Abstract: The automobile water pump is the heart of the automobile engine cooling system and the power source of the automobile forced circulation water cooling system. Once the performance of the water pump decreases, it will lead to a decrease in the coolant pressure and flow rate in the system. The article lists common failure modes of pump impellers, analyzes the impact of cavitation and erosion on pump performance, and proposes corresponding preventive measures.
Key words: automobile water pump; Failure mode; Cavitation; abrasion
During the long-term operation of an automotive water pump, due to the two-phase flow of water flow and particles in the water, cavitation and erosion often occur, resulting in the gradual wear of components such as the impeller of the automotive water pump due to this impact. The working efficiency and water output of the water pump decrease, and the energy consumption per unit time of the water pump increases, even leading to the failure and damage of key components such as the impeller before the pump reaches its designed service life. This article analyzes the common failure modes and causes of automotive water pumps, their impact on pump performance, and discusses preventive measures such as replacing materials and enhancing protective coatings to improve pump operation efficiency and service life.
1. Analysis of Common Failure Modes of Water Pump Impeller
Cavitation and erosion are the most common failure modes of water pumps. Cavitation and erosion of water pumps are mainly concentrated in the impeller ring gap, impeller inlet, and impeller outlet. The failure state is usually a regional honeycomb pit or fish scale pit, and serious cases can also cause local perforation of the pump blade, significantly shortening the actual service life of the pump, and reducing the working performance indicators of the pump. Generally, the flow rate at the pump outlet is reduced by about 30%, Work efficiency will also be reduced by between 20% and 30%. There are significant differences in the degree of failure between the positions of the water pump blades. The root of the leading edge of the blade on the water inlet side is relatively slightly damaged, and the direction of the leading edge towards the end face of the blade is relatively severe. The grooves on the water inlet side of the blade are damaged pits in the shape of fish scales, while the back surface of the blade at the outlet presents deep fish scales, and the edge of the blade presents irregular blade shaped notches.
1.1 Water pump impeller cavitation
Cavitation is a cave like damage phenomenon caused by the contact between the surface of the water pump blade and the gas under the change of water flow pressure. When the automobile water pump is in normal operation, the water pressure gradually decreases from the water pump inlet position to the impeller inlet position. When the absolute pressure of the water is lower than the saturated steam pressure, the liquid undergoes vaporization, and many small bubbles are formed. These small bubbles accumulate in the low-pressure region and reach the high-pressure region of the water pump as the water flows. Cavitation usually occurs in the high-speed decompression zone at the end of a water pump blade. Under pressure, the small bubble high-pressure zone ruptures and produces a large impact pressure, which destroys the protective film on the metal surface, accelerating the corrosion rate of the blade. Cavitation is characterized by the formation of multiple small pits on the metal surface of the blade, which gradually expand and eventually form caves. Cavitation includes the process of bubble formation, growth, rupture, and erosion of the surface of the water pump impeller material. Bubbles formed in the low pressure region of the water pump usually have a large impact force on the surface when they rupture in the high pressure region of the water pump. Cavitation can instantly generate impact stresses of hundreds to thousands of atmospheric pressures. The impact stress strikes the metal surface of the water pump blade at high frequencies, causing the microcracks of the water pump impeller to gradually develop into fatigue cracks, The propagation of fatigue cracks will further cause the metal grains to fall off and form early pitting shaped cavitation pits on the surface of the water pump blades. The cavitation process usually acts in the negative pressure zone on the back of the pump blade, and also affects the outlet position of the pump blade. After continuous cavitation, the resulting blade failure characteristics are pitted, honeycomb, and hole shaped cavitation areas. The micro cracks in the metal surface of the water pump blade are mainly caused by defects in the metal material itself, impact, and penetration of particles. During cavitation, the energy of bubble rupture transfers to the small particles in the water, causing the particles to impact the water pump blade at a certain angle. The particles entering the cavitation pit will cause reciprocating damage to the formed cavitation pit, which leads to further expansion and consolidation of the cavitation pit, Cavitation pit walls become smooth under the action of micro cutting, and when multiple adjacent cavitation pits intersect, a knife edge shaped cavitation pit will be formed.
1.2 Water pump impeller wear
Erosion wear is another mode of failure of water pump blades. Erosion refers to the wear phenomenon that occurs when the water pump blades are impacted by small and loose particles in the water, causing damage to the surface of the water pump blades. It can be understood as the wear caused by the metal surface contacting with water containing solid particles and moving in opposite directions. Because erosion is the result of the friction action of small particles in water on the surface of the water pump blade, erosion usually occurs on the upstream side of the water pump blade. The particles rub at high speed on the surface of the blade, causing the particle layer on the metal surface of the water pump blade to be ground by sand particles, causing damage to the surface of the water pump blade after a long and repeated grinding process. In addition, the high-speed impact of water containing particles on the surface of the water pump blade will form a local vortex zone. Under long-term continuous action, the water pump blade will form a bumpy surface. Due to the contact between the sharp corners of particles in some water flows and the surface of the water pump blade, the pressure on the surface of the blade will be greater under the same impact force, resulting in local small plastic deformation on the surface of the water pump blade during the combined action with local cavitation, When the deformation reaches the plastic ultimate strength of the metal material, erosion of the water pump blade occurs. Erosion usually forms fish scale shaped groove corrosion, which is a metal failure caused by relatively high velocity relative movement between the surface of the pump blade and the water flow. Erosion is a destructive failure mode, especially when there are many particles in the water flow, and the damage is more severe in two-phase flow, resulting in a significant reduction in the service life of automotive water pumps.
When the automobile water pump is operating, particles with sharp corners will have a micro cutting effect on the water pump blade, while non sharp particles will have a squeezing and plowing effect on the water pump blade. Generally, the cutting effect of sharp particles is more destructive than the plowing effect of non sharp particles. The micro cutting effect of particles causes debris to form on the surface of the water pump blade grain, Extrusion and ploughing extrude and crimp the surface material of the water pump blade grain, and the crimp continues to peel off under continuous cutting, extrusion, and ploughing. Cutting, extrusion, and ploughing can simultaneously produce a locally hardened thin layer of grains on the surface of the water pump blade, which to some extent reduces the grinding rate. However, due to the discontinuous grain hardened layer on the surface of the water pump blade, the locally hardened thin layer often peels off along with the curls and chips generated in the non hardened area. During the erosion process of water pump blades, there is also a phenomenon of metal corrosion. Erosion refers to the separation of metal materials from the metal surface in the form of solid particles, and corrosion refers to the separation of corrosion products from the metal surface in the form of ions. The amount of material loss caused by erosion and corrosion is not only a simple superposition of the two, but also a result of the synergy and mutual promotion of mechanical and chemical processes.
2. Effect of cavitation and erosion of water pump impeller on its performance
The interaction between cavitation and erosion of the water pump impeller usually causes a certain degree of wear on the sealing ring and other flow passage components of the water pump. As the wear intensifies, the gap between the sealing rings will increase, and the performance of the water pump during operation will gradually decrease. As the energy consumption per unit time increases, the water flow will decrease in reverse. The water pump impeller and flow passage components will be gradually eroded and damaged, resulting in a shortened normal service life of the water pump, Faults such as abnormal noise at the water pump and high water temperature occur.
2.1 Abnormal noise at the engine water pump
When the water pump blades are damaged due to cavitation erosion and wear, such as honeycombs, pits, or grooves, the resistance coefficient of the water flow will become larger. When the automobile engine is at a certain rotational speed, the water pump may generate continuous noise and vibration due to resonance and other factors. The power source of the automobile engine water pump is the engine crankshaft. The crankshaft pulley located at the front end of the crankshaft drives the water pump pulley through a V-belt, and the water pump pulley is connected to the water pump shaft through a flange plate, The water pump shaft drives the impeller to rotate, and the engine crankshaft serves as a kinetic energy output to convert mechanical energy into hydraulic energy. The noise generated by the failure of the water pump blade is a rotational friction noise, which can accelerate with the increase in engine speed and change in volume. The noise generally becomes more and more obvious as the severity of the fault increases, resulting in an increase in NVH of the entire vehicle.
2.2 Increased coolant temperature
The cooling cycle system of an automobile engine is a water cooling system, which utilizes a water pump to increase the pressure of the coolant in the cycle system and force the coolant to circulate in the cooling system composed of the automobile engine and water tank. The automobile engine cooling system is mainly composed of a cylinder block water jacket, a cylinder head water jacket, a water pump assembly, a water tank radiator, an electronic cooling fan, and a thermostat. The automobile water pump is the heart of the automobile engine cooling system. Once the performance of the water pump decreases, it will lead to a decrease in the pressure and flow of the coolant in the cooling system. The heat generated by the operation of the automobile engine cannot be taken away in a timely manner, and the water temperature in the cooling system will gradually increase. When the fluctuation range of the water temperature exceeds the extent that the engine can withstand, such as the driver failing to pay attention to the alarm information on the water temperature gauge in a timely manner, forcing the vehicle to drive, It may lead to serious faults such as cylinder pulling in the automobile engine, and ultimately lead to engine scrapping.
3 Measures to prevent the failure of water pump impeller
In order to improve the reliability of automotive water pump impellers and achieve their durability throughout the entire life cycle of the vehicle, necessary measures need to be taken for their failure modes. Currently, the commonly adopted measures can be divided into two categories: one is to select erosion resistant materials to manufacture water pump impellers, and the other is to use thermal spraying technology to prepare coatings on the surface of water pump impellers and their components.
3.1 Change the material of the water pump impeller
In order to adapt to more stringent national fuel consumption regulations, passenger cars have increasingly high requirements for fuel economy, and automotive engines generally adopt miniaturization and lightweight design. In order to achieve lightweight engine, automotive water pump housings can be made of aluminum alloy materials, and water pump impellers can be made of engineering plastics. With the continuous progress of engineering plastics technology, the high-temperature resistance of water pump impellers is becoming stronger and stronger, and the strength of the impellers is also increasing. Compared to traditional metal impellers, engineering plastic impellers adopt injection molding, which can easily produce more complex blade shapes. Pump design can be further optimized. The blade shape adopts a backward curved semi circular arc shape, double circular arc shape, or multi circular arc shape. To improve pump efficiency, the blade is aligned with the direction of water flow, significantly improving pump performance and efficiency. The chemical properties of engineering plastic materials determine their good corrosion resistance. Nonmetallic plastic materials are typically suitable for small and medium-sized engines due to their low density, small rotational inertia, low noise, small vibration, and small impeller imbalance during operation. However, their disadvantages are low mechanical strength, and they are generally suitable for small and medium-sized engines.
3.2 Precoating the blades
Thermal spraying technology has been widely used in related fields such as aerospace, national defense, energy, metallurgy, and petrochemical industry. This technology can effectively reduce energy consumption, improve pump operation efficiency, increase pump service life, and improve reliability by surface treatment of automotive water pump impeller components. In order to improve the cavitation resistance of water pump impellers, a method of pre coating water pump impellers can be used in engineering. This method uses powder spraying or polyether polyurethane coating and other spraying techniques to spray non-metallic materials with impact and wear resistance on the surface of the impeller, including epoxy emery materials, composite nylon coatings, epoxy resin polymer materials, polymer composites, wear-resistant ceramics, etc. The high-performance wear-resistant material on the pump impeller can effectively reduce the rate of cavitation wear, which can extend the service life of the pump by more than twice compared to the same period last year.
4 Conclusion
The failure mode of an automobile water pump not only depends on the wear resistance and cavitation resistance of the material of the water pump impeller itself, but also has a certain correlation with factors such as the installation conditions of the automobile water pump, vehicle operating conditions, pump assembly accuracy, and impeller processing and manufacturing quality. Further failure mode analysis requires analysis and research of these non major factors. With the continuous development of materials science and technology, more and more new materials and technologies are emerging and applied to automotive water pumps. In order to further improve the performance and durability of automotive water pumps, it is necessary to conduct more in-depth research and analysis on water pumps.
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2023-04-08