More and more trailers are adding aerodynamic devices leading to more and more questions concerning aerodynamics and fuel economy.
So what do these devices actually do?
In order to answer that question, it’s important to understand a bit about aerodynamics. In its simplest form, it’s the science of controlling air flow. Air has mass and when air pushes anywhere on the truck it creates forces that have to be overcome. To overcome this, it requires power which comes from fuel. Obviously, the more force, the more power and fuel it requires to overcome.
These aerodynamic devices help manage airflow by reducing drag. They decrease pressure on forward facing surfaces and increase the pressure in the rear. Pushing harder on the rear doors counteracts the air pressure pushing on the front of the truck and trailer.
These aerodynamic devices usually fall into two categories, fairings and flow control devices. Fairings are surfaces that cover and shield non-aerodynamic shapes like suspensions or even the front of a trailer. Flow control devices are surfaces that modify air flow passing over vehicles. The skirts you see on an increasing number of trailers are fairings that stop air traveling under the truck and pushing on the bogie system.
Do these types of aerodynamic devices really work?
How well these devices work varies depending on the designs and locations of these devices. There are eight zones on a combination vehicle where drag is an issue. They are the front of the truck, the area behind the truck, under the truck, the front of the trailer, the area behind the trailer, under the trailer, the wheels and any surfaces that are not “smooth.”
For trailers, skirts can be effective if they extend to the trailer wheels. Even a plate that runs across the top rear of the trailer to deflect air over the rain gutter can increase mpg up to 2%. Vortex generators can add 1-2% to mpg depending on design and placement. Additionally, wheel covers can minimize turbulence at the deep wells of the wheels, improving fuel economy 1-2%.
What about cross winds?
Cross winds enter gaps between aerodynamic devices and the vehicles themselves. The greater the angle and wind speed, the less efficient most devices will be. And as gap increases, so do the negative effects of cross winds.
Some devices might work better alongside other installed devices. For example, some boattails may be more effective if the trailer is equipped with vortex generators at the rear. Benefits of bogie fairings can diminish faster than skirts as cross winds increase.
What’s important to remember is that aerodynamic devices are tested under ideal weather and road conditions. Obviously, if conditions vary, so will results. When choosing a device or combination of devices, operators should determine what test conditions were and ask the vendor how the device would operate in their world.