Expose the true face of 'abnormal noise' in the cylinder

Analyze the 'abnormal noise' in the diesel engine and engine cylinder, which can be summarized as the sound of piston knocking on the cylinder, piston pin knocking, piston top hitting the cylinder head, piston top hitting, piston ring knocking, and valve knocking. and cylinder knocking sounds, etc. So HaixingGenerator setThe company analyzes the true nature of these 'abnormal noises' for everyone.
1. The sound of impact between piston top and cylinder head
The abnormal sound when the piston top hits the cylinder head is a 'click-click' continuous metal knocking sound, which is especially obvious at high speeds. The source of the abnormal sound is in the upper part of the cylinder, the sound is solid and powerful, and the cylinder head vibrates. The main reasons are as follows.
(1) The crankshaft bearing, connecting rod bearing and piston pin hole are severely worn, and the matching clearance seriously exceeds the standard. At the moment when the piston stroke changes, the top of the piston hits the cylinder head under the action of inertial force.
(2) When the piston is replaced, other pistons of similar specifications are mistakenly installed, or 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. When the piston reaches the top dead center, it collides with the cylinder head due to the excessive height. .
If you encounter such a situation while driving, the first aid method is to remove the cylinder head and add a cylinder gasket to raise the cylinder head to prevent further collision. However, when there are conditions for repair, repairs should be carried out immediately to restore its good technical condition.
2. Abnormal noise at the piston ring area
Abnormal noises in the piston ring area mainly include the metal knocking sound of the piston ring, the sound of air leakage from the piston ring, and abnormal sounds caused by excessive carbon deposits.
(1) Metal knocking sound of piston rings. After the engine has been operating for a long time, the cylinder wall is worn, but the upper part of the cylinder wall that is not in contact with the piston ring almost maintains the original geometric shape and size, which creates a step on the cylinder wall. If an old cylinder gasket is used or a new one is too thin, the working piston ring will collide with the cylinder wall step, making a dull 'puff' metal impact sound. If the engine speed increases, the abnormal noise will also increase. In addition, if the piston ring is broken or the gap between the piston ring and the ring groove is too large, it will also cause a loud knocking sound.
(2) The sound of air leakage from the piston ring. The elasticity of the piston ring is weakened, the opening gap is too large or the openings overlap, and the cylinder wall has grooves, etc., which will cause the piston ring to leak. The sound is a 'drinking' or 'hissing' sound, and when there is a serious air leak, it makes a 'puffing' sound. The diagnostic method is to stall the engine when the water temperature reaches above 80°C. At this time, you can inject a little fresh and clean engine oil into the cylinder, crank the crankshaft for a few turns, and then restart the engine. If it appears, it can be concluded that the piston ring is leaking.
(3) Abnormal sounds due to excessive carbon deposits. When there is too much carbon deposit, the abnormal noise coming from the cylinder is a sharp sound. Because the carbon deposit is burned red, the engine has symptoms of premature ignition and is difficult to stall. The formation of carbon deposits on the piston ring is mainly due to the lax sealing between the piston ring and the cylinder wall, excessive opening clearance, reverse installation of the piston ring, overlapping of the ring ports, etc., which causes lubricating oil to escape upward and high-temperature and high-pressure gas to escape downward, causing the piston to accumulate carbon deposits. The ring part burns, causing carbon deposits to form and even sticking to the piston ring, causing the piston ring to lose its elasticity and sealing effect. Generally, this fault can be eliminated after replacing the piston ring with a suitable specification.
3. The sound of knocking on the cylinder
Cylinder knocking refers to the 'dang-dang' or 'dang-dang' or 'dang-dang' sound produced by the piston swinging in the cylinder when the piston starts its working stroke, or when the piston moves upward. Its head and skirt collide with the cylinder wall. Abnormal clicking sound. If there is a 'dang-dang' sound, it is mostly caused by poor lubrication of the cylinder wall. At this time, you can drip a little oil into the cylinder and restart the engine. If the abnormal sound is reduced or disappears, it means that the abnormal sound is indeed caused by poor lubrication. . If there is a 'clicking' sound and blue smoke is emitted from the exhaust pipe, it is usually due to the gap between the piston and the cylinder wall being too large. The main reasons for the above situation are as follows:
(1) If this phenomenon only occurs after starting a cold car. It will disappear automatically when the water temperature reaches normal operation. This is because the matching gap between the piston and the cylinder wall is too large. When the car is cold, the piston shrinks, which further increases the matching gap between the two, resulting in an obvious knocking sound. After the machine temperature rises, the piston expands and the gap tends to the normal value, so the abnormal noise disappears. This situation will not cause major problems in the short term.
(2) The grade of the engine oil does not meet the requirements. When the engine is restarted after stalling for a long time, the engine oil is thick and has poor fluidity. A good oil film cannot be formed on the cylinder wall in a short period of time. The piston and the cylinder wall directly collide, causing cylinder knocking. After running for a period of time, the viscosity of the lubricating oil will be normal, an oil film will form on the cylinder wall, and the abnormal noise will weaken or disappear. As long as the lubricating oil is properly selected, the engine crankcase is preheated before starting, and the engine crankcase is lubricated with mixed oil, then the starting lever should be stepped on several times before starting to allow more lubricating oil to adhere to the machine parts. In this case, Avoidable.
(3) The mixture entering the cylinder cannot burn normally, causing premature ignition or deflagration, or the moment the accelerator is pressed hard when the engine is under no load, the sound of the piston hitting the cylinder wall will be produced. The engine should be kept running at normal temperature, use gasoline that meets the octane number requirements, and adjust the ignition time appropriately.
(4) The piston skirt is worn and the cylindricity error is too large. When the piston moves upward, its top will hit the cylinder wall. The piston and the cylinder are severely worn, and the gap between the two is the wall. It can be inspected after disassembly and corresponding repair measures can be taken according to the situation.
(5) Due to the bending and twisting of the connecting rod, the deflection of the piston pin and the pin hole, the non-parallel axes of the crank pin and the piston pin, etc., it will also cause the piston to deflect in the cylinder and hit the cylinder wall. In this case, it can only be inspected after disassembly, and the corresponding parts can be replaced after diagnosis.
4. The knocking sound of piston pin
If the clearance between the piston pin and the piston pin hole, the piston pin and the connecting rod small end bushing (or bearing) is too large, a sharp, crisp, and high-pitched metal knocking sound will be emitted. It sounds like a hammer hitting an iron drill. The rule is that the engine does not make any noise when it is cold and starts, but it makes noise when the temperature rises, and the higher the temperature, the louder it becomes. If the ignition time is adjusted forward, the sound will speed up and become louder. If the spark plug is cut off, the sound will weaken or disappear.
In some engines, not only the gap between the piston pin and the pin hole is large, but also the gap between the piston pin and the small end bushing (or bearing) of the connecting rod. In this case, the knocking sound is more complicated, and there will be a continuous 'click' sound. 'sound.
The cause of this failure is not only the wear and tear during work that enlarges the gap, but also the fact that the piston pin, piston pin hole and connecting rod small end bushing are made of steel, aluminum and copper respectively during assembly, and their expansion coefficients are very different. If you are a little careless, the gap between the three will become larger due to different thermal expansion during work, causing collisions with each other. Moreover, when the gap increases, the axial movement of the piston pin also increases, and the cylindrical end surface of the pin will hit the pin ring, adding an abnormal sound in the cylinder.
5. Abnormal noise of valve
(1) The abnormal sound of exhaust valve leakage can be heard at the exhaust pipe muffler, such as the 'squeaking' sound when a tire is seriously leaking. Valve leakage can be heard at the air filter on the upper part of the carburetor, and the sound is like a 'shush' sound when a child whistles. The reason is that the working surfaces of the valve and the seat ring are severely worn or ablated, with grooves and spots appearing and cannot be tightly sealed. In some cases, the gap between the valve stem and the duct is too large or the valve stem is bent, causing the valve head to be unable to be centered and skewed, causing air leakage. If the elastic force of the valve spring is weakened or broken, the valve cannot fit tightly with the working surface of the seat ring, which will also cause air leakage. From an assembly point of view, if the gap between the valve stem and the duct is too small and the valve is stuck due to thermal expansion during operation, or if the valve gap is adjusted too small and the valve stem is stretched by heat and the valve is pushed open by the tappet or rocker arm, the valve will be damaged. This will cause the valve to fail to close completely and cause air leakage.
(2) The abnormal sound when the valve spring breaks varies depending on the valve placement method. After the side-mounted valve spring is broken, it will make a 'clicking' sound during operation. If the valve chamber cover is removed, it will be heard more clearly. After the overhead valve spring breaks, the valve automatically sinks, collides with the piston, and makes a 'dang-dang' knocking sound. If the latter is not turned off in time, it will cause the piston to be broken, the connecting rod to be broken, and even more serious losses.
(3) Too much carbon deposit on the valve will also cause abnormal noises. Because there is too much carbon deposit, the carbon layer will be hot, the piston temperature will be high, and the gaps between the piston ring groove and the piston pin hole will increase, thus emitting a 'chatter' 'The strange sound. This fault is often accompanied by phenomena such as the engine temperature being too high and the engine being difficult or unable to stall. The reason is that the proportion of engine oil in the mixed oil exceeds the standard or a large amount of oil from the oil pan escapes into the combustion chamber. The specific cause should be further identified and eliminated.
6. Abnormal noises from the combustion chamber
When the engine load increases, some engines will make a 'clicking' sound from the cylinder, which is different from the sound during normal operation, and may even be accompanied by deterioration in acceleration performance. This kind of abnormal noise is loud and can be heard 5-6m away from the engine. The abnormal noise is especially obvious during sudden acceleration, which can be regarded as a sign of engine knocking. When the combustible mixture burns too quickly in the cylinder, it instantly releases a large amount of heat, causing the pressure in the cylinder to rise sharply. The high-pressure gas strongly impacts the piston top, cylinder head and cylinder wall, causing knocking. The main reasons for knocking include engine overheating, premature ignition time, excessive carbon deposits in the combustion chamber, low octane number of gasoline, low calorific value of the spark plug used, etc.