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Measures to Prevent Diesel Engine Pulling and Burning Pistons
1. Pistons and cylinders must have a reasonable fit clearance for the pistons to maintain normal movement in the cylinders, but also have good airtightness, requiring a reasonable fit clearance between the piston and the cylinder. At present, when assembling a diesel engine, there are roughly the following methods for selecting the clearance between the piston and the cylinder: (1) Calculation method The calculation method considers only the temperature-induced thermal expansion, and actually the status change of the diesel engine during working hours is Very complicated, such as piston, cylinder thermal deformation and other factors are not taken into account, so the calculation method is only theoretically studied, there are still some problems in practical application, so this method has not been widely used. (2) Tensile force selection method The pull force selection method is to reverse the piston into the cylinder, select a certain size of the thin gauge to insert the piston skirt and the cylinder wall perpendicular to the direction of the piston, and then use a scale to pull the thick gauge The reasonable range of fit clearance is determined by measuring its frictional resistance. The more groups of pistons and cylinders are grouped together, the smaller the value of group-wise radial clearance is, the better the fit clearance can be guaranteed. Foreign manufacturers take this approach. (3) Group Matching Method The grouping method is to divide the processed piston and cylinder bore into different diameters, and then to cooperate with the same group. (4) Baseline system selection method The so-called base shaft system is to machine a hole based on an axis. The repair unit determines the machining size of the cylinder bore based on the clearance between the purchased piston and the cylinder. This method is simple and easy, but if the material of the purchased piston is different from that of the original manufactured piston, the coefficient of expansion will be different. This will ensure that the working gap between the piston and the cylinder in the hot state is not guaranteed. If the purchase of the piston's expansion coefficient is less than the original piston's expansion coefficient, it will make the working state of the gap becomes larger, the occurrence of knocking and reduce the overhaul interval mileage; if the purchased piston its expansion coefficient is greater than the original piston The expansion coefficient is bound to reduce the gap in the working state, and even the piston is locked in the cylinder, resulting in pulling the cylinder and burning the piston. Therefore, when using this method, it is necessary to compare the thermal expansion of the original piston of the purchased piston, calculate the respective expansion coefficients, and then use the empirical formula of the calculation method to check and correct, and finally determine the theoretical cooperation between the piston and the cylinder. Gap, and actual control and measurement of the gap can be combined using the above methods. 2. Strictly control the working temperature Because the diesel engine is working under high temperature, the geometrical dimensions of the piston are constantly changing under the influence of temperature. The higher the temperature is, the more the piston expands than the cylinder, and the smaller the clearance during working is. The formation conditions of the lubricant film are also worse, the greater the frictional heat generated, and the expansion will be further increased; such a vicious cycle will result in pull cylinders, and even lock up and burn the piston, especially after the new machine and overhaul. The above phenomenon is more likely to occur during the period when the mileage is not long. In a certain temperature range, the reserved clearance can effectively prevent the occurrence of cylinder pull and piston burning. Therefore, when the diesel engine is in operation, it is very important to strictly control the working temperature (cooling water temperature) of the diesel engine. However, if the diesel engine is often operated at a low temperature, the gap in the working state will be larger and a knock will be generated, and a large amount of heat energy will be absorbed by the cooling system, so that the combustion of the mixture will be incomplete, resulting in reduced power and economical deterioration. Cylinder wall erosion increases. At present, there are many operators who are afraid that the temperature is too high to cause cylinder pull. Therefore, the temperature of the engine is often rather low and not high. In fact, this is very undesirable, and normal operating temperature should be maintained. The main factor influencing the working temperature of the diesel engine is the efficiency of the diesel cooling system. Therefore, in the use and maintenance of the diesel engine, it is necessary to ensure that the cooling system has a reliable control over the temperature of the diesel engine. In addition, the premature injection of fuel and the lean mixture can increase the temperature of the engine and even cause the top of the piston to ablate or pull the cylinder. Such as 12150L diesel engine, the injection advance angle is 34, if this is too early (fuel injection advance angle is greater than 35), the engine will obviously knock when working, the source of the knock is generally generated behind the injector opposite the piston 1/4 edge at the top. During the overhaul, two 12150L diesel engines were found to have caused the piston to hit the top due to the explosion of the above-mentioned parts. One of the 12150L diesel engines even caused severe piston burn and the cylinder was locked. If the mixture is too dilute, the combustion rate will be slow, and the temperature will rise. As a result of the combustion of coke, localized ignition will occur at the top of the piston, causing the temperature to rise further and the working conditions of the piston in the cylinder to deteriorate. As a result, the piston will be severely burned and pulled. Cylinder.