The Relationship Between Drag and Fuel Economy
|Lined up and ready to head out on track at the 2022 Green Grand Prix, an annual fuel economy competition held the Friday of opening weekend at Watkins Glen International.|
We all know that reducing drag can improve fuel economy. It’s intuitive but can be proven mathematically: when your car has less force acting against it, it takes less fuel (gas, diesel, or electric) to move. But what is the exact relationship between aero drag and economy? Can we measure it? Or predict it? Figuring this out can help us plan modifications to our cars with the aim of improving economy, as well as tell us if fuel economy is an accurate measure of changes in drag.
|How unrealistic? This unrealistic. There hasn’t been a car this light on the American market in decades.|
He suggests that if “average aerodynamic drag coefficient
could be reduced by 2% say, the engine power requirement would be reduced by
about 0.5%,” and writes that engine power is “directly proportional” to fuel
consumption. In reality this isn’t quite true, as power per fuel consumption
can vary quite widely between engines and between operating conditions of the
same engine. You can see at what load and RPM a gas engine is most efficient by
looking at its brake-specific fuel consumption, or BSFC, map.
|BSFC maps for the 2nd (left) and 3rd (right) generation Toyota Prius engines (image credit: Ramli, W.R.B.W. et al, "Organic Rankine Cycle Waste Heat Recovery for Passenger Hybrid Electric Vehicles," in Energies 13(17): 4532).|
More recently, RH Barnard wrote about the relationship between aerodynamic drag and fuel consumption in Road Vehicle Aerodynamic Design (2009). Using the example of a mid-size European sedan with a 1.5L engine and CD = 0.35, he points out that any reduction in fuel consumption will be proportional to the change in total drag force at a steady speed, not the change in aero drag coefficient. (This is an important distinction, as many online commenters try to use the 10%:5% rule with CD rather than FD). Reduce the car’s drag coefficient to 0.25, a 28% reduction, and you change the total drag force—which includes aerodynamic and mechanical drag, remember—by only 22% at 75 mph, which decreases fuel consumption by the same percentage. Also keep in mind that fuel consumption is not measured in MPG but its inverse, volume per distance (usually L/100km, as most of the world outside the USA uses).
|69.0 mpg at the 2022 Green Grand Prix, up from 59.2 mpg in 2019. That suggests that the (measured) changes I made to this car in between are working, but it’s not possible to say just how much (and how much was weather, wind, and my driving).|