When a diesel generator is operating normally, it emits regular and rhythmic sounds. When mechanical parts are severely worn, the fit clearances change, maintenance and adjustments are incorrect, and connecting parts become loose, abnormal noises often accompany the components, such as the metallic knocking sound, abnormal combustion sound, and air leakage sound during the operation of the diesel generator. The presence of such abnormal noises indicates that there is a fault in the diesel generator, and it should be shut down for inspection and repair; otherwise, the accident will be further aggravated. When there are abnormal phenomena during the operation of the diesel generator, it usually emits abnormal noises. Usually, by “listening” to the diesel generator, one can tell whether there is a fault in it. If any abnormal noises are heard, the diesel generator should be immediately shut down for inspection to identify the cause.
1. Common Phenomena of Abnormal Noises
（1）. When the diesel generator suddenly changes its load, a short, dull knocking sound of “Dong, Dong, Dong” can be clearly heard near the crankcase (within the range from the middle to the lower part of the cylinder block). It changes drastically with the increase of load or rotational speed, and the sound becomes louder. The sound can be heard more clearly when the oil filler cap is opened, and it is not obvious when there is no load. This is caused by the excessive wear of the connecting rod bearings, resulting in an overly large fit clearance between the connecting rod bearings and the crankshaft connecting rod journals. If the fuel supply to a certain cylinder of the diesel generator is cut off and this abnormal sound disappears immediately, it indicates that the abnormal sound is coming from the connecting rod bearings of that cylinder. （
（2）. When a rhythmic “Hua La, Hua La” sound is emitted from the front side of the engine and the timing gear chamber, and the sound is louder when the engine is running at a low speed, this is due to the excessive wear clearance between the timing gear shaft and its bushings. When the diesel generator is idling, an unrhythmic “Ga La, Ga La” sound is emitted from the timing gear chamber and the side of the camshaft. The sound is more prominent at medium and low speeds, and it becomes chaotic and accompanied by a cracking sound at high speeds. If the noise intensifies under heavy load, this is caused by an overly large meshing clearance of the timing gear, the mutual impact of the gear teeth, or the abnormal meshing of the two gears due to the wear of the bushings. When the diesel generator generates a violent vibrating sound during operation and is difficult to operate, this is caused by incorrect installation of the timing gear marks, incorrect weight of the balance blocks, or severe wear of the bearings of the balance shaft.
（3）. A “Ca, Ca, Ca” sound is emitted from the cylinder head, accompanied by an air flow sound of “Xu, Xu, Xu”. The sound is more obvious when the engine is about to stall. This is the impact sound between the valve seat ring, the valve, and the cylinder head after the valve seat ring becomes loose. When a “Ca Ca” sound similar to the cracking of broken tiles is emitted from the outer cover of the valve chamber of the diesel generator, or a sound similar to the impact of two springs is heard, and at the same time, explosions are intermittently emitted from the exhaust pipe, the diesel generator works unstably and emits black smoke. This is caused by the breakage of the valve spring. At the joint between the cylinder head and the engine block, if a rough and sharp “Wee, Wee, Wee” sound is emitted, accompanied by gas leakage and smoking phenomena, this is caused by the damage of the cylinder gasket. If a “Xu, Xu, Xu” sound is heard at the air filter, and the diesel generator is difficult to start, or if it feels hot to the touch when the intake pipe is touched by hand after running for a while after starting, this is mostly caused by the burning (erosion) of the valve, carbon deposits, and air leakage due to poor sealing. Near the injector on the upper part of the engine, a clear and relatively loud “Dang, Dang” impact sound is heard, which changes with the change of rotational speed, and both starting and combustion are abnormal. This may be caused by the falling of the swirl chamber insert. During the operation of the engine, a low-pitched “Kuang Dang, Kuang Dang” and “Ga, Ga” sounds are emitted, and the sound is much louder than the sound caused by an overly large clearance of the main shaft bearings, especially when the rotational speed suddenly changes. This is caused by the loosening of the fastening nut of the flywheel.
（4）. When a heavy and continuous knocking sound is heard at the lower part of the crankcase of the diesel generator, the sound is clearly distinguishable at a low speed and is more obvious under heavy load. Meanwhile, the oil pressure decreases accordingly. This is caused by an overly large clearance of the main shaft bearings of the crankshaft. When the engine suddenly changes its rotational speed at a low speed, a “Dang, Dang” sound is emitted from the upper part of the cylinder. This is caused by an overly large fit clearance between the piston pin and the connecting rod bushing or the piston pin seat. If a muffled “Ge Deng, Ge Deng” impact sound is emitted when the engine is running at a high speed, especially when the throttle is suddenly decreased, this is caused by an overly large axial clearance of the main shaft bearings. A roaring, dull “Kong Dong, Kong Dong” knocking sound is heard along the entire length of the cylinder, and bubbles are found when the oil filler cap is opened (indicating air leakage). This is caused by an overly large fit clearance between the piston and the cylinder sleeve. If a “Ga, Ga, Ga” impact sound is heard slightly closer to the crankshaft direction in the same area, this is the knocking sound caused by the swinging of the connecting rod due to its distortion during operation. The latter two sounds can be more clearly heard when the rotational speed is changed and are easier to distinguish. A sound similar to the light tapping of a small hammer on an iron block is heard at various points along the upper and lower parts of the cylinder. The exhaust pipe of the diesel generator emits blue smoke (accompanied by oil burning phenomenon). It is easier to hear by placing a long-handled screwdriver against the cylinder block. It can be concluded that this is caused by an overly large side clearance of the piston ring or the breakage of the piston ring.
2. Solutions to Abnormal Noise Faults
(1). If the fuel injection time of the diesel generator is too early or too late, and a rhythmic, clear metallic knocking sound or a dull and indistinct knocking sound is emitted inside the cylinder, the fuel injection advance angle should be adjusted at this time.
(2). When the connecting rod bearings are worn, resulting in an overly large fit clearance, and a mechanical part impact sound is heard inside the crankcase during operation, and a heavy and powerful impact sound can be heard when the rotational speed of the diesel generator is suddenly reduced. In this situation, the bearings should be disassembled and inspected, and replaced if necessary.
(3). If the fit between the piston pin and the small end bushing hole of the connecting rod is too loose, and a light and sharp sound is produced during the operation of the diesel generator, and this sound is especially clear during constant-speed operation. Replace the small end bushing of the connecting rod to make it within the specified clearance range.
(4). If the clearance between the piston and the cylinder sleeve is too large, and an impact sound is heard on the outer wall of the cylinder during the operation of the diesel generator, and this impact sound intensifies when the rotational speed increases. At this time, the piston should be replaced or the cylinder sleeve should be replaced depending on the wear situation.
(5). If the valve spring of the diesel generator is broken, the tappet is bent, and the push rod sleeve is worn, and a rhythmic, slight knocking sound is emitted at the entire cylinder. At this time, the parts of the diesel generator should be inspected and the damaged parts should be replaced, and the valve clearance should be adjusted.
(6). If the radial clearance of the rolling main bearings of the crankshaft is too small, an extremely sharp sound is emitted during the operation of the diesel generator, and this sound is even clearer when the throttle is increased. If the radial clearance of the rolling main bearings of the crankshaft is too large, a “Huo Huo” sound is emitted during operation. At this time, the diesel generator should be shut down to inspect the rolling main bearings that are making the noise, and replaced if necessary.