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A Practical Guide to Aerodynamic Modification

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Updated August 15, 2023 Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many peo...
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EPA Fuel Economy as a Function of Vehicle Parameters

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Another semester is over and once again, I can’t stop thinking about systems design. Specifically, as I did last semester , I find myself gathering data and trying to apply analysis techniques from aerospace systems design to try and learn something about the physical limits of fuel efficiency in cars.   This time, I started with the EPA’s current list of battery electric (BEV), plug-in hybrid (PHEV), and hybrid cars (HEV)—a comprehensive accounting of fuel economy ratings for several hundred vehicles—to try and get an idea of what the current state of alternative-fuel cars looks like. You can find these at www.fueleconomy.gov . For this analysis, I limited data to MY2025 cars but these included all  2025 BEV, PHEV, and HEV with EPA ratings in the US today with a few exceptions (cars for which I was unable to find other information, such as the Ferrari SF90 and 296, which are plug-in hybrids but for which Ferrari does not publish data such as curb weight). I also excluded ...
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Investigating Separation

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Nearly a year ago, I was sitting in an incompressible flows class when the professor said something I had never considered before. Loss of lift over an airfoil after stall, she claimed, is caused by a pressure increase due to flow separation over the upper surface.   This was contrary to everything I thought I knew about separated and attached flow. Keep flow attached, the conventional wisdom goes, over a tapered shape for greatest pressure recovery and lowest drag (and “zero lift” on a ground vehicle, if you believe some people online, which you shouldn’t); if the flow separates, the pressure drops behind the separation point. Yet in this case, it must be true that separation increases pressure—otherwise, airfoils would not lose lift in stall.   Conventional wisdom, as is so often the case, does not tell us the whole story. The effect of separation on surface static pressures and, consequently, lift and drag can be complicated. To investigate, I decided to run some tests. Yo...
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Why Don't Cars Have Long Tails?

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The drag force cars create as they move through air is generated largely by a pressure imbalance. Cars are bluff bodies as opposed to streamlined; that is, airflow separates over them and, in modern cars, this separation usually occurs most prominently at the very back and creates a large wake (even fully streamlined shapes like wings have wakes, too, but they tend to be a lot smaller). This large wake results in negative gauge pressure acting on whatever body surface is exposed to it which, since it’s the back of the car, tends to be a large vertical surface (we call this the base )—so the low pressure ( base pressure ) results in a force acting almost directly backward.   One way to reduce the drag of cars is to make this wake area and its resulting pressure drag smaller by tapering the rear part of the car in a long tail; this allows the flow to recover pressure so long as it does not detach prematurely and reduces the vertical surface area exposed to low gauge pressure. Alt...
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Light Vehicle Efficiency Over Time

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In the first chapter of Aerodynamics of Road Vehicles you will find, regardless of edition, an excellent overview of the history of passenger vehicle aerodynamics written by Wolf-Heinrich Hucho. In it is this chart,  showing the general trend of drag coefficients over time : This is from the most recent edition, published 2016. This gives us an idea of progress in reducing drag coefficients (at least in European cars, but I imagine a chart of American or Asian cars would show the same), which you can see sometimes moves in fits and starts before plateauing for a while. Generally , the trend over time is that drag coefficients get smaller, and it is assumed that cars become more efficient as a result. But is this really what’s happened? Sure, cars are more efficient now—but how much more efficient? And is all of that attributable to a decrease in drag coefficients? I found myself wondering this over the last several weeks as I’ve played around with various models simulating roa...
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The Problem of EV Sizing: Weight, Battery Capacity, and Required Range

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Weight is a problem for electric vehicles: almost invariably, EVs are heavier than their ICE counterparts of similar size. For example, the Chevrolet Blazer EV weighs 5163 lb in its lightest trim, 1245 lb more than the lightest ICE Blazer. Weight increases force and power required at speed, hinders maneuverability and negatively impacts driving dynamics, increases particulate matter pollution ( from both tires and brake pads ), and reduces efficiency —not to mention requiring greater energy inputs to construct commensurate with the greater mass of material needed to build the heavier car in the first place. This Hyundai Ioniq 6 weighs 1000 lb more than the Sonata, despite the two having similar dimensions. The reason EVs are heavy is because they require large batteries to have acceptable range; however, while total range increases with battery size, efficiency (range per unit of energy) goes down as more weight is added by the increased battery size, which requires more energy per u...
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