Contributed by Bob Chabot
“Suite” Success: Key-Off and Soak
A systems approach optimizes thermal management
One of the biggest concerns automotive thermal engineers face during design is how to manage thermal transients after the vehicle has stopped running. Typically referred to as “key-off and soak,” during this transitory thermal state, the underhood surface temperatures usually increase since there is little or no cooling airflow, unlike while the vehicle is operating.
The risks of rising surface temperatures range include damage to underhood components, premature part failures, and even fires. MOTOR met with Exa Corp. to learn about the issues automotive engineers face every day attempting to accurately predict and manage thermal conditions during thermal transients.
According to Exa Corp., increasingly stringent federal regulations, part costs and cumbersome late-stage physical testing make it difficult for thermal engineers to find the right balance for thermal management. For example, the CO2 and fuel economy charts above demonstrate how fast vehicles are changing due to regulations. (Image— U.S. EPA)
Between a Rock and a Hard Place
“There are a number of key factors driving the automotive industry that affect thermal engineers,” shared Dr. Hudong Chen, Exa Corporation's Chief Scientific Officer. “To mitigate the impact of these high temperatures, thermal engineers must first identify the peak underhood temperatures. There’s no simple solution — such as relocating heat-sensitive components — due to competing demands, such as vehicle safety, packaging, aesthetic design and performance. In addition, traditional testing methods limit underhood thermal design optimization, as it is time-intensive, very expensive and often performed too late in the design process to have a meaningful impact.”
Chen cited three drivers of note:
- Rising fuel economy/CO2 emissions regulations — Rapidly increasing fuel economy and CO2 requirements are driving improvements in aerodynamics that pressure thermal engineers to eliminate any excess cooling airflow into the engine compartment as well as reduce the size of the engine compartment itself.
- Vehicle lightweighting — In addition to aerodynamics, fuel economy improvements are driving the industry to reduce the weight of the vehicle. Lightweighting is driving the use of thinner materials and plastics that are less thermal resistant than the materials they are replacing.
- Increasing vehicle particulate emissions standards — Particulate tailpipe emissions impact air quality and cause related health risks. Government regulatory agencies continue to lower the amount of allowable vehicle particulate emissions. This is driving vehicle manufacturers to incorporate additional after-combustion exhaust treatment devices such as close-coupled catalytic converters and particulate filters that run at extremely high temperatures.
“Key-off and soak challenges are directly related to these key drivers, and engineers must evaluate them early in the design process,” he explained. “Cumbersome late-stage physical testing makes it exceedingly tough for thermal engineers to find the right balance between fuel economy and CO2 requirements, part costs and particulate emissions standards during vehicle design — all while managing the thermal performance of the vehicle. Accurate measurement, let alone prediction, of heat transfer and fluid flow behaviors is impossible to achieve with this methodology.”
In the traditional design process, thermal engineers use prototype testing in climatic wind tunnels to measure vehicle underhood and underbody temperatures. While this prototype testing provides some component temperature data, it is insufficient to fully understand, let alone predict, heat transfer and fluid flow conditions. Due to these physical test limitations, engineers lack the insights they need to improve the design to meet the temperature requirements. (Image — Exa Corp.)
Tradition Test Methods are Inadequate
“Vehicle thermal design traditionally depends heavily on prototype testing in thermal wind tunnels or on-road testing with hundreds of thermocouples, both late in the manufacturing process," added Dr. Ales Alajbegovic, Exa’s Vice President for Ground Transportation Applications. “The testing process is very expensive, time consuming, and inflexible.”
“Testing involves thermocouple instrumentation, which requires test engineers to estimate a-priori where thermal problems might occur,” he noted. “But the highly turbulent nature of underbody flows makes this very difficult or impossible to predict. For example, relying on redesign and retesting is an expensive hit-or-miss process that often fails to identify the highest temperature locations. In addition, the inherently transient nature of turbulent flow is almost impossible to measure or visualize in wind tunnel testing.”
Key-off and soak conditions challenges faced by thermal engineers include:
- Limitation of physical testing — Thermal testing on a physical prototype in a wind tunnel is difficult; it’s also often performed far too late in the process to make a design impact because it requires a physical prototype. Physical testing also requires time-intensive preparation, calibration and proper shielding of thermocouples from radiation when running the tests. Measurements can be inaccurate and range at least ±10°C from the actual underhood part temperatures. Additionally, engineers cannot visualize flow or heat transfer during key-off and soak using a wind tunnel, making it hard to understand what is happening.
- Late-stage design failures — Physical prototypes are built late in the development cycle, when design changes are difficult to implement and deploying these modifications is expensive (or even impossible) and time-consuming. Most solutions available in the late development stage add significant cost and (sometimes) weight to the vehicle as thermal engineers are forced to add heat shields or switch components to higher temperature-rated materials.
- Warranty issues — High temperatures during key-off and soak increase the risk that vehicle components degrade or fail faster, causing automakers to spend more on warranty and recall expenses. For instance, in 2014, Edmunds.com reported that roughly 1 million vehicles were recalled in the United States due to engine and cooling problems, which represents only a small fraction of the worldwide total. These warranty or recall problems damage an automaker’s bottom line and lower its J.D. Power rating.
PowerFLOW simulations enable thermal engineers to accurately predict a vehicle’s underhood temperature during key-off and soak early in the design cycle, when design changes are more easily accommodated and long before physical prototypes are built, according to Exa. These simulations, coupled with the ability to visualize the flow and component temperatures, help thermal engineers gain insight on how to improve the vehicle design and propose and analyze new designs cheaper and faster. The image above displays the hot air rising in the engine compartment after the vehicle is parked. (Image — Exa Corp.)
Simulation and Visualization Provide the Solution
“Complex heat transfer and fluid flow behaviors must be understood in order to optimally locate and protect components,” Exa CEO Stephen Remondi advised. “Temperature is a function of the complex interaction between conduction, radiation, and convection in the surrounding fluid, especially for very hot components. Accurately predicting this is extremely challenging.”
“Given that there is increasing pressure from the marketplace to speed up and improve the vehicle development process, it is clear that a more effective method is required to address thermal protection early in the vehicle design process. A high-fidelity simulation is necessary to capture the relevant physics in order to solve thermal protection problems.”
"At Exa, thermal engineers use proprietary PowerFLOW and PowerTHERM simulations to accurately predict real world flow problems and find out exactly how underhood components will perform before physical prototypes are built,” explained James Hoch, Exa Vice President of Software Development. "Unlike traditional physical tests, these simulations provide the accuracy and visual insights that thermal engineers can use to learn about key-off and soak earlier in the design cycle. These insights empower thermal engineers to quickly implement data-driven design changes to verify key-off and soak performance improvements with confidence."
“The unique combination of PowerFLOW’s transient flow and heat transfer capabilities with PowerTHERM’s capabilities to simulate heat conduction in solids and radiation effects, allows thermal engineers to streamline the vehicle design process,” Hoch elaborated. “Together, they enable thermal engineers to first accurately predict the peak underhood temperatures during key-off and soak, and then visualize the flow and temperature fields for the entire vehicle. Because thermal engineers are able to better understand flow and thermal conditions that cause problems related to key-off and soak, they can take the necessary steps to reduce these issues without significant investments in costly, time-intensive physical prototypes."
“Post simulation, Exa PowerVIZ application provides thermal engineers with fast, interactive visualizations of simulation data sets and offers them the ability to easily combine different techniques, all within the same scene,” Chen shared. Engineers can move slices, point probes and streamline rakes and particle sources throughout the fluid domain to observe how flow patterns evolve and change over extended periods of time. This helps engineers “see” their simulation data to determine how to thermally optimize their vehicles throughout the design process. This ability to visualize the flow and heat transfer also facilitates cross-team communication and collaboration.”
In the video above,
Dr. Andrea Shestopolov, Exa's Director of Optimization, discusses how Exa's optimization helps automotive OEMs improve designs and meet performance targets by using simulation to help determine the best compromise for the various design requirements. (Video — Exa Corp.)
A Systems Approach Optimizes Thermal Issues
“The Exa product suite — PowerFLOW, PowerTHERM and PowerVIZ — allows thermal engineers to better understand exactly what is happening,” Dr. Andrea Shestopolov, Director of Optimization at Exa. “The suite provides thermal engineers the ability to conduct fully detailed simulations that they can use to identify problems and assess various potential solutions of thermal issues during key-off and soak.”
Shestopolov described the major benefits that simulation and visualization offered, when compared to traditional testing:
- Simulations enable engineers to calculate fluid flows and analyze heat transfers to confidently predict thermal behavior relating to key-off and soak.
- Thermal engineers can then determine early in the design process whether heat shields, insulating materials or better component placement are viable options to safeguard sensitive components against overheating.
- By understanding airflow and temperatures during thermal transients much earlier in the design process, late-stage vehicle expensive design problems are avoided.
- Automakers also benefit from reduced warranty costs and recall expenses associated with key-off and soak that would damage both revenues and reputations.
“The Exa product suite offers thermal engineers the ability to evaluate key-off and soak in an accurate, streamlined manner that is impossible to duplicate with physical testing,” she continued. “The suite gives thermal engineers the ability to collect up front knowledge and make design decisions based on actionable, accurate insights — empowering them to produce better vehicle designs. In today’s marketplace, to meet the multiple goals of an automaker, we can no longer change one or two attributes at a time,” she stressed. “Simulation allows us to examine all of the attributes simultaneously, saving time, money and more.”
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