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Reasons for Why Piston of Engine Cylinder Has Abnormal Sound

Feb. 19, 2019

The abnormal noise in engine cylinder can be summarized as piston knock, piston pin knock, piston top impact cylinder head, piston top impact, piston ring knock, valve knock and cylinder knock, and so on, which can be classified into several types of noise: piston knock, piston pin knock, piston top impact, piston ring knock, valve knock, cylinder knock, and so on.

Reasons for Why Piston of Engine Cylinder Has Abnormal Sound.jpg

1. Impact sound of piston head and cylinder head

The abnormal sound of the piston head striking the cylinder head is a continuous metal knock, especially at high speed. The abnormal sound source is in the upper part of the cylinder, its sound is firm and powerful, and the cylinder head has vibration. The main reasons are as follows.


A. Crankshaft bearings, connecting rod bearings and piston pin holes are severely worn, and the matching clearance is seriously exceeded. At the moment of piston stroke change, the piston impacts the cylinder head under the action of inertial force.


B. The distance between the center line of the piston pin hole and the top surface of the piston is larger than that of the original piston, so that when the piston reaches the upper stop point, it will collide with the cylinder head because of the super high.


In case of such a situation while travelling, the first aid method is to remove the cylinder head and add a cylinder cushion so that the cylinder head rises without further collision. But when there is repair condition, should carry on repair immediately, restore its good technical condition.


2. Abnormal noise in Piston Ring


The abnormal sound of piston ring mainly includes metal percussion of piston ring, leakage sound of piston ring and abnormal sound caused by excessive carbon deposit.


A. The metal percussion of the piston ring. After working for a long time, the cylinder wall is worn out, but the original geometry and size are almost maintained where the upper part of the cylinder wall is not in contact with the piston ring, which makes the cylinder wall produce a step. If an old cylinder pad is used or the new one is thin, the piston ring will collide with the steps of the cylinder wall, making a dull puff of metal. If the engine speed increases, the abnormal noise will also increase. In addition, if the piston ring breaks or the gap between the piston ring and the ring groove is too large, it will also cause a great beating sound.


B. The sound of air leakage from the piston ring. The elastic force of piston ring is weakened, the opening gap is too large or the opening is overlapped, and the cylinder wall is pulled with grooves, which will cause the gas leakage of the piston ring. The sound is a "drink" or a "hissing" sound, and a "puff" sound when there is a serious leak. The diagnostic method is to extinguish the engine when the water temperature is above 80 ℃. At this time, a little fresh and clean oil can be injected into the cylinder, then the engine can be restarted after turning the crankshaft for several laps. If the abnormal noise disappears at this time, it will soon appear again. It can be concluded that the piston ring is leaking.


C. An abnormal sound of too much carbon. When the product is too much carbon, the abnormal sound from the in-cylinder is a sharp sound, because the product carbon is burned red, the engine has the symptoms of premature ignition, and is not easy to extinguish. The formation of the product carbon in the piston ring is mainly caused by the tight seal between the piston ring and the cylinder wall, the excessive clearance of the opening, the reverse rotation of the piston ring, the coincidence of the ring opening and the like, so as to cause the piston ring to lose the elasticity and the sealing action. This fault can be ruled out after a proper replacement of a suitable piston ring.


3. The knocking sound of the piston pin


"Piston pin and piston pin hole, piston pin and connecting rod small end bushing (or bearing) gap is too big, will make a sharp, crisp, very high pitch" click "metal knock, similar to the small hand hammer tapping iron drill." The rule is that the engine cold car does not sound when it starts, but when the temperature rises, it will ring, and the higher the temperature is, the louder it will be. If the ignition time before the adjustment, the sound will be accelerated and increased. If the spark plug breaks the fire, the noise will weaken or disappear.


The engine not only has a large clearance between the piston pin and the pin hole, but also the clearance of the piston pin and the small-head bush (or the bearing) of the connecting rod is large, and the knocking sound emitted in the case is more complex, and a continuous “to have a hand in hand” sound can be generated.


The reason for this failure is that apart from the wear and tear in work, the gap between the piston pin, the piston pin hole and the small end bushing of the connecting rod are steel, aluminum and copper respectively, and the expansion coefficient is quite different, slightly sloppy. In the work, the three gaps will become larger because of the different thermal expansion, resulting in the impact between each other. Moreover, when the gap increases, the axial momentum of the piston pin increases, and the cylinder end surface of the pin will impact the pin ring, which increases a kind of abnormal sound in the cylinder.



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