## 5 Methodology
### Vehicle HVAC System Model  
The heating, ventilation and air conditioning (HVAC) system is one of the most power-intensive systems in the vehicle. [PERSON] et al. found that HVAC and other electrical subsystems can contribute up to more than half of total vehicle consumption at low speeds in different scenarios [69]. Before introducing the simulation setup, we would like to give an overview of the HVAC system [70]. A complete cooling and heating system works in two basic ways. The first is cabin heating, for which a water heating system is commonly used in today's modern vehicles. A water heating system uses the engine's cooling water system and transfers the heat to the cabin via the water/air heat exchanger and the available hot air ducts. The advantage of water heating is that the engine and cab are heated together. The system is usually installed in the engine compartment and connected to the cooling water system. The heat energy is dissipated by the vehicle's heat exchanger and the warm air is gently blown into the cab through the air duct. The other case is cooling, where the primary objective is to transport the warm air inside the cabin to the outside environment. The refrigerant gas circulating in the system passes from the evaporator to the gas compressor in the engine compartment, where it is compressed to a higher pressure, resulting in higher temperatures. The hot, compressed refrigerant vapor is then at a temperature and pressure that can be condensed and passed through a condenser, usually located in front of the car radiator. The air is blown through the evaporator, often after filtration by the cabin air filter, by a variable-speed electric centrifugal fan, causing the liquid part of the cold refrigerant to evaporate, further reducing the temperature. The hot air is thus cooled and stripped of moisture (which condenses on the evaporator coils and is exhausted outside the vehicle). It is then passed through a heatingbox in which the engine coolant circulates, where it can be heated to a user-selected degree or even a specific temperature, and then released into the passenger compartment through adjustable vents. Another way of setting the desired air temperature, this time by adjusting the cooling capacity of the system, is to precisely control the speed of the radial fan so that only the airflow strictly required is cooled by the evaporator. The user also has the option of closing the vehicle's external air vents to achieve even faster and more powerful cooling by recirculating the already-cooled air in the passenger compartment back to the evaporator. To complete the cooling cycle, the refrigerant vapor is returned to the compressor. The hotter the air entering the evaporator, the higher the pressure of the vapor mixture leaving it, and therefore, the greater the load on the compressor and hence the motor to circulate the refrigerant through the system. The compressor load is also proportional to the condensing temperature. The compressor can be driven either by the vehicle engine (e.g., a belt-driven, electronically controlled compressor can be driven directly by a belt without the need for a clutch or magnet) or by an electric motor. The HVAC model we use is based on the HVAC Vehicle Model (Figure 10) available in the Matlab example [71]. Necessarily, in order to fit the model to the characteristics of the test vehicle, we had to modify the simulation parameters. Some of the input parameters for this model came from the measurement data, like the environmental pressure, environmental temperature and desired temperature in the vehicle cabin. Some other inputs are from the base simulation system, like the vehicle motor speed and the calculated vehicle speed. For power calculation, we use the calculated mechanical power requirement of the system.