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Exa's simulation can help optimize placement of exterior sensors on autonomous vehicles; this example is of a "baseline" rear-diffuser setup. (Exa).

Simulation rules: scrubbing heat, sound and dirt from autonomous EVs

Engineering simulation software company Exa, now part of French Dassault Systèmes, has some advice for non-automotive companies that may be considering venturing into electric vehicle (EV) and autonomous EV R&D—and eventually production:

  • Don’t build prototypes: expensive and lacking insight, physical prototype testing delivers the ‘what’ but not the ‘why,' and often too late to make significant product changes.
  • Make aerodynamics a priority: for every 10% reduction in drag, range can be increased by 5%. Designing an EV from scratch provides flexibility to optimize aerodynamic efficiency from day one.
  • Make it look good: give designers time and tools to deliver a beautiful product within a given timeframe.
  • Feel the heat: thermal is a pass or fail scenario for new car development—test early how new battery technology reacts to extreme temperatures.
  • Check the noise: EVs don’t have an engine to ‘mask’ other noises (tire, road and wind). Identify elements of the exterior which impact cabin noise and fix them before design is finalized.

This early warning list may look daunting to wannabe car makers, but for simulation specialists it signals a burgeoning appetite for information that they will be expected to satisfy.

Dassault  Systèmes, also a simulation-software specialist for product engineering, states that following its $400-million acquisition of  Exa Corp. in November, 2017, its 3DEXPERIENCE platform now delivers Lattice Boltzmann fluid simulation technologies plus Exa’s fully industrialized solutions such as PowerFLOW. These can be applied to help solve challenging multiphysics and multiscale simulation fluids problems across auto-industry disciplines. These include aerodynamics, aeroacoustics and thermal management—all very much in the minds of EV developers.

At Dassault Systèmes, Dr. Brad Duncan, Exa’s Director of Aerodynamic Applications, has a worldwide remit working closely with OEMs. He spoke with Automotive Engineering in detail about the importance of finely-targeted simulation work in a rapidly changing automotive landscape:

“Electrification and autonomy are two of the most important emerging technologies leading the automotive industry revolution,” he said.

Challenges associated with these technologies for vehicle manufacturers include fresh competition in the form of new entrants with less experience who may be nimbler and be acutely "technology-aware." There are also new regulations—manufacturer claims about their products must be matched by real-world performance or face fines and/or consumer backlash. Consumer demands increase with new technology, including vehicle range, options and aesthetics, while new processes must reduce prototypes yet meet targets, increase process efficiency and integrate new platforms.

Simulaton for EV improvement

Duncan underlined that digital simulation is playing an important role in the ongoing improvement of EV technology, helping engineers discover new ways to improve efficiency, save energy, and boost range: “Aerodynamic, thermal, and system simulations can provide accurate results to measure performance at the earliest possible design stage and throughout the design process. Opposed to prototypes, simulation can be used to test many more options and to front-load that testing earlier in the design process. Simulation helps you determine cause and effect because you have a lot more data and insight. You can look at multiple attributes and optimize for those, and you can do the entire process in a more real-world, realistic environment.”

As an example, he said Exa demonstrated, using simulations, how to tailor the design of ICEs for electric derivatives. Simply adding a smooth EV underbody and new fascia resulted in an approximate drag reduction of ΔCD = 0.025, while improving other aspects of the vehicle design could lead to a reduction of ΔCD =  0.070.

He explained: “Smooth, high-speed flow is a prominent feature of an EV underbody and determines the design cues at both the front and rear of the vehicle. But it also makes other design elements more challenging, such as how airflow interacts with the wheels and tires. EV underbody design is a constant area of investigation with our customers and simulation allows them to experiment with sculpted shapes and aero devices which can help manage tire wakes more effectively and prevent the high-speed flow from simply slamming into components around the wheels!”

Changes to wheel wakes will also have a big impact on soiling, both at the rear and front of a vehicle and he emphasized a particular emerging aspect: “Simulation powers advancements in autonomous driving by evaluating shape changes to keep externally-mounted cameras and sensors clear of the dirt, dust, and water that could obstruct their views.

“The latest version of Exa PowerFLOW introduces new tracking features to visualize the interactions between particles and airflow. This helps to identify upstream sources of vehicle soiling – especially in places critical to cameras and sensors.”

Exa is working with manufacturers to help them position sensors in places where they are less prone to soiling as the most effective way to ensure autonomous systems remain fully operational. “We believe that regulatory control over sensor design, placement and soiling management is something that will happen in the future,” added Duncan.

Autonomous technologies and ADAS (advanced driver assistance systems) require many cameras and sensors to operate. Whether they are substitute cameras for rear visibility, or sensors for autonomous emergency braking and lane-keeping, the key to their unhindered operation is cleanliness.

Cameras and lidar sensors certainly must to be clean to operate correctly. In normal driving, autonomous technology encounters dirt, salt, water, snow, tire debris and other contaminants. When these deposit on cameras or other sensors, their performance is hindered. To prevent this, they must either be positioned in locations which are shielded from contamination or include some form of self-cleaning system.

Optimal sensor placement

Duncan said the goal is to create vehicles where sensors are placed optimally for minimal dirt deposition. Sensors must not be placed where they are exposed to large droplets of water: “Exa PowerFLOW has the capability to simulate the lifecycle of raindrops as they approach the vehicle, splash on its surface, are entrained into the wake, and become deposited on the vehicle’s surfaces. This capability liberates carmakers to position sensors early in the design process to avoid costly cleaning devices.

Acoustics can be a difficult area for EVs/hybrids as sound switches from ICE to electric drive sources and other noise impinges on cabin ambience. Run-flat tires, particularly after reaching about a third of their mileage life, may become noisier.

Exa acoustics’ specialist Dr. Sivapalan Senthooran dealt with a popular misconception: “It is that EVs are quieter than ICE cars due to the absence of combustion-powertrain noise, but this is not strictly true. At normal cruising speeds where wind noise and tire noise become dominant, EVs are subject to as much interior noise pollution as ICE vehicles. Management of battery and electronics temperatures in EVs can actually introduce additional noise sources from the extra cooling fans required for safe operation.”

When excessive noise is discovered on a physical prototype, he said the quickest solution was to literally cover up the problem, typically via adding heavy wheel-arch and carpet insulation, laminated or thicker glass and additional door trim and headliner insulation, which may bring extra manufacturing costs but, even more importantly, add anything up to 50 kg to a vehicle’s weight, which (with the notable exception of batteries) cuts range.

“Using simulation-driven design, based on a form of CFD, car makers can identify and optimize elements of exterior design which impact on cabin noise, eliminating them as noise sources early in the design process,” added Senthooran.

Most end users would regard thermal management to pertain essentially to HVAC and engine coolant, but it plays a significant part in EV development. Asked what improvements have been made in the past six years, Duncan explained what can be expected in the near- to medium term, particularly regarding vehicle use in low/ultra-low temperatures:

“Lithium-ion batteries don’t like to be too cold and they don’t like to be too hot. Achieving this equilibrium is a challenge, but ensuring batteries are always operating at the right temperature is aided by simulation.

Battery operational temperature limits are quite different from the ambient air temperature or temperatures of other components. Hybrid and electric vehicles require large amounts of systems’ packaging, making cabins smaller and reducing cargo areas. Batteries may have to be positioned in very confined spaces with little room for cooling systems.”

He added that batteries are a rapidly evolving technology, with each generation rapidly improving in benefits, but that the effort to engineer competitive packs is increasing.

And he warned that the engineering knowledge base for traditional battery cooling no longer applies, with simulation now widely regarded as a key aspect of working with modern battery technology, facilitating evaluation of thermal performance at cell, module, pack and full-vehicle levels.

Simulation can predict the two-dimensional temperature distribution on the cell as a function of battery current, giving a more accurate prediction of heat rejection. Duncan stressed: “At the battery module and pack level, this detailed simulation of cells helps to accurately show how well the cooling works, so the entire system can be optimized for cooling efficiency, minimal peak temperatures, and minimal temperature gradients. Such analysis can extend to the entire vehicle, allowing evaluation of its thermal interactions with other components on the vehicle and with electronic systems that control battery operation.”

But he warned that autonomous vehicles bring additional thermal challenges. Sensors placed in parts of the car exposed to the sun, for example, need to be properly cooled to ensure reliable operation: “Today, the sensor and control electronics for autonomy require kilowatts of power to process the sensor information and make the decisions required to safely drive the vehicle. These electronics will need to dissipate this power in an economic and unobtrusive manner. This is a new thermal challenge that will need to be managed.”

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