Diesel engine failures are complex and variable, but there are patterns to be found. The principles of fault diagnosis can be summarized one sentence: "Understand the phenomena, familiarize with the principles, distinguish the phenomena, conduct thorough analysis, start with the simple and proceed to the complex, proceed from the surface to the interior, make accurate diagnoses, and minimize disassembly."
1. Clarify all the phenomena of the fault
A fault is manifested through certain phenomena. Therefore, before analyzing the fault, one should start with investigation and collect as much information about the fault signs as possible to provide a basis for the analysis. Inquiry is an important means to understand the cause of the fault. One should ask the operator about various situations before and after the diesel engine failed. For example, combining with checking the diesel engine's file, keeping records of regular data is an essential task for the manager. It is necessary to understand the service life of the diesel engine, the load working volume, and the regular maintenance and upkeep situation; whether maintenance, adjustment, or replacement of parts was carried out before the fault occurred; the detailed process of the phenomenon changes during the fault occurrence; and what inspections, disassemblies, etc. were conducted after the fault occurred.
2. Analyze the fundamental causes of the malfunction
Whenever a malfunction occurs, it is always caused by one or two fundamental reasons. For example, the malfunction of a diesel engine emitting black smoke, the fundamental cause is that the diesel cannot be completely burned in the combustion chamber. Therefore, when analyzing the cause of the malfunction, one must focus on the key factors such as oil, gas, and mixture. To deepen the understanding of the fundamental causes of the malfunction, one must be familiar with the internal structure and working principle of each system of the diesel engine, and master the conditions necessary for each system to work normally. One must understand the structural characteristics of various models, the differences between different models, which parts are wear-prone, and where are the weak links in the structure. Practice has proved that the more familiar one is with the structural principle of the diesel engine, the stronger the ability to analyze and judge the malfunction will be, and the faster one can grasp the regularity of the malfunction.
3. Determine the true cause of the malfunction
A comprehensive malfunction is often caused by several specific reasons, which requires inspection and diagnosis to identify the true cause. Whether a complex malfunction can be quickly resolved depends crucially on whether one can proficiently master the inspection methods. During the inspection process, one should try to avoid disassembling or minimize the disassembly. Blindly disassembling and reassembling will not only result in waste of manpower, material resources and time, but also disrupt the normal cooperation between parts, making the malfunction more complicated.
Fault diagnosis includes two aspects: first, use simple methods to quickly narrow down the scope of the malfunction, and then determine whether each part within the faulty section is in good or bad condition, with both having differences and being interrelated. The following introduce several commonly used diagnostic methods in production:
1. Isolation Method
The method of partially isolating or disconnecting the operation of a certain system or component, and determining the scope of the fault by observing changes in symptoms, is called the isolation method. Generally, after isolating or disconnecting a certain part, if the fault symptoms disappear immediately, it indicates that the fault occurs at this location; if the fault symptoms still exist, it indicates that the fault is elsewhere. When diagnosing the problem of diesel engine black smoke emission, if the black smoke emission disappears immediately after cutting off the fuel supply to a certain cylinder, it indicates that the fault occurs in this cylinder. Then, further inspection should be conducted on this cylinder.
2. Trial Method
The method of determining whether certain parts within the fault range are normal by conducting tentative elimination or adjustment measures is called the trial method. This method is based on thorough analysis. When conducting tentative adjustments, it is necessary to consider the possibility of restoring the original state and confirm that no adverse consequences will result. Also, it is necessary to avoid conducting several tentative adjustments at the same time or several tentative adjustments at the same part to prevent confusion and misperception.
3. Comparison Method
The method of replacing a component suspected of being faulty with a functioning identical component and observing the changes in symptoms to determine if there is a fault is called the comparison method. For example, when suspecting that the fuel injector of a certain cylinder is not functioning properly, this cylinder's fuel injector can be replaced with that of another functioning cylinder and installed. If the fault symptoms shift to the other cylinder, it indicates that the suspicion is correct; if the fault symptoms remain unchanged, it means the fault is elsewhere. Similarly, when the oil temperature is too high, if it is suspected that the cooling system's efficiency has declined, the working normal cooling system can be swapped with it. If the oil temperature returns to normal after replacing the component, it indicates that the original cooling system is faulty. Comparison of replacement parts is a measure taken when it is impossible to accurately determine the technical status of each part. Therefore, blind disassembly and replacement should be minimized. In fact, in all diagnostic methods, there are certain comparison components, not just limited to the comparison of replacement parts.
4. Experience Method
This method requires managers to have extensive experience. It mainly relies on the senses of the operator's ears, eyes, nose, and body to determine the technical condition of each part. This method is called the experience method. Common methods include:
(1) Auditory Inspection: Generally, a high-pitched metal rod is used. On ships, a copper rod with a copper ball at one end is commonly used. Based on the abnormal periodic characteristics of the sound, pitch, volume, and sound during the operation of the diesel engine, by listening with the ear or by touching the corresponding part with a metal rod, the location of the fault can be determined. Judging the technical condition of the mating parts based on the characteristics of the sound produced by the diesel engine (such as periodic changes in pitch and volume) is called auditory inspection. When the diesel engine is operating normally, the sound has a regular pattern. From the sounds emitted by each component during operation, it is possible to roughly determine whether the operation is normal. When an abnormal sound is heard, there will be an abnormal feeling. Obvious abnormal sounds can be identified directly by the ear; for mixed and difficult-to-distinguish abnormal sounds, a stethoscope can be used to assist in hearing. Therefore, through the auditory inspection method, faults can be detected early and eliminated promptly.
Due to the differences in size, shape and material of the parts, the sounds vary as well. They also change with the temperature of the diesel engine, the speed, the load, and the lubrication conditions. Some sounds are clear, sharp and short, while others are low, dull, rough, or slight; some are rhythmic interrhythmic sounds, while others are continuous knocking; some are severe at high speeds, while some are prominent at idle; some are clear at the moment of speed change, while some are obvious at a stable speed; some occur when the engine is cold and just started, while some get worse as the temperature rises; some become more severe when the engine is idling, and some are serious at high loads; and some sounds appear accompanied by heating phenomena. No matter what kind of sound it is, when the lubrication is poor, the sound becomes more severe. During the diagnosis, it is necessary to carefully distinguish the sound under different conditions such as cold engine, idle speed, medium speed, sudden speed increase, or idling, spinning, and under load, sometimes close to the sound source, and sometimes it is necessary to listen from a distance from the sound source.
(2) Observation: Through visual inspection, identify the fault symptoms. For instance, observe the obvious defects and damage characteristics of the external components and assemblies of the diesel engine; check for any water leakage, oil leakage, or air leakage; observe the air emission from the crankcase vent holes; observe changes in instrument readings and oil indicators; check the color, viscosity, and amount of metal debris in the oil; observe the color of the exhaust smoke; observe whether the rotational speed is uniform and stable; observe the degree of damage and deformation of the parts; observe the atomization quality of the fuel injectors, etc. Based on the observed abnormal symptoms, analyze the fault.
(3) Smelling: This involves detecting and identifying faults in certain parts by smelling the exhaust smoke odor, burnt smell, or pungent smell of raw diesel oil, etc., promptly.
(4) Touching: This involves touching or manipulating mechanical components with hands, and judging whether the working temperature or clearance of the components is normal by the sensation of the hands. After operating under load for a certain period of time, touching the temperature of the corresponding parts of the bearings can reveal whether they are overheated. Generally, when the hand feels the component is hot, the temperature is around 40°C; when it feels hot but can still be touched for a few minutes, the temperature is between 50-60°C; if it is immediately felt to be unbearably hot upon contact, the component temperature has reached above 80-90°C. Checking the exhaust port of the indicator cock by turning the crankshaft can sense whether the cylinder compression is good, touching and shaking the moving parts can determine whether the fasteners are loose or jammed, and whether there is rubbing. For a diesel engine that has just started, touching the exhaust branch pipes of each cylinder with the hand can compare the temperature differences among them and compare them with the oil supply pulsation felt by the hand of each cylinder's high-pressure oil pipe. This can roughly understand whether the working conditions of each cylinder are normal. When the fingers touch the high-pressure oil pipe and feel a solid and powerful vibration, it indicates that the piston pair and the oil outlet valve pair have good technical conditions; if it feels empty and weak, it indicates that the piston pair and the pressure reduction ring of the oil outlet valve are worn; if the high-pressure oil pipe has a large vibration resistance and a rebounding sensation upon touch, it indicates that the needle valve of the injector is stuck, does not spray oil, or sprays very little oil; if the hand touches the high-pressure oil pipe and feels empty and weak, with a large resistance and an impact sensation, it indicates that the needle valve and the needle valve seat are not tightly sealed. In the absence of measuring tools, experienced people can roughly judge whether the valve clearance and bearing clearance are appropriate by the sensation of the hands.
The empirical method has certain practical value. However, judging the technical condition based on sensory organs requires continuous experience, exploration and summarization in long-term practice, so as to gradually enrich one's own experience.
5. Instrumentation Method
By using lightweight instruments and meters, the parameters of the diesel engine are measured to analyze the combustion and power generation conditions of each cylinder. Without disassembling or with minimal disassembly, this method enables a relatively accurate understanding of the internal condition of the diesel engine. This method is called the "instrumentation method". By using instruments to check the technical status of each part, qualitative and quantitative analyses of the technical status of each part can be conducted. To prevent and promptly detect faults, it is necessary to have a clear understanding of the technical status of each system of the diesel engine, that is, not only to know whether the technical status is good or bad, but also to know the specific degree of good or bad. In this way, merely relying on experience for inspection is often insufficient, while using instruments for inspection can achieve qualitative and quantitative analysis more accurately. For example, the supply pressure of a well-sealed piston pair can reach over 50 megapascals. When the piston supply pressure drops to 15-18 megapascals, it will cause difficulties in starting and insufficient power. At this time, if the normal fuel injector with normal fuel pressure is placed outside the cylinder for fuel injection, and the sealing of the piston is checked, since the conditions outside the cylinder are different (mainly referring to the different air pressure), the fuel injector can still spray normally. Therefore, people often do not suspect that there is a problem with the piston, and blindly disassemble other parts. When using a 60 megapascal pressure three-way valve to check the sealing of the piston, by observing the pointer reading, the problem can be easily identified. Additionally, combined with locomotive maintenance work, the cylinder pressure and the resistance of the oil filter element passing through can be regularly measured. When the pressure of a certain cylinder drops rapidly or the resistance of the filter element passing through is very small, the fault can be promptly detected and eliminated at the initial stage.
In recent years, various instruments have been developed in different places, including Micro Inspection Instrument for Diesel Engine Fuel System, Micro Inspection Instrument for Diesel Engine Compression System, BQ-III Portable Injector Tester, QMY Valve Sealing Inspection Instrument, XCG-I Fuel Consumption Meter, BS-90 Portable Multi-functional Diesel Engine Detector and Diesel Engine Operating Condition Electronic Instrument.