Electric Bus Modelling with VeSyMA, TiL and Human Comfort

We have just completed another Bus modelling demo using the VeSyMA suite of libraries that we develop at Claytex. This time the objectives were related to energy management of the overall vehicle and understanding the losses within the powertrain and driveline. This gave the base to studies that involved propulsion and transmission sizing for particular markets and duty cycles and more importantly including the impact of often neglected attributes such as cabin thermal management and human comfort of passengers.

This blog post shows the range of methodologies that we use to study the vehicle systems and how they interact.

Flexible Vehicle Architecture

We used the VeSyMA suite of libraries to provide the vehicle and experiment architecture; its templates with replaceable objects provide flexibility.

Figure 1. Experiment diagram layer clearly laid out into main subsystems. All objects are replaceable so that the same template can be used for a variety of roads, vehicles, drivers, and atmospheric conditions.
Figure 1. Experiment diagram layer clearly laid out into main subsystems. All objects are replaceable so that the same template can be used for a variety of roads, vehicles, drivers, and atmospheric conditions.

The bus’ mechanical subsystems were based on models from the VeSyMA suite, including suspension, wheels, driveline, brakes, and body. These multibody models include animation object to easily visualise the results of the experiments.

Figure 2. Built-in visualisation of the bus, bus subsystems and road during an energy usage experiment. Horizontal arrows at each rear wheel show the contact patch traction force.
Figure 2. Built-in visualisation of the bus, bus subsystems and road during an energy usage experiment. Horizontal arrows at each rear wheel show the contact patch traction force.

The electric motors and energy storage systems were also from the VeSyMA suite which reduced the need for co-simulation with other tools outside of Dymola.

The VeSyMA suite provides a structure that allows customised subsystem and components to be slotted into the vehicle model. This permitted the heating, ventilation and air conditioning (HVAC) models using TiL and Human Comfort libraries to be easily integrated into the bus model.

By using the systems integration approach, we can demonstrate the impact of the HVAC loads on the battery (see figure 3).

For these experiments we are using a 30degC ambient with 40% relative humidity, 22degC target cabin temperature, 300W/m2 direct solar radiation and 100W/m2 diffuse solar radiation.

Figure 3. Plots of  battery state of charge vs. time with no HVAC system integrated (blue) and with HVAC system integrated (red). Plots for speed time trace are identical for both experiments.  Mean cabin temperature is only shown for the integrated HVAC system experiment.
Figure 3. Plots of battery state of charge vs. time with no HVAC system integrated (blue) and with HVAC system integrated (red). Plots for speed time trace are identical for both experiments. Mean cabin temperature is only shown for the integrated HVAC system experiment.

Thermal Management

To integrate the cabin thermal management, we initially integrated models built with the TiL suite from TLK and then for a more detailed approach we can integrate a Human Comfort library based cabin model.
The cabin models within the TiL suite and Human Comfort libraries allow the users to build up partitions and glazing where the overall thermal performance is derived from the material properties and geometries specified.

Figure 4. Diagram layer of the bus powertrain and chassis model using VeSyMA with integrated HVAC and cabin models from TiL and Human Comfort lib.
Figure 4. Diagram layer of the bus powertrain and chassis model using VeSyMA with integrated HVAC and cabin models from TiL and Human Comfort lib.
Figure 5. Diagram layer of the TiL suite HVAC model with e-compressor, twin evaporators, and cabin model.
Figure 5. Diagram layer of the TiL suite HVAC model with e-compressor, twin evaporators, and cabin model.
Figure 6. Diagram layer of the TiL suite bus cabin model with front, centre and rear zones with solid and glazed partitions.
Figure 6. Diagram layer of the TiL suite bus cabin model with front, centre and rear zones with solid and glazed partitions.

The inclusion of the thermal model of the cabin allowed events like opening and closing the bus doors to be analysed to see their effect on the bus’s battery charge.

Figure 7. Top: SOC plots for considering no HVAC (blue), HVAC (red), HVAC and door opening when at bus stops (green). Middle: Vehicle longitudinal velocity. Bottom: Mean cabin temperatures.
Figure 7. Top: SOC plots for considering no HVAC (blue), HVAC (red), HVAC and door opening when at bus stops (green). Middle: Vehicle longitudinal velocity. Bottom: Mean cabin temperatures.

We were also able to test the HVAC system capacity for ambient temperature excursions taking this up to 40degC from the 30degC baseline. The study allowed us to confirm through simulation that the system is in fact undersized for cooling the bus at these ambient conditions. Depending on how often these occur, it might be a compromise the operator is willing to take. Alternatively an upscaling of the system is required.

Figure 8. We have added the cabin temperature plot for the 40degC ambient conditions (bottom: Magenta plot no longer matches the average cabin target of 22degC.
Figure 8. We have added the cabin temperature plot for the 40degC ambient conditions to the plot in Figure 7. The magenta signal in the bottom plot no longer matches the average cabin target of 22degC.

CFD within Dymola, no co-simulation required!

For more detailed cabin simulation, the Human Comfort library from XRG includes a Dymola CFD-based approach (no co-simulation required!).

Although the implementation of the CFD-based approach to the cabin modelling is not yet complete for this demo, we are able to show an example of what this can look like from a similar project.

Figure 7. Bus geometry in ParaView.
Figure 9. Bus geometry in ParaView.

Figure 8. Diagram layer incorporating the CFD based approach (central grid), glazing and solid partitions (left), inlet flow positions and oriantations (right).
Figure 10. Diagram layer incorporating the CFD based approach (central grid), glazing and solid partitions (left), inlet flow positions and orientations (right).
Figure 9. Flow visualisation in Dymola for the complete bus cabin
Figure 11. Flow visualisation in Dymola for the complete bus cabin

Using the Human Comfort library, you can visualise the flow around the bus cabin within the Dymola environment (Figure 11). A more detailed visualisation of the flows and temperatures can be seen below using the XRG Excel plug-in Score (Figure 12).

Figure 10. Visualisation of flows and temperatures using the XRG Excel plug-in "Score"
Figure 12. Visualisation of flows and temperatures using the XRG Excel plug-in “Score”

Process Automation

As part of the methodology for efficient modelling and analysis, we applied the structure shown below which allowed a rapid setup of the vehicles and experiments as well as simulation execution and results analysis.

Figure 11. 1. Definition of vehicle variants, 2. Generation or extension of existing experiments, 3. Automated generation of test libraries for each vehicle variant. 4. Parallel running of all selected experiments, 5. Analysis of results with built-in scripts
Figure 13. 1. Definition of vehicle variants, 2. Generation or extension of existing experiments, 3. Automated generation of test libraries for each vehicle variant. 4. Parallel running of all selected experiments, 5. Analysis of results with built-in scripts.

Vehicle Modelling and Simulation

Please do not hesitate to get in touch if you are looking to design and analyse vehicle systems to any level of detail. For those who would prefer not to invest in a new software or additional libraries and are interested in just running vehicle models themselves we can also provide executables that are representative of your vehicle that allow you to modify parameters and initial conditions so that you can experiment using such models, analyse the results and distribute them freely within your company or team.

Further publications of our other work using VeSyMA, TiL suite and Human Comfort library can be found here: https://www.claytex.com/services/publications/

Written by Hannah Hammond-Scott and Alessandro Picarelli – Modelica Project Leader and Engineering Director

Please get in touch if you have any questions or have got a topic in mind that you would like us to write about. You can submit your questions / topics via: Tech Blog Questions / Topic Suggestion

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