The present disclosure relates to drag reduction devices for land vehicles, such as trucks, tractor-trailer rigs, vans, buses, recreational vehicles and similar vehicles having a large frontal area.
Fuel economy is a persistent concern for all land vehicles and is particularly acute for large vehicles such as trucks and tractor-trailer rigs. Fuel economy improvements have been achieved by innovation in engine design and improvements in fuel composition. However, the size and shape of the vehicles plays a substantial role in fuel economy. Ultimately, drag is the greatest enemy to fuel economy, with as much as 70% of the engine power devoted to cutting through the air in front of the vehicle.
Drag is a force that resists the movement of a body through a fluid, whether the body is a baseball and the fluid is air, or the body is a swimmer moving through water. Drag is a function of two components—friction drag and pressure drag. Friction drag is a force that acts tangential to a surface of the body. Friction drag is increased by irregularities or roughness on the surface and decreased by making the surface more slippery. A clean truck cuts through the air more efficiently and with less friction drag than a dirty truck.
Pressure drag is a force that acts perpendicular to a surface and is a function of the surface area in the direction of travel as well as the velocity or speed at which the body is traveling. Pressure drag increases as the square of velocity so that doubling vehicle speed actually creates four times more pressure drag. On the other hand, pressure drag is directly related to surface area so that a ten percent reduction in surface area leads to a ten percent decrease in pressure drag.
For aerodynamically configured vehicles, such as airplanes, friction drag contributes more heavily to overall drag than pressure drag. However, for land vehicles this relationship is reversed significantly. For a typical tractor-trailer, pressure drag can be as much as ten times greater than friction drag due to the large frontal surface area of the truck. Unfortunately, the size of these types of vehicles is dictated by their function—hauling products or materials. Unlike passenger vehicles, the box-like shape of trucks cannot be significantly altered. A smaller frontal surface area means a smaller truck, which means less cargo that can be hauled. Pressure drag in land vehicles, and especially in trucks, is increased by pressure “hot spots”, such beneath the undercarriage, behind the rear of the trailer or between the tractor and the trailer. These hot spots are generally regions of low pressure, which causes air flowing over the vehicle to deviate from a streamlined path around the vehicle. Vortices can form in these hot spots that significantly increase the pressure drag.
In quantitative terms, if a square body has a drag coefficient (CD) of 1.00, elongating the body into a rectangular shape reduces CD to 0.80. Adding a rounded nose cuts the coefficient in half to 0.40. Adding a “boat tail” or a conical tail decreases CD further to 0.20. The typical boattail configuration includes plates projecting from the rear of the vehicle and angled inwardly at an angle of 10-15°. An elliptical body tapered at both ends produces a drag coefficient less than 0.05, but the shape significantly reduces available cargo space and is difficult to produce.
It has been estimated that a 20% reduction in drag yields at least a 10% increase in fuel economy at highway speeds. For truckers and trucking companies, this increase in fuel economy means significantly reduced fuel costs year in and year out. For the environment, increases in fuel economy mean fewer deleterious emissions. A significant amount of effort has been expended in developing drag reduction technology for trucks. These efforts include streamlining the tractor, introducing seals, air deflectors or vortex generators in the gap between the tractor and trailer, and adding undercarriage skirts, guide vanes, air deflectors and boat tails to the trailer. Each of these modifications contributes in some measure to the overall drag reduction, so a fully optimized rig will incorporate a number of these improvements.
Presently, the typical drag reduction device utilizes “mechanical” redirection of the air flow to reduce drag. For instance, one type of device utilizes spoilers or fairings mounted to the top trailing edge of a vehicle or trailer to redirect the airflow and attempt to reduce turbulence. Another drag reduction device is the boat tail device that includes boat tail plates extending rearward from the rear of the trailer. Boat tail devices can reduce drag by up to ten percent. The typical boat tail is a large shell that is mounted over the rear doors of the trailer. Such devices are cumbersome to install and remove. Moreover, the large unitary shell is difficult to store when access to the rear doors is desired, such as to unload the trailer.
Other “mechanical” drag reduction devices require intervention by the vehicle operator to deploy and stow the device, while still other mechanical approaches require the vehicle operator to manipulate the drag reduction device to access the rear doors of the vehicle or trailer. Some mechanical devices are mounted directly to the vehicle/trailer doors which can eliminate the need to manipulate the device in order to access the trailer door. However, these devices are bulky and prevent the trailer/vehicle door from being fully opened flat against the side wall of the trailer/vehicle. This discrepancy can pose significant risks at shipping/receiving locations where trucks are backed into closely spaced loading docks. A door that projects too far away from the side of the trailer can be struck by an adjacent vehicle as it attempts to back into an adjacent loading dock.
A more recent development in drag reduction are non-mechanical devices, meaning devices that do not mechanically turn or redirect the air flow, like a spoiler, but instead generate a force that acts on the air flow. One such device is a plasma flow actuator that provides a “body force” to the air flow as it passes over a surface to accelerate the air, thereby stabilizing the boundary layer and causing the air to remain “attached” or conform to the surface to reduce or eliminate flow separation, turbulence and, ultimately, drag. One type of plasma flow actuator is electronically controlled for activation at appropriate vehicle speeds and for precise operation to optimize the drag reducing effect.
Whether the actuator is mechanical or non-mechanical, there is a persistent need for a drag reduction device that does not impede the ability to open the rear doors of a trailer or truck on which the device is mounted. Moreover, in order to ensure that the drag reduction device is consistently used, the device should be “automatic”, meaning that it does not require any intervention by the vehicle operator to deploy or to position the device clear of the vehicle/trailer doors when access is desired for loading or unloading the vehicle. There is also a need for a drag reduction device that has a lower profile than existing devices—i.e., that does not extend excessively beyond the end of the vehicle—to lessen the chance for damaging contact when the vehicle is moved in reverse.