Basic knowledge of car air conditioning

1. The role of car air conditioning

Car Air ConditionerIt has the functions of adjusting indoor temperature, changing air supply, dehumidifying, purifying and filtering air, and satisfying passengers' comfort.
1.Interpretation
fromcarIn terms of the function of air-conditioning, the indicators of car air-conditioning failures are represented by the corresponding indoor temperature, air supply, dehumidification, and air purification four abnormal conditions. Then, draw the conclusion of diagnostic significance: the first step of the diagnostic procedure, when the owner describes the car air-conditioning failure phenomenon, which one or several of the above four failure indicators should be converted?
What is the role? The effect is the effect of the performance of the system function. We know that car air conditioning functions are cooling, heating, blowing, purifying, operation and control functions.
2.Knowledge points
(1) Dehumidification method: The air is first cooled by the refrigeration system to make the water vapor in the air condense water droplets, and then the water vapor-removed air is heated by the heating system to achieve the purpose of dehumidification. Therefore, dehumidification is the effect of the system function, and it cannot be called the system function. Draw the conclusion of diagnostic significance: "When the car air conditioner is normal in cooling and heating, the dehumidification must be normal."
(2) Methods of purifying and filtering air: general car air conditioners use activated carbonAir filterTo achieve the purpose of purifying and filtering the air. Purification relies on activated carbon, and filtration relies on filter paper.Usually through regular maintenance and replacementFilterOr use a cleaning and deodorant to clean the air duct to remove peculiar smell. It should be noted that we can regard it as a unit of the blast system or a single system, and it does not affect our diagnosis. Draw a conclusion of diagnostic significance: "For this type of car owner's complaint, it is enough to replace the car air-conditioning filter and maintain the air duct." It is just a very simple special case of car air-conditioning failure.
(3) Check according to the system function: the system function manifests as two types of faults, one is whether there is any problem with the function output, for example, the car air conditioner is not cooling. The other is the problem of abnormal function output, such as the poor cooling effect of car air conditioners. According to the fault type of automobile air-conditioning function and system function, the diagnosis meaning is concluded: "The second step of the diagnostic procedure is to check each system function of the automobile air-conditioning. Determine whether there is any problem with the output of each system function and the problem of output difference." The purpose is to judge whether the owner's description or reflection of the fault phenomenon is correct and comprehensive. To give an example: a car owner repaired the air conditioning and refrigeration system here, and the effect was very good. But the next day complained about poor cooling. According to the function inspection, the author found that the temperature adjustment operation position is in the heating position. When the author adjusts it to the cooling position, the cooling effect is achieved. Another purpose is to determine which system has a problem, and then diagnose the system to prove the cause and location of the failure. There are two inspection schemes: one is to inspect the functions of each system of the car air conditioner one by one, and it is ordered according to the manufacturer's specifications, optimization schemes, and maintenance season principles. The other is to rely on maintenance experience and experience, and to check one by one according to the fault indicator principle determined in the first step of the diagnostic procedure.
(4) Inspection rules: General automobile air conditioner inspection has certain rules, and the inspection method and method depend on the type and form of the automobile air conditioner operation panel. The air conditioner operation panel includes manual type, automatic type and intelligent type. The difference in air-conditioning control methods determines the difference in the operation specification and process refinement of automobile air-conditioning fault diagnosis technology and methods. The control modes of automobile air-conditioning include pull-wire type, vacuum type, electric type, and electric control type. For example, if a cable is used to control a temperature door, it can be observed whether the cable is connected or not, and the position of the temperature door can be changed artificially to determine the specific cause and location of the fault.Another example is the use of a vacuum-type temperature control door, by operating the vacuumswitchYou can sense whether there is vacuum. If you can change the door position several times, the vacuum system is normal and can be stored. If you don’t change the door position and start the engine, the door position can be changed, indicating that the vacuum control pipeline is normal and changed.ThrottleThe opening degree, if the position of the door changes, it means that the vacuum check valve is abnormal.For another example, if the automatic air conditioner is faulty, you can check whether the manual state is normal, and then you can determine whether it issensorProblem. In short, the difference between the operation mode and the control mode of the air conditioner panel can be seen in the diagnosis skills and doorways, because it does not affect the diagnosis procedure, but only reflects the difference in process refinement.
(5) Related knowledge of automobile air conditioning and thermodynamics: The thermodynamic performance of indoor air is the heat load. It is the medium for heat exchange. The thermodynamic parameters of indoor air are: temperature, humidity, heat, volume, pressure, specific heat, etc. Familiar with thermodynamics knowledge and basic concept meaning is of great help to diagnosis. For example, generally, the heat load of humid air is greater than that of dry air at the same temperature, and the more humid the heat load, the greater the heat load. In actual maintenance, most maintenance personnel often confuse the physical meaning of temperature and heat, and they don't know the meaning. Why is the cooling effect of the evaporator frosting poor? The explanation is not very clear. For another example, if the high-pressure pipeline is blocked in the air-conditioning refrigeration system, people often think that the pressure will increase due to the blockage of the high-pressure pipeline. In fact, it is not understood that the refrigeration system is a closed circulation system. . However, what we commonly use is an open-loop system. It is a fact that the high-pressure pipeline is blocked and the pressure will be higher for the open-loop system. Therefore, it is necessary to master related knowledge of thermodynamics and hydraulics in order to better solve the fault.
(6) Significance of system diagnosis: The system mentioned earlier, then what is a system? Simply put, the system has external independence and internal relevance, forming a complete functional unit. Each functional unit is a subsystem. Because the system is independent to the outside, the influence of other systems on the system is not considered when analyzing the system; due to the correlation within the system, it is necessary to analyze each functional element (unit) in the system one by one. Diagnose and decompose according to the system, and the failure state or phenomenon decomposed to the smallest maintenance unit is the cause or location of the failure.

2. Characteristics of automobile air conditioning system
(1) Adopt an independent refrigeration system, powered by the engine that drives the vehicle, and drive the air conditioner through a belt drivecompressor. Explain that the air conditioning and refrigeration system is related to the engine.When the engine water temperature is too high, cut off the refrigeration systemcompressor, The purpose is to reduce engine load. It is sensed by the engine coolant temperature signal or the engine oil temperature signal.Engine idle speed control mode, for small displacement engines, generally cut offcompressorWork to meet idling stability.It is to provide the engineRotating speedThe signal senses the idling state; for large-displacement engines, there is no need to cut off the compressor of the refrigeration system, but to increase the engine idling speed to achieve idling control. It is sensed through the air conditioning demand signal. Therefore, it tells us such information: When a system does not work or changes state, it may be a problem with the system itself, or it may be a need to protect other systems. For example, when the compressor of the air-conditioning refrigeration system is not working, the protection function of the engine should be considered.
(2) Use engineCooling systemHeat source and passWater pumpThe air conditioning heating system that is provided by the circulation and the engine cooling system in parallel is the heating system.The prerequisite for the operation of the air-conditioning heating system is that the working temperature of the engine cooling system is normal.ThermostatThe air-conditioning heating system starts to circulate when it is turned on. Some large-displacement models have engine overheating protection. When the engine cooling system is overloaded, the air-conditioning heating system will be used for protection. Even in the air-conditioning refrigeration state, the air-conditioning heating system will work to participate in the engine's heat dissipation.

Third, the composition of the air-conditioning system
From the perspective of diagnosis, the composition of automobile air conditioning system should be composed of refrigeration system, heating system, blast system, operation and control system. The composition and function of each subsystem are explained below from the perspective of diagnosis.
1.Cooling System
By generating cycle power and compressing the heat-absorbing gaseous refrigerant into an air-conditioning refrigerant that can release heatCondenser, The liquid refrigerant that has released heat is expanded and converted into an expansion valve that can absorb heat in a mist of liquid refrigerant. The mist of liquid refrigerant that can absorb heat undergoes heat exchange to absorb heat to evaporate and convert to vaporization of the gaseous refrigerant that absorbs heat. Box, as well as a blower to achieve heat absorption and an electricFan, There are also the medium for exchange and circulation, that is, the heat load air exchanged indoors, the atmosphere exchanged outside the car, and the refrigerant circulating inside the refrigeration.
From the composition of the refrigeration system, what makes us think of is that it contains the following principles: heat absorption and release balance, efficiency matching, two external heat exchanges, four internal cycle conversions, and the working principle of the refrigeration system. There are also physical phenomena and laws that apply compression, expansion, evaporation, and condensation. Therefore, we need to have a considerable grasp and understanding of several principles and laws of physical phenomena in order to correctly perform fault diagnosis and air-conditioning operations.
2.Heating system
Since the heating system uses the heat source of the engine cooling system and is connected in parallel with the engine cooling system, its diagnostic significance is determined. The water jacket that generates the heat source by the engine, the centrifugal water pump that realizes circulation power, the actuator heater of heating function, the thermostat for heating circulation control, the water valve or temperature door for heating regulation, and the blower that realizes heat exchange, and The exchange and circulation media are the heat load air exchanged inside and outside the room and the cooling liquid circulating inside.
The heating system also has the principle of heat absorption and heat release balance, efficiency matching principle, external heat dissipation exchange principle, internal circulation conversion principle, heating system working principle, and applied thermodynamic parameters and laws.
3.Blowing system
Similarly, interpret the composition of the blast system from the same visual angle. The blower that provides the power of the blower system, depends on the change of speed to form a certain amount of air supply, is driven by a permanent magnet DC motor to drive the centrifugal fan, the air source door that realizes the internal circulation or the external circulation control, and the cold source provided by the evaporator of the refrigeration system It is composed of a modulator composed of a heat source provided by the heater of the heating system, a temperature door that adjusts or controls the temperature, and a functional door that controls and changes the air supply volume of different air outlets.
The blower and three types of doors (air source door, temperature door, functional door) are provided by the air blowing system itself. The cold source and heat source (ie evaporator box and heater) are the executive functional components of the refrigeration system and the heating system, which are two The interface part of the system, the interface part provides an interface diagnosis method for diagnosis. The interface is a test point for diagnostic significance.
4.Operation and control system
The operating part is the air-conditioning panel, which includes the following levels: The air-conditioning panel operation is divided into manual, automatic, and intelligent modes; according to the operation keys, it is divided into push-pull, sliding, rotary, rocker, touch, etc. Method: According to the operation items, it is generally divided into AC switch, temperature adjustment, wind speed adjustment, air outlet control, internal and external circulation control, etc.
The control part also includes the following levels: According to the control mode, it is divided into pull-wire type, vacuum type, electric type, and electric control type. ①Pull-wire type: the purpose of manual operation is realized through mechanical pull-wire transmission and driving, generally to drive the position of the function door, air source door, temperature door of the blower system, and the opening and closing of the water valve of the heating system; ②Vacuum type: engine working HourValveThen, the intake vacuum negative pressure is generated, and the vacuum that controls the vacuum negative pressure energy is usedmotorThe position of the function door, air source door, temperature door, and the opening and closing of the heating system water valve of the driving air blowing system; ③Electrical type: by switch orRelayIt is composed of a single loop or a main loop and a control loop to achieve control; ④Electronic control type:sensor, The logic circuit composed of the control unit and the actuator realizes program control.
According to the control content, it is divided into the compressor control of the refrigeration system, the water valve control of the heating system, the blower speed control of the air blowing system, the function door position control, the temperature door position control, and the air source valve position control.
According to the composition of the air conditioning system, it is composed of four subsystems. Then the faults determined by the system are the refrigeration system fault, heating system fault, blower system fault, operation and control system fault. Draw the conclusion of diagnostic significance: "The third step of the diagnostic procedure is to diagnose the four subsystems according to the optimized sequence. The complex operation and control system can be assumed to be normal, and the operation and Whether the control system is normal. As long as you have a certain understanding of the system, you can use the system method to diagnose, measure the test points and diagnostic parameters of the system, and determine the cause and location of the fault.

Four, summary
Cognitive meaning is the name, function, type, assembly location and relationship of cognitive components. Cognition is a prerequisite for diagnosis, it is to understand, understand or master the objective things that need to be diagnosed. It is the source of collecting diagnostic data and information. This is the end of the basics of car air conditioning in this issue. There may be some readers who can read it, and there may be some readers who are more difficult to understand as they read it. Finally, I will give you a summary, the purpose is to develop a good study habit, be good at summarizing, and be good at sublimation when studying in the future.
This lecture first talks about the role of automobile air conditioners. According to the role of automobile air conditioners, the indicator of automobile air conditioner faults can be determined. Based on the diagnosis principle, the indicators of dehumidification and purifying air are simplified into special cases without the need for necessary diagnostic procedures. Can be solved. The role of automobile air conditioning determines the first step of the diagnostic procedure, which is to convert the fault described by the car owner into a fault indicator according to the phenomenon, and express the fault according to the fault indicator. For example, describing the fault according to the phenomenon is equivalent to describing the various symptoms of the condition as the patient sees the doctor. The fault indicator is equivalent to what kind of disease the patient feels or which department the doctor initially confirms. According to the relationship between the function and the function, execute the second step of the diagnostic program to conduct an orderly inspection according to the function. At the same time, it explained the characteristics of automobile air-conditioning, the purpose is to tell us to consider the correlation effect of other systems on the diagnosis system when diagnosing. Finally, the composition of the automobile air conditioning system is explained, and the diagnosis rule is determined according to the system failure. It is clear that the automobile air conditioner has four system failures, namely, the refrigeration system failure, the heating system failure, the blower system failure, and the operation control system failure. Then proceed to the third step of the diagnostic procedure according to the system failure rule, diagnose according to the system, decompose and simplify according to the system, and finally find the cause and location of the failure.