At present, the requirements for energy conservation and emission reduction of passenger vehicles at home and abroad are becoming higher and higher. High efficiency and low emission have become an important research topic in the domestic automobile industry. In the process of engine combustion, due to the gap between the cylinder wall and the piston, part of the gas will leak into the engine crankcase and crankcase system through the gap, so as to reduce the combustion efficiency. On the premise of ensuring the normal operation of the piston, reducing the air leakage can effectively reduce the energy loss of the engine and improve the fuel economy of the engine.
There are three main ways for engine air leakage: the clearance between the outer circle of the piston ring and the cylinder wall, the clearance between the piston ring groove and the side of the piston ring, and the clearance between the opening of the piston ring. When the air leakage of the engine increases, the main improvement direction is to optimize the piston and piston ring, and improve the air leakage by adjusting the shape, opening clearance and tension of the gas ring. Such improvements are costly and require re validation of reliability and durability. Previous studies rarely considered the influence of cylinder wall casting on air leakage, and the graphite shape of cylinder wall is also an important factor. By controlling the proportion of pig iron in the molten iron smelting process, increasing the superheating temperature and time, the graphite morphology of the metallographic structure of the cylinder wall can be effectively improved, and the coarse graphite can be obviously refined. Thus, the abnormal separation of graphite in the friction process between the cylinder wall and the piston is avoided, resulting in the existence of abnormal holes in the cylinder wall. The improvement of the holes in the cylinder wall effectively reduces the air leakage, cylinder hole wear and fuel consumption of the engine, and improves the reliability of the engine.
1.Analysis of abnormal air leakage
During the 800H cyclic load bench durability test of an engine, it is found that the air leakage of the piston increases abnormally when it runs to 500h. The air leakage reaches 47l / min at the speed of 4100r / min, exceeding the design standard (air leakage ≤ 40L / min). The abnormal change trend of engine air leakage is shown in Figure 1. Abnormal wear was found on the cylinder wall after the engine was disassembled from the lower frame. The wear state is shown in Figure 2. After replacing the new cylinder block, it returned to normal when retesting the air leakage, so the influence of the piston and ring was eliminated, and it was preliminarily judged that the air leakage of the piston was related to the abnormal wear of the cylinder block.
After dissecting the cylinder wall of the failed cylinder block, the mechanical properties were tested. It was found that the strength and hardness of the cylinder wall met the design requirements. Then the microstructure was examined by electron microscope. It was found that there were many holes in the cylinder wall reticulation, as shown in Fig. 3 and Fig. 4. The preliminary judgment is that the matrix falls off during the wear process of the cylinder wall, forming holes, which aggravates the wear and leads to the increase of air leakage. The metallographic structure of cylinder wall section was tested, and it was found that there was coarse graphite in the metallographic structure of cylinder wall, as shown in Figure 5. The coarse graphite falls off in the process of cylinder hole honing and piston ring wear, resulting in cylinder wall holes.
2. Influencing factors of graphite morphology
In the production process of cast iron cylinder block, molten iron is melted with pig iron and scrap as the main raw materials. In the melting process, the graphite structure will breed different forms with the change of time and temperature. The following factors affect the size of graphite.
(1) Heredity of raw material graphite: the main raw materials for cast iron production are scrap steel and pig iron, among which the graphite form in pig iron is larger. When the melting temperature of molten iron is not enough or the holding time is short, the coarse graphite in pig iron is difficult to melt in time, and becomes a crystalline core during solidification, which continues to grow, making it easier to produce coarse graphite.
(2) Content of C and Si in molten iron: the two elements that play a major role in promoting the process of inoculating graphite are C and Si. When the content of these two elements is too high, the morphology of graphite will change and produce C-type and F-type graphite, which are prohibited from being used in the casting process of cylinder block.
(3) Overheating temperature and time: when the overheating temperature is low and the overheating time is insufficient, the graphite in pig iron cannot be fully melted, which is not conducive to the refinement of graphite.
(4) Cooling time: in the process of molten iron solidification, if the cooling is too fast, it is easy to produce D-type graphite and E-type graphite, but too slow cooling will further promote the inoculation of graphite, resulting in excessive inoculation of graphite, out of control growth process and coarse graphite morphology.
3. Improvement measures and effect verification
Through the analysis of the factors affecting the graphite morphology, combined with the actual production process of a cylinder block, this paper puts forward a scheme to improve the graphite state of the cylinder block: reduce the proportion of pig iron in the raw material from 20% to 5%, and increase the proportion of ordinary carbon scrap from 5% to 20%, so as to reduce the graphite content brought in by pig iron and the content of C element in molten iron, which is increased from 3.5% 5% – 3.7% down to 3.5% 2% ~3. 4%。
The metallographic structure of the improved molten iron cylinder block was tested after dissection. It was found that the graphite morphology after improvement was significantly thinner than that before improvement, and the coarse abnormal graphite disappeared. The comparison of graphite morphology before and after improvement is shown in Figure 6.
In order to verify the influence of the change of graphite state on the air leakage before and after improvement, two cylinder blocks before and after improvement were selected for installation verification. Test the changes of air leakage under different working conditions after the cylinder blocks in two states operate for 100h, 300h, 400H and 500h respectively. The test data are shown in Figure 7 ~ Figure 10 below.
The test data show that the air leakage of the improved cylinder block has decreased under various working conditions. After 500h of operation, the problem of abnormally high air leakage has been significantly improved, and the maximum air leakage under various working conditions is less than the design standard of 40L / min.
(1) The air leakage of the engine is related to the graphite state of the cylinder wall. When the cylinder wall is worn during cylinder hole processing and engine operation, the coarse graphite will fall off from the cylinder wall, forming holes in the cylinder wall, further aggravating the wear of the cylinder wall and increasing the air leakage. In serious cases, it will also lead to excessive air leakage and increased engine fuel consumption.
(2) Controlling the content of pig iron in raw materials and appropriately reducing the content of C element in molten iron in the casting process of cylinder block can effectively improve the graphite morphology of cylinder wall and avoid the generation of coarse graphite, so as to reduce air leakage and improve the fuel consumption of engine.