Failure Analysis of Electric Vehicle Air Conditioner Compressor - Lack of Oil and Lubrication

Electric vehicle air conditioning compressor is a complex machine running at high speed. Ensuring sufficient lubrication of compressor crankshaft, bearings, connecting rods, pistons, and other moving parts is a basic requirement for maintaining the normal operation of the machine. For this reason, the compressor manufacturer requires the use of designated grades of lubricating oil and regular inspection of the lubricating oil level and color. However, due to negligence in the design, construction, and maintenance of refrigeration systems, insufficient lubrication of moving parts is common due to compressor oil shortage, oil coking, return dilution, refrigerant flushing, and the use of inferior lubricating oil. Insufficient lubrication can cause wear or scratches on the bearing surface, and in severe cases, it can cause the shaft and piston to get stuck in the cylinder, as well as resulting in rod bending and fracture accidents.
1. Lack of oil
Oil shortage is one of the most easily identified compressor faults. When the compressor is short of oil, there is little or no lubricating oil in the crankcase. The compressor is a special air pump, where a large amount of refrigerant gas is discharged while also entraining a small amount of lubricating oil (called running oil or running oil). Compressor oil leakage is unavoidable, but the oil leakage speed is different. There is approximately 2-3% lubricating oil in the exhaust gas of a semi hermetic piston compressor, while 0.5-1% in a scroll compressor. For a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase oil storage capacity of 6 liters, 3% oil flow means about 0.3-0.8 liters per minute of oil flow, or the compressor operates for more than ten minutes without oil return.
If the lubricating oil discharged from the compressor does not return, the compressor will run out of oil.
There are two ways to return oil from the compressor, one is to return oil from the oil separator, and the other is to return oil from the return pipe. The oil separator is installed on the exhaust pipe of the compressor and can generally separate 50-95% of the running oil. The oil return effect is good and the speed is fast, greatly reducing the amount of oil entering the system pipeline, thereby effectively extending the operation time without oil return. It is not uncommon for cold storage refrigeration systems with particularly long pipelines, full liquid ice making systems, and freeze-drying equipment with very low temperatures to not return oil or to have very little oil return after ten or even dozens of minutes of startup. Poorly designed systems can cause the compressor to shut down due to low oil pressure. Installing a high-efficiency oil separator in this refrigeration system can greatly extend the compressor's oil return free operation time, enabling the compressor to safely pass the crisis stage of no oil return after starting up.
The lubricating oil that has not been separated will enter the system and flow with the refrigerant in the pipe, forming an oil circulation. After the lubricating oil enters the evaporator, on the one hand, due to low temperature and low solubility, a portion of the lubricating oil is separated from the refrigerant; On the other hand, low temperature and high viscosity make the separated lubricating oil easily adhere to the inner wall of the pipe, making it difficult to flow. The lower the evaporation temperature, the more difficult it is to return oil. This requires that the design and construction of the evaporation and return pipelines must be conducive to the return of oil. The common practice is to adopt a downward type pipeline design and ensure a larger airflow speed. For refrigeration systems with particularly low temperatures, such as - 85 ° C and - 150 ° C medical cryogenic tanks, in addition to selecting a high-efficiency oil separator, special solvents are usually added to prevent lubricating oil from blocking the capillary tubes and expansion valves, and to help return oil. In practical applications, oil return problems caused by improper design of evaporators and return lines are not uncommon. For R22 and R404A systems, it is very difficult to return oil from a full liquid evaporator, and the design of the system return pipeline must be very careful. For such a system, using efficient oil separation can greatly reduce the amount of oil entering the system pipeline, effectively extending the non return time of the return pipe after startup. When the compressor is higher than the evaporator, a return bend on the vertical return pipe is necessary. The oil return bend should be as compact as possible to reduce oil storage. The spacing between oil return bends should be appropriate. When the number of oil return bends is large, some lubricating oil should be added. The return line of the variable load system must also be careful. When the load decreases, the return air speed will decrease, and too low a speed is not conducive to oil return. In order to ensure oil return under low load, the vertical suction pipe can use double risers.
Frequent starting of the compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stopped, and there was no time to form a stable high-speed air flow in the return pipe, so the lubricating oil could only remain in the pipe. If the return oil is less than the discharge oil, the compressor will run out of oil. The shorter the operation time, the longer the pipeline, and the more complex the system, the more prominent the oil return problem. For fully enclosed compressors (including scroll compressors and rotor compressors) and partially semi enclosed compressors that do not have an oil pressure safety switch, frequent startup causes more damage.
Compressor maintenance is equally important. During defrosting, the temperature of the evaporator increases, and the viscosity of the lubricating oil decreases, making it easy to flow. After the defrosting cycle, the refrigerant flow rate is high, and the remaining lubricating oil will be concentrated and returned to the compressor. Therefore, the frequency and duration of the defrost cycle should also be carefully set to avoid large fluctuations in the oil level or even oil shocks. When there is more refrigerant leakage, the return air speed will decrease, and too low a speed will cause the lubricating oil to stay in the return air pipeline and cannot quickly return to the compressor.
The return of lubricating oil to the compressor housing does not equal the return to the crankcase. For compressors using the negative pressure oil return principle of the crankshaft cavity, if the piston leaks due to wear, etc., the pressure in the crankcase rises, and the oil return one-way valve automatically closes due to the differential pressure effect. The lubricating oil returned from the return pipe stays in the motor cavity and cannot enter the crankcase. This is the internal oil return problem, which can also cause oil shortage. In addition to the occurrence of such accidents in worn old machines, starting with liquid caused by refrigerant migration can also cause difficulties in internal oil return, but the time is usually short, with a maximum of more than ten minutes. When there is an internal oil return problem, it can be observed that the oil level of the compressor continues to decrease until the oil pressure safety device acts. After the compressor was shut down, the oil level in the crankcase quickly recovered. The root cause of the internal oil return problem lies in the leakage of the cylinder. The worn piston assembly should be replaced in a timely manner. The oil pressure safety device will automatically shut down when there is a lack of oil, protecting the compressor from damage. Fully enclosed compressors (including rotors and scroll compressors) and air-cooled compressors without oil sight glasses and oil pressure safety devices have no obvious symptoms when oil is lacking, and they do not shut down. The compressor may be inadvertently worn and damaged. Compressor noise, vibration, or excessive current may be related to a lack of oil. Accurate judgment of the operating conditions of the compressor and system is very important. Low ambient temperature may cause some oil pressure safety devices to malfunction and cause compressor wear.
The wear caused by compressor oil shortage is generally relatively uniform. If there is little or no lubricating oil, severe friction will occur on the bearing surface, and the temperature will rise rapidly within a few seconds. If the power of the motor is large enough, the crankshaft will continue to rotate, and the crankshaft and bearing surfaces will be worn or scratched. Otherwise, the crankshaft will be locked by the bearings and stop rotating. The reciprocating motion of the piston in the cylinder is the same. Lack of oil can cause wear or scratches, and in severe cases, the piston can become stuck in the cylinder and cannot move.
2. Insufficient lubrication
The direct cause of wear is insufficient lubrication. Lack of oil can definitely lead to insufficient lubrication, but insufficient lubrication is not necessarily caused by lack of oil. Insufficient lubrication can also be caused by three reasons: the inability of lubricating oil to reach the bearing surface; Although the lubricating oil has reached the bearing surface, its viscosity is too small to form an oil film of sufficient thickness; Although the lubricating oil has reached the bearing surface, it has decomposed due to overheating and cannot function as a lubricant.
Blockage of the oil suction screen or oil supply pipeline, or failure of the oil pump can affect the delivery of lubricating oil, and the lubricating oil cannot reach the friction surface away from the oil pump. The oil suction screen and oil pump are normal, but bearing wear and excessive clearance can cause oil leakage and low oil pressure, making the friction surface away from the oil pump inaccessible, causing wear and scratches.
Liquid return is a common system problem, and one of the major hazards of liquid return lies in diluting the lubricating oil. After diluted lubricating oil reaches the friction surface, its viscosity is low and it cannot form a protective oil film of sufficient thickness, which can cause wear over time. When the return liquid volume is relatively large, the lubricating oil will be very thin, not only unable to play a lubricating role, but also will dissolve and wash the original oil film, causing refrigerant flushing.
Due to various reasons (including the compressor startup stage), the temperature of the friction surface where no lubricating oil is obtained will rapidly rise, and the lubricating oil will begin to decompose after exceeding 175 ° C. "Insufficient lubrication - friction - high surface temperature - oil decomposition" is a typical vicious cycle, and many malignant accidents, including connecting rod holding shaft and piston clamping cylinder, are related to this vicious cycle. Insufficient lubrication and oil shortage can be seen in disassembled compressors. Lack of oil generally manifests itself in large areas of relatively uniform surface damage and high temperatures, while lack of lubrication is more commonly manifested in wear, scratches, and high temperatures at specific locations, such as the bearing surface away from the oil pump.
When the piston moves up and down, the load of the piston pin is rotated between the upper and lower parts of the bearing surface, which allows the lubricating oil to evenly brush the piston pin and provide sufficient lubrication. If the exhaust valve plate is bent or broken, or the compressor operates at a high pressure ratio for a long time, it will cause insufficient lubrication and wear on one side of the piston pin, resulting in increased porosity. If the piston pin has a shaking gap, the piston will be thrown out at the top dead center and hit the valve plate and valve plate, producing a knocking sound. Therefore, when replacing the valve plate, check the wear of the piston pin.
Conclusions and recommendations
Lack of oil can cause serious lubrication deficiencies. The root cause of lack of oil is not the amount and speed of compressor oil flow, but rather the poor oil return of the system. Installing an oil separator can quickly return oil and extend the compressor's operation time without oil return. The design of the evaporator and return line must take into account the return oil. Maintenance measures such as avoiding frequent starts, defrosting regularly, replenishing refrigerant in a timely manner, and replacing worn piston components in a timely manner also help to return oil.
Return liquid and refrigerant migration will dilute the lubricating oil, which is not conducive to the formation of oil film; Failure of the oil pump and blockage of the oil circuit can affect the oil supply and pressure, leading to a lack of oil on the friction surface; The high temperature of the friction surface will promote the decomposition of the lubricating oil, making the lubricating oil lose its lubricating ability. Insufficient lubrication caused by these three problems often leads to compressor damage. The root cause of oil shortage lies in the system. Therefore, only replacing the compressor or some accessories cannot fundamentally solve the problem of oil shortage.