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
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.
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.
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.
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.