Test The Limits

The importance of real-world simulation

By Exa

May 19 2016

Tesla has stated that with the new Model 3 electric car, it wants to reduce the drag coefficient to an incredibly slippery 0.21 CD (210 counts), down from 0.24 on the Model S.


But the maker also wants to ensure that’s these figures are representative of the real-world, not just the wind tunnel, so that its customers can achieve the car’s stated range. If they can’t, it’s a black mark against the wider acceptance of electric cars in general.


There’s a great incentive for manufacturers of internal combustion engine cars to achieve good emissions results in the laboratory. Around the world, they and their customers are rewarded with tax advantages for focusing on cars with a low carbon dioxide emission rating.


The downside of designing cars purely to do well in these lab-based emissions tests is that customers find they aren’t achieving the stated fuel economy figures, as we all know too well.


This is contributing to the general dissatisfaction with traditional car makers. So much so that they’re now having to deflect this negative publicity. PSA Peugeot Citroen, for example, is publishing real-world fuel economy figures for its European cars showing a 30-40 percent penalty in everyday driving compared to lab data.


This might be tolerable for buyers of IC vehicles since filling up with gas is so easy, but EV buyers need a reliable indication of when they need to charge again. To achieve that, Tesla has worked with Exa to simulate real-world air behavior on digital prototype models, aiming to incorporate the effects of wind, traffic turbulence, and other effects out on the road.


These effects can seriously compromise an otherwise sound aero design. For example, Exa demonstrated, with a premium manufacturer, how the real-world effects on a front air dam, designed to deflect air around the front wheels, may impact a device’s performance differently that wind tunnel data. When the air was coming from straight ahead, it improved aerodynamic drag by 16 counts, approximately 5% of the total vehicle drag. With real-world conditions taken into account, the improvement dropped to just 8 counts.


These advanced aero simulator tools have proven most popular with manufacturers whose customers are the most demanding when it comes to achieving real-world economy or range figures. Trucking firms, for example, care far more about the bottom line than claimed figures.


EV makers are another. Tesla reckoned that moving from 0.30 CD to 0.24 for the Model S earned it another 50 miles per charge. Improving to 0.21 CD for the Model 3 could unlock another 10-20 miles.


Traditional car makers are facing the harsh glare of negative publicity over questionable lab figures. It would help their case enormously if legislators around the world changed the official fuel economy tests to encourage these companies to give buyers figures that are based in reality.