Abstract:
Systems, methods and media for reducing the aerodynamic drag of vehicles are disclosed. More particularly, a control system for a pressure drag reduction system for a vehicle is disclosed. Embodiments may include a machine-accessible medium containing instructions to receive input relating to a vehicle and to determine an improved pressure drag reduction system configuration based on the received input. Received input may include pressures, vehicle speed, user inputs, or other information. Embodiments may also include instructions to change the configuration of the pressure drag reduction system based on the determined improved configuration. In some embodiments, the operation of fans or ducts of the pressure drag reduction system may be controlled.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     Pursuant to 35 USC § 121, this divisional application claims priority to and benefit of U.S. Utility patent application Ser. No. 10/883,389, filed Jul. 1, 2004, on behalf of inventor Khosrow Shahbazi, entitled “SYSTEMS, METHODS, AND MEDIA FOR REDUCING THE AERODYNAMIC DRAG OF VEHICLES.” 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is in the field of vehicles and, in particular, a method and apparatus for reducing the aerodynamic drag and/or improving the fuel efficiency of a vehicle. More particularly, the present invention relates to a control system for a drag reduction system to reduce the aerodynamic drag of a vehicle in motion by reducing the pressure drag associated with the vehicle.  
       BACKGROUND  
       [0003]     A significant portion of the operational costs of a modern motor vehicle arises from the consumption of fuel. Motor vehicles with internal combustion engines typically burn fossil fuels such as gasoline. Gasoline and other fossil fuels are relatively inexpensive at the present time in the United States as compared to the prices in other countries, but the prices are subject to fluctuations and the potential of future increases in the event of political instability in oil-producing regions, increased taxes by governmental entities, or shortages of oil. Fuel can be very expensive for commuters, vacationers, cross-country truckers, or anyone driving a fossil fuel-powered vehicle. Increased fuel prices can also have a significant detrimental impact on the economy, as they can deter travel, increase the cost of transportation or manufacture of goods, etc. The relative usage of fuel by a motor vehicle is known as fuel efficiency, and is also known as fuel economy or gas mileage.  
         [0004]     Motor vehicles, such as automobiles, sports utility vehicles (SUVs), vans, and trucks, require a propulsive force in order to move the vehicle. This propulsive force must overcome the drag of the vehicle in order for the vehicle to move forward. Drag on a vehicle originates from a number of sources, including aerodynamic drag and tire drag (resulting from frictional forces caused by the tire-road interaction). Propulsive force is typically generated by some sort of engine, such as internal combustion engine, fuel cell, electrical engine, etc.  
         [0005]     Aerodynamic drag includes both frictional drag and pressure drag. Frictional drag derives from friction between the fluid (air) and the surfaces of the vehicle over which it is flowing. The velocity of the air moving over the surfaces of the vehicle is known as the free stream velocity. The free stream velocity is based on the speed of the vehicle and the prevailing wind. For example, if a vehicle had a speed of 60 miles per hour (“MPH”) and was heading directly into a 15 MPH headwind, the free stream velocity would be 75 MPH. Because of the air moving over the surfaces of the vehicle, a boundary layer is typically formed along the surface of the vehicle, serving as a transition between air at zero velocity right at the surface and air at the free stream velocity at the edge of the boundary layer. The boundary layer may contain both laminar (smooth) flow and turbulent flow. Often, near the rear of a vehicle, the flow “separates”, meaning that the boundary layer separates from the surface, resulting in eddies and fully turbulent flow. The drag caused by the air in the boundary layer creates frictional drag.  
         [0006]     Pressure drag (also known as form drag) results from the difference in pressure between the front of the vehicle and the rear of the vehicle. Accordingly, it depends on the size and shape of the vehicle. When the airflow separates on the vehicle, as described above, lower pressures are created behind the vehicle than would exist in the absence of separation, resulting in increased pressure drag. This problem is exacerbated on less aerodynamic vehicles, such as SUV&#39;s and trucks, as their relatively blocky shapes cause additional separation and thus additional pressure drag. The pressure behind a vehicle may drop low enough, particularly when moving at high speeds, to create vacuum-like conditions behind the vehicle, resulting in very high pressure drag.  
         [0007]     Aerodynamic drag is directly proportional to the coefficient of drag, frontal area of the motor vehicle, and the square of the velocity of the motor vehicle. The coefficient of drag, also known as the drag coefficient, (Cd), is a number that describes the characteristic amount of aerodynamic drag caused by fluid flow for a particular shape, such as a vehicle. The Cd includes the effects of drag caused by pressure drag, frictional drag, and induced drag (drag caused by positive or negative lift). Lower Cd&#39;s result in lower drag and thus improved fuel efficiency. A typical modern automobile has a Cd of between 0.30 and 0.35. SUVs and other larger, boxier automobiles, have Cd&#39;s of about 0.35 to 0.45. Tractor-trailer combinations can have Cd&#39;s of 0.6-0.9. For modern motor vehicles, the majority of the Cd is now based on pressure drag, as both frictional drag and induced drag have been greatly reduced by utilizing various technical advances. Reduction of pressure drag can therefore have a significant impact on the Cd and, thus, the fuel efficiency of a vehicle.  
         [0008]     Vehicle pressure drag results from separation of the airflow around the vehicle and the resultant wake or separation bubble formed on the backside of the vehicle. Typically, when an airflow moves around the top, bottom, or side of a vehicle, the airflow will transition from a attached flow (often smooth and laminar) to a separated flow when boundary layer separation occurs. A vehicle with an optimized rear end, such as a sports car with a tapered rear, will allow for the boundary layer to stay attached for as long as possible, minimizing the separated flow behind the vehicle (and reducing pressure drag). A vehicle such as an SUV with a squarish design and flat rear end will have significantly increased separated flow behind it, as the boundary layers and thus airflows will separate near the rear of the vehicle, causing a larger region of low pressure. Depending on the particular design and conditions (e.g., vehicle speed, Cd, etc.), this lower pressure region can be close to a vacuum and extend a relatively large distance behind the vehicle.  
         [0009]     In response to pressure from consumers and mandates from governmental entities (such as the Corporate Average Fuel Economy, or CAFE, standards in the United States), motor vehicle manufacturers have attempted to reduce the Cd and improve the fuel efficiency of their vehicles through a variety of technical solutions. While manufacturers have been somewhat successful in improving fuel efficiency, further reductions utilizing these technical solutions are likely to have significant aesthetic, cost, or performance disadvantages. For example, vehicle manufacturers have created aerodynamic shapes that have reduced drag significantly. Reducing the drag substantially further, however, will result in reduction of useful interior space, particularly in the rear of the vehicle. This downside is even more significant in trucks, vans, or SUVs where cargo capacity is a major selling point.  
         [0010]     As another example, reduction of vehicle weight does reduce the tire drag of a vehicle (and thus improves the efficiency), but vehicles have been getting heavier over time as a result of increased safety functionality and other features. Reductions in weight would require elimination of features or more expensive, lighter materials, neither of which is desirable to either manufacturers or consumers.  
       SUMMARY OF THE INVENTION  
       [0011]     The problems identified above are in large part addressed by a method and apparatus for the aerodynamic drag and/or improving the fuel efficiency of a vehicle by reducing the pressure drag created by the vehicle. One embodiment provides a fan attached to a vehicle, wherein the fan is positioned to direct air towards the lower pressure region created behind the vehicle when the vehicle is moving forward. Air directed into the lower pressure region raises the pressure in the lower pressure region, lowering the pressure drag partially created by the lower pressure region.  
         [0012]     In one embodiment, a duct is adapted to be attached to the vehicle and to direct air to the fan. The duct may be located on an outer surface of the vehicle body, may be integral to the body, or a combination thereof. In an alternative embodiment, multiple fans and/or multiple ducts may also be used.  
         [0013]     In another alternative embodiment, a pressure sensor is provided and positioned so as to provide an indication of the pressure in the lower pressure region. A controller may also be provided to alter an output characteristic of the fan based at least partially on the indicated pressure.  
         [0014]     In another alternative embodiment, a method for reducing the pressure drag is provided. The method generally involves driving a vehicle at a speed high enough to create a lower pressure region behind the vehicle and directing air from one part of the vehicle to the lower pressure region.  
         [0015]     In a further embodiment, a vehicle with a chassis and body is provided, where the vehicle includes an internal duct. The internal duct may be in communication with a fan, which is positioned to direct air towards the lower pressure region.  
         [0016]     In another embodiment, a method for reducing the pressure drag of a vehicle is provided. The method generally involves capturing air from the airflow around the vehicle, directing air from a first location to a second location, and forcing the captured air from the second location into a lower pressure region created in response to movement of the vehicle in a direction.  
         [0017]     In another embodiment, a machine-accessible medium containing instructions to cause a machine to perform operations. The instructions generally involve receiving input relating to a vehicle, determining an improved pressure drag reduction system configuration at least partially based on the input, and changing the configuration of the system based on the determined improved configuration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which like references may indicate similar elements:  
         [0019]      FIG. 1  depicts a side view of a prior art motor vehicle subjected to an airflow;  
         [0020]      FIG. 2  depicts a rear view of a motor vehicle equipped with a fan and ducts according to one embodiment;  
         [0021]      FIG. 3  depicts a top view of the vehicle of  FIG. 2 ;  
         [0022]      FIG. 4  depicts a side view of the vehicle of  FIG. 2 ;  
         [0023]      FIG. 5  depicts a rear view of a vehicle equipped with fans and ducts according to an alternative embodiment;  
         [0024]      FIG. 6  depicts a rear view of a vehicle equipped with a fan, side ducts and top ducts according to an alternative embodiment;  
         [0025]      FIG. 7  depicts a top view of the vehicle of  FIG. 6 ;  
         [0026]      FIG. 8  depicts a side view of the vehicle of  FIG. 6 ;  
         [0027]      FIG. 9  depicts a rear view of a vehicle equipped with fans according to an alternative embodiment; and  
         [0028]      FIG. 10  depicts a flowchart for optimizing the pressure drag reduction system according to one embodiment. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0029]     The following is a detailed description of example embodiments of the invention depicted in the accompanying drawings. The example embodiments are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The detailed descriptions below are designed to make such embodiments obvious to a person of ordinary skill in the art.  
         [0030]     Generally speaking, a method and apparatus for reducing the aerodynamic drag of a vehicle is disclosed. More particularly, a method and apparatus is disclosed for increasing the pressure of air behind the vehicle so as to reduce pressure drag. One embodiment provides one or more fans attached to a vehicle, wherein at least one fan is positioned to direct air towards the lower pressure region created behind the vehicle when the vehicle is moving forward. Air directed into the lower pressure region raises the pressure in the lower pressure region, lowering the pressure drag partially created by the lower pressure region. In one embodiment, one or more ducts are adapted to be attached to the vehicle and to direct air to the at least one fan.  
         [0031]     Turning now to the drawings,  FIG. 1  depicts a prior art motor vehicle  100  subjected to an airflow, such as is created when moving forward. Motor vehicle  100  is shown moving left in  FIG. 1 , with rear surface  104  being located at the rear of the motor vehicle  100 . When the motor vehicle  100  is moving forward, air flows along the outer surface of the motor vehicle (from left to right on the page and as shown by the arrows), as depicted by the air streams  102  in  FIG. 1 . The air streams  102  begin as parallel streams in front of the vehicle  100 , progress and bend along the surface, and usually separate from the surface near the rear of the vehicle  100 . For motor vehicle  100  of  FIG. 1 , the air streams  102  separate approximately near the top of the rear surface  104  and become roughly parallel again well behind the vehicle. A lower pressure region  106  is created between the rear surface  104  and the separated air streams  102  and is generally located behind the vehicle  100 . The pressure in the lower pressure region  106  is lower than the pressure near the front of the motor vehicle  100 , resulting in a net drag force on the vehicle  100 . This pressure drag ultimately results in lower fuel efficiency and increased fuel consumption for a motor vehicle  100 .  
         [0032]     The effect of pressure drag is magnified in vehicles that have poor aerodynamic shapes, such as SUVs, vans, trucks, pick-up trucks, etc. A rear surface  104  that is generally perpendicular to the ground is usually indicative of a poor aerodynamic shape, especially when compared to sports cars or other vehicles with relatively tapered rear ends. For these vehicles with poor aerodynamic shapes, the lower pressure region  106  has a lower pressure (closer to a true vacuum) than in smaller, more aerodynamic vehicles, resulting in a larger pressure (and force) differential—and thus more drag.  
         [0033]     The size of the lower pressure region  106  and its actual pressure (i.e., how close it is to a vacuum) depend on many factors. Besides vehicle  100  shape, the lower pressure region  106  will enlarge, and the pressure will go down, as vehicle  100  speed increases. Other variables, such as air pressure, wind speed and direction, weather, etc. will also impact the physical configuration of the lower pressure region  106 .  
         [0034]      FIGS. 2-4  depict a rear view, a top view, and a side view, respectively, of a motor vehicle equipped with a fan and ducts according to one embodiment. Vehicle  200  has a body with a top surface  202 , rear surface  204 , bottom surface  206 , side surfaces  208 , and a front surface  218 . Vehicle  200  typically has a chassis (not shown) with a plurality of wheels  216  rotatably attached to the chassis so as to facilitate movement. An optional rear bumper  214  may be included to provide protection in the event of an accident or for aesthetic purposes.  
         [0035]     Vehicle  200  includes a pressure drag reduction system  230 , which includes a fan  210  and one or more air ducts  212  in the embodiment depicted in  FIG. 2 . In one embodiment, the fan  210  is used to direct air into the lower pressure region  106  that is formed behind a vehicle  200  when in motion. Air ducts  212  may optionally provide additional airflow for the fan  210 . The air from the fan  210  enters the lower pressure region  106  and spreads out, serving to attenuate or eliminate the lower pressure region  106 . Attenuation of the size or difference in pressure associated with the lower pressure region  106  will reduce the pressure drag of the vehicle, thus reducing the Cd and increasing fuel efficiency.  
         [0036]     Vehicle  200  may be any type of vehicle, such as automobile, truck, SUV, tractor-trailer rig, boat, aircraft, hydrofoil, train, etc. Vehicle  200  may be propelled by any sort of engine, such as an internal combustion engine, natural gas powered vehicle, fuel cell powered vehicle, solar-powered vehicle, etc., or a vehicle without propulsion, such as a glider. The pressure drag reduction system  230  is suitable for any shape of the vehicle  200  that incurs any amount of pressure drag when moving through a fluid such as air or water. The pressure drag reduction system  230  may also be designed to reduce pressure drag created from any anticipated direction of movement of the vehicle  200 .  
         [0037]     In one embodiment, the fan  210  includes a fan blade rotatably mounted on a fan body. Air enters the fan  210  through an optional fan inlet and is then propelled by the fan blade through a fan outlet. The optional fan inlet directs air towards the fan blade while the fan outlet, which is also optional, directs the airflow from the fan blade away from the fan  210 . The fan  210  may be powered by any source, such as a motor powered by the battery of the vehicle  200 , by power generated from the engine of the vehicle  200 , by internal battery or other power source, solar power, fuel cell, etc. The fan  210  may also include a shroud or the like to minimize any safety risk associated with the rotating blade and/or to further channel or direct air into lower pressure region  106 . A shroud may also improve the aesthetic qualities of the fan  210 .  
         [0038]     The fan  210  may be attached to the vehicle  200  in any way, such as a permanent mounting, temporary mounting, welding, bolting, magnets, glue, etc. The fan  210  may desirably be mounted to the rear surface  204 , facing generally away from the rear surface  204 . In an alternative embodiment, the fan  210  may be attached to the top surface  202 , side surfaces  208 , bottom surface  206 , bumper  214 , to a spare tire, spoiler, etc., as long as the fan  210  is able to potentially direct air towards lower pressure region  106 .  
         [0039]     In one embodiment, the fan  210  is positioned to direct air towards the center of the lower pressure region  106 . The center, or pinnacle, of the lower pressure region  106  may serve as a rough approximation of the most effective location in which to direct air from the fan  210 . The fan  210  may be located flush with a vehicle surface, such as the rear surface  204 , or may alternatively be located at enough distance so that ducting may be attached to the fan  210 . Any distance between the fan  210  and vehicle may be used, though the fan  210  is desirably near the rear surface  204  so as to maximize the effect of directed air. If the fan  210  is used without ducting, the fan  210  may desirably located in a location with sufficient ambient air for adequate operation of the fan  210 .  
         [0040]     In another alternative embodiment, the fan  210  may be mounted within the bumper  214  so as to provide a lower profile alternative. In this embodiment, the fan blade and inlet may be mounted within the bumper  214 , hiding them from view and reducing the chances of an injury from the fan blades, while the fan outlet (if any) would be pointing away (either behind the vehicle, upwards, or another direction) from the bumper  214 . The fan outlet could also be within the bumper  214 .  
         [0041]     The fan  210  may also have the ability to direct air in any direction. In one embodiment, the fan  210  is able to pivot to point in a variety of directions. If the fan  210  has an outlet, the outlet may pivot instead in order to provide the ability for the fan  210  to direct air in different directions. This may be useful, for example, in conjunction with a control strategy that optimizes fan direction based on vehicle speed, sensed regions of lower pressure, etc. In one embodiment, the fan  210  is directed towards the center of the lower pressure area  106  in order to maximize the effect of the fan  210 . In alternative embodiments, the fan  210  may be pointed in any direction that will direct air into the lower pressure area  106  and serve to increase the pressure.  
         [0042]     The optimal location or locations to direct air within the lower pressure region  106  depends on a variety of factors. As a vehicle accelerates, for example, the center of the lower pressure region  106  moves backwards. A vehicle  200  navigating a curve or subjected to a crosswind might have an offset lower pressure region  106 , and the fan  210  that can be directed towards either side of the vehicle  200  may offer improved performance. The fan  210  with the ability to pivot in different directions provides many options to optimize performance under any conditions.  
         [0043]     The fan  210  may also have the ability to have different rotational speeds for the fan blade. The fan  210  may have a pre-defined fixed speed, a user-configurable fixed speed, a controllable speed, etc. In one embodiment, a driver or passenger could choose different speeds for the fan  210 , such as by a switch located in the cabin. In another embodiment, a controller (such as a microprocessor-based controller) may control the fan speed based on any type of input, such as vehicle speed, sensed air pressure, fan performance, etc. In yet another embodiment, the physical configuration of the fan  210 , such as fan blade angle of attack, may be changed in order to optimize or improve performance.  
         [0044]     The fan  210  may be composed of any materials. It is desired that the fan  210  be made of a durable, inexpensive, lightweight, rustproof, etc. An optional fan cover (not shown) may also be included to cover the fan  210  when not in use. In particular, the fan cover could advantageously be deployed or retracted depending on whether the fan  210  is in use or not.  
         [0045]     In  FIGS. 2-4 , two air ducts  212  are attached to the top surface  202  of the vehicle  200 . Each air duct  212  includes an inlet  222 , a body  220 , and an outlet  224 . Air is brought into the duct  212  through the inlet  222  and transported to the outlet  224  via the body  220 . In one embodiment, the air duct inlet  222  is substantially perpendicular to the flow of air when the vehicle  200  is moving forward so as to minimize additional drag created by the air duct  212 . The air duct outlet  224  may be attached to (and in fluid communication with) the inlet of the fan  210  so as to facilitate the smooth transition of air from the air duct  212  to the fan  210 . A filter to trap particulates or other objects may optionally be included within the air duct  212 . The air ducts  212  may be attached to the vehicle  200  in any way, such as by bolts, magnets, welding, glue, rope, etc., or any method or system that is stable and secure at the maximum speed of the vehicle  200 . Air ducts  212  may be integral to the body of the motor vehicle  212  and built into the sheet metal or other material, or may be added at a later time.  
         [0046]     The use of the fan  210  to control airflow entering the lower pressure region  106  may be controlled automatically, by a user, by optimized control algorithms, etc. In one embodiment, the fan is turned on when a certain condition is reached. For example, the fan could turn on at a certain velocity (such as from the speedometer) or a certain pressure (as provided from a pressure sensor that would measure the pressure of the lower pressure region  106 ). A velocity-based trigger may be useful as the fan  210  will be potentially more effective at higher speeds, as the pressure drag generally increases with velocity. Similarly, a pressure-based trigger allows for the fan  210  to be used only when the benefit of the fan  210  outweighs the extra drag and power associated with operation of the fan  210  and duct  212  system. In another embodiment, a person in the vehicle  200  may turn any fans  210  on or off or control any other aspect of the fans  210  via a user control, such as a switch.  
         [0047]     It is believed that the coefficient of drag may be reduced by five to ten percent utilizing one or more fans  210  to direct airflow into the lower pressure region  106 . The exact reduction in coefficient of drag will depend on many factors, including the number and type of fans, shape of the vehicle, vehicle speed, etc.  
         [0048]     Other aspects of the pressure drag reduction system  230  may also be controlled or optimized. In one embodiment, the aspects of the fan  210  such as fan speed or the fan blade angle may be changed to optimize performance. In another embodiment, the airflow through the ducts  212  may be controlled (such as moving ducts or interior surfaces) to provide an optimal amount of air to the fan  210 . Optimization may include any aspects of the pressure drag reduction system  230 , including fan speed, fan size, fan direction, fan blade angle of attack, duct configuration, number of fans turned on, duct configuration, etc. Optimization may be taken into account during initial design or construction, and is advantageously performed during operation. Optimization or any controls during operation may be controlled by an on-board controller, vehicle computer/control system, remote control, controls within the vehicle  200  cabin, etc.  
         [0049]     One control example may be based on vehicle speed. While the vehicle  200  is moving at a slow speed (such as stuck in traffic or in the city), the lower pressure region  106  may be relatively small, and the power cost, complication, noise, etc. of the fan  210  may outweigh the benefits of running the fan  210 . But as the motor vehicle  200  gets on the highway, the size and pressure differential of the lower pressure region  106  increases, making the ability of the fan  210  to direct air into the lower pressure region  106  more advantageous. In this embodiment, a typical vehicle speed may be calculated and the fan  210  may advantageously be turned on when the vehicle reaches that speed.  
         [0050]     In another example, a pressure sensor could be located so as to sense a pressure indicative of the pressure of the lower pressure region  106 . In this embodiment, when the pressure in the lower pressure region  106  falls below a certain pressure (such as when the vehicle  200  accelerates), one or more fans such as the fan  210  could be turned on. In this way, the fans need only be turned on when there is a benefit to doing so. Alternatively, the number of fans or the particular configuration of fans (when there are two or more fans like fan  210 ) may also be optimized based on the sensed pressure. In an alternative embodiment, a similar control scheme could be based on a pressure differential between the lower pressure region  106  and ambient air pressure, pressure from the front of the vehicle  200 , a speed of a crosswind, a radius or angle of a curve on which vehicle  200  is traveling, and/or any other measurement indicative of the characteristics of the lower pressure region  106 .  
         [0051]      FIG. 5  depicts a rear view of a vehicle equipped with a pressure drag reduction system  530  according to an alternative embodiment. As shown in  FIG. 5 , vehicle  200  has two fans  510  for directing air behind the vehicle. In one embodiment, each fan  510  has a separate fan inlet and receives air from the outlet  524  of the corresponding air duct  212 . In another embodiment, the two fans  510  share a fan inlet, and air from the outlets  524  of both air ducts  212  enters the joint fan inlet for use in the two fans  510 . In an alternative embodiment, more than two fans  510  may be used to direct air behind the vehicle  200 . For example, a series of smaller fans  510  could provide advantages in system power consumption, effectiveness, noise, aesthetics, cost, redundancy, etc. A larger number of fans  510  could also provide finer control over the amount of air being blown through each fan  510 . Any combination of number, size, outlet direction, type, material, speed, or other design considerations for the fans  510  may be used and be within the scope of the invention.  
         [0052]      FIGS. 6-8  depict a rear, top, and side view, respectively, of a vehicle equipped with a pressure drag reduction system  630  including a fan, side ducts and top ducts according to an alternative embodiment. The vehicle  200  of  FIGS. 6-8  includes both top air ducts  212  and side air ducts  602 . Both the top air ducts  212  and side air ducts  602  may feed into the inlet of fan  210 . By utilizing both top and side air ducts, each duct may be sized to smaller dimensions, which may result in lower drag created by the ducts themselves as well as an improved aesthetic effect. Side air ducts  602  may be attached to the side surface  208  in any way and may extend any distance along side surface  208 . In an alternative embodiment, top air ducts  212  and side air ducts  602  merge before reaching the fan  210  so that fewer ducts need to interface with the inlet of fan  210 . Any combination of ducts may be utilized, including combinations of top, side, bottom, and internal ducts.  
         [0053]      FIG. 9  depicts a rear view of a vehicle equipped with pressure drag reduction system  930  including two fans and no ducts according to another alternative embodiment. In this embodiment, fans  910  draw air from the space between the fans  910  and the rear surface  204  of the vehicle  200 . Air may be located adjacent the rear surface  204  even when the lower pressure region  106  exists behind the vehicle  200 . In this embodiment, fan speed is preferably limited so as to minimize over-rotation of the fan blade in the event that insufficient air is present, as over-rotation could result in burnout of the motor of the fans  910 .  
         [0054]     In an alternative embodiment, air is transferred to the inlet of the fans  910  via internal ducting instead of external ducts. In this embodiment, an air inlet could be located on, for example, the front surface  218  of the vehicle. Air could be ducted from the inlet to the inlet of the fans  910  via internal ducts, ducts that may be much less obtrusive than external ducts. Internal ducts may also have reduced drag when compared to external ducts, as they do not add additional area to create drag. In one embodiment, air may be ducted from a radiator near the front surface  218  or any other source of air from within the vehicle  200 .  
         [0055]     In another alternative embodiment, a vehicle  200  may use internal ducts to deliver air to one or more fans  910  utilizing the bumper  214 . In this embodiment, internal space within the bumper  214  may be used to contain the internal ducts and/or the inlets of the fans  910 . This embodiment may provide structural and/or aesthetic advantages, as the fan/duct interface may be hidden within the bumper  214 .  
         [0056]     In another alternative embodiment, one or more fans  910  may be located within any of the ducts  212 , whether external or internal. This may provide the benefits of reducing any drag associated with the fans as well as minimizing any safety risk. In this embodiment, the air duct outlet  224  may serve as the outlet for the fans  910 . The fans  910  in this embodiment must be small enough to fit within the duct  212 , limiting the physical size of each of the fans  910 .  
         [0057]     The method and apparatus for reducing pressure drag of a vehicle disclosed herein may be provided as part of a vehicle when manufactured, bought or leased. In this embodiment, the pressure drag reduction system, such as pressure drag reduction system  230 , including fans and/or ducts, may be provided as an option for the vehicle  200 . This may allow the fan and/or ducts to be more fully integrated into the vehicle  200 , such as by using internal ducts, integrating the ducts or fans with body work, matching colors and materials, etc. Alternatively, any fans may be designed to appear as a bumper guard (such as a black or chrome bumper guard) in order to provide a more “rugged” appearance. Integration with the vehicle  200  is a particularly attractive option for manufacturers of SUVs, trucks, vans, tractor-trailer rigs, or other high-Cd vehicles, where fuel efficiency is increasingly desired (and where the benefits of reducing pressure behind the vehicle are greater) and any aesthetic disadvantage may be reduced.  
         [0058]     In another embodiment, the pressure drag reduction system  230  may be provided as a retrofit system. In this case, for example, a vehicle owner could purchase pressure drag reduction system  230  in order to improve the fuel efficiency of their vehicle  200 . An easily attachable/detachable pressure drag reduction system  230  may be particularly attractive to a rental agency, who could rent them to drivers who are, say, about to embark on a cross-country trip and want to improve their vehicle&#39;s fuel efficiency. For a retrofit system, it may be desirable to provide an independent power source, such as a battery, small fuel cell, solar cells, etc., but the vehicle  200  power source may also be used.  
         [0059]     As will be recognized, the pressure drag reduction system  230  may add complexity, cost, weight, and drag to a vehicle  200 , resulting in a higher upfront cost and a weight/drag penalty. The operation of the pressure drag reduction system  230 , however, will serve to reduce operating costs of the vehicle  200  dramatically by overcoming the added weight and drag associated with the system by reducing the pressure drag of the vehicle  200 .  
         [0060]      FIG. 10  depicts a flowchart  1000  for optimizing the pressure drag reduction system  230  of a vehicle  200  according to one embodiment. The method of  FIG. 10  may be implemented by any type, or combinations of type, of processor, such as hardware, software, firmware, etc. Flowchart  1000  begins with optional step  1002 , receiving input from the vehicle  200 . If input is received from the vehicle  200 , it may include vehicle speed, time of day, pressures from different locations on the vehicle, environmental conditions, vehicle angle (e.g., such as tilt during a turn), duct configuration, air velocity from different locations near the vehicle, vehicle type or configuration, ambient noise level, etc. Input from the vehicle could also include user inputs such as power on commands, power off commands, requests for fan speed or fan configuration, pre-defined profiles (e.g., a profile optimized for low noise or city driving, etc.), or any other type of information.  
         [0061]     Flowchart  1000  continues in optional step  1004 , receiving input from sensors. The input from sensors may include any type of information discussed in relation to step  1002 , as well as information derived from sensors related to the pressure drag reduction system  230 , such as fan speed, fan blade angle of attack, fan direction, pressure, duct configuration, noise level, or any other type of information.  
         [0062]     Flowchart  1000  continues in step  1006 , determining an improved pressure drag reduction system  230  configuration. In this step, an improved or optimized configuration is determined based on the inputs received in step  1002  and/or step  1004 . The improved or optimized configuration may include turning fans on or off, changing the direction of fans, changing the speed or blade angle of attack for fans, changing the duct configuration, or any other aspect or output characteristic of the system. A determination may be made by calculations, algorithms, look-up tables, or any other means. As one example, a decision to turn on a fan  210  may be made in step  1006  if a vehicle speed over a predefined amount is received as an input in step  1002 .  
         [0063]     The flowchart  1000  then proceeds to step  1008 , transmitting a command based on the determined improved system configuration to the pressure drag reduction system  230 . The flowchart may then terminate, or redirect to step  1002  for repeated improvement or optimization cycles.  
         [0064]     Unless the context clearly requires otherwise, throughout the description and the claims, words using the singular or plural number also include the plural or singular number, respectively. Additionally, the words “herein,” “hereunder,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portion. While certain aspects of the invention are presented below in certain claim forms, the inventors contemplate the various aspects of the invention in any number of claim forms. Accordingly, the inventors reserve the right to add additional claims after filing the application.