###### Abstract  
Experimental work with heating systems installed in public transport vehicles, particularly for optimisation and control design, is a challenging task due to cost and space limitations, primarily imposed by the heating hardware and the need to have a real vehicle available. In this work, a hybrid hardware-in-the-loop (HIL) test bench for heating systems in conventional coaches is introduced. This approach consists of a hardware system made up of the main heating components, assembled as a lab experimental plant, along with a simulation component including a cabin thermal model, both exchanging real-time data using a standard communication protocol. This scheme presents great flexibility regarding data logging for further analysis and easily changing the experimental operational conditions and disturbances under different scenarios (i.e., solar irradiance, outside temperature, water temperature from the engine cooling circuit, number of passengers, etc.). Comparisons between the hybrid system's transient and steady-state responses and those from selected experiments conducted on an actual coach allowed us to conclude that the proposed system is a suitable test bed to aid in optimisation and design tasks. In this context, several closed-loop test experiments using the test bench were additionally carried out to assess the performance of the proposed control system. coach heating system; cabin thermal model; hardware-in-the-loop (HIL); modelling; simulation; automotive +
Footnote †: journal: Applied  
Modelling of actual HVAC system components (valves, heat exchangers, etc.) is a lengthy and costly task that requires, on the basis of several assumptions, taking into account the physical properties of different fluids (air, water, etc. ), the involved energy balance equations, the component geometries and materials or the estimation of thermal parameters from experimental data, among other aspects [3,4]. Different approaches can be found in the literature to obtain the thermal model of a vehicle (car, van or coach) cabin. The most detailed and computationally expensive models are those based on CFD (Computational Fluid Dynamics), which have been used to accurately obtain the evolution of indoor air flows and temperature distribution in order to determine the occupants' comfort level [5, 6, 7, 8], to analyse the air quality [9, 10, 11] or to study the influence or effect of certain conditions, such as the thermal insulation properties of the cabin [12] or the opening of doors and windows [13,14]. By contrast, there are lumped models that treat the whole cabin as a single homogeneous zone, primarily intended to assess the impact of the different thermal loads [15] or passenger comfort [16]. Finally, between the two previous types of models, there are those that segment the cabin into several zones, which exchange air volumes, defining lumped models for each one as a simplification of CFD models [17, 18, 19]; therefore, such exchange mechanisms between adjacent zones must also be mathematically defined. These latter models are a well-suited option for bus or coach cabins, which can be easily partitioned into clearly distinguished air zones, with an optimal balance between accuracy and simplicity with a reasonable computational cost when they are used in the design of control strategies and for real-time simulation. Therefore, the use of a hybrid HIL test bench made up of the real HVAC system \"connected\" to a coach cabin thermal model could provide a less costly setup to test control systems and compare different prototypes under the same conditions (number of occupants, solar irradiance, etc.) or the same prototype under different conditions, reducing and improving validation steps. Within the context of HIL test benches for vehicles, several papers can be found in the literature. For example, in [20,21], HIL test benches are employed to validate their proposed electric vehicle battery state-of-charge (SoC) estimation.