Auscultation of engine abnormal sound is the simplest, convenient and most commonly used method to judge engine fault. It does not need to disassemble the engine, but can judge the technical condition of the engine with the help of human ears or simple tools. Let’s take Steyr’s engine as an example to tell how to use auscultation to judge engine failure.
The coefficient of thermal expansion of some mating pairs is large, and the abnormal sound is related to the temperature of the engine. For example, the piston pin knocks, and the piston pin impact sound appears when the temperature is low, and will weaken or disappear when the temperature is high. The abnormal noise caused by too low roundness of piston pin and too small clearance of piston ring will appear after the temperature increases, and will weaken or disappear when the temperature is low. Matching pairs with small coefficient of thermal expansion, such as crankshaft, connecting rod bearing and valve, have nothing to do with engine temperature.
Many abnormal noises are closely related to engine speed. Some abnormal noises occur during rapid acceleration, such as crankshaft main bearing, connecting rod bearing, etc. Some abnormal noises are obvious when the engine accelerates rapidly, such as the abnormal noise caused by loose piston pin bushing and broken crankshaft. There is also abnormal noise during low-speed operation, which weakens or disappears when the speed increases. Such as piston knock, piston pin, valve tappet, etc. Therefore, it can be judged by the relationship between these abnormal sounds and the change of speed.
Many abnormal noises during engine operation are related to engine load, so such abnormal noises can be diagnosed according to load changes. It is usually judged by single cylinder fire break method or simultaneous fire break method of adjacent cylinders, such as abnormal sound caused by piston knocking and loose connecting rod bearing. When the cylinder is off, the abnormal sound will weaken or disappear. The abnormal noise caused by the loose copper sleeve of piston pin will be aggravated when the cylinder is disconnected. As another example, the abnormal sound caused by the looseness of crankshaft bearing will weaken or disappear when the fire of adjacent cylinders is cut off at the same time. In addition, some abnormal noises are independent of load, such as valve noise, camshaft bearing noise and timing gear noise. Usually, our Steyr cars are running under overweight load. We can diagnose it by this method.
During the operation of the engine, due to different parts, different parts and different working conditions, the vibration generated by the sound source of abnormal sound is different, so the abnormal sound is different in tone, pitch, audio frequency, sound intensity, location, etc. Using the characteristics and laws of these abnormal noises, the abnormal noises of the engine can be diagnosed under certain diagnostic conditions. This method requires a long time of experience accumulation, and should be confirmed with other methods.
Rhythmic sound quality: the piston pin sound is very sharp and crisp, with high tone and obvious metal knocking sound. It is double continuous sound, light and heavy; The sound of crankshaft bearing occurs at the lower part of cylinder block, which is heavy, strong, blunt and accompanied by vibration. The greater the engine load, the more obvious the sound, the faster the speed, and the greater the sound.
The abnormal sound generated by each mechanism of the engine is sometimes obviously related to the working cycle of the engine, such as the knocking sound caused by the excessive clearance between the piston and the cylinder wall. Most of the abnormal noise caused by engine accessory failure has nothing to do with the working cycle. It is generally clearance noise, such as knocking caused by damage to air compressor, generator, crankshaft drive pulley, water pump, generator, etc.
To cut off the fire is to stop the work of the tested cylinder and relieve the load of the cylinder. This will reduce the mutual impact of relevant parts of the inoperative cylinder, and the abnormal sound will also be reduced or even disappear.
Due to the advance or delay of ignition, it has a response to the abnormal sound of crank connecting rod mechanism, but has no response to the sound of valve mechanism. This method is helpful to diagnose the abnormal sound of crank connecting rod mechanism.
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