1. Functions and Characteristics of Automotive Air Conditioning

1.1 Functions of Automotive Air Conditioning

Automotive air conditioning refers to a device that regulates the air inside the car's cab and passenger compartment. It can adjust parameters such as temperature, humidity, flow rate and cleanliness of the air in the vehicle compartment, making the driver and passengers feel comfortable, and preventing or removing fog, frost, ice and snow on the windshield to ensure the health of the driver and passengers and driving safety. The specific functions of automotive air conditioning are summarized as follows:

1.1.1 Temperature Regulation

Temperature regulation is the main task of automotive air conditioning. The temperature range that people feel comfortable in daily life is about 22℃—28℃ in summer and 16℃—20℃ in winter, and the temperature required for the head is slightly lower than that for the feet, with a temperature difference of about 2℃. The cooling of automotive air conditioning in summer is completed by the refrigeration device;

In winter, cars and small and medium-sized vehicles generally use engine cooling water as the heat source of the heating system, while most large passenger cars or vehicles in severe cold areas use independent heaters for heating.

1.1.2 Humidity Regulation

Air humidity refers to the content of water vapor in the air. The relative humidity range that people feel comfortable is about 50%—70% in summer and 40%—60% in winter. Humidity has a great impact on the comfort of vehicle occupants. If the air humidity is too low, occupants will feel dry mouth and tongue; if the air humidity is too high, occupants will feel stuffy and breathless. Automotive air conditioning can cool and remove moisture from the air through the refrigeration device, and then heat the air through the heating device to reduce the relative humidity of the air;

Since ordinary cars are not equipped with independent humidifiers, humidity can only be adjusted by opening windows or introducing outside air through ventilation devices.

1.1.3 Air Flow Rate Regulation

The flow rate and direction of air have a great impact on human comfort. The wind speed that people feel comfortable is about 0.1—0.2m/s. Generally, the wind speed should be higher in summer and lower in winter. According to human physiological characteristics, the head is more sensitive to cold, while the feet are more sensitive to heat, so the air outlet mode of "cool head and warm feet" can be adopted.

1.1.4 Air Filtration and Purification

Maintaining air cleanliness and timely ventilation in the car is also a requirement for comfort. Since the car interior is filled with chemical decomposition products released by various materials, as well as dust, turbid air and smoke exhaled by occupants, a set of air filtration and purification devices needs to be equipped.

1.2 Characteristics of Automotive Air Conditioning

1.2.1 Compact Structure and Light Weight

Due to the structural limitations of the car itself, the automotive air conditioning is required to have a compact structure that can be installed in the limited space of the car, and its weight should be light to meet the requirements of vehicle lightweighting.

1.2.2 Strong Cooling and Heating Capacity

Since the car works outdoors, it is directly affected by the external weather, and the passenger density in the compartment is high, resulting in a large amount of heat generation and high heat load in summer, and a large demand for heat in winter. Therefore, the automotive air conditioning is required to adjust the temperature inside the car to an appropriate value in a short time.

1.2.3 Strong Impact Resistance

The car is subject to severe and frequent vibration and impact during driving, which is likely to cause loosening of the air conditioning system pipelines, leading to refrigerant leakage, affecting the operation of the entire air conditioning system, and even damaging the components of the refrigeration system. Therefore, the automotive air conditioning system must have strong impact resistance, and all pipeline connections must be firm and sturdy.

1.2.4 Power from Engine or Battery Pack

For traditional fuel-powered vehicles, the power of the automotive air conditioning comes from the engine. Among them, for cars and small and medium-sized passenger cars, the type of air conditioning where the power required for the air conditioning and the power to drive the car both come from the same engine is called non-independent air conditioning; for large and medium-sized passenger cars, due to their large cooling and heating capacity requirements, a dedicated engine is needed to provide power, and this type of air conditioning is called independent air conditioning. For pure electric vehicles, the power required for the automotive air conditioning comes from the power battery pack.

2. Composition of Automotive Air Conditioning System

An automotive air conditioning system is generally composed of a refrigeration system, a heating system, an air distribution system and a control circuit, as shown in Figure 1. The function of the refrigeration system is to cool or dehumidify the air inside the car or the fresh air entering the car from the outside, making the air inside the car cool and comfortable. Its main components include compressor, condenser, receiver drier, expansion valve, evaporator, etc. The heating system is mainly used for heating, and heats the air inside the car or the fresh air entering the car from the outside to achieve the purpose of heating and defrosting. The heating system is mainly composed of heater, water valve, water pipe, engine coolant, etc. The air distribution system draws fresh air from the outside into the car to play the role of ventilation and air exchange.

At the same time, ventilation also plays a good role in preventing fogging of the windshield. The air distribution system is composed of air intake mode damper, blower, air mixing mode damper, air flow mode damper, air guide pipe, etc. The control circuit controls the temperature and pressure of the refrigeration system and heating system, and manipulates the temperature, air volume and flow direction of the air inside the car to improve various functions of the air conditioning system. The control circuit mainly includes power switch, A/C switch, electromagnetic clutch, blower switch and speed regulating resistor, various temperature sensors, high and low pressure switches, temperature controller (or thermostat), air supply mode control device, various relays, etc.

3. Development Trends of Automotive Air Conditioning

At present, from the development direction of automobiles, it is necessary to take the route of intelligence, lightweighting and environmental protection. The comfort and efficiency of automotive air conditioning need to be further improved to reduce its use cost; in addition, from the perspective of market demand, the functions of automotive air conditioning should be more powerful, and the operation should be simpler and more reasonable.

3.1 Development towards Fully Automatic Direction

Automotive electronic technology is also applied to automotive air conditioning, making the performance of air conditioning equipment higher and higher. Traditional automotive air conditioners are mostly manual air conditioners, which require manual adjustment of temperature, blower speed and air outlet mode, etc., and frequent operations are needed to adjust the comfort of air conditioning use. Automotive automatic air conditioners use sensors to detect changes in the internal and external environment of the car such as internal and external temperature and solar radiation, automatically adjust the blower speed, air outlet mode and compressor operation, keep the temperature and humidity inside the car at the optimal value, greatly improve comfort, and can also automatically detect and diagnose faults, shortening the detection and maintenance time.

3.2 Development towards High Efficiency and Energy Saving Direction

The power unit of the automotive air conditioning refrigeration system is the compressor. Traditional air conditioning compressors are fixed displacement compressors, whose displacement increases proportionally with the increase of engine speed. They cannot automatically change power output according to refrigeration demand, and have a relatively large impact on engine fuel consumption. Variable displacement compressors can automatically adjust power output according to the set temperature. Due to their advantages of compact structure, light weight and energy saving (energy consumption can be reduced by more than 30%), variable displacement compressors have gradually become the main development trend of automotive air conditioning compressors.

3.3 Adoption of More Environmentally Friendly Materials

The refrigerant used in early automotive air conditioners was R12, which caused extremely serious damage to the ozone layer and was banned after 2000. Later, R134a was used to replace R12 as the refrigerant for automotive air conditioners, and a series of problems such as automotive air conditioning matching and materials were solved. In terms of refrigerant selection, it should generally comply with the requirements of relevant regulations, not only meet environmental protection requirements, but also follow the principles of safety and stability, and cost control. The ideal state of refrigerant should be that ODP (Ozone Depletion Potential) is 0, GWP (Global Warming Potential) is 0, chemical and physical properties are relatively stable, non-flammable and non-toxic, thermodynamic properties are superior, raw material cost is low and sources are abundant and extensive.

3.4 New Energy Vehicle Air Conditioning

In the development process of China's energy conservation and environmental protection strategy, new energy vehicles are also an important part, and their development trend is to continuously improve energy utilization efficiency. As the main part of energy consumption of new energy vehicles, automotive air conditioning should also comply with this development trend. Different from traditional fuel-powered vehicles, new energy vehicles, especially pure electric vehicles, have canceled the engine, so the energy of the air conditioning system comes from the power battery pack. The refrigeration system and heating system are the two major energy-consuming parts of the new energy vehicle air conditioning system. The energy consumption of the refrigeration system is mainly on the compressor, and new energy vehicles adopt electric compressor air conditioning systems.

To improve the high efficiency and energy saving of the air conditioning system, improvements can be made in the following aspects:

First, develop more efficient and energy-saving variable displacement DC scroll compressors;

Second, use more efficient parallel flow condensers;

Third, develop silicon electronic expansion valves with more precise control and higher energy saving;

Fourth, improve the microchannel structure of the evaporator, optimize the evaporator performance through energy efficiency advanced design, and make the evaporation of refrigerant more uniform.

Since pure electric vehicles do not have an engine, the heating system of the automotive air conditioning cannot use the waste heat of the engine for heating. Generally, new energy vehicles adopt PTC heating devices, but their energy consumption is huge, leading to a significant reduction in the driving range of the car. To this end, it can be improved by researching heat pump heating technology. The heat pump air conditioning system for electric vehicles is shown in Figure 2.

The working principle of the refrigeration condition is: the refrigerant starts from the electric compressor, passes through the four-way valve to the external condenser for heat dissipation, then enters the internal evaporator to absorb heat and evaporate through the electronic expansion valve 1, and then returns to the compressor. The working principle of the heating condition is: the refrigerant starts from the electric compressor, passes through the four-way valve to the internal condenser for heat dissipation, then enters the external condenser to absorb heat and evaporate through the electronic expansion valve 2, and then returns to the compressor through the solenoid valve.