A diesel engine exhaust valve failure analysis 1, exhaust valve working conditions Marine diesel exhaust valve operating conditions are very poor, the valve bottom and high temperature combustion products in direct contact with the valve is also open during the high temperature ( 900 ~ 1000 ° C) and corrosive gases (up to 600m / s), the center temperature of the valve is as high as 700 ~ 800 ° C, the middle of the transition arc between valve disc and valve stem is 600 ~ 700 ° C , Exhaust valve operating temperature distribution shown in Figure 1-1. Excessive temperature will reduce the mechanical properties of metal materials, thermal deformation of the material. When the valve surface seal is not strict, it will cause high temperature gas on the valve surface burning. When the valve is seated, the inertia force of the valve and the valve seat and the spring force exert a considerable impact on the alternating load, the valve beating or valve clearance increases, the load will be obvious increase. Valve and valve seat impact, easy to form the sealing surface deformation and severe wear. Due to the vast majority of marine diesel engines for the supercharged diesel engine, due to the fresh air pressure within the intake blocked from the valve guide to obtain the possibility of oil, so prone to dry friction between metals. However, in the general diesel valve and the exhaust valve seat of the supercharged diesel engine alloy surface will always be a layer of lubricating oil or grease and other lubricants. In addition, there is wear between the stem and the conduit, and the top of the stem is struck and wobbled by the rocker arm. 2. Influences of Additional Factors Due to rising fuel prices and fierce competition in the shipping market, shipowners use low-priced and inferior fuel for the purpose of lowering their costs and achieving more profits. These fuels have high viscosities, long shelf life, and high levels of vanadium, sodium and sulfur. When this fuel is burned in a diesel engine, only a portion of the emissions (fuel ash) contained in the residue leaves the machine together with the exhaust gas and the remainder remains in the engine at some elevated temperature (497-797 ° C) On the parts. For example, the exhaust vent and piston crown form deposits that cause so-called "high temperature corrosion." So far, there is no economically rational process to remove the corrosive elements from the residual oil, even the advanced alloy steel and the surfacing exhaust valve steel are also corroded by the fuel. In the diesel engine running violates the provisions of car maintenance, low temperature start the diesel engine, low temperature forced loading, diesel cylinder combustion temperature abrupt changes in the diesel engine load condition, sharp change handle position, the combustion engine cylinder deterioration, a large number of poor atomized heavy heavy oil Particles into the cylinder, resulting in serious afterburning and incomplete combustion, serious coke valve surface to the exhaust valve is also contaminated by coke, and even lead to the difficulty of starting the host, which became the next host to drive shortly after Oil head and exhaust valve failure of the hidden dangers, so these bad habits of diesel engine maintenance, maintenance, diesel engine also caused the valve failure factor. Second, the exhaust valve common fault analysis 1, exhaust valve burning Exhaust valve burning is the most common fault exhaust valve. The main reason is the tightness of the exhaust valve, resulting in high temperature gas leaks, causing severe overheating and melting of metal materials. The main reasons for the poor sealing of the exhaust valve are as follows: (1) As the shape and thickness of the different parts of the valve disc are different, the temperature distribution on the valve disc circumference is not uniform due to the different conditions of heating and cooling. The center temperature is higher than the surrounding temperature, Valve radial temperature difference between the valve plate, too much temperature difference will result in valve disc deformation resulting in the generation of gas leakage. ⑵ marine fuel containing impurities in the combustion chamber after a variety of complex thermal process in the exhaust valve disc and valve seat sealing cone deposited as a layer of carbon mixed glassy hard brittle material mixed with Sodium sulfate, calcium sulfate, iron oxide and other substances. When this layer of glassy sediment deposition thickness is too large, the valve will be cracked under the impact of the valve closing, repeated impact and then developed into flaking, forming a high-temperature gas injection channel burning valve. ⑶ ordinary exhaust valve sealing cone hardness at work temperature is not very high, the deposition of hard combustion products in the valve closing the impact of the valve, the sealing surface can be pitted to form a leak. 2, the exhaust valve temperature corrosion Currently in the shipping market commonly used in poor quality fuel contains a lot of vanadium, sodium and sulfur and other elements. In the process of burning. Sulfur, vanadium and sodium and other elements form sulfur oxides, vanadium pentoxide and sodium oxide (the chemical composition of these oxides depends on excess oxygen and combustion temperature). Oxide to react between, but also with the oil in the reaction of calcium, the formation of low melting point salts, sodium sulfate, calcium sulfate and different components of sodium vanadate and so on. The melting point of these salts mixture is generally about 535 ° C, at the same time has a strong corrosive. When the part temperature is above 550 ° C, it is enough for the vanadium and sodium compounds to be melted and adhere to the part surface. When the exhaust valve is in operation, it is deposited as a liquid on the transition surfaces of the valve disc and valve seat, as well as the stem and valve face, due to venting (valve temperatures up to 650-800 ° C). At this moment, even a very hard alloy steel that is resistant to corrosion will be corroded, and as a result, pitting and pitting will be formed on the sealing conical surface. Pits connected may cause air leaks. Since the above corrosion is generated under high temperature, it is called "high temperature corrosion". In the above high-temperature corrosion of harmful elements to the greatest harm of vanadium. 3, valve sealing cone worn too fast In the combustion chamber under the pressure of the valve seat and the valve disc are under elastic deformation, the valve seat impact can cause valve seat and valve disc elastic deformation, it will disc cone Repeated wedge surface, the sealing cone relative movement, resulting in sealing cone wear. Valve clearance is too large, valve disc and seat stiffness is not enough, valve and valve seat material performance does not meet the requirements or mismatch, heavy oil contains more vanadium, sodium, sulfur and other harmful elements, high load operation or combustion deterioration , Poor cooling, valve stem and catheter gap is too large, valve mechanism vibration so that the valve seat speed is too large, can make the wear rate increases. 4, the valve plate and valve stem in the transition between the valve disc and the corner of the valve stem and the installation of the Ministry of the groove block, due to the stress concentration of these parts easily, when the stress concentration to a certain extent, there will be fatigue fracture damage. Causes of fracture are: the stem and the catheter gap is too large; valve plate and valve seat deformation so that the local stress is too large; valve clearance is too large, severe fatigue damage; valve mechanism vibration. Stem installation card block groove is the weakest part of the valve, if the groove processing poor process or valve closing impact will produce fatigue fracture. 5, the valve stuck dead valve is mainly due to the gas valve stem and the gap between the catheter is too small, when the gap after the expansion of the gap between the two card jamming phenomenon. On the other hand, when the valve stem is bent and deformed, the valve stem can also be stuck in the catheter. 6, the valve spring rupture valve spring itself unreasonable structure, internal defects, improper processing or use of the distortion or fatigue limit in the work will lead to fracture. Valve spring rupture directly destroy the diesel engine normal work, serious, the valve may fall into the cylinder. Third, the diesel engine valve repair 1, the valve and valve seat repair before the major technical maintenance, measuring the bottom surface of the valve and the cylinder head cover the distance is greater than the provisions. If the difference between this value is too large, it will affect the compression ratio of the diesel engine and the valve and piston impact, it is necessary to replace the valve seat pad. Overhaul, the valve after long-term work should be carried out the following measurements: After the valve stem wear roundness error and cylindrical error, the error can be measured with an outer diameter micrometer obtained. When these two errors exceed the limit of 50% of the catheter with the gap should be scrapped replacement; inspire the cone on the outer surface of the stem radial runout should be less than 0.003mm. Otherwise it should be scrapped replacement. Check the valve will be placed on the top of the lathe spindle and tail thimble between the top of the two thimble valve in the thimble hole in the hole, the dial indicator seat block, and the contact of the table and the gas Valve cone contact. Valve rotation of a week, the maximum reading of the dial indicator is the radial runout. At this point the valve holding state as shown above; on the valve crack check, pits and pits and other checks. The outer surface of the stem allows 10 and less than 20mm length of the hairline, the cone and the valve stem lock clamp should be carried out magnetic inspection and testing degaussing. Cracked persons should be scrapped. (1) Valve and valve seat welding process Relieved from the host of the exhaust valve and the valve seat has a lot of grease, pitted pits, ablation or burn through, you must clean. Cleaning work can be machined in the lathe surface dirt and the original spray layer (the thickness of the weld layer can be sprayed from the basement process and the base metal of different colors, or car scrap iron is flaky and coil Judge it). Spray-welding layer is hard, the iron chips for the small, while the parent metal mostly filamentous. Parts of the serious erosion of the local grinding wheel with dirt, stain-free flaw detection, the welding can be repaired. The electrode can be made of high-chromium nickel austenitic stainless steel electrode, the effect will be better with molybdenum, generally 200 ° C before welding, preheating for 1.5 hours, applying a small current, multi-layer multi-track welding, Between the temperature of about 200 ° C. For the part of the substrate above the grinding wheel worn off, no flaw detection after the flaw detection weld spray. (2) valve and valve seat grinding process Disc and valve seat grinding angle must be strictly adjusted and measured in accordance with instructions. In particular, the sealing surface of the valve stem grinding angle must be carefully adjusted before dialing with a dial to find the time, should make the top of the dial gauge for the contact surface of the site, find the correct circle to the wheel and sealing surface all contact, Requirements of the maximum deviation should be within 0.05mm, this value is the maximum amount of wear away. Surface grinding standards are: Grinding wheel must be evenly across the entire sealing surface of the broadband and wear away the material. Check the stem of the wear layer should be cleaned before the stem; measured in the circumferential direction of the stem diameter, not exceeding the lower limit specified in the instructions; check the chrome layer of network cracks in the valve head by a slight chrome layer Cracks are allowed, but chrome plating should never allow peeling. After grinding should be sealed to check: valve cone surface with soft pencil to draw equidistant lines, and the valve seat fitting, gently remove the 1/3 weeks later. If you find that every pencil line is wiped in the same position in the middle, it means the seal is good. In addition, the valve device can also be assembled as required, pour kerosene from the exhaust port, observe the 10 minutes, if the valve at no leak marks, but also shows a good seal. 2, the remaining defects and treatment (1) the main reason for rupture of the valve rupture valve is due to uneven temperature distribution on the valve caused by thermal stress is too large, or valve disc (seat) warpage, to withstand greater The bending stress. Therefore, in the use of maintenance, to strictly ensure the valve stem and the catheter with the gap, wear and tear should be timely replacement of catheter. Because too large with the gap will lead to poor heat dissipation, resulting in leakage at the stem, the exhaust valve stem leaking more likely to cause oil caking valve stuck; too much gap also makes the valve lateral vibration increased, the valve Slippage of the sealing surface of the disc and the seat increases, wear increased; may also cause the valve seat on one side, which often results in valve disc and valve stem transition fillet rupture. Of course, the gap between the catheter and the stem can not be too small, otherwise it will lead to the valve jamming. Stem assembly in the catheter if by weight, slowly decline as well. Long-term work wear, hand pushing the valve stem from the side, there is shaking, loose feeling, can be judged to have exceeded the wear limit. (2) valve spring defect detection and treatment should pay attention to the valve spring surface cracks, corrosion and other defects inspection. Cracked, scrapped. Corrosion spots should be polished and polished with emery cloth to avoid stress concentration. Spring on the platform test found that bending and distortion of the overqualified, should be replaced. Lack of elasticity of the valve will result in lax valve closure caused by leakage. Valve body drive parts out of contact, resulting in percussion, resulting in increased wear and tear. Serious may lead to the spring lock device off and a major accident occurred. Overhaul, it should be taken to compare the standard spring and the relative deformation, overshoot, should be scrapped. Fourth, the conclusion Exhaust valve is an important part of the diesel engine, its performance is a direct impact on the normal operation of the host. Therefore, the turbine manager must pay great attention to the work of the exhaust valve. Requirements to do the following: (1) tour carefully check the navigation, pay attention to changes in row temperature. Always pay attention to exhaust valve drive mechanism for oil pressure. In the external sea conditions and the host load unchanged under the premise, such as a cylinder of the exhaust gas temperature appears a gradual upward trend, abnormal sound, etc., should be promptly and correctly determine the fault where decisive measures can be effectively control the accident occur. (2) Weekdays to strengthen the maintenance of the injector to ensure that the injector is working properly. If the nozzle is poorly atomized, when the exhaust valve is hoisted out, it is obvious that the inner side of the valve seat closest to the nozzle can be seen. A large amount of mixture of coking and unburned fuel accumulates in a diagonal pattern, And down to seal the coupling surface, leading to carbon formation, these carbon is the "culprit" burning valve, these two may be the valve burned specific parts. (3) Engineers must regularly inspect the exhaust valve in strict accordance with the instructions of the ship diesel engine. (4) Both the valve plate and valve seat are covered with heat resisting alloy, the thickness is about 3mm. When dismantling the valve, it should be measured according to the instructions. When the maximum grinding amount Gt> 2.0mm, G3≤O Valve can not be used. (5) In order to avoid the phenomenon that the surface of the valve disc vibrates due to the fluctuation during grinding, the exhaust valve should be grinded as far as possible when the ship sails or breaks down. If possible, try to polish the wharf or professional manufacturers. (6) The best host interval for a period of time, in and out of port motor vehicles, the use of light oil to flush the fuel system is conducive to improving the jet pump and injector conditions, improve combustion quality. At the same time, engineers should diligently pay attention to the cooling of the combustion chamber parts, so that the part temperature is kept below 550 ° C, but the cooling water temperature can not be adjusted too low while the thermal stress is too large cracks.