Inside the huge body of construction machinery, the engine is like a beating "heart", continuously providing power for the operation of the equipment. The injection pump, on the other hand, is like the "pacemaker" of the engine, precisely regulating fuel injection and playing a crucial role in the engine's performance. Whether it is an excavator on a construction site or a loader in a mining operation, the injection pump silently ensures the efficient operation of the machinery. This article will deeply explore the working principle, types, common faults, and maintenance points of construction machinery injection pumps.
I. Working Principle of the Injection Pump
The core mission of the injection pump is to raise the fuel pressure to a sufficiently high level and inject the fuel into the combustion chamber of the engine according to the working sequence of the engine, at the precise time and in the appropriate amount. Taking the plunger-type injection pump, which is widely used, as an example, its working process can be divided into three stages:
- Fuel Suction Process: The plunger moves downward under the drive of the cam on the camshaft. At this time, the volume of the oil chamber increases, the pressure decreases, and a negative pressure is formed. When the radial oil inlet hole on the plunger sleeve is exposed, the fuel in the low-pressure oil chamber flows into the pump chamber along the oil inlet hole under the action of the pressure difference, preparing for the subsequent fuel pumping.
- Fuel Pumping Process: As the cam rotates, its raised part pushes the plunger upward. The volume of the pump chamber gradually decreases, and the fuel is squeezed, resulting in a sharp increase in pressure. When the pressure reaches a certain level, the fuel will first flow back to the low-pressure oil chamber along the radial oil hole on the plunger sleeve. When the plunger moves upward to completely block the radial oil hole, the pressure in the pump chamber rises rapidly until it overcomes the pre-tightening force of the delivery valve spring, and the delivery valve is opened. At this time, the high-pressure diesel oil enters the high-pressure oil pipe through the delivery valve and is finally sprayed into the cylinder through the injector. It mixes with air and burns, releasing huge energy to push the piston to move.
- Fuel Return Process: The plunger continues to move upward. When the inclined groove on the plunger communicates with the radial oil hole on the plunger sleeve, the fuel in the pump chamber flows back to the low-pressure oil chamber through the axial oil passage and the inclined oil passage on the plunger and the oil hole on the plunger sleeve. This connection causes the pressure in the pump chamber to drop sharply, and the fuel pumping process stops immediately. By cleverly designing the position and shape of the inclined groove on the plunger, the fuel return time can be precisely controlled, and thus the accurate adjustment of the fuel injection amount can be achieved.
II. Types of Injection Pumps
- Plunger-Type Injection Pump: The plunger-type injection pump has a long history and is extremely widely used. It has a high degree of reliability, which benefits from its precise processing technology and mature design. In the plunger-type injection pump, the plunger and the plunger sleeve are a pair of super-precision processed couples with an extremely small fit clearance, which can ensure the sealing and precise delivery of high-pressure fuel. Each set of plunger systems corresponds to one cylinder. For example, in an in-line injection pump, if the engine has 4 cylinders, 4 sets of plunger systems will be equipped. This type of injection pump has a relatively complex structure and a large volume, but it can generate a high injection pressure and is suitable for large and medium-sized construction machinery with high requirements for power output, such as heavy trucks and large excavators.
- Injection Pump - Injector: The injection pump - injector combines the injection pump and the injector into one and is directly installed on the engine cylinder head. The biggest advantage of this integrated design is to eliminate the adverse effects brought by the high-pressure oil pipe. In the traditional fuel injection system, factors such as the length, inner diameter and internal resistance of the high-pressure oil pipe will all affect the pressure and time of fuel injection, resulting in inaccurate fuel injection effects. However, the injection pump - injector can inject fuel into the cylinder more quickly and accurately due to the shortened fuel delivery path, improving the combustion efficiency. However, this design requires an additional set of drive mechanisms on the engine to meet its working needs, which increases the complexity and cost of the engine to a certain extent. It is usually applied to engines with extremely high requirements for fuel injection accuracy, such as some high-performance diesel engines.
- Rotary Distributor-Type Injection Pump: The internal structure of the rotary distributor-type injection pump is relatively simple, with only a pair of plungers. During operation, it mainly relies on the rotation of the rotor to achieve the pressurization and distribution of fuel. During the rotation of the rotor, the plunger reciprocates under the action of the cam ring, sucking in and pressurizing the fuel from the oil inlet and then distributing it to the corresponding injectors in sequence according to the working sequence of each cylinder of the engine. This type of injection pump has significant advantages such as small size, light weight, convenient use and low cost, but it is obviously limited by the number of cylinders and the maximum speed. It is generally used in diesel engines of cars and light vehicles and is also applied in some small construction machinery, such as small loaders and agricultural machinery, which can meet their requirements for power and space layout.
III. Common Faults and Cause Analysis
- Abnormal Fuel Supply: Excessive or insufficient fuel supply is one of the common faults of the injection pump. When the fuel supply is excessive, the fuel cannot be fully burned, which will cause the engine to emit black smoke, increase fuel consumption, reduce power, and even cause dangerous situations such as "runaway". Common causes include governor faults. For example, too much oil in the governor will affect the normal operation of the governor and cause the fuel supply to increase uncontrollably; the fuel quantity adjustment mechanism is out of order. For example, the rack of the rack-type fuel quantity adjustment mechanism is stuck or worn, or the fork of the fork-type fuel quantity adjustment mechanism is loose or deformed, which will all lead to inaccurate control of the fuel supply. Insufficient fuel supply will make the engine lack power, difficult to start, and run unstably. This may be due to severe wear of the plunger couple, resulting in a decrease in sealing performance and partial fuel leakage, reducing the actual fuel supply; the delivery valve is not tightly sealed, and the high-pressure fuel leaks back to the low-pressure oil chamber through the delivery valve during the fuel pumping process, which also causes insufficient fuel supply.
- Inaccurate Injection Timing: Premature or late injection timing will have a serious impact on the engine performance. If the injection timing is too early, when the fuel burns in the cylinder, the piston has not reached the optimal position, which will cause the engine to work roughly, produce knocking sounds, and at the same time reduce power and increase fuel consumption. If the injection timing is too late, the fuel cannot burn at the best time, resulting in incomplete combustion, an increase in the exhaust temperature of the engine, and a weakening of the power output. The main reasons for inaccurate injection timing include camshaft wear. The wear of the camshaft will change the profile of the cam, resulting in a change in the movement law of the plunger and thus affecting the injection timing; drive mechanism faults, such as wear and looseness of the timing gears, improper chain tension, etc., will cause a deviation in the drive time of the injection pump, and finally lead to inaccurate injection timing.
- Insufficient Injection Pressure: Insufficient injection pressure will cause poor fuel atomization, inability to mix well with air, and also result in incomplete combustion, reduced engine power, and increased fuel consumption. Insufficient injection pressure may be due to the weakening or breakage of the plunger spring, so that the plunger cannot obtain sufficient force during the fuel pumping process and cannot raise the fuel pressure to the normal level; the delivery valve spring is damaged or the pressure is adjusted improperly, which will also affect the opening and closing pressure of the delivery valve, resulting in unstable or insufficient injection pressure; in addition, the rupture and leakage of the high-pressure oil pipe will cause excessive pressure loss of the high-pressure fuel during the transportation process, and finally the injection pressure cannot meet the requirements.
IV. Maintenance Points
- Regular Inspection and Adjustment: Regularly checking the fuel supply advance angle of the injection pump is of great importance. When installing the injection pump on the diesel engine, be sure to carefully check and adjust the fuel supply advance angle to ensure that it meets the technical requirements of the engine. After the diesel engine has run for a certain period of time (such as 500 hours), due to factors such as component wear, the fuel supply advance angle may change. At this time, it is necessary to check and adjust it again to ensure that the injection pump works in the best state. At the same time, regularly check whether the fuel supply of the injection pump is uniform, and the deviation of the fuel supply among each cylinder should be controlled within the specified range. If the fuel supply is found to be uneven, it can be corrected by adjusting the fuel quantity adjustment mechanism.
- Keep Fuel Clean: Construction machinery should use light diesel oil that meets national standards and ensure that the diesel oil is fully precipitated and filtered. It is generally recommended that the diesel oil be precipitated for 48 - 96 hours before use, which can effectively remove impurities and water in the diesel oil. When impurities and water enter the injection pump, they will exacerbate the wear of precision components such as the plunger couple and the delivery valve couple, shorten the service life of the injection pump, and may also cause the injector nozzle to be blocked, affecting the fuel injection effect. Usually, pay attention to the cleaning of the diesel tank and the diesel filter, and regularly replace the filter element of the diesel filter to prevent impurities from entering the injection pump.
- Proper Maintenance of the Lubrication System: If there is a skeleton oil seal between the injection pump and the governor, an appropriate amount of engine oil should be added separately to ensure the good lubrication of each moving part. At the same time, pay close attention to the oil level and oil quality of the engine oil. If the oil level is found to rise due to oil dilution, it is very likely that there is a serious oil leakage in the injection pump and the fuel transfer pump. At this time, the fault should be promptly investigated and repaired to avoid aggravated component wear due to poor lubrication. In addition, replace the engine oil according to the specified cycle to keep the lubrication system clean and in normal operation.
- Timely Repair of Faults: Once a fault is found in the injection pump, such as abnormal fuel supply, inaccurate injection timing, insufficient injection pressure, etc., stop the machine immediately for repair. During the repair, professional technicians should operate, use appropriate tools and testing equipment, accurately judge the cause of the fault, and take corresponding repair measures. For severely worn precision components such as the plunger couple and the delivery valve couple, they should be replaced with original factory parts or parts that meet quality standards in a timely manner to ensure the performance and reliability of the injection pump. At the same time, after the repair, conduct a comprehensive debugging of the injection pump to ensure that its various performance indicators return to normal.
The construction machinery injection pump, as a key component of the engine, its performance directly affects the power, economy, and reliability of construction machinery. Understanding the working principle, type characteristics of the injection pump, and mastering the diagnosis and maintenance methods of common faults are of great significance for ensuring the normal operation of construction machinery, extending the service life of equipment, and improving work efficiency. Only by doing a good job in the maintenance and management of the injection pump can the "steel giant" of construction machinery continue to play a stable role in various engineering construction.
