内燃机爆震中共晶硅铝合金活塞材料损坏机理

发布时间：2019-05-26 22:39

【摘要】：内燃机爆震是限制其燃料经济性提高的关键结构参数,提高压缩比是当今内燃机节能减排的重要途径。因此,阐明缸内爆震对活塞等部件破坏的机理,找出发生爆震的结构影响因素,避免其对活塞等的破坏,为进一步提高发动机压缩比以改善内燃机热效率提供理论支持,本研究工作主要集中在以下几个方面。第一,在发动机上进行了燃用纯甲醇燃料的爆震燃烧的试验研究。在一台压燃式发动机基础上搭建试验台架,运行超速超负荷的强化工况,记录爆震发生过程中气缸压力。试验中观察到纯甲醇发动机发生爆震时,发动机燃烧室内的爆发压力迅速升高,而且变化幅度剧烈,形成爆轰压力波。当该压力波作用在活塞上,其表面及内部快速发生热氧化腐蚀,直至发生表面被穿孔,说明了爆震发生对发动机部件的强烈损坏作用。第二,对破坏后的活塞进行宏观和微观结构的金相分析。试验发现硅铝合金活塞表面在燃烧室内爆震波作用下,宏观损伤由侵彻穿孔和凹坑构成,以侵彻穿孔为主,背面产生了带裂纹的鼓包及崩落;在压缩波和反射拉伸波的作用下,表现出分层损伤和崩落破坏。硅铝合金活塞合金在冲击波下被挤压变形,形成高密度的位错以及非晶和微晶。在远离侵彻穿孔部位,合金变形减小,缺陷以微裂纹和微孔洞为主。硅铝合金活塞微观组织的绝热剪切带由沿剪切方向的宽度为15~45nm的拉长组织构成,具有较高位错密度,剪切带中心部位由大量低位错密度的直径为40~80nm的晶粒组成,具有典型的再结晶组织特征,再结晶过程表现为晶粒机械碎化及晶界迁移、亚晶粗化共同作用的结果。通过电子显微镜扫描进一步分析,发现失效的活塞表面发生了绝热剪切熔孔。采用XRD成分确定后,观察到失效活塞表面的晶型改变;根据分析结果,可以说明纯甲醇压燃式发动机发生爆震破坏活塞失效的形式是热力耦合所造成。第三,对发动机发生爆震在燃烧室内形成超温和超压现象进行了数值分析。以二维数值模拟为基础,研究了锥顶型燃烧室内的冲击波发展的过程,得到作用于活塞不同位置处的超压分布。模拟结果表明:由于燃烧室结构的独特性,导致冲击波能在特定区域进行汇聚,致使该区域超压明显高于其他区域。将该模拟结果与实际破坏失效的活塞进行对比,发现冲击波汇聚区域为活塞被破坏的地方,数值模拟结果和实际破坏结果相同,这为设计燃烧室形状以避免冲击波对活塞造成破坏提供了理论依据。通过本研究获得的结果,可以得出爆震对活塞等材料破坏的机理是:爆震产生的震荡燃烧,在一定条件下转化为具有破坏性的爆轰波,爆轰波燃烧室中汇聚并产生作用于部件表面的超温和超压的条件,导致爆震压力波对活塞表面的破坏。本研究的结果揭示爆震损坏活塞等部件内部结构特征,给出了爆震在燃烧室形成超温和超压的模型,提出了爆震损坏活塞的基本形式,初步阐明爆震对发动机活塞等材料的破坏的机理,为发动机燃烧系统结构设计和运转因素控制提出了重要的理论依据。
[Abstract]:Internal combustion engine knock is a key structural parameter that limits its fuel economy, and it is an important way to improve the energy-saving and emission reduction of the internal combustion engine. therefore, the mechanism of the failure of the in-cylinder knocking on the parts such as the piston and the like is explained, the structural influencing factors of the knocking are found, the damage to the piston and the like is avoided, the theoretical support is provided to further improve the compression ratio of the engine to improve the thermal efficiency of the internal combustion engine, The work of this study is mainly focused on the following aspects. First, a test study on the knock combustion of pure methanol fuel is carried out on the engine. A test-bed frame is built on the basis of a compression-ignition engine, and the intensified working condition of the over-speed overload is operated to record the cylinder pressure during the knocking generation process. In the test, the explosion pressure in the combustion chamber of the engine is rapidly increased when the pure methanol engine is knocking, and the variation amplitude is severe to form the detonation pressure wave. When the pressure wave acts on the piston, the surface and the inside of the piston are rapidly thermally oxidized and corroded until the surface is perforated, and the strong damage effect of the knocking on the engine part is explained. Secondly, the macroscopic and microscopic structure of the damaged piston is analyzed. It is found that under the action of detonation wave in the combustion chamber of the surface of the silicon-aluminum alloy piston, the macroscopic damage is formed by the penetration of the penetrating hole and the pit, and the penetrating hole is the main body, and the back surface produces the drum bag with the crack and the sublevel caving; under the action of the compression wave and the reflected tensile wave, the layer damage and the level caving are shown. The silicon-aluminum alloy piston alloy is extruded and deformed under the shock wave to form high-density dislocations and amorphous and micro-crystals. The deformation of the alloy is reduced and the defects are mainly micro-cracks and micro-holes at the location away from the penetration hole. the adiabatic shear band of the micro-structure of the silicon-aluminum alloy piston is composed of an elongated tissue with a width of 15-45 nm in the shearing direction, has higher dislocation density, The recrystallization process is the result of grain mechanical crushing and grain boundary migration and grain-grain coarsening. A further analysis of the surface of the failed piston was found by a further analysis of the electron microscope. The change of the crystal form of the surface of the failed piston is observed after the determination of the XRD composition, and the form of failure of the piston of the pure methanol compression-ignition engine can be explained according to the analysis result, which is caused by the thermal coupling. In the third part, a numerical analysis of the occurrence of super-mild over-pressure in the combustion chamber is carried out for the occurrence of knocking of the engine. Based on the two-dimensional numerical simulation, the process of the development of the shock wave in the cone-type combustion chamber is studied, and the overpressure distribution at different positions of the piston is obtained. The simulation results show that, due to the uniqueness of the combustion chamber structure, the shock wave can be collected in a specific area, so that the overpressure in the region is obviously higher than that of other regions. The simulation result is compared with the actual failure of the piston, and it is found that the shock wave convergent region is the place where the piston is damaged, the numerical simulation result and the actual damage result are the same, which provides a theoretical basis for designing the shape of the combustion chamber to avoid the damage of the shock wave to the piston. As a result of this study, it can be concluded that the mechanism of the destruction of the material such as the knock to the piston is that the shock generated by the knocking is converted into a condition with a destructive detonation wave, a detonation wave combustion chamber and a super-moderate overpressure which acts on the surface of the component under certain conditions, Resulting in a failure of the detonation pressure wave to the piston surface. The results of this study revealed that the internal structure of the parts such as knocking damaged the piston and so on. The model for forming the super-mild overpressure in the combustion chamber was given. The basic form of the knock-damaged piston was put forward, and the mechanism of the destruction of the material such as the engine piston and the like was clarified. The important theoretical basis for the structure design and operation factor control of the engine combustion system is put forward.
【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.21;TK401

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