The sealing performance of the cylinder head has a great influence on the technical condition of the engine. When the cylinder head is not tightly sealed, the cylinder will be leaked, resulting in insufficient cylinder compression pressure, temperature reduction and air quality reduction. When the cylinder leaks seriously, the engine power will be significantly reduced or even unable to work. Therefore, if there is a power drop failure during engine operation, in addition to finding the cause of the failure that affects the engine power drop, it is also necessary to check whether the cylinder head sealing performance is good. The following small series will analyze the main reasons affecting the sealing performance of the engine cylinder head for reference.
1. Cylinder head is used and installed incorrectly
The cylinder head gasket is installed between the engine cylinder block and the cylinder head. Its function is to ensure the sealing of the combustion chamber and prevent leakage of gas, cooling water and lubricating oil. Therefore, the use and installation of the cylinder head gasket is not satisfactory, which directly affects the sealing reliability of the cylinder head and the service life of the cylinder head gasket.
In order to ensure the quality of the seal, the selected cylinder head gasket must be the same size and thickness as the original cylinder. The surface should be flat, the edging fits firmly, and there are no scratches, dents, wrinkles and rust. Otherwise, it will affect the quality of the seal of the cylinder head.
2. Slightly beat of the cylinder head
The slight runout of the cylinder head is the result of the cylinder head trying to separate from the cylinder under compression and combustion pressure. These pressures elongate the cylinder head attachment bolts, thus causing the cylinder head to have a slight bounce relative to the cylinder block. This slight jump will cause the cylinder head gasket to relax and compress, thereby accelerating the damage of the cylinder head gasket and affecting its sealing performance.
3. The cylinder head connecting bolt does not reach the specified torque value.
If the cylinder head connecting bolts are not tightened to the specified torque value, the cylinder head wear caused by this slight jump will occur faster and more severely. If the connecting bolt is too loose, the amount of runout of the cylinder head relative to the cylinder will increase. If the connecting bolt is tightened too much, the force on the connecting bolt will exceed its yield strength limit, causing the connecting bolt to extend beyond its design tolerance, which will also increase the runout of the cylinder head. Accelerate the wear of the cylinder head gasket. By using the correct torque ratings and tightening the connecting bolts in the correct order, the cylinder head relative to the cylinder can be minimized to ensure the seal quality of the cylinder head.
4, the cylinder head or cylinder flatness is too large
Warpage and twisting are common problems in cylinder heads and are the main cause of repeated burning of cylinder head gaskets. In particular, the aluminum alloy cylinder head performs more prominently because the aluminum alloy material has a high heat transfer efficiency, and the cylinder head is smaller and thinner than the cylinder block, and the temperature of the aluminum alloy cylinder head rises rapidly. . When the cylinder head is deformed, it will not be tightly engaged with the plane of the cylinder block, and the sealing quality of the cylinder will be lowered, causing air leakage and burning of the cylinder head gasket, thereby further deteriorating the sealing quality of the cylinder. If the cylinder head is severely warped, it must be replaced.
5, the cylinder surface is unevenly cooled
Uneven cooling of the cylinder surface creates a localized hot spot. Localized hot spots can cause excessive expansion of the metal in the cylinder head or small areas of the cylinder block, which can cause the cylinder head gasket to be crushed and damaged. Leakage and corrosion are caused by damage to the cylinder head gasket and are eventually burned through.
If the cylinder head gasket has been replaced before the cause of the local hot spot has been found, this will be useless because the replacement cylinder head gasket will eventually be burnt out. Local hot spots can also cause additional internal stresses in the cylinder head itself, which can result in cracks in the cylinder head. If the operating temperature exceeds the normal temperature, local hot spots will also have a serious negative impact. Any overheating can cause permanent distortion of the cylinder block cast iron.
6. Related problems in additives in coolant
When a coolant is added to the coolant, bubbles may be generated. Air bubbles in the cooling system can cause the cylinder head gasket to fail. When there are air bubbles in the cooling system, the cooling fluid cannot be normally circulated in the system, so the engine will not be uniformly cooled, and local hot spots will occur, causing the cylinder gasket to be damaged, resulting in poor sealing. Therefore, in order to achieve uniform cooling of the engine, air must be exhausted from the engine when coolant is added.
Some operators use antifreeze in winter and water in summer, which is considered to be economical. In fact, this is endless, because the minerals in the water are prone to scale and stick to the water jacket, radiator and water temperature sensor, causing the engine temperature to be misaligned and causing overheating, and even causing engine cylinder gasket to be broken and the cylinder head to warp and deform. Faults such as pulling cylinders and burning tiles. Therefore, antifreeze should also be used during the summer.
7, diesel engine repair, poor assembly quality
The poor maintenance and assembly quality of the engine is the main reason for the quality of the cylinder head seal of the engine, and is also the main factor causing the cylinder head to burn. For this reason, when repairing the engine, it is necessary to strictly follow the relevant requirements, and correctly disassemble the cylinder head.
When disassembling the cylinder head, it should be carried out in the cold state. It is forbidden to disassemble in the state of the heat engine to prevent the cylinder head from warping. When disassembling, it should be gradually released from the two sides to the middle symmetrically. If the cylinder head and the cylinder block are combined to be difficult to remove, it is strictly forbidden to strike with a metal object or with a sharp hard object embedded in the slot. (The effective method is to use the starter to drive the crankshaft to rotate or crank the crankshaft to rotate. The high pressure gas pushes it open to prevent scratching the joint surface of the cylinder block and the cylinder head or damaging the cylinder head gasket.
When assembling the cylinder head, first remove the oil surface, carbon particles, rust and other impurities in the joint surface of the cylinder head and the cylinder and the bolt holes in the cylinder block, and blow it off with high pressure gas. In order to avoid the insufficient pressing force of the bolt to the cylinder head. When tightening the cylinder head bolts, it should be symmetrically tightened from the middle to the two sides in 3-4 times. The last time to reach the specified torque, and the error is ≯2%. For the cast iron cylinder head, after the temperature of the hot machine reaches 80 °C, it should be pressed. Re-tighten the connecting bolts by specifying the torque. For bimetallic engines, this retightening operation should be performed after the engine has cooled down.
8, choose inappropriate fuel
Due to the different types of diesel engines, there are different requirements for the cetane number of diesel. If the selected fuel is not satisfactory, it will not only cause economic and power degradation, but also cause a large amount of carbon deposits or abnormal combustion of the diesel engine, resulting in excessive local temperature of the body, causing ablation of the cylinder mat and the body. The sealing performance of the cylinder head is lowered. Therefore, the selection of the diesel cetane number of the diesel engine must meet the requirements of the use regulations.
9. Improper use of diesel engines
Some operators are afraid of the engine stalling, so when starting the engine, they always slam the throttle continuously, or let the engine run at high speed when the engine starts, in order to maintain the working state of the engine; The gear is turned off and the gear is forced to start the engine. The engine operating in such a situation not only increases the wear of the engine, but also causes the pressure in the cylinder to rise sharply, which is extremely easy to damage the cylinder head gasket, resulting in a decrease in sealing performance. In addition, the engine often works excessively (or prematurely ignited), and burns for a long time, causing partial pressure and temperature in the cylinder to be too high. At this time, the cylinder head gasket is also damaged, so that the sealing performance is degraded.