Why do gasoline engines experience cylinder scuffing?

Why do gasoline engines experience cylinder scuffing?

During operation, the piston and piston rings move at high speed in a reciprocating motion within the cylinder. Under the influence of the huge expansion thrust generated by the combustion of the fuel-air mixture, the piston and piston rings are in close contact with the inner wall of the cylinder through the oil film formed by the engine oil. Under normal circumstances, due to the isolation and buffering effect of the oil film, the piston and piston rings do not come into direct contact with the cylinder. However, in certain specific conditions, they may come into direct contact, causing sliding friction and generating a large amount of heat. If the heat dissipation conditions are not effectively improved, the metal surfaces of the piston, piston rings, or cylinder inner wall may melt, causing the sliding friction surfaces to fuse together. During the high-speed movement of the piston, longitudinal scratches are produced, and in severe cases, the two may seize, which is known as the cylinder scuffing phenomenon. 

The main reasons are as follows: 

1. Insufficient initial break-in of the engine and cylinder pulling caused by high-speed operation

For a newly assembled engine, the seemingly smooth inner surface of the cylinder and the outer surfaces of the piston and piston rings are actually composed of countless microscopic protrusions of different heights. Before the engine can be operated freely, the mating surfaces must undergo initial break-in to remove the sharp peaks of the protrusions, making the surfaces smooth, evenly stressed, and preventing them from biting each other during sliding, ensuring smooth operation. Therefore, a newly assembled engine must undergo a certain degree of break-in. Otherwise, the higher protrusions on the mating surfaces of the piston, piston rings, and cylinder will bear concentrated loads, with extremely high pressure per unit area. After friction, the temperature rises, the protrusions soften and melt, leading to cylinder pulling.

2. Cylinder pulling caused by overheating of the piston

When the piston overheats, local abnormal expansion occurs. Due to the deformation of the piston, the piston ring grooves become wavy, and the piston rings and piston ring grooves bite together briefly or for a long time, causing the piston rings to fail to function properly. At this point, the edge of the piston ring comes into direct contact with the inner wall of the cylinder at an extremely high pressure per unit area, not only losing its function of sealing the combustion gas but also destroying the oil film on the inner wall of the cylinder, thereby causing cylinder pulling. 

The main causes of excessive heat in the piston are: 

① Poor quality of gasoline leads to abnormal combustion of the fuel-air mixture and generates high temperatures.

② Improper fuel supply system, ignition setting or adjustment causes abnormal combustion of the fuel-air mixture and results in high temperatures.

③ Poor design of the piston body or low density of the aluminum alloy used in the piston leads to insufficient heat transfer from the piston to the cylinder, causing the piston to overheat.

④ Insufficient external cooling of the engine causes overheating. 

3. Cylinder Scuffing Caused by Improper Piston Ring Clearance

During engine operation, carbon deposits are often inevitably formed in the combustion chamber. Although engine oil and piston rings have the function of removing carbon deposits, it is very difficult to remove the carbon deposits accumulated in the piston ring grooves, especially in the first gas ring groove. As the carbon deposits in the piston ring grooves gradually increase, they will cause improper side clearance and back clearance for the piston rings, hindering their movement and applying abnormal pressure on the piston rings, causing them to directly contact and rub against the inner wall of the cylinder, thereby leading to cylinder scuffing.

The main reasons for the formation of carbon deposits in the combustion chamber are as follows:

① Poor quality of the gasoline used;

② Abnormal combustion of the mixture;

③ Excessively rich mixture in the fuel supply system;

④ Ignition system faults, especially spark plug ignition faults;

⑤ Excessive residual engine oil on the inner wall of the cylinder;

⑥ Engine oil leakage from the cylinder head into the combustion chamber. 

4. Cylinder Scuffing Caused by Improper Clearance between Piston and Cylinder

Due to poor machining accuracy of the cylinder bore and the outer diameters of the piston and piston rings, the clearance between the piston and the cylinder is improper, causing cylinder scuffing. When the clearance is too small, the contact pressure between the piston and the cylinder increases, and the piston may even seize. When the clearance is too large, the swing of the piston head increases, and the piston head and skirt or the edge of the piston ring come into contact with the cylinder, resulting in extremely high pressure per unit area and causing cylinder scuffing.

5. Cylinder Scuffing Caused by Poor Surface Shape of Piston and Cylinder

During engine operation, the ideal shape of the contact surfaces of the piston and cylinder should be cylindrical to ensure uniform contact and prevent excessive pressure on large areas. Due to thermal expansion and contraction, at room temperature, the shape of the piston skirt is barrel-shaped in the longitudinal direction and elliptical in the transverse direction. This complex geometric shape has very high precision requirements. However, due to design, manufacturing, or material issues, the shape may be suboptimal, leading to cylinder scuffing.

The main reasons are:

① Insufficient machining accuracy of the piston skirt causing shape deformation;

② Poor machining of the inner surface shape of the cylinder (excessive roundness and cylindricity);

③ Insufficient machining accuracy of the piston pin outer diameter or the piston pin hole inner diameter, resulting in too small a clearance between them, or foreign matter trapped between them, causing the piston pin to expand and seize in the pin hole after engine operation, hindering the expansion of the piston skirt in the pin hole direction and preventing it from achieving the ideal shape;

④ Poor design of the piston skirt structure leading to insufficient rigidity, causing the front and rear force-bearing surfaces of the skirt to deform and sink under pressure, resulting in increased local pressure on the 45° side of the skirt and causing cylinder scuffing;

⑤ Poor design or material of the cylinder liner structure causing cylinder deformation, resulting in tight fit between the piston and cylinder near top dead center, increasing friction and causing cylinder scuffing.

6. Cylinder Scuffing Caused by Poor Surface Treatment

From a material perspective, the piston, piston rings, and cylinder are in high-speed motion, generating heat through friction. In severe cases, the metal surfaces may melt and stick together. Therefore, various surface treatments are generally applied to the piston, piston rings, and even the inner surface of the cylinder, such as chrome plating, nickel plating, molybdenum spraying, nitriding, phosphating, and PVD (Physical Vapor Deposition), to form a high-melting-point material layer on the surface, preventing high-temperature fusion. However, if the surface treatment is poor, such as insufficient coating thickness or poor adhesion, the piston and rings will fuse with the cylinder at high temperatures, which is inevitable.

7. Cylinder Scuffing Caused by Failure of Engine Oil Lubrication

During engine operation, engine oil forms a film between the cylinder, piston, and piston rings, effectively preventing direct contact and friction. However, the following situations can prevent the engine oil from fully exerting its lubricating effect, leading to cylinder scuffing:

① Insufficient engine oil. It is obvious that when the engine oil is not added in sufficient quantity and is continuously consumed during engine operation without timely replenishment, a thick enough oil film cannot be formed, resulting in cylinder scuffing.

② Engine oil emulsification. Especially in winter, when the engine is turned off and the temperature drops, water from the air in the lower part of the cylinder and the crankcase (inlet through the ventilation pipe) condenses and mixes with the engine oil, gradually causing emulsification. At the same time, the leaked mixture from the combustion chamber continuously mixes with the engine oil, promoting emulsification. Emulsified engine oil becomes thinner and deteriorates, making it difficult to form an effective oil film between the cylinder, piston, and piston rings.

③ Improper selection of engine oil grade. Appropriate engine oil grades should be selected based on the region and season. If the selection is incorrect, the engine oil will be too viscous and have poor fluidity in winter or too thin in summer, both of which can lead to poor lubrication. 

8. Cylinder Scuffing Caused by Foreign Objects and Impurities

When there are hard foreign objects between the cylinder and the piston or piston rings, these objects act as abrasive materials, accelerating the wear of their surfaces. In severe cases, it can lead to cylinder scuffing. 

The main sources of foreign objects are as follows: 

Foreign substances such as dust brought in due to the engine parts not being cleaned thoroughly; 

② Burrs remaining on engine parts; 

③ Iron and aluminum filings ground off during the engine's operation; 

④ Dust that enters with the air due to poor filtration by the air filter; 

⑤ Carbon deposits resulting from incomplete combustion of engine oil and fuel mixture; 

When the muffler was sandblasted, iron sand remaining in the muffler was sucked back into the combustion chamber.