clockwise to position the piston of the first cylinder at the top dead center of the compression stroke. When the measuring rod of the piston stroke dial indicator aligns with the 90° mark on the timing instrument scale, set the push rod stroke dial indicator to zero.
(3) Rotate the crankshaft counter - clockwise. When the top dead center mark of the first to sixth cylinders moves to about 10 mm from the calibration point on the scale, move the piston stroke dial indicator to compress its measuring head by about 5 mm, and then fix it. Slowly rotate the crankshaft. When the piston returns to the top dead center position, set the piston stroke dial indicator to zero.
(4) Continue to rotate the crankshaft counter - clockwise. When the measuring rod of the piston stroke dial indicator aligns with the 45° mark on the scale (corresponding to the crankshaft being 45° before the top dead center), rotate the crankshaft clockwise until the reading on the piston stroke dial indicator meets the specifications. Adjust the thickness of the tappet pin shaft cover gasket according to the measured difference to ensure that the injection timing meets the requirements.
Inspection and Adjustment of the Mechanical Plunger - Type Fuel Injection System
Although Cummins mainly uses the PT fuel system, maintenance personnel may also encounter mechanical plunger - type fuel injection systems, and the inspection and adjustment of these systems are equally important.
Inspection and Adjustment of the Injection Advance Angle
The injection advance angle is an important adjustment parameter for the mechanical plunger - type fuel injection pump fuel system. After disassembling the fuel injection pump or when certain faults occur in the diesel engine, it is generally necessary to inspect and adjust the injection advance angle.
There are two issues related to the injection advance angle: an excessively large injection advance angle (too much advance) and an excessively small injection advance angle (too little advance). If the injection advance angle is too large (i.e., too much advance), the compression pressure and temperature in the cylinder are relatively low, which is not conducive to the mixing and combustion of fuel and air. This will lead to problems such as rough operation of the diesel engine, abnormal exhaust smoke (white smoke at low speeds and black smoke under heavy loads), insufficient power, and fuel dripping from the exhaust pipe.
If the injection advance angle is too small, it will result in severe after - burning in the diesel engine, high overall engine temperature, a red - hot exhaust pipe, black exhaust smoke, and insufficient power.
Steps for on - engine fuel supply timing inspection:
Step 1: Confirm the top dead center of the first cylinder (rotate the crankshaft to the top dead center of the compression stroke of the first cylinder).
Step 2: After confirming the top dead center, convert the injection advance angle unit (α - crankshaft rotation angle) into a length unit (L, mm). Starting from the top dead center, mark the length unit corresponding to the injection angle in the direction of crankshaft rotation.
The calculation formula is: L = 3.14Dα/360
Where D is the diameter of the pulley or damper in mm; α is the injection advance angle and crankshaft rotation angle in degrees (°); L is the arc length of the injection advance angle (OT - FB) in mm.
Step 3: Rotate the crankshaft in the reverse direction to about 90° before the compression top dead center of the cylinder.
Step 4: Remove the high - pressure fuel pipe of the first cylinder on the fuel injection pump and install a special capillary glass tube (if no special tool is available, direct observation can be used).
Step 5: Move the throttle lever to the maximum throttle position and use the hand pump to fill the internal oil chamber of the fuel injection pump with diesel.
Step 6: Slowly rotate the crankshaft clockwise and observe the change in the liquid level in the capillary tube. When the liquid level in the capillary tube rises instantaneously, it indicates the start of fuel supply. At this time, observe the position of the pointer on the flywheel (or pulley, damper). If it is not the desired fuel supply advance angle, adjustment is required.
Step 7: Loosen the fastening bolt of the fuel injection pump drive shaft on the fuel injection pump coupling. Rotate the crankshaft clockwise to the corresponding fuel supply angle position.
Step 8: Rotate the fuel injection pump drive shaft clockwise until the liquid level in the capillary tube starts to rise instantaneously, and then tighten the fastening bolt of the fuel injection pump drive shaft.
Step 9: Recheck whether the injection advance angle is within the standard range. If it is appropriate, the adjustment is completed; otherwise, readjust.
Inspection and Maintenance of Injectors
Injectors should be inspected and adjusted after approximately 700 hours of operation. If the opening pressure is more than 1 Mpa lower than the specified value or there is carbon deposition on the needle valve head, remove the needle valve, place it in clean diesel, scrape off the carbon deposits with a wooden piece, and use a fine wire to clear the injection holes. After installation, conduct a test.
The injection pressure difference between cylinders of the same machine must be less than 1 Mpa.
Inspection of Injection Pressure
When inspecting the injection pressure, first release the air in the test bench oil circuit. Open the three - way switch to allow the fuel injection pump to supply fuel to both the injector and the pressure gauge simultaneously. Slowly press the handle (10 - 20 times/min). When the injector starts to inject, the reading on the pressure gauge is the injection pressure of the injector.
If it does not meet the specifications, adjustment is required. Remove the injector pressure - regulating nut, loosen (decrease) or tighten (increase) the pressure - regulating screw to make the injection pressure meet the requirements, and then tighten the pressure - regulating nut.
The injection pressure should be adjusted according to the technical conditions of the fuel injection pump and the injector. If the technical conditions are good, the injection pressure should be adjusted to the upper limit of the specified value. Conversely, if the fuel injection pump and injector are severely worn, the injection pressure should be adjusted to the lower limit of the specified value.
Inspection of Spray Quality
When the injector is within the standard pressure range, press the handle at a speed of 60 - 80 times/min. There should be a clear and crisp sound during injection. If the sound is hoarse, it indicates poor atomization of the injector or inflexible movement of the needle valve.
Visually, the oil mist should be uniform, without any dispersed oil streams or individual oil droplets.
Inspection of Spray Cone Angle
Place a white paper 150 - 200 mm directly below the injector. After injection, check the spray cone angle of the injector based on the diameter of the oil stain on the white paper and the injection distance.
For single - hole pintle injectors, the oil stain should be a uniform circle. For multi - hole symmetric injectors, the shapes of the oil stains sprayed from each injection hole should be the same, without any missing, distorted, or differently sized individual oil stains.
Common Faults and Solutions
Solutions for Excessive Smoke from the PT Pump
Excessive smoke from the Cummins PT pump is a common problem and can be solved by the following methods:
(1) Lower the fuel tank so that the fuel level in the fuel tank is lower than the PT injector.
(2) Install a valve between the fuel tank and the PT fuel pump. When the machine stops running, disconnect the main oil circuit to prevent fuel from the fuel tank from dripping into the cylinder through some injectors during the metering period.
(3) Install a float - type fuel tank between the high - pressure fuel tank and the PT fuel pump. Use it to automatically control the pressure fuel from the high - pressure fuel tank to prevent it from dripping into the cylinder through some PT injectors during the metering period after the machine stops running. Otherwise, a large amount of smoke will be generated when the machine is restarted, and detonation may occur during startup.
Sticking of the Injector Needle Valve
The sticking of the injector needle valve is caused by the blockage of the gap between the needle valve and the valve seat by residual sticky substances, which makes the needle valve move sluggishly and unable to open normally, thus affecting the normal fuel injection volume.
After the injector needle valve sticking fault occurs, the engine may experience difficulties in starting, unstable idling, and poor acceleration. The main cause of injector sticking is the use of low - quality gasoline/diesel, as the paraffin and gum in low - quality fuel can cause the injector needle valve to stick.
Precautions for Installation and Adjustment
1. Appropriate adjustment of the plunger seating pressure: The seating pressure of the injector plunger on the barrel head should neither be too large nor too small. Excessive pressure can cause deformation of the injector driving components, while too little pressure can lead to carbon deposition due to the sintering of residual fuel in the conical cavity of the barrel head at the end of injection, overheating of the barrel head, and even the detachment of the front end of the barrel head.
2. Strictly tighten to the specified torque: When using the torque adjustment method, a calibrated torque wrench must be used, and the adjusting screw and lock nut must be tightened to the torque values specified in the Cummins technical specifications.
3. Pay attention to part cleanliness: The injector needle valve assembly has high matching precision, and the injection hole diameter is very small. Clean diesel of the specified grade must be selected according to seasonal changes; otherwise, the injector will not work properly.
4. Prevent damage to precision parts: When cleaning the injector needle valve assembly, avoid collisions with other hard objects and prevent it from falling to the ground to avoid scratches and damage.
5. Treatment of new assemblies: When replacing the injector needle valve assembly, first soak the new assembly in hot diesel at 80°C for about 10 seconds to fully dissolve the anti - rust oil. Then, move the needle valve back and forth in the valve body in clean diesel to clean it. This can prevent the needle valve from sticking due to the dissolution of anti - rust oil during injector operation.
The correct installation and adjustment of various series of Cummins injectors are crucial for ensuring engine performance. As professional maintenance personnel, mastering the torque and lift adjustment techniques for PT injectors, as well as the inspection and adjustment methods for injection timing, is of great significance for improving maintenance quality.