Development Of Automotive HVAC Under The Trend Of Electrification
Under the wave of vehicle electrification and intelligence, electric vehicles have developed rapidly. The sales of electric vehicles have grown rapidly in the past few years, but "range anxiety" and "safety anxiety" are still troubled by many consumers. The problem of cruising range is particularly obvious at low temperatures. When an electric vehicle is driven at low temperatures, the cruising capability of the vehicle may be reduced by more than 40% due to the use of heater air conditioning and reduced battery performance. From a safety perspective, the battery and Thermal management of power appliances in the car is also particularly important. These problems require a higher-performance automotive thermal management system to solve. Simply put, the automotive thermal management system mainly includes related parts such as HVAC, battery thermal management, motor electronic control, and high-power electrical appliance thermal management.
Among them, vehicle HVAC (Heating, Ventilation, and Air Conditioning, HVAC) refers to the system or related equipment responsible for heating, ventilation, and air conditioning in the vehicle. Specifically, HVAC is mainly divided into related components such as refrigeration devices, heating devices, and human machine interface (HMI).
Next, we will look at the refrigeration device, heating device and HMI in HVAC respectively, and focus on the technical solutions and development trends of HVAC for electric vehicles.
1. Refrigeration device
The refrigeration device of automobile HVAC is similar to our common air conditioner, which uses the state cycle of refrigerant compression, condensation, expansion and evaporation to achieve temperature control. Heat transfer occurs primarily through changes in the state of the refrigerant. The main components of a refrigeration unit responsible for the above functions are the compressor, condenser, expansion valve, receiver/dryer and evaporator.
As shown in the figure, the liquid refrigerant absorbs heat from the passenger compartment and evaporates into a gaseous state in the evaporator. The vapor is then drawn out of the evaporator by a compressor, which compresses the vapor to increase its pressure. The high-pressure, high-temperature steam generated by the compressor is then cooled by outside air in the condenser and condensed into high-pressure liquid. The liquid expands and decompresses in the expansion valve before entering the evaporator. The above process constitutes a continuous cycle. The evaporation process absorbs heat from the surrounding air to achieve a cooling effect, and the cold air is blown into the passenger compartment through a fan.
2. Heating device
For internal combustion engine vehicles, it is easier to heat the vehicle than to cool it. The conversion efficiency of the internal combustion engine is low (about 30%), and it generates more heat during operation. This heat flows into the heater core through the coolant, and then through simple control, controllable hot air can be blown into the cabin through a fan. , keeping the temperature of the cabin constant. The process is very economical and simple and can be carried out as long as the internal combustion engine is working. It does not require additional fuel and will not increase fuel consumption. But for electric vehicles, heating is a relatively complicated operation. Currently, the more mainstream heating solutions are PTC and heat pumps.
PTC (Positive Temperature Coefficient, Positive Temperature Coefficient Resistor) is a typical semiconductor resistor with positive temperature sensitivity. When energized, it will generate heat, which can be used for air conditioning heating. When the PTC is first powered on, its resistance will show a slowly decreasing trend as the temperature increases, that is, its calorific value at room temperature is low; when the temperature exceeds the Curie temperature, its resistance value will decrease with the temperature. The increase is a stepwise increase, and the actual performance is that it automatically stops working. As a PTC element for heating, its automatic constant temperature feature can eliminate the need for a complex temperature control circuit. In electric vehicles, the PTC is directly powered by the high-voltage battery pack, and then its heating status is controlled by a simple PWM switch to achieve a very simple and reliable heating system.
Although PTC has the characteristics of simple structure, durable materials, and good heating effect, the system structure of being directly powered by a high-voltage battery pack causes the heating process to affect the cruising range of electric vehicles. Research shows that the use of PTC will reduce the battery life of electric vehicles by approximately 24%.
Another solution is a heat pump. As a type of resistive element, the limit of PTC's COP (Coefficient of Performance, heating efficiency) is 100%, that is, electrical energy can only be converted into the same amount of heat energy at most, while a heat pump can be as high as 300%. The principle of a heat pump is similar to that of a household air conditioner. The refrigerant flows bidirectionally in the evaporator and condenser of the air conditioner through a four-way valve, transferring heat energy from a low-level heat source to a high-level heat source, thereby achieving the effect of heating or cooling. This "transfer of heat" Compared with the PTC mode, the non-"heat-generating" process can significantly save electric energy, thereby effectively extending the cruising range of electric vehicles and becoming an important application trend of HVAC for electric vehicles.
3. HMI
If the engine makes the car move, gives the car life, and is the trunk of the car, then the HMI gives the car wisdom and thought, and is the soul of the car. Therefore, HMI system solutions have always been valued by automotive OEMs. OEMs must provide drivers and passengers with safe, flexible, and comfortable navigation and entertainment experiences. At the same time, this is also an important factor in shaping product differentiation. In recent years, with the electrification of automobiles and the drive of major new power car manufacturers, HMI has gradually developed towards centralization, screenization and intelligence, similar to the development of smartphones. Many times HVAC no longer has a completely independent HMI interface, but is shared with other functions.
