BACKGROUND 
     1. Technical Field 
     The disclosure relates to active aerodynamic fairings or shrouds for vehicles, and more particularly to a power source for controlling the extension and retraction of such shrouds. 
     2. Description of the Problem 
     Aerodynamic drag contributes greatly to fuel consumption by tractors pulling trailers at highway speeds. The gap between the tractor and trailer tends to trap air, creating a low-pressure wake behind the tractor, resulting in a net pressure difference creating drag. It also exposes portions of the front of the trailer to the direct impact of air further contributing to decreased fuel economy. The gap distance between the tractor-trailer combination has been dependent on the position of what is known in the art as a fifth wheel. The fifth wheel is a bearing and serves as a coupling between the tractor and trailer. The position of the fifth wheel is adjusted to accommodate different weight distributions based on the load of the trailer. The gap distance typically varies from about 36 to 48 inches. The drag coefficient and effective frontal area of the trailer is dependent on the gap distance. More specifically, the larger the gap distance, the larger the drag coefficient and exposed frontal area of the trailer, and conversely, the smaller the gap distance, the smaller the drag coefficient. 
     The gap causes the formation of a turbulent air mass creating a low-pressure wake behind the tractor, decreasing the fuel efficiency of the vehicle. The goal in aerodynamic design is to produce shapes that delay air separation. Maximum efficiency is achieved when airflow remains attached and moves parallel to the side of the vehicle such that air separation from the tractor is reduced and a constant and smooth airflow along the side of the vehicle is achieved. 
     One solution has been to close the gap between the sides of the cab and the sides of the trailer using cab extenders or fairings. Fixed cab extenders are planar members which extend aft longitudinally from a back end of the tractor substantially parallel with the sides of the tractor and upper fairings which slope upwardly from the cab roof to the front leading edge of the trailer. Ideally, the fixed cab extenders would extend across the entire gap between the tractor and trailer, which, as stated above, typically varies from 36 to 48 inches, to obtain the maximum aerodynamic gain. However, if the fixed cab extenders were configured as described, as the tractor turned relative to the trailer, the trailer could collide with impact and damage the extenders. Various solutions to this problem have been proposed. 
     Some previously developed fixed cab extenders were positioned to leave substantial clearance between the rearmost vertical edge of the fixed cab extender and a front end of the trailer. This clearance between the front end of the trailer and the fixed cab extenders prevented contact during sharp, typically slow speed turns. Of course, the wider the gap left, the less effective the extender is, though any extension provided some benefit. Other methods of addressing the issue were proposed, including various types of deployable cab extender allowing changing the clearance space between the tractor and trailer at high speeds so that it would fully deployed at highway speeds to improve fuel economy, and retracted at low speeds to allow the tractor trailer truck to maneuver. It was also recognized that a need existed for a cab extender that was adjustable in length to accommodate the variability in spacing encountered between the tractor and trailer. 
     One previously developed attempt at fulfilling some of these needs is disclosed in U.S. Pat. No. 3,711,146 issued to Madzsar (hereinafter “Madzsar”). Madzsar teaches adjusting the length of the cab extenders based upon a sensed amount of articulation between a tractor and trailer. 
     U.S. Pat. No. 6,846,035 issued to Wong, et al. provided an adjustable length cab extender disposed in a gap located between a tractor and a trailer. The dynamic cab extender was configurable between a deployed position, a stowed position, and an extended position. In the deployed position, the dynamic cab extender was oriented substantially coplanar with a side of the vehicle in a gap extending between a back end of the tractor and a front end of the trailer. In the stowed position, the dynamic cab extender was stowed behind the back end of the tractor. In the extended position, the dynamic cab extender was oriented substantially coplanar with the side of the vehicle in the gap. A length of the dynamic cab extender when in the extended position exceeds the length of the dynamic cab extender when stowed. Automatic adjustment of the length of the cab extender was provided in response to changes in vehicle speed. This included automatically adjusting a longitudinal length of the dynamic cab extender to selectively position a trailing edge of the dynamic cab extender a selected distance from a front end of the trailer when the sensed speed of the tractor exceeds a selected speed. 
     SUMMARY 
     A power source for an extensible fairing for an articulated vehicle is autonomous of the vehicle&#39;s engine. The power source is based on fluid power system disposed on the vehicle to be exposed to airflow around the vehicle resulting from forward movement of the vehicle. The fluid power system may take one of several forms including systems for generating rotary mechanical energy or a pneumatic system. A coupling mechanism responsive to exposure of the fluid power system to airflow for extending the extensible fairing. Springs may be used to bias the extensible shrouding toward a retracted position. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The contribution to the art believed novel is set forth in the appended claims. The preferred mode of use will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is perspective view of a shroud system for a truck/trailer combined vehicle where the shroud is deployed to close the cab between the trailer. 
         FIGS. 2A  and B are schematic illustration of a shroud for one side of the vehicle featuring inside and outside faces of a shroud and its modifications to incorporate a shroud deployment and retraction system. 
         FIGS. 3A-C  are schematics for a power system for a segmented, extensible aerodynamic fairing for an articulated vehicle. 
         FIG. 3D  is a view of a rotary power system. 
         FIGS. 4A-E  illustrate operation of the shroud system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and in particular to  FIG. 1  a tractor/trailer combination  100  is shown comprising a tractor  10  towing a trailer  12 . Disposed on the tractor  10  between the back of a tractor cab  16  and a front wall of the trailer  12  is a shroud system  14 . Shroud system  14  includes extensible shrouds/fairings  15 A, B, which are shown as deployed. Shrouds  15 A, B deploy by extending from the cab  16  toward the front wall of the trailer as the tractor/trailer combination increases in speed. As the shrouds  15 A, B extend they progressively close more of the gap between the cab  16  and the trailer  14  and thereby reduce wind resistance by causing air to flow more smoothly along the side of the vehicle as indicated by the letter A. Shrouds  15 A, B retract with decreasing speed, anticipating that the operator of the vehicle might be slowing the vehicle in preparation for a turn which could result in the trailer hitting a shroud as the vehicle articulates. 
       FIGS. 2A-B  illustrate a representative segmented, extensible shroud  15  comprising four segments, a base segment  20 , a leading segment  22 , a first trailing segment  24  and a second trailing segment  26 . Segments  20 ,  26 ,  24  and  22  are elongated panels oriented with their long axes perpendicular to the ground. In the preferred embodiment each successive segment shows an increase in height from the base segment  20 , which is the shortest, to the second trailing segment  26  to the first trailing segment  24  to the leading segment  22  which is the tallest. This is done to accommodate the difference in height of the trailer and the cab. Trailers are typically, though not always taller than cabs. The base segment is attached to the back of cab  16  and in the preferred embodiment is positionally fixed. The leading segment  22  is typically the first segment to be extended upon deployment of the shroud  15 , followed by the first trailing segment  24  and lastly the second trailing segment  26 . The order in which the segments  26 ,  24  and  22  are extended and withdrawn depends on the configuration of the particular fluid power system used. The segments nest when the system is stowed with the leading segment  22  falling to the outside relative to the vehicle&#39;s longitudinal center line, the first trailing segment  24  being located just to the inside of the leading segment, the second trailing segment  26  being located just to the inside of the first trailing segment and the base segment  20  being the innermost of all the segments. 
       FIG. 2A  illustrates the interior side of a representative segmented shroud  15 . A fluid power system (See  FIGS. 3A-B ) is used to power deployment of the shrouds  15 A,  15 B. The fluid power system may be implemented in a number of ways. Here, for example, cylinders  31 ,  33  and  35  receive air to extend shroud segments  22 ,  24  and  26  as extension arms  30 ,  32  and  34  are pushed out of the cylinders. Cylinder  35  is mounted to base segment  20  and, upon introduction of air to the cylinder, the air drives an extension arm  34  outwardly from the cylinder. The extension are is connected to segment  26  while the cylinder is mounted to segment  20  and thus segment  26  is extended from segment  20 , rearwardly away from the cab  16 . Cylinder  35  may include a compression spring to bias extension arm  34  toward a retracted position. Loss of air pressure in cylinder  35  thus would result in retraction of the segment  26 . Similarly a cylinder  33 , extension arm  32  combination is connected between segments  26  and  20  and a cylinder  31  and extension arm  30  combination is attached between segments  24  and  22 . Alternative extension and retraction mechanisms are of course possible, including a crank system based on a wind turbine or an air screw exposed to air flow around the vehicle. Such a rotary power system  270  is illustrated in  FIG. 3D . It would generate rotary mechanical energy. Pitch reversal of the blades of the turbine or screw, or use of a reverse gear system  280  would allow such a system to be used to extend or retract the fairings/shrouds  15 . Such a system would allow precise positioning of the fairings  15 . The operation of the fluid power controller  156  is described below with reference to  FIG. 3B . 
       FIG. 2B  illustrates an outside face of the representative segmented shroud  15 . Excluding the base segment  20 , the other segments  22 ,  24  and  26  can be configured to ride on inlay tracks  60 ,  61 ,  62 ,  63 ,  64  and  65  on relatively trailing adjacent segments, i.e. segments  20  (the base segment),  26  and  24 . Segments that deploy relatively further aft are mounted progressively further to the outside of the vehicle relative to a longitudinal center line. When retracted segments  20 ,  26  and  24  are nested within segments  26 ,  24  and  22 , respectively. 
       FIG. 3A  is a simplified schematic of an automatically deploying shroud system for improving vehicle aerodynamics. In a preferred form the system operates using air flowing around the vehicle and thus the system is essentially passive. Air flow  47  can be used both as an indicator of vehicle speed, determined by a sensor  56 , and as a power source for the fluid power system  70 . A fluid power system responsive to the sensor  56  controls the degree of deployment of the shrouds  15 A, B. When the truck is operating at slow speed the shroud system remains nested. When the truck slows to execute a turn the shroud system retracts. As speed increases the sensor activates the fluid power system  70  and the shrouds are extended to close the gap between cab and trailer, thereby improving the aerodynamics of the truck. There are a number of ways the fluid power system  70  can be made to utilize air flow around a vehicle. In some embodiments the sensor  56  may be merged with the fluid power system  70  to achieve the greatest degree of passivity. For example, the fluid power system  70  may be a mechanically cranked system driven by a wind driven fins. At speeds above a certain threshold, the fins may be set at a pitch to extend the shrouds. At speeds falling below the threshold the fins&#39; pitch may be reversed to drive the system in the opposite direction. When the shrouds  15 A, B are fully extended or retracted the fins may be set to neutral. A ram air or pressure plate system would in effect be its on air speed indicator with the degree of shroud extension following purely from frontal air pressure on the vehicle. Spring resistance may be used with such a system to limit the degree of extension of the shrouds  15 A, B and to bias the system toward a retracted state. 
       FIG. 3B  introduces additional nuances to the control system by providing a fluid power controller  156  which monitors shroud extension (displacement), the degree of truck to trailer articulation  157  and directly measure vehicle speed. Vehicle speed measurements taken from wind speed may be dependent upon air density and wind direction and thus deployment speed could increase at low air density as might be encountered at high altitudes or if the vehicle enjoyed a following wind. Deployment speeds would decrease if the vehicle encountered a head wind. If vehicle speed measurements are received from another source, for example a drive shaft tachometer  158 , this issue does not arise. A more nuanced, active control scheme can be implemented where a fluid power controller  156  can be programmed to take direct account of the degree of extension of the shrouds, directly measured vehicle speed and the degree of truck/trailer articulation  157 . For example, if actual vehicle articulation is measured it is possible to configure the fluid power system  70  to withdraw the shroud on only the side of the vehicle affected. In addition, vehicle compressed air may be tapped to provide air  47  rather than introducing a drag inducing modification to the vehicle. However, the modifications introduced for this embodiment require interaction with the vehicle&#39;s electronic control system at the cost of some additional complication to the vehicle. Employment of a control system would allow the easy addition of valves to the fluid power system which in turn would allow control over the order in which segments were extended or retracted. 
     It is also possible to configure the system to be completely passive. In the embodiment of  FIG. 3C  a ram air induction system  147  feeds air to a manifold  170  which directs air into each of six cylinders  31 ,  33 ,  35 ,  36 ,  38  and  40 . Increasing air pressure in cylinders  31 ,  33 ,  35 ,  36 ,  38  and  40  results in extension arms  30 ,  32 ,  34 ,  37 ,  39  and  41  being pushed outwardly against the resistance of springs  130 ,  132 ,  134 ,  137 ,  139  and  141 , respectively. If air pressure drops the springs  130 ,  132 ,  134 ,  137 ,  139  and  141  urge the arms  30 ,  32 ,  34 ,  37 ,  39  and  41  to a retracted position, retracting the shrouds connected to the arms. In this embodiment the degree of extension of the shrouds is always directly proportional to air pressure, and the degree of extension is controlled by the springs  130 ,  132 ,  134 ,  137 ,  139  and  141 . It would be possible to make the springs adjustable to some extent to accommodate changing environments of application of the vehicle. It is also possible that one cylinder (two in total) could be used for each shroud connected between the base segment  20  or cab  16  and the lead segment  22  for each shroud  15 . Trailing segments would simply drop in place as they reach the limit of their travel on their respective tracks. 
       FIGS. 4A-E  illustrate progressive extension and retraction of a shroud  15 B on the driver&#39;s side of a vehicle. The order of extension of the segments is to be taken as representative, as the different embodiments may operate differently. For example, in the fully passive system of  FIG. 3C  it might be expected that the shroud segments  26 ,  24  and  22  would extend together and retract synchronously. Where a control system and valves are provided the order of extension and retraction can be controlled. As illustrated here the shroud  15 B is initially retracted with forward segments nested within the segments which are deployable aft from the cab  16 . Segments  20 ,  26  and  24  are inside of lead segment  22 . As the shroud  15 B is deployed rearward, the lead segment  22  moves to the rear. As its extension progresses the base segment  20 , the second trailing segment  26  and the first trailing segment  24  are progressively revealed until the shroud is fully deployed, as shown in  FIG. 4D . In  FIG. 4E  the beginning of reversal of the process is shown as the lead segment is withdrawn. 
     The present invention provides a simple, deployable aerodynamic shroud or fairing for the sides of a truck trailer combination. 
     While only a few preferred embodiments are described here in detail, the claims are not limited thereto, but are intended to extend to various changes and modifications thereof.
A power or motive source for an extensible fairing for an articulated motor vehicle is based on a fluid power system. The fluid power system is disposed on the vehicle to be exposed to airflow around the vehicle resulting from forward movement of the vehicle. A coupling mechanism responsive to exposure of the fluid power system to airflow for extending and retracting the extensible fairing responsive to changes in vehicle speed.