Thermal management in the vehicle
Human sensitivity for computers: Future of climate comfort optimization lies in virtual application
by Michael Ellinger, Group Leader CFD & Thermal Analysis,
and Max Hauk, Senior Expert Thermal Management at ARRK Engineering
With the introduction of electromobility, the issues of air conditioning and climate comfort are becoming massively more important. This is because unlike the combustion engine, which can always provide sufficient waste heat for heating and operates all electrical consumers in the interior via the generator, the driver of an e-car pays dearly for any heating or cooling power required with potential range. In addition, due to increasing competition in the industry, thermal comfort in the vehicle cabin is increasingly coming into focus as a differentiating feature. This makes it all the more important to optimize thermal management in terms of its quality and efficiency. To date, this has required lengthy development processes, the results of which are strongly influenced by the subjective perception of the application engineers involved. These ultimately evaluate the thermal comfort in the vehicle. Consequently, a way must be found to objectify the development of air conditioning and climate comfort.
ARRK Engineering is therefore working to simulate the complex thermal processes using hardware and software dummies, as well as to create a comfort specification. Based on these results, the application work is to be virtualized to a large extent in the future, thus not only achieving a more objective result, but also saving development time and ‑costs.
According to the LeasePlan Mobility Insights Report of February 2021, in which a total of around 5,400 people from 21 European countries and the USA were interviewed, 34 percent of participants said that the limited range reduced their interest in an e-car. In Portugal and Germany, even more than six out of ten respondents suffer from what is known as "range anxiety" with regard to electromobility. Whereas in conventionally powered vehicles, all electrical consumers are generally operated via the alternator or, like the air conditioning system, directly by the combustion engine and thus do not usually noticeably reduce the range of a tank of fuel, the e-car supplies all functions - from acceleration to on-board infotainment - from the same energy source. It is therefore essential for electromobility to make the thermal management systems in the vehicle as efficient and "intelligent" as possible in order to maximize the range actually available.
In addition to this topic, another phenomenon can be observed in recent years: Partly due to the growing e-mobility start-up scene, competition within the automotive industry is rising rapidly. In order to increase competitiveness, developers and customers alike are placing more and more emphasis on climate comfort in the vehicle interior, particularly in the Asian market, but recently also in Europe and the USA. Last but not least, autonomous driving will revolutionize previous air-conditioning concepts, with occupants sitting facing each other instead of facing forward, for example, resulting in the need for a completely different distribution of air flows in the passenger compartment. For these reasons, it is necessary to develop a virtualization strategy in order to be able to map and optimize new thermal management concepts much faster and better at the same time.
More efficient development processes thanks to virtualization
To a certain extent, virtualization has long since found its way into thermal management development. Nevertheless, a large part of the comprehensive fine-tuning of the interior climate control still depends on the individual sensibilities of the application engineers involved, who have so far only been able to rely on objective measurement results to a very limited extent when evaluating comfort. The application tests required for this purpose currently not only provide mainly subjective results, they can also only take place late in the development process, since large parts of the hardware and software must already have been defined and integrated. In addition, these tests involve running numerous load scenarios in different climatic environments. For this purpose, elaborate test trips are made to sometimes distant locations such as South Africa or Death Valley with test vehicles and personnel, which is lengthy and cost-intensive.
In order to be able to meet market demands for a high level of comfort in the vehicle in the future from an economic point of view, a development process for air conditioning and climate comfort is therefore necessary which - instead of being based on classic test drives - is primarily based on dynamic models and simulations. In this way, expensive application work is to be reduced to a minimum in the long term and largely replaced by fully transient calculations.
The specialists at ARRK Engineering have designed a general development process that takes into account the individual steps of climate comfort development: starting with benchmark studies, through the design of the circuits and the functional and comfort design of the cabin air conditioning with development of the climate control strategy, to optimization and validation. The focus of the current work on the process is on the question of which prerequisites must be created overall in order to be able to implement the desired virtualization, and which elements must be worked out in more detail in the simulation.
Air and heat dynamics in the passenger compartment
To close the gaps that still exist, ARRK engineers are currently in the process of developing different models to represent all the factors involved. This includes, for example, modeling the HVAC component, which is the core component for controlling the air conditioning and air distribution in the vehicle cabin. The focus here is on the one hand on mapping the thermal behavior of the HVAC to transiently determine the inlet and outlet temperatures of the cabin air on the simulation model.
On the other hand, the focus is on mapping the hydraulic behavior of the HVAC in order to obtain a statement about the air distribution in the individual ducts as a function of the individual flap positions in the HVAC and thus to be able to map the air distribution in the vehicle interior. In order to determine the information required for this, the HVAC module is measured in detail in test bench setups.
This task is performed by an experienced cooperation partner, IPETRONIK GmbH & Co. KG, with whom ARRK Engineering has been working closely on thermal management for 10 years. In addition to the HVAC module, the measurement of all other relevant components, such as those of the refrigeration circuit, can also be carried out by IPETRONIK GmbH.
In order to transfer the hardware measurements to the simulation, a detailed model of the vehicle cabin and occupant is also required, which is created using ARRK's proprietary simulation software THESEUS-FE. To represent the occupants, the tool includes a complex human model based on the Fiala occupant model. It imitates the human body functions relevant for air and heat exchange such as breathing, blood circulation, sweating as well as cold shivering and considers different clothing situations. In order to provide detailed boundary conditions on all parts of the body for thermal comfort statements with the human model, the tool enables a very fine and automated discretization of the total air volume into individual air zones with the newly developed "pseudo-3D approach". Furthermore, the simulation speed is enormously increased by the "pseudo-3D approach", which is essential for the highly dynamic transient simulations. Within the air zones, detailed statements can be made in each case about the air velocity, temperature and humidity as well as about the long-wave and short-wave radiation. Of course, in summer load cases, the position of the sun and the areas illuminated or shaded by it are automatically determined as a function of the vehicle and window geometry as well as the vehicle orientation. Since the air and surface temperatures influence each other, THESEUS-FE considers radiation, flow and heat conduction processes on a defined component as a coupled system.
Finally, the comfort index according to ISO 14505-2, which describes the thermal sensation in vehicle cabins on the basis of the equivalent temperature and is implemented in THESEUS-FE, represents the basis for the evaluation of the measured and modeled thermal comfort. With the help of this index, objective statements about the influence of different dynamic factors on the climate comfort in the vehicle interior shall be made. Finally, these should be available for the definition of general comfort and air conditioning targets - and consequently for the creation of universal specifications.
Development of a comfort use case for transient climate load cases
Once the individual models have been correlated, the resulting simulations will have to be compared with real driving data in the coming months and any sources of error and inaccuracies in the calculations identified and compensated for.
To this end, test drives have already been carried out with the ARRK dummy developed in-house. This is equipped with 31 sensors evenly distributed over the body for measuring air temperature and ‑humidity, long and short wave radiation and wind speed. A revised version of the dummy is currently under development. In the future, it will not only have around three times the number of sensors, but will also be able to record additional values such as contact heat flows.
The effects that interact during such journeys are caused on the one hand by the external climate, the course of the road intraurban or out of town, and changing direct and indirect solar radiation from different angles and directions. In addition to these external factors, which affect the occupants and the thermal comfort they perceive depending on their age, physical condition and body mass, the respective number of people in the passenger compartment also plays a role.
Once the simulations have been verified, the focus shifts to the final project: the numerous factors influencing the vehicle interior climate and thus the comfort value are to be evaluated and objectified in all their facets.
The future of thermal comfort assessment and ‑application
As soon as ARRK Engineering can simulate these dynamic journeys with sufficient precision and speed, the last major step in the development process for air conditioning and climate comfort will be tackled. After all, when all the pieces of the puzzle necessary for calculating the transient load cases can be reproduced virtually, this puts the specialists in the promising position of being able to derive concrete effects on the climate-related well-being of the occupants from these values. Accordingly, objectified action instructions for the thermal management system with integrated control of the HVAC system can follow.
Once these dynamic processes are recorded in a comfort specification, the actual application work in the test vehicle can be reduced to a minimum, namely only the fine-tuning on the final vehicle. In this way, the use of the virtual application, which can be used to address changed load cases and boundary conditions early, quickly and easily, will result in significantly reduced development time compared to manual application. This will fundamentally change the future approach to thermal comfort assessment and ‑application in automotive development.