How Vehicle Cabin Model Order Reduction Can Optimize Passenger Comfort and Range

Why Cabin Modelling?  

Efficient cabin modelling has become crucial for the new age battery electric vehicles (BEVs) as every bit of energy that can be conserved will help increase the range of a BEV. The goal of a design engineer is to achieve an optimal balance between passenger comfort and the energy consumption of the vehicle for various ambient conditions like hot or cold weather in different geographical locations. In the latest GT-SUITE v2025 release, Gamma Technologies has introduced a new feature that can automatically generate a physics-based, real-time capable 1D reduced order model (ROM) from a detailed 3D cabin model. This new feature will help the users to quickly explore various cabin geometries and develop accurate cabin comfort control strategies.  

Main Components in the HVAC Circuit of a Vehicle 

The complexity of a modern vehicle HVAC system has increased recently as the refrigerant loop not only has to provide cooling and heating to the cabin in a heat-pump operation but also helps in maintaining the temperature of critical powertrain components like batteries, electric motors, and power electronics. 

A typical HVAC circuit consists of the following components: 

• Compressor – compresses the refrigerant to the required condenser pressure 
• Condenser – rejects the heat taken from the cabin air 
• Expansion Valve (TXV/EXV) – throttles the refrigerant to the evaporator pressure 
• Evaporator – cools down the hot cabin air 
• HVAC Door – controls the air recirculation rate 
• Blower – creates air flow in the system 
• Heater – heats up the cold cabin air 
• Cabin – accounts for the thermal inertia of the system 

The multi-physics simulation platform GT-SUITE accurately models the interaction of these components, like in a real-world vehicle. These simulations allow engineers to develop an optimal design on the individual component-level as well as on the entire system-level. An illustration of a simple HVAC system in GT-SUITE is shown in Figure 1. To learn more about the advanced features in the HVAC modelling, please visit this webpage.  

Figure 1: A simple HVAC system of a vehicle in GT-SUITE

Different Cabin Modelling Fidelities in GT-SUITE  

In GT-SUITE, cabin modelling can be performed at different levels of fidelity depending on the design need. On the bottom left of Figure 2, we have the 0D/1D cabin modelling capabilities using mono and multi-zones. This is a system-focused approach. As we move towards the right side in Figure 2, it becomes more comfort-focused with high-fidelity results. 

In GT-SUITE, 3D cabin comfort modelling is possible through co-simulation with GT-TAITherm. In this approach, GT-SUITE solves the fluid domain inside the cabin, while GT-TAITherm solves the thermal solid structures of the cabin and the passenger comfort. To learn more about the GT-TAITherm modelling, please visit this webpage. 

Figure 2: Different Cabin Modelling Fidelities in GT-SUITE

These cabin modelling methodologies in GT-SUITE are multiple orders of magnitude faster than performing a conventional 3D CFD. At the same time, GT-SUITE modelling offers the required accuracy to design an efficient and integrated HVAC system. Some of the key advantages of GT-SUITE cabin modelling and simulations are:   

• Predict if new components in the vehicle meet comfort and energy use targets 
• Improve control strategies in the HVAC systems 
• Evaluate new technologies like low emissivity glass coatings and localized cooling/heating 
• Optimize global and local passenger comfort 
• Size different components such as a compressor, evaporator, blower and heater 
• Investigate different boundary conditions like air recirculation rate, flap positions and inlet temperatures 

Cabin Model Order Reduction   

As shown in Figure 2, this new streamlined workflow can automatically create a fast-running 1D ROM from a detailed 3D GT-TAITherm cabin model. This feature can automatically extract all the required properties from the 3D model and generate the ROM using the flow and thermal primitives. It can also automatically prepare the ROM for calibration based on the 3D simulation results and automatically create plots to check the calibration results. This fast-running ROM can be easily integrated into system-level models and used in real-time applications.  

A cabin is usually made up of different layers of solid structures. As shown by the illustration in Figure 3, each layer in a cabin part is modelled as a lumped thermal mass. The air inside the cabin is represented as a flow volume. The physics-based ROM gives accurate results by capturing the following modes of heat transfer: 

• Conductive heat transfer between different layers in all the solid parts 
• Convective heat transfer between the cabin solid parts and cabin air 
• Convective heat transfer between the cabin solid parts and surroundings 
• Radiative heat transfer between different solid parts 
• Effects of solar heat flux 

Figure 3: Illustration of the Automatic Generation of a ROM from a 3D GT-TAITherm Cabin Model

A case study has been performed to investigate the cabin ROM in heat up and cool down scenarios. The 3D GT-TAITherm cabin model used for this case study is shown in Figure 4. The cabin model consists of various parts representing the solid structures and a human manikin. Each solid structure in this cabin model is composed of multiple layers. 

Figure 4: The 3D GT-TAITherm Cabin Model used for the Case Study

The boundary conditions for the cool down scenario are shown in Figure 5. The ambient temperature is 50 degrees Celsius and cold air is blown into the cabin through the dashboard vents. The total duration of the simulation is 60 minutes.

Figure 5: Boundary Conditions used for the Cool Down Scenario

Simulation Results from the ROM

The temperature results of the 3D Model and ROM for the windshield, doors, floor, and cabin air are shown in Figure 6. The solid structure temperatures and cabin air temperature of the ROM match closely with the 3D results. In this workflow, the tool automatically prepares the ROM for calibration based on the 3D simulation results to further improve the accuracy. During the calibration process, various multipliers corresponding to the solid material properties and convective heat transfer coefficients (HTCs) were varied using the integrated design optimizer in GT-SUITE. The aim of the optimization was to minimize the difference between the 3D simulation and ROM results for three quantities, namely cabin air temperature, ambient side temperature of the solid parts, and cabin air side temperature of the solid parts. To learn more about the Integrated Design Optimizer and Machine Learning platform in GT-SUITE, please visit this webpage. 

Figure 6: Temperature Results of the ROM

Values Delivered by the Innovative Cabin Model Order Reduction Workflow  

The key values delivered to the users by the cabin model order reduction workflow are listed below and illustrated in Figure 7. 

1. Automatic data extraction from the 3D GT-TAITherm vehicle cabin model 
2. Automatic creation of a physics-based ROM in the order of seconds allowing quick model setup for various geometries and levels of details 
3. Automatic preparation of the ROM for calibration based on the 3D results and automatic creation of plots to the check the calibration results 
4. Applicable for both the cool down and heat up scenarios 
5. Simulation speed in the order of 100x faster than real time 
6. Seamless integration of the ROM with a system-level model 

Figure 7: Values Delivered by the Innovative Cabin Model Order Reduction Workflow

Learn More about our Vehicle Thermal Management Simulation Solutions 

If you’d like to learn more or are interested in trying GT-SUITE and GT-TAITherm for thermal management and cabin comfort simulations, please view this webpage. To speak with a GT expert, contact us here!