Field
The present embodiment relates to an apparatus for the reduction of aerodynamic drag on vehicles having wind-exposed wheels of a wheel assembly mounted underneath the vehicle body, such as on large commercial trucks.
Description of Prior Art
Inherently characteristic of rotating vehicle wheels, and particularly of spoked wheels, aerodynamic resistance, or parasitic drag, is an unwanted source of energy loss in propelling a vehicle. Parasitic drag on a wheel includes viscous drag components of form (or pressure) drag and frictional drag. Form drag on a wheel generally arises from the circular profile of a wheel moving though air at the velocity of the vehicle. The displacement of air around a moving object creates a difference in pressure between the forward and trailing surfaces, resulting in a drag force that is highly dependent on the relative wind speed acting thereon. Streamlining the wheel surfaces can reduce the pressure differential, reducing form drag.
Frictional drag forces also depend on the speed of wind impinging exposed surfaces, and arise from the contact of air moving over surfaces. Both of these types of drag forces arise generally in proportion to the square of the relative wind speed, per the drag equation. Streamlined design profiles are generally employed to reduce both of these components of drag force.
The unique geometry of a wheel used on a vehicle includes motion both in translation and in rotation; the entire circular outline of the wheel translates at the vehicle speed, and the wheel rotates about the axle at a rate consistent with the vehicle speed. Form drag forces arising from the moving outline are apparent, as the translational motion of the wheel rim must displace air immediately in front of the wheel (and replace air immediately behind it). These form drag forces arising across the entire vertical profile of the wheel are therefore generally related to the velocity of the vehicle.
As the forward profile of a wheel facing the direction of vehicle motion is generally symmetric in shape, and as the circular outline of a wheel rim moves forward at the speed of the vehicle, these form drag forces are often considered uniformly distributed across the entire forward facing profile of a moving wheel (although streamlined cycle rims can affect this distribution somewhat). This uniform distribution of pressure force is generally considered centered on the forward vertical wheel profile, and thereby in direct opposition to the propulsive force applied at the axle, as illustrated in FIG. 17.
However, as will be shown, frictional drag forces are not uniformly distributed with elevation on the wheel, as they are not uniformly related to the speed of the moving outline of the wheel rim. Instead, frictional drag forces on the wheel surfaces are highly variable and depend on their elevation above the ground. Frictional drag must be considered separate from form drag forces, and can be more significant sources of overall drag on the wheel and, as will be shown, thereby on the vehicle.
Vehicles having wind-exposed wheels are particularly sensitive to external headwinds reducing propulsive efficiency. Drag force on exposed wheels increases more rapidly on upper wheel surfaces than on vehicle frame surfaces, causing a non-linear relation from rising wind speeds between net drag forces on vehicle frame surfaces versus net drag forces on vehicle wheel surfaces.
Since upper wheel surfaces are moving against the wind at more than the vehicle speed, the upper wheel drag forces contribute more and more of the total vehicle drag as external headwinds rise. Thus, as external headwinds rise, a greater fraction of the net vehicle drag is shifted from vehicle frame surfaces to upper wheel surfaces.
Moreover, upper wheel drag forces must be overcome by a propulsive counterforce applied at the axle. Such propulsive counterforces suffer a mechanical disadvantage against the upper wheel drag forces, since each net force is applied about the same pivot point located at the bottom where the wheel is in stationary contact with the ground. This mechanical advantage that upper wheel drag forces have over propulsive counterforces further augments the effective vehicle drag that exposed upper wheels contribute under rising headwinds. As a result of these magnified effects of upper wheel drag on resisting vehicle propulsion, vehicle drag is more effectively reduced by reducing the aerodynamic pressure on the upper wheel surfaces while leaving the lower wheel surfaces exposed to impinging headwinds.
Furthermore, shielding the lower wheel surfaces can cause a net increase in vehicle drag, and a loss in propulsive efficiency. Not only does the propulsive counterforce applied at the axle have a mechanical advantage over the lower wheel drag forces, but shielding the lower wheel surfaces using a deflector attached to the vehicle body shifts the drag force from being applied at the lower wheel to an effective higher elevation at the axle, thereby negating any mechanical advantage of a propulsive counterforce applied at the axle has over the lower wheel drag force. As a result, aerodynamic trailer skirts in widespread use today are unnecessarily inefficient, since they generally extend below the level of the axle.
Nevertheless, extended height trailer skirts have been shown to improve propulsive efficiency, since they reduce the aerodynamic pressure on the upper wheel surfaces, which cause the vast majority of wheel drag and virtually all of the loss in vehicle propulsive efficiency due to wheel drag. However, the extended skirts shown in the art also impact the aerodynamic pressure on the lower wheel surfaces, where propulsive counterforces delivered at the axle have a mechanical advantage over lower wheel drag forces.
As mentioned, diverting wind from impinging on the lower wheel surfaces actually increases overall vehicle drag, reducing propulsive efficiency. Deflecting wind from impinging on these lower wheel surfaces transfers the aerodynamic pressure from these slower moving surfaces also suffering a mechanical disadvantage, to faster moving vehicle body surfaces having no mechanical advantage over propulsive counterforces, thereby increasing vehicle drag.
Nevertheless, numerous examples in the art demonstrate the current preference for aerodynamic skirts extending to below the level of the axle. For example, in U.S. Pat. No. 7,942,471 B2, US 2006/0152038 A1, U.S. Pat. Nos. 6,974,178 B2, 8,303,025 B2, 7,497,502 B2, 8,322,778 B1, 7,806,464 B2, US 2010/0066123 A1, U.S. Pat. Nos. 8,342,595 B2, 8,251,436 B2, 6,644,720 B2, 5,280,990, 5,921,617, 4,262,953, 7,806,464 B2, US 2006/0252361 A1, U.S. Pat. No. 4,640,541 all make no mention of the differing relationships between upper wheel drag forces and lower wheel drag forces affecting vehicle propulsive efficiency. Most of these patents depict figures showing skirts extending well below the level of the axle. And an examination of leading trailer skirt manufacturers shows the prevalence for extended height skirts currently for sale and needed to meet California carbon emission requirements.
Furthermore, a recent in-depth wind tunnel study sponsored the US Department of Energy and conducted at a pre-eminent research institution of the United States government, Lawrence Livermore Laboratory was published Mar. 19, 2013, “Aerodynamic drag reduction of class 8 heavy vehicles: a full-scale wind tunnel study”, Ortega, et. al, and concluded that trailer skirts are one of the most effective means to reduce drag on large tractor-trailer trucks. A large number of trailer skirt configurations were tested in this study, which employed traditional techniques for measuring total drag on the vehicle. Due to the nonlinear effects of upper wheel drag in rising headwinds, such techniques can produce inaccurate measurements of gains in propulsive efficiency for vehicles having wheels exposed to headwinds. Thus, as yet this important relationship of upper wheel drag more predominately affecting overall vehicle drag—and especially over lower wheel drag which is often comparatively negligible and suffers a mechanical disadvantage against propulsive counterforces applied at the axle—has gone unrecognized.
And in the patent art cited above, several patents such as U.S. Pat. Nos. 4,262,953, 4,640,541, US 2006/0252361 A1, U.S. Pat. Nos. 7,806,464 B2, 8,322,778 and others depict wind-deflecting panels generally spanning the lateral width of the trailer, thereby inducing unnecessary drag by blocking air otherwise funneled between the wheels. Funneled air into the rear of the vehicle can reduce pressure drag on the vehicle. In the art, there are numerous other examples of devices attempting to enhance this vehicle drag reducing effect.
Also in the cited art above, several patents such as US 2010/0066123 A1, U.S. Pat. Nos. 8,342,595 B2 and 8,251,436 B2 depict wind deflecting panels where aligned in front of the wheels of the trailer extending to well below the level of the axle, thereby inducing unnecessary vehicle drag by transferring drag from the slower moving lower wheel surfaces having a mechanical disadvantage, to the faster moving vehicle body and frame surfaces. And in the art, there are numerous other examples of devices attempting to enhance this wheel drag reducing effect.
And in the art, several attempts have been made to reduce the pressure drag induced on the body of the vehicle. For example, the oscillating system in U.S. Pat. No. 9,487,250—intended to reduce pressure drag on the vehicle itself—introduces considerable complexity over more common fixed drag-reduction means, since it generally includes a moving diaphram that must be tuned for the specific operating configuration of the vehicle. And the oscillating mechanism is generally attached at the rear of the trailer, behind the rear wheels.
And the adjustable skirts in U.S. Pat. No. 9,440,689, as well as the skirts in U.S. Pat. No. 8,783,758, both being located rearward of the trailer wheel assembly, do not induce air to flow in-between the trailer wheels to yield a reduction in pressure drag on the vehicle. Instead, the aforementioned skirts prevent air flow from flowing laterally under the body of the vehicle. For example, as disposed the combination of the dual adjustable skirts of U.S. Pat. No. 9,440,689 directs air away from the ‘pocket’ of air formed immediately behind the trailer. And the skirts of U.S. Pat. No. 8,783,758 prevent air from flowing laterally inward under the rearmost portion of the trailer body.
And many trailer skirts in the art are generally disposed largely along the lateral sides of the trailer, and therefore do not induce air to flow generally in-between the wheel sets to thereby reduce pressure drag on the trailer body. Indeed, early configurations of trailer skirts were often disposed wholly along the outer lateral sides of the trailer body. However, more recent configurations include the forwardmost ends thereof being disposed slightly inset toward the longitudinal centerline of the vehicle body, since it has been found through testing that this outwardly slanted configuration further decreases overall vehicle drag.
As taught by prior inventions by the present applicant, one reason for this somewhat better performance is due to this outwardly slanted configuration providing improved shielding of the trailing wheels from impinging headwinds. And as discussed herein, in order to minimize vehicle drag, it is critically important to shield the uppermost portions of otherwise exposed wheels from headwinds while leaving lowermost wheel surfaces exposed to headwinds. The slanted skirts—extending laterally outwards toward the rear—generally partially shield the upper wheels, but also shield much of the lower wheels, thereby not optimally minimizing drag on the vehicle. And these outwardly slanted skirts also present a serious liability issue for trucks, since the outwardly directed air from the skirts can destabilize adjacent cyclists—especially bicycle riders—from passing trucks.
With the numerous embodiments for shielding open wheels of the vehicle—which include prior inventions by the present applicant in U.S. Pat. No. 9,567,016 as well as in U.S. Pat. No. 9,796,430—teaching the critical importance of specifically shielding the critical drag-inducing upper wheel using a minimal drag-inducing wheel fairing, only further reinforces in the art the preference by skilled artisans for even further deepening the outwardly slanting arrangement of conventional trailer skirts to provide even more effective shielding of the trailing wheels from headwinds. As such, skilled artisans have had no motivation to consider a contrary arrangement further exposing the rearward wheels to headwinds, since such a contrary arrangement would be known to substantially increase drag on the vehicle.
For example, in U.S. Pat. No. 9,809,260 air deflectors are used in some embodiments to direct air outward away from the undercarriage components—and thereby away from flowing in-between the wheel sets—in order to reduce drag on these components. As such, it has remained generally unappreciated in the art that any increased drag induced on these undercarriage components could be insufficient to offset the overall drag reduction gains achievable simply by instead redirecting substantial air flow in-between the trailer wheel sets to thereby substantially reduce pressure drag on the vehicle.
Other previous attempts to reduce pressure drag induced on the body of the vehicle employed an air capture system to redirect air from the front to the rear of the vehicle, often including air ducts. For example, in U.S. Pat. No. 9,527,534 air ducts are used to capture air impinging near the front of the vehicle and communicating the thus captured air to rear of the vehicle through these ducts. The air ducts are generally directed either over the top or underneath the vehicle, while also generally extending rearward of the trailer wheel assembly. And such, these lengthy air ducts have substantial surface areas, introducing considerable friction drag thereon—on surfaces thereof both within and without the duct itself—to thereby limit any reduction in overall vehicle drag gained from any reduction in pressure drag on the vehicle itself.
And in U.S. Pat. No. 9,403,563 much smaller air ducts were used on the rear of the trailer, which still introduce considerable friction drag for their relatively small size, especially when considering that the their smaller size severely limits the potential amount of redirected air, thereby further limiting their effectiveness in increasing the effective pressure developed in the relatively large volume of reduced pressure zone located immediately behind the trailer. Thus, these smaller air ducts redirecting smaller volumes of air also have limited potential to reduce the overall pressure drag on the vehicle.
For these multiple reasons, a different approach is needed to reduce pressure drag on the vehicle, by using a minimal drag-inducing air diverting means to substantially increase the effective air pressure developed immediately behind the vehicle.