## 2 Materials and Methods  
### Experimental Plant  
The designed experimental plant, shown in Figure 1 and integrated into the proposed test bench, is located in the Research Laboratory of Automatic Control at the University of Cordoba. The plant primarily consists of an HVAC unit and a water pump, both belonging to a real coach, and a system to emulate the hot water coming from the internal combustion engine cooling circuit, obviously not available in a laboratory. The experimental setup only includes the heating system elements for the driver's zone because the control of its interior air temperature is more challenging than in the passengers' zone, for two main reasons: Firstly, the driver discomfort caused by the air and heat exchanges between both zones, with a strong influence of the passengers' zone on the driver's one, considering the much smaller air volume in the latter; secondly, the complexity of the control equipment in the driver's zone is higher (e.g., the valve regulating the hot water flow from the engine cooling circuit is a proportional valve in this zone, while it is an on/off valve in the passengers' zone). Furthermore, in actual coaches, the HVAC unit installed in the driver's zone includes security actions involved in glass dehumidification, which is isolated from the passenger system. A complete set of sensors, more than those that are usually present in actual vehicles, were added to monitor the behaviour of the variables of interest, such as temperatures, flows and pressures. In addition, part of the software needed to run the experimental tests was implemented in a PLC (Programmable Logic Controller). Specifically, the controller was based on a Siemens S7-1512 CPU, which was connected to a PC where the variables of interest were monitored and automatically registered by the SCADA system (created with Siemens SIMATIC WinCC). The low-level operation algorithms of the valve and fans, among other elements, were also implemented in the PLC, and they reproduced exactly what exists in actual vehicles. Furthermore, Matlab/Simulink software (Mathworks, Inc.) was used to implement high-level control strategies and data registering and, of course, the model of the coach thermal dynamics, which will be described in the next section. Finally, the communication between PLC and PC was implemented using the OPC UA protocol. Figure 2 shows the P&ID (Process & Instrumentation Diagram) of the plant with their main elements and sensor locations. The system incorporated a flowmeter that measured the HVAC unit's inlet hot water flow rate, and another flowmeter was located at the output hot airflow. The HVAC unit presents a crossflow arrangement, i.e., the ambient air moves perpendicularly to the hot water, which flows through the heat exchanger tubes. Regarding the temperature measurements, four sensors were located at the inlets and outlets of the HVAC unit and a fifth sensor measured the tank temperature, as shown in the diagram. Table 1 lists the brand, model and other details of the sensors used. In the following section, the main components of the experimental plant are described in detail. Figure 1: Experimental plant. Figure 2: P&ID of the experimental plant. * Engine emulation: This subsystem consists of a gas boiler heating unit (24 kW) and a buffer tank (100 L), which allows the emulation of the heat produced by the vehicle's internal combustion engine through its cooling circuit, being able to produce a constant and continuous hot water flow of 12 L/min at 80 \({}^{\circ}\)C. A regulated three-way mixing valve was included at the outlet of the tank to control inlet temperature to the HVAC unit and thus reproduce the behaviour of the engine at start-up, emulating the transitory regime until the steady-state operating temperature is reached. In addition, a water hydraulic pump (the same model of 24 VDC installed in coaches) was used to impulse the hot water to the HVAC unit.