Patent Publication Number: US-8973633-B2

Title: Tire inflation system with discrete deflation circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/369,159, which was filed on Jul. 30, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to the art of tire inflation systems. More particularly, the invention relates to tire inflation systems for heavy-duty vehicles such as trucks and tractor-trailers or semi-trailers, which can operate as the vehicles are moving. Still more particularly, the invention is directed to a tire inflation system which includes a tire deflation circuit that is discrete or separate from an inflation Circuit, thereby enabling control of tire deflation based on specific predetermined conditions. 
     2. Background Art 
     Heavy-duty vehicles typically include trucks and tractor-trailers or semi-trailers. Tractor-trailers and semi-trailers, which shall collectively be referred to as tractor-trailers for the purpose of convenience, include at least one trailer, and sometimes two or three trailers, all of which are pulled by a single tractor. All heavy-duty vehicles that are trucks or tractor-trailers include multiple fires, each of which is inflated with a fluid or gas, such as air, to an optimum or recommended pressure. This optimum or recommended tire pressure typically is referred to in the art as the target inflation pressure or the target pressure. 
     However, it is well known that air may leak from a tire, usually in a gradual manner, but sometimes rapidly if there is a problem with the tire, such as a defect or a puncture caused by a road hazard. As a result, it is necessary to regularly check the air pressure in each tire to ensure that the tires are not significantly below the target pressure and thus under-inflated. Should an air check show that a tire is under-inflated, it is desirable to enable air to flow into the tire to return it to the target pressure. Likewise, it is well known that the air pressure in a tire may increase due to increases in ambient air temperature, so that it is necessary to regularly check the air pressure in each tire to ensure that the tires are not greatly above the target pressure and thus over-inflated. Should an air check show that a tire is over-inflated, it is desirable to enable air to flow out of the tire to return it to the target pressure. 
     The large number of tires on any given heavy-duty vehicle setup makes it difficult to manually check and maintain the target pressure for each and every tire. This difficulty is compounded by the fact that trailers of tractor-trailers or trucks in a fleet may be located at a site for an extended period of time, during which the tire pressure might not be checked. Any one of these trailers or trucks might be placed into service at a moment&#39;s notice, leading to the possibility of operation with under-inflated or over-inflated tires. Such operation may increase the chance of less-than-optimum performance and/or reduced life of a tire in service as compared to operation with tires at the target pressure, or within an optimum range of the target pressure. 
     Moreover, should a tire encounter a condition as the vehicle travels over-the-road that causes the tire to become under-inflated, such as developing a leak from striking a road hazard, or over-inflated, such as increasing pressure from an increased ambient air temperature, the life and/or performance of the tire may be significantly reduced if the under-inflation or over-inflation continues unabated as the vehicle travels. The potential for significantly reduced tire life typically increases in vehicles such as trucks or tractor-trailers that travel for long distances and/or extended periods of time. 
     Such a need to maintain the target pressure in each tire, and the inconvenience to the vehicle operator to manually check and maintain a proper tire pressure that is at or near the target pressure, led to the development of prior art tire inflation systems. In these prior art systems, an operator selects a target inflation pressure for the vehicle tires. The system then monitors the pressure in each tire and attempts to maintain the air pressure in each tire at or near the target pressure by inflating the tire when the monitored pressure drops below the target pressure. These prior art tire inflation systems inflate the tires by providing air from the air supply of the vehicle to the tires by using a variety of different components, arrangements, and/or methods. In prior art systems that are also capable of deflation, the system deflates the tire when the monitored pressure rises above the target pressure by venting air from the tires to atmosphere. 
     While being satisfactory for their intended functions, tire inflation systems of the prior art may experience disadvantages in certain situations. More particularly, many prior art tire inflation systems are not capable of deflation. As a result, when the air pressure in a tire increases to a level that is greatly above the target pressure due to increases in ambient air temperature, these systems are not able to reduce the pressure in the tires. As a result, such prior art tire inflation systems may allow the tires to operate in a significantly over-inflated condition, which undesirably decreases performance of the tires and in turn decreases the life of the tires. 
     In addition, in those prior art tire inflation systems having a deflation capability, the systems generally inflate and deflate the vehicle tires through the same components, circuit or path of pneumatic conduit, valves and the like that extend from the vehicle air supply to the tires, which is referred to herein as a circuit. Use of the same circuit for inflation and deflation functions has been achieved in the prior art by employing electronically-controlled systems that include electronically-actuated solenoid valves. With a solenoid valve, when it is desired to inflate the tires, an electronic controller actuates the valve to move the valve to a position that enables air to flow from the air reservoir to the vehicle tires. When it is desired to deflate the tires, the electronic controller actuates the valve to move the valve to a position that exhausts air from the tires to atmosphere. Such prior art deflation-capable tire inflation systems have certain disadvantages. 
     First, prior art tire inflation systems only maintain the inflation pressure in the tires at the target pressure, and lack the ability to accommodate an increased tire pressure based on operating conditions. More particularly, the desired target inflation pressure typically is selected by the vehicle operator based on what is known in the art as a cold inflation pressure or cold pressure, which is the inflation pressure of the tires when the vehicle remains parked. In many cases, the tire manufacturer recommends a target pressure that is at a cold pressure setting for a specific axle load. 
     However, as the vehicle operates and travels over-the-road, the energy and forces associated with the travel cause the temperature of each vehicle tire to increase. When the temperature of the tire increases, the air inside the tire expands. Because the volume of the tire is limited, the expansion of air causes the air pressure inside the tire to increase above the cold inflation pressure. This increased air pressure is typically referred to as the operating pressure of the tires. By way of example, the operating pressure may be about fifteen (15) pounds per square inch (psi) greater than the cold pressure of each tire in a typical heavy-duty vehicle dual-wheel configuration. In fact, the National Highway Traffic Safety Administration (NHTSA) recommends adding about 15 psi to a cold pressure setting when checking pressure while the tires are at their operating temperature. The increase to the operating pressure is desirable, as tire manufacturers typically rely on the increase to compensate for lower side wall stiffness of the tire as its temperature increases during over-the-road travel, and thus often design heavy-duty vehicle tires to provide optimum performance at the operating pressure. 
     Because the vehicle operator typically selects a target inflation pressure for the tires which is at the cold inflation pressure, prior art tire inflation systems inflate or deflate the tires as needed to arrive at this cold target pressure. However, as described above, as the vehicle operates, the air pressure in the tires increases from the cold pressure to the higher operating pressure. Because prior art tire inflation systems only maintain the inflation pressure in the tires at the target pressure, as the air pressure in the tires increases during vehicle operation, the systems deflate the tires down from the optimum operating pressure to the lower cold target pressure. Due to this lack of ability to accommodate an increased tire pressure based on operating conditions, prior art tire inflation systems often maintain the inflation pressure of the tires at a level that is below the optimum operating pressure, which decreases tire performance, and thus vehicle performance. 
     In the event that the vehicle operator attempts to prevent a prior art inflation system from deflating the tires down from the optimum operating pressure to the lower cold target pressure by selecting a target inflation pressure which is at the higher operating pressure, undesirable demands may be placed on the system. More particularly, because the operating pressure is higher than the cold pressure, the operating pressure may approach or be at a pressure level that is not available in the vehicle air supply, or which would require the vehicle air supply to be maintained at an undesirably high level. The requirement of maintaining such a pressure level in the vehicle air supply places undesirable demands on the tire inflation system, which in turn reduces the performance and/or the life of the system. As a result, it is not practical to attempt to prevent prior art systems from deflating the tires down from the optimum operating pressure to the lower cold target pressure by selecting a target inflation pressure that is at the operating pressure. 
     A second disadvantage of prior art tire inflation systems is that most systems which are capable of both inflation and deflation are electronically controlled, which is undesirably expensive, complex, and potentially undependable. For example, electronically-controlled systems typically involve electronically-operated solenoid valves, electronic controllers, and other electronic components, which are expensive and are often complex to install and configure. In addition, these electrical components require the use of the electrical system of the vehicle, which may be unreliable or even non-functional at times, and in turn renders the operation of the tire inflation system unreliable and potentially non-functional. 
     A third disadvantage of prior art tire inflation systems is that the electronic systems are not constant-pressure systems. More particularly, when the system is not performing inflation, the pneumatic conduit of the system is exhausted to atmosphere and thus does not actively monitor tire pressure. In such a system, without air pressure in the pneumatic conduit, electronic controls are employed to periodically check tire pressure, and to in turn trigger or commence inflation. Because such prior art systems are capable of only providing a periodic check of tire pressure, any inflation to bring the tires to the target pressure only takes place following the periodic check. This lack of ability of prior art systems to continuously monitor tire pressure and dynamically respond to pressure changes undesirably reduces the ability of the system to actively or quickly respond to reduced tire pressure conditions, such as in the case of an air leak. Moreover, as mentioned above, the electronic controls that are employed by prior art tire inflation systems to determine when it is necessary to trigger or commence inflation are expensive, complex, and require power from the electrical system of the vehicle, which may be unreliable. 
     A fourth disadvantage of prior art tire inflation systems occurs in certain pneumatically-controlled systems which are constant-pressure systems, that is, systems that maintain air pressure at all times in a pneumatic conduit extending between the vehicle air reservoir and the tires. Some of these constant-pressure systems include a wheel valve that is capable of deflation, which keeps the inflation path from the air reservoir to the tires open. As is known to those skilled in the art, when a vehicle is parked for an extended period of time, the pneumatic pressure in the air reservoir may drop or bleed down due to small air leaks that are typical in any pneumatic system. Because prior art constant-pressure systems that include a wheel valve which is capable of deflation keep the inflation path from the air reservoir to the tires open, when the pneumatic pressure in the air reservoir drops, the pneumatic pressure in the tires also drops. This pressure drop may be up to 25 psi or more, at which point the wheel valve typically closes to eliminate an even greater pressure drop. 
     However, when the vehicle is started up to prepare for over-the-road travel, the tire inflation system must re-inflate each tire up to or near the target pressure, which may thus involve adding about 25 psi to each one of eight or more tires. This re-inflation process typically takes a great deal of time and places repeated demands on the tire inflation system, which may reduce the life of the system. In addition, the vehicle operator may not wait for the tires to be re-inflated to the target pressure before operating the vehicle, which in turn causes the tires to be operated in an under-inflated condition until the target pressure is reached. Such operation reduces the life of the tires. As a result, it is desirable for a constant-pressure tire inflation system to optionally include a feature that would isolate the tires from the air reservoir and other components of the system when the vehicle is parked, thereby minimizing pressure loss from the tires and in turn minimizing the subsequent time and demand on the system that is required to provide significant re-inflation of the tires. 
     A fifth disadvantage of prior art tire inflation systems occurs in certain pneumatically-controlled, constant-pressure systems that do not include a wheel valve that is capable of deflation. More particularly, without a wheel valve that is capable of deflation, such prior art systems cannot respond to excessive increased tire pressure from an increased ambient air temperature, as described above for prior art systems that are not capable of deflation. As a result, such prior art tire inflation systems may allow the tires to operate in a significantly over-inflated condition, which undesirably decreases performance of the tires and in turn decreases the life of the tires. 
     As a result, there is a need in the art for a tire inflation system that overcomes the disadvantages of the prior art by providing control of the conditions under which deflation occurs, by providing a system that has the ability to accommodate an increased tire pressure due to operating conditions, does not employ electronic components and thereby is more economical, simpler, more dependable and more efficient than tire inflation systems of the prior art, and which is a constant-pressure system that is capable of deflation and optionally includes a feature that enables isolation of the tires from the air reservoir and other components of the system when the vehicle is parked to minimize pressure loss. The tire inflation system with discrete deflation circuit of the present invention satisfies this need, as will be described in detail below. 
     BRIEF SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a tire inflation system that includes control of the conditions under which deflation through the system occurs. 
     Another objective of the present invention is to provide a tire inflation system that has the ability to accommodate an increased tire pressure, which is due to operating conditions. 
     Yet another objective of the present invention is to provide a tire inflation system that does not employ electronic components, and thus is more economical, simpler, more dependable and more efficient than tire inflation systems of the prior art. 
     Still another objective of the present invention is to provide a tire inflation system that is a constant-pressure system which is capable of deflation. 
     Yet another objective of the present invention is to provide a tire inflation system that optionally includes a feature which enables isolation of the tires from the air reservoir and other components of the system when the vehicle is parked in order to minimize pressure loss. 
     These objectives and others are obtained by the tire inflation system with discrete deflation circuit of the present invention. By way of example, the vehicle tire inflation system includes an air supply source in fluid communication with a plurality of tires of the vehicle. A pneumatic conduit extends between and is in fluid communication with the air supply source and the tires. Means are fluidly connected to the pneumatic conduit for enabling selective inflation and deflation of the tires. The means include a first pneumatic circuit for inflation of the tires, and a second pneumatic circuit for deflation of the tires. The second pneumatic circuit is discrete from the first pneumatic circuit and is common to more than one of the tires. The means provide controlled deflation of the tires in the second pneumatic circuit based upon a predetermined condition, enabling the tire inflation system to accommodate an increased pressure in the tires. 
     These objectives and others are obtained by the tire inflation system with discrete deflation circuit of the present invention. By way of additional example, the vehicle tire inflation system includes an air supply source in fluid communication with a plurality of tires of the vehicle. A pneumatic conduit extends between and is in fluid communication with the air supply source and the tires. A tire isolation pilot valve is in fluid communication with the pneumatic conduit and is equipped with means for monitoring a condition of the vehicle. The tire isolation pilot valve interrupts the fluid communication between the air supply source and the tires to pneumatically isolate the tires when the vehicle is in a parked condition, which minimizes a pneumatic pressure loss of the tires. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The preferred embodiments of the present invention, illustrative of the best mode in which Applicants have contemplated applying the principles, are set forth in the following description and are shown in the drawings, and are particularly and distinctly pointed out and set forth in the appended claims. 
         FIG. 1A  is a schematic diagram of a first exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention, shown indicating an inflation mode; 
         FIG. 1B  is a schematic diagram of the embodiment of the tire inflation system shown in  FIG. 1A , but shown indicating a deflation mode; 
         FIG. 2A  is a schematic diagram of a second exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention, shown indicating an inflation mode; 
         FIG. 2B  is a schematic diagram of the embodiment of the tire inflation system shown in  FIG. 2A , but shown indicating a deflation mode; 
         FIG. 3A  is a schematic diagram of a third exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention, shown indicating an inflation mode; 
         FIG. 3B  is a schematic diagram of the embodiment of the tire inflation system shown in  FIG. 3A , but shown indicating a deflation mode; 
         FIG. 4A  is a schematic diagram of a fourth exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention, shown indicating an inflation mode; 
         FIG. 4B  is a schematic diagram of the embodiment of the tire inflation system shown in  FIG. 4A , but shown indicating a deflation mode; 
         FIG. 5  is a schematic diagram of an optional tire isolation system of the tire inflation system, shown incorporated into a representative tire inflation system with discrete deflation circuit of the present invention; 
         FIG. 6A  is a schematic diagram of a fifth exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention, incorporating a deflation circuit similar to the fourth embodiment tire inflation system shown in  FIG. 4A  with the optional tire isolation system shown in  FIG. 5 , and shown indicating an inflation mode; 
         FIG. 6B  is a schematic diagram of the embodiment of the tire inflation system shown in  FIG. 6A , but shown indicating a deflation mode; 
         FIG. 7  is a schematic representation of a cross-sectional view of an exemplary relieving regulator for use in the first and third exemplary embodiments of the tire inflation system with discrete deflation circuit of the present invention, shown in  FIGS. 1A-1B  and  3 A- 3 B; and 
         FIG. 8  is a schematic representation of a cross-sectional view of another exemplary relieving regulator for use in the first and third exemplary embodiments of the tire inflation system with discrete deflation circuit of the present invention, shown in  FIGS. 1A-1B  and  3 A- 3 B. 
     
    
    
     Similar numerals refer to similar parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a tire inflation system which includes a deflation circuit that is at least partially discrete or separate from an inflation circuit. The discrete deflation circuit enables controlled deflation of tires, based on specific predetermined conditions, which will be described below in the exemplary embodiments of the invention. Use of these specific predetermined conditions prevents deflation of the tires until the vehicle is parked, or limits the amount of deflation of the tires, which in turn prevents the tire pressure from falling below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer. It is to be understood that reference hereinbelow to the term target pressure means the desired target inflation pressure as selected by the vehicle operator based on the cold inflation pressure or cold pressure of the vehicle tires. 
     Turning now to  FIG. 1A , a first exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention is indicated generally at  10 .  FIG. 1A  shows tire inflation system  10  in an inflation mode, and the direction of air flow is generally indicated by arrows I. Tire inflation system  10  includes a vehicle air supply or source  12  of pressurized or compressed air. Vehicle air supply  12  typically is a reservoir tank and will be referred to hereinbelow for the purpose of convenience as a supply tank. Supply tank  12  is connected, by components to be described in detail below, to vehicle tires  14 . For the purpose of convenience, only a single tire  14  is illustrated in  FIG. 1A , but it is to be understood that tire inflation system  10  typically is utilized with multiple tires. A pneumatic conduit represented generally by the letter C, and including specific conduit sections or portions to be described in greater detail below, extends between and interconnects components of tire inflation system  10 . 
     More particularly, a first section  15  of pneumatic conduit C extends between and is fluidly connected to supply tank  12  and to a supply valve  18 . A second section  16  of pneumatic conduit C is fluidly connected to and extends from supply valve  18 . Supply valve  18  preferably is a mechanically-operated regulator that is mechanically adjustable to a setting that actuates or opens the valve at the target pressure. Preferably, valve  18  is biased to a typically closed position, and when the air pressure in second pneumatic conduit section  16  drops below the target pressure, supply valve  18  opens to enable air to flow through the valve, as known to those skilled in the art. Once supply valve  18  is open, air is delivered from supply tank  12 , through first pneumatic conduit section  15 , and to second pneumatic conduit section  16 . 
     Once the target pressure is reached in second pneumatic conduit section  16 , supply valve  18  closes, as known in the art. The setting at which supply valve  18  opens and closes to achieve the target pressure is adjustable by mechanical means, such as by rotation of a knob, set screw, stem, and the like by a technician or vehicle operator, depending on system requirements. Also based upon system requirements, the means for adjusting supply valve  18  may be placed in a convenient location inside the vehicle cab (not shown), or outside of the vehicle cab, such as on the trailer of a tractor-trailer. Supply valve  18  preferably also includes a flow switch (not shown), which can detect an air flow leak, as known in the art. As will be described in greater detail below, in the event that the target pressure is exceeded in second pneumatic conduit section  16 , supply valve  18  vents to atmosphere. It is to be understood that supply valve  18  may be any mechanically-operated valve known to those skilled in the art which is suitable for controlling air flow in pneumatic conduit C. 
     When supply valve  18  is in an open position, pressurized air flows through the valve to second pneumatic conduit section  16 , through a first tee fitting  34  of a deflation circuit  24 , which will be described in greater detail below, and through a third section  17  of pneumatic conduit C that extends between and is fluidly connected to the first tee fitting and a first check valve  22 . First check valve  22  is also part of deflation circuit  24 . It is to be understood that deflation circuit  24  employs certain sections or portions of pneumatic conduit C and other components that are also used for inflation, as will be described in greater detail below. After flowing through first check valve  22 , air flows through a fourth section  19  of pneumatic conduit C that extends between and is fluidly connected to the first check valve and a second tee fitting  36  of deflation circuit  24 . 
     After flowing through second tee fitting  36  of deflation circuit  24 , air flows through a fifth section  20  of pneumatic conduit C that extends between and is fluidly connected to the second tee fitting of deflation circuit  24  and an isolation pilot valve  26  of an optional tire isolation system  130 . Optional tire isolation system  130  and isolation pilot valve  26  will also be described in greater detail below. 
     Once pressurized air flows through isolation pilot valve  26  of optional tire isolation system  130 , it proceeds to a mechanically-operated wheel valve  28  through a sixth section  21  of pneumatic conduit C, which extends between and is fluidly connected to the isolation pilot valve and the wheel valve  28 . Wheel valve  28  preferably is a diaphragm valve that isolates each tire  14  from the rest of tire inflation system. More particularly, wheel valve  28  preferably is spring biased and actuates or opens the wheel valve at a selected pressure setting or pressure level that is below the target pressure, thereby enabling maximum air flow to tires  14  from tire inflation system  10 . This selected pressure setting or pressure level is less than the minimum pressure that would be expected to be utilized as a target tire pressure. In this manner, wheel valve  28  remains open during all normal operating conditions of the vehicle and the tire(s)  14 , and closes in the event of an extreme condition, such as low or insufficient pressure in sixth pneumatic conduit section  21 . 
     A seventh section  30  of pneumatic conduit C is fluidly connected to and extends between wheel valve  28  and a tire valve  32 . Tire valve  32 , which preferably is a Schrader valve, is pneumatically connected to tire  14  as known in the art. Tire valve  32  typically is spring-biased to a closed position, and typically is open only when mechanical means are employed to hold it open. Preferably, seventh pneumatic conduit section  30  includes a fitting (not shown) that holds tire valve  32  open by mechanical means while the seventh pneumatic conduit section is connected to the tire valve to enable inflation of tire  14 . In this manner, when tire inflation system  10  is in an inflation mode, air flows via pneumatic conduit C from supply tank  12 , through supply valve  18 , isolation pilot valve  26  of optional tire isolation system  130 , wheel valve  28 , tire valve  32 , and into tire  14 . 
     Turning now to  FIG. 1B , first embodiment tire inflation system  10  is shown in a deflation mode, and the direction of air flow is generally indicated by arrows D. Deflation circuit  24  of first embodiment tire inflation system  10  employs a fixed differential deflation pressure as the condition under which deflation of tire  14  occurs, thereby enabling controlled deflation of the tire. More particularly, deflation circuit  24  is pneumatically connected to and includes certain sections or portions of pneumatic conduit C. 
     By way of example, in one type of configuration, deflation circuit  24  includes first and second pneumatic fittings  34  and  36 , respectively, which preferably are tee fittings. First and second fittings  34  and  36  are spaced apart from one another and are fluidly connected to pneumatic conduit C. As described above, first tee fitting  34  is fluidly connected to and extends between second pneumatic conduit section  16  and third pneumatic conduit section  17 , while second tee fitting  36  is fluidly connected to and extends between fourth pneumatic conduit section  19  and fifth pneumatic conduit section  20 . First check valve  22  is disposed between first and second tee fittings  34  and  36 , and is fluidly connected to third pneumatic conduit section  17  and fourth pneumatic conduit section  19 . First check valve  22  enables air to flow in the direction from supply tank  12  to tires  14 , but prevents air from flowing in the opposite direction, that is, from the tires to the supply tank. 
     Deflation circuit  24  further includes a deflation pneumatic conduit  38 , which in turn includes a first deflation conduit section  38   a  and a second deflation conduit section  38   b . First deflation conduit section  38   a  includes a first end  42  and a second end  44 . First end  42  of first deflation conduit section  38   a  is fluidly connected to first tee fitting  34 , which provides fluid communication between second pneumatic conduit section  16  and the first deflation conduit section. Second deflation conduit section  38   b  includes a first end  46  and a second end  48 . First end  46  of second deflation conduit section  38   b  is fluidly connected to second tee fitting  36 , which provides fluid communication between fifth pneumatic conduit section  20  and the second deflation conduit section. 
     Second end  44  of first deflation conduit section  38   a  is fluidly connected to a second check valve  40 , and second end  48  of second deflation conduit section  38   b  is also fluidly connected to the second check valve. In this manner, second check valve  40  is fluidly connected to and extends between first deflation conduit section  38   a  and second deflation conduit section  38   b . Second check valve  40  enables air to flow in the direction from tires  14  to supply tank  12 , but prevents air from flowing in the opposite direction, that is, from the supply tank to the tires. In addition, second check valve  40  is biased to only allow air to flow from the direction of tires  14  to supply tank  12  when the pneumatic pressure in second deflation pneumatic conduit section  38   b  is at least a fixed differential or predetermined amount greater than the target pressure. This fixed differential or predetermined amount is referred to herein as X. 
     An example of a preferred fixed differential X is the difference between the cold pressure of the tires and the operating pressure of the tires. As described above, when the heavy-duty vehicle has been parked for a period of time, the air pressure in the tires of the vehicle moves to a pressure level that is referred to as the cold pressure. The cold pressure typically is the recommended pressure from the tire manufacturer for a specific axle load. Then, as the vehicle travels over-the-road, the energy and forces associated with the travel cause the temperature of each vehicle tire to increase. When the temperature of the tire increases, the air inside the tire expands. Because the volume of the tire is limited, the expansion of air causes the air pressure inside the tire to increase. This increased air pressure is typically referred to as the operating pressure of the tires. Often, the operating pressure of the tires of a typical heavy-duty vehicle dual-wheel configuration is about fifteen (15) pounds per square inch (psi) greater or higher than the cold pressure of the tires, as NHTSA recommends adding about 15 psi to a cold pressure setting when checking pressure while the tires are at their operating temperature. As a result, a preferred fixed differential X is the difference between the cold pressure and the operating pressure, that is, about 15 psi. 
     Of course, other pressure amounts or levels that account for the difference between the cold pressure and the operating pressure of a specific tire or tire arrangement are contemplated by tire inflation system of the present invention  10 , without affecting the concept or operation of the invention. 
     The desirable effect of the use of fixed differential X in deflation circuit  24  of first embodiment tire inflation system  10  is illustrated by the operation of the system. More particularly, as described above, the vehicle operator or a technician selects a target pressure by adjusting supply valve  18  using means that are placed in a convenient location inside the vehicle cab, or outside of the vehicle cab, such as on the trailer of a tractor-trailer, depending on system requirements. As shown in  FIG. 1A , when inflation of tires  14  is required, supply valve  18  is opened or actuated, enabling air to flow from supply tank  12 , through first pneumatic conduit section  15 , through the supply valve and to second pneumatic conduit section  16 , first tee fitting  34  of deflation circuit  24  and third pneumatic conduit section  17 . First check valve  22  ensures that air continues to flow from third pneumatic conduit section  17  through fourth pneumatic conduit section  19  to second tee fitting  36  of deflation circuit  24 , through fifth pneumatic conduit section  20  and to isolation pilot valve  26  of optional tire isolation system  130 . Air then flows through sixth pneumatic conduit section  21 , wheel valve  28 , seventh pneumatic conduit section  30 , and into tires  14 . Second check valve  40  ensures that air flows through second, third, fourth and fifth pneumatic conduit sections  16 ,  17 ,  19  and  20 , respectively during the inflation process, rather than flowing through deflation conduit  38 . Once the target pressure is reached, supply valve  18  closes. Because tire inflation system  10  is a constant-pressure system, pneumatic pressure remains in second, third, fourth, fifth, sixth and seventh pneumatic conduit sections  16 ,  17 ,  19 ,  20 ,  21  and  30 , respectively, and tires  14 . 
     If the pneumatic pressure in tires  14  increases, deflation of the tires may be necessary. In the prior art tire inflation systems that are not capable of deflation, tires  14  may operate in a significantly over-inflated condition, which undesirably decreases their performance and in turn decreases the life of the tires. In tire inflation systems of the prior art that are capable of deflation, the lack of ability to accommodate an increased tire pressure causes the systems to deflate tires  14  down from the optimum operating pressure to the lower cold-tire target pressure, which also undesirably decreases tire performance. However, deflation circuit  24  of first embodiment tire inflation system  10  limits deflation of tires  14  below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer, which optimizes tire performance. 
     More specifically, as shown in  FIG. 1B , first check valve  22  prevents air from flowing in the direction from tires  14  to supply tank  12 . Thus, when the pneumatic pressure in tires  14  increases, the pressure increases in seventh, sixth, fifth and fourth pneumatic conduit sections  30 ,  21 ,  20  and  19 , respectively, to first check valve  22 . First check valve  22  prevents the increased pressure from proceeding directly through third and second pneumatic conduit sections  17  and  16 , respectively, to supply valve  18 . In this manner, first check valve  22  prevents supply valve  18  from exhausting air from second, third, fourth, fifth, sixth and seventh pneumatic conduit sections  16 ,  17 ,  19 ,  20 ,  21  and  30 , and thus tires  14 , down to a pressure that is below a recommended level. 
     Rather than reaching supply valve  18 , air flows through second deflation conduit section  38   b  to second check valve  40 . Second check valve  40  only allows air to pass or flow through it if the pneumatic pressure is fixed differential X greater than the target pressure. For example, using a fixed differential X of 15 psi, which is the difference between the cold pressure and the operating pressure of tires  14 , second check valve  40  only allows air to flow through it when the pneumatic pressure is greater than the target pressure plus 15 psi. When the pneumatic pressure is greater than the target pressure plus 15 psi, air flows through second check valve  40 , through first deflation conduit section  38   a , through second pneumatic conduit section  16  and to supply valve  18 . Supply valve  18  then exhausts air until the pressure in second pneumatic conduit section  16  drops below a level of the target pressure plus 15 psi, which then causes second check valve  40  to close and thus prevent further deflation. 
     In this manner, first embodiment tire inflation system  10  provides a constant-pressure system that includes discrete deflation circuit  24 . Discrete deflation circuit  24  accommodates an increased tire pressure due to operating conditions by enabling deflation of tires  14  to be controlled, employing fixed differential deflation pressure X to prevent deflation of the tires below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer. In addition, by being a constant-pressure system and using mechanical components that are mechanically and/or pneumatically actuated, rather than components that are electrically actuated and rely on the electrical system of the trailer, first embodiment tire inflation system  10  is more reliable, more economical, and is easier to install and use than the electrically-actuated and electrically-controlled systems of the prior art. 
     It is to be understood that deflation circuit  24  of first embodiment tire inflation system  10  has been described with reference to the use of separate check valves  22 ,  40 , tee fittings  34 ,  36 , and conduit sections  16 ,  17 ,  19 ,  20 ,  38   a ,  38   b  for the purposes of clear illustration of the invention. Preferably, check valves  22 ,  40  are incorporated into a single or integrated valve body with corresponding passages in the valve body, thereby eliminating one or more of tee fittings  34 ,  36  and conduit sections  16 ,  17 ,  19 ,  20 ,  38   a ,  38   b , without affecting the overall concept or operation of the invention. 
     In addition, as described above, check valve  40  is biased to allow air to flow from the direction of tires  14  to supply tank  12  when the pneumatic pressure in second deflation pneumatic conduit section  38   b  is at least fixed differential X greater than the target pressure. Preferably, rather than employing supply valve  18  in combination with separate first check valve  22  and second check valve  40 , the use of fixed differential X by deflation circuit  24  is accomplished through the use of a relieving regulator with a built-in hysteresis for the supply valve. Such a construction eliminates check valves  22 ,  40  and associated tee fittings  34 ,  36  and conduit sections  17 ,  19 ,  38   a , and  38   b , without affecting the overall concept or operation of the invention. A relieving regulator with a built-in hysteresis for supply valve  18  can be accomplished using several types of structures. 
     A first exemplary relieving regulator with a built-in hysteresis  200  is shown in  FIG. 7 , and includes a body  202 . A supply chamber  204  is formed in body  202 , and is in selective fluid communication with an outlet chamber  206  that is also formed in the body. Disposed between supply chamber  204  and outlet chamber  206  is a supply check member  208 . A pneumatic relief piston  210  selectively mechanically contacts supply check member  208 , and is mechanically connected to a diaphragm  212  and a primary main spring  214 . Adjustment of primary main spring  214  is provided by adjustment of a pressure adjustment screw  216 . In an inflation mode, the pneumatic pressure in outlet chamber  206  is not sufficient to overcome the bias of primary main spring  214 , so that the primary main spring moves diaphragm  212  in a downward direction. Downward movement of diaphragm  212  in turn moves pneumatic relief piston  210  and supply check member  208  downwardly, thereby enabling air to flow from supply chamber  204  past the supply check member to outlet chamber  206 , and out of regulator  200 . 
     Relieving regulator  200  also employs a secondary main spring  218  with a standing height, indicated by d 1 . Secondary main spring  218  resists diaphragm  212  when the diaphragm moves from a neutral position to a relieving position. More particularly, in a deflation or relieving mode, air enters regulator  200  through outlet chamber  206  and causes diaphragm  212  to move in an upward direction when the pneumatic pressure overcomes the bias of primary main spring  214  and secondary main spring  218 . Upward movement of diaphragm causes relief piston  210  to move upwardly, which creates a gap between the relief piston and supply check member  208 . Air then flows through the gap between relief piston  210  and supply check member  208 , through a central bore  211  formed in the relief piston, and through an exhaust passage  220 . As a result, with the use of secondary main spring  218 , the force that is required to relieve pneumatic pressure is greater than the force that is required to deliver pneumatic pressure. By adjusting the spring rate of secondary main spring  218 , the hysteresis can be controlled. Preferably, secondary main spring  218  does not extend to pressure adjustment screw  216 , so that adjustment of primary main spring  214  by the pressure adjustment screw does not affect the secondary main spring. 
     A second exemplary relieving regulator with a built-in hysteresis  222  is shown in  FIG. 8 , and is similar in construction and operation to first exemplary relieving regulator  200  ( FIG. 7 ). However, rather than employing secondary main spring  218 , relieving regulator  222  employs a supply check member  226  that includes a supply check poppet  224 , which is mechanically attached to the supply check member. Supply check poppet  224  is aligned with the central bore of relief piston  210 , as shown in  FIG. 8 . Alternatively, supply check poppet  224  may surround the outside diameter of relief piston  210 . Supply check poppet  224  requires diaphragm  212  to move an upward distance or displacement indicated by d 2  before allowing air to flow through central bore  211  of relief piston  210  and through exhaust passage  220 . By requiring diaphragm  212  to move distance d 2  from a neutral position before regulator  222  starts to relieve air, supply check poppet  224  in turn requires the force to relieve pneumatic pressure to be greater than the force to deliver pneumatic pressure. By requiring movement of distance d 2 , supply check poppet  224  essentially provides resistance against movement of diaphragm  212  that is in addition to the initial resistance provided by primary main spring  214  to relieve pneumatic pressure. 
     With reference now to  FIG. 2A , a second exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention is indicated generally at  50 .  FIG. 2A  shows tire inflation system  50  in an inflation mode, and the direction of air flow is generally indicated by arrows I. Second embodiment tire inflation system  50  is generally similar in structure and operation to first embodiment tire inflation system  10 , with the exception that the second embodiment tire inflation system employs a deflation circuit  52  and a predetermined condition that are different from the first embodiment tire inflation system. As a result, only the differences between second embodiment tire inflation system  50  and first embodiment tire inflation system  10  will be described below. 
     Second embodiment tire inflation system  50  employs a variable deflation pressure as the condition under which deflation occurs, as opposed to fixed differential deflation pressure X employed by first embodiment tire inflation system  10 . More particularly, deflation circuit  52  is pneumatically connected to and includes a portion of pneumatic conduit C. By way of example, in one type of configuration, deflation circuit  52  also includes a first check valve  54 , which is fluidly connected to second pneumatic conduit section  16 . Because second pneumatic conduit section  16  extends to first check valve  54  without a tee fitting, second embodiment tire inflation system  50  eliminates third pneumatic conduit section  17  ( FIG. 1A ), which is employed in first embodiment tire inflation system  10 . 
     First check valve  54  of second embodiment tire inflation system  50  enables air to flow in the direction from supply tank  12  to tires  14 , but prevents air from flowing in the opposite direction, that is, from the tires to the supply tank. Fourth pneumatic conduit section  19  is fluidly connected to and extends between first check valve  54  and a pneumatic fitting  56 , which preferably is a tee fitting. Fifth pneumatic conduit section  20  is fluidly connected to and extends between tee fitting  56  and isolation valve  26  of optional tire isolation system  130 . 
     Deflation circuit  52  further includes a deflation pneumatic conduit  58 . Deflation pneumatic conduit  58  includes a first end  60  and a second end  62 . First end  60  of deflation pneumatic conduit  58  is fluidly connected to tee fitting  56 , which provides fluid communication between fifth pneumatic conduit section  20  and the deflation pneumatic conduit. Second end  62  of deflation pneumatic conduit  58  is fluidly connected to a second check valve  64 . 
     Turning now to  FIG. 2B , second embodiment tire inflation system  50  is shown in a deflation mode, and the direction of air flow is generally indicated by arrows D. Second check valve  64  allows air to flow in the direction from tires  14  to the second check valve to exhaust air directly to atmosphere  66  upon reaching a predetermined condition. More particularly, second check valve  64  is adjustable by mechanical means, such as by rotation of a knob, set screw, stem, and the like, to a setting that actuates or opens the check valve at a predetermined pressure level. The means for adjusting second check valve  64  may be placed in a convenient location inside the vehicle cab (not shown) or outside of the vehicle cab, such as on the trailer of a tractor-trailer, depending on system requirements. This pressure level is a predetermined level, referred to herein as Y. Predetermined level Y is adjustable by a vehicle operator or technician for a specific vehicle load and/or travel conditions through adjustment of second check valve  64 , and thus is a variable deflation pressure employed by deflation circuit  52 . For example, a preferred pressure level Y is the cold-tire target pressure plus 15 psi, so that if the target pressure is 100 psi, Y would be 115 psi. 
     The desirable effect of the use of variable deflation pressure Y in deflation circuit  52  of second embodiment tire inflation system  50  is illustrated by the operation of the system. More particularly, as described above, the vehicle operator or a technician selects a target pressure, which is based on a cold inflation pressure, by adjusting supply valve  18 . As shown in  FIG. 2A , when inflation of tires  14  is required, supply valve  18  is opened or actuated, enabling air to flow from supply tank  12 , through first pneumatic conduit section  15 , through the supply valve and to second pneumatic conduit section  16 . First check valve  54  ensures that air continues to flow through second pneumatic conduit section  16  to fourth pneumatic conduit section  19 , tee fitting  56 , fifth pneumatic conduit section  20 , optional tire isolation pilot valve  26 , sixth pneumatic conduit section  21 , wheel valve  28 , seventh pneumatic conduit section  30 , and into tires  14 . Once the target pressure is reached, supply valve  18  closes. Because tire inflation system  50  is a constant-pressure system, pneumatic pressure remains in second, fourth, fifth, sixth and seventh pneumatic conduit sections  16 ,  19 ,  20 ,  21  and  30 , respectively, and tires  14 . 
     If the pneumatic pressure in tires.  14  increases, deflation of the tires may be necessary. In the prior art tire inflation systems that are not capable of deflation, tires  14  may operate in a significantly over-inflated condition, which undesirably decreases their performance and in turn decreases the life of the tires. In tire inflation systems of the prior art that are capable of deflation, the lack of ability to accommodate an increased tire pressure causes the systems to deflate tires  14  down from the optimum operating pressure to the lower cold-tire target pressure, which undesirably decreases tire performance. However, deflation circuit  52  limits deflation of tires  14  below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer, which optimizes tire performance. 
     More specifically, as shown in  FIG. 2B , first check valve  54  prevents air from flowing in the direction from tires  14  to supply tank  12 . Thus, when the pneumatic pressure in tires  14  increases, the pressure increases in seventh, sixth, fifth and fourth pneumatic conduit sections  30 ,  21 ,  20  and  19 , respectively, to first check valve  54 . First check valve  54  prevents the increased pressure from proceeding directly through second pneumatic conduit section  16  to supply valve  18 . In this manner, first check valve  54  prevents supply valve  18  from exhausting air from second, fourth, fifth, sixth and seventh pneumatic conduit section  16 ,  19 ,  20 ,  21  and  30 , and thus tires  14 , down to a pressure that is below a recommended level. 
     Rather than reaching supply valve  18 , air flows through deflation pneumatic conduit  58  to second check valve  64 . Second check valve  64  only allows air to pass or flow through it if the pneumatic pressure in deflation pneumatic conduit  58  is predetermined level Y psi. When the pneumatic pressure is greater than predetermined level Y, which is greater than the cold-tire target pressure, air flows through second check valve  64  and is exhausted to atmosphere  66  until the pneumatic pressure is reduced to predetermined level Y psi. Once the pneumatic pressure in deflation pneumatic conduit  58  drops below a level of Y psi, second check valve  64  closes and thus prevents further deflation. 
     In this manner, second embodiment tire inflation system  50  provides a constant-pressure system that includes discrete deflation circuit  52 . Discrete deflation circuit  52  accommodates an increased tire pressure due to operating conditions by enabling deflation of tires  14  to be controlled, employing variable deflation pressure Y to prevent deflation of the tires below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer. In addition, by being a constant-pressure system and using mechanical components that are mechanically and/or pneumatically actuated, rather than components that are electrically actuated and rely on the electrical system of the trailer, second embodiment tire inflation system  50  is more reliable, more economical, and is easier to install and use than the electrically-actuated and electrically-controlled systems of the prior art. 
     It is to be understood that deflation circuit  52  of second embodiment tire inflation system  50  has been described with reference to the use of separate check valves  54 ,  64 , tee fitting  56 , and conduit sections  16 ,  19 ,  20 ,  58  for the purposes of clear illustration of the invention. Preferably, check valve  54 ,  64  are incorporated into a single or integrated valve body with corresponding passages in the valve body, thereby eliminating tee fitting  56  and/or one or more conduit sections  16 ,  19 ,  20 ,  58 , without affecting the overall concept or operation of the invention. In addition, as described above, second check valve  64  is mechanically adjustable to exhaust air directly to atmosphere  66  upon reaching predetermined condition Y. Preferably, rather than employing supply valve  18  in combination with separate first check valve  54  and second check valve  64 , the adjustability to achieve predetermined pressure level Y is accomplished by combining the mechanical adjustment of second check valve  64  in supply valve  18 , with a common mechanical drive for the supply valve and the second check valve. Because of the difference between cold tire target pressure and operating pressure, as described in detail above, the target pressure and the predetermined pressure level preferably are adjusted simultaneously. 
     Turning now to  FIG. 3A , a third exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention is indicated generally at  70 .  FIG. 3A  shows third embodiment tire inflation system  70  in an inflation mode, and the direction of air flow is generally indicated by arrows I. Third embodiment tire inflation system with discrete deflation circuit  70  is generally similar in structure and operation to first and second embodiments tire inflation system  10 ,  50 , respectively, with the exception that the third embodiment tire inflation system employs a deflation circuit  72  and a predetermined condition that are different from the first and second embodiments of the tire inflation system. As a result, only the differences between third embodiment tire inflation system  70  and first embodiment tire inflation system  10  will be described below. 
     Third embodiment tire inflation system  70  employs a fixed differential deflation pressure similar to fixed differential deflation pressure X of first embodiment tire inflation system  10 , and further includes monitoring of the supply pressure. More particularly, deflation circuit  72  is pneumatically connected to and includes a portion of pneumatic conduit C. By way of example, in one type of configuration, similar to deflation circuit  24  of first embodiment tire inflation system  10  ( FIG. 1A ), deflation circuit  72  of third embodiment tire inflation system  70  includes first and second tee fittings  34  and  36 , which are spaced apart from one another and are fluidly connected to pneumatic conduit C. First tee fitting  34  is fluidly connected to and extends between second pneumatic conduit section  16  and third pneumatic conduit section  17 , while second tee fitting is fluidly connected to and extends between fourth pneumatic conduit section  19  and fifth pneumatic conduit section  20 . First check valve  22  is disposed between first and second tee fittings  34  and  36 , and is fluidly connected to third pneumatic conduit section  17  and fourth pneumatic conduit section  19 . First check valve  22  enables air to flow in the direction from supply tank  12  to tires  14 , but prevents air from flowing in the opposite direction, that is, from the tires to the supply tank. 
     Deflation circuit  72  further includes a deflation pneumatic conduit  74 , which in turn includes a first deflation conduit section  74   a , a second deflation conduit section  74   b , and a third deflation conduit section  74   c . First deflation conduit section  74   a  includes a first end  76  and a second end  78 . First end  76  of first deflation conduit section  74   a  is fluidly connected to first tee fitting  34 , which provides fluid communication between second pneumatic conduit section  16  and the first deflation conduit section. Second end  78  of first deflation conduit section  74   a  is fluidly connected to a supply override valve  84 , which will be described in greater detail below. 
     Second deflation conduit section  74   b  includes a first end  80  and a second end  82 . First end  80  of second deflation conduit section  74   b  is fluidly connected to second tee fitting  36 , which provides fluid communication between fifth pneumatic conduit section  20  and the second deflation conduit section. Second end  82  of second deflation conduit section  74   b  is fluidly connected to second check valve  40 , similar to deflation circuit  24  of first embodiment tire inflation system  10 . 
     Third deflation conduit section  74   c  includes a first end  86  and a second end  88 . First end  86  of third deflation conduit section  74   c  is fluidly connected to second check valve  40 , and second end  88  of the third deflation conduit section is fluidly connected to supply override valve  84 . In this manner, third deflation conduit section  74   c  extends between second check valve  40  and supply override valve  84 . 
     Similar to deflation circuit  24  of first embodiment tire inflation system  10 , second check valve  40  prevents air from flowing in the direction from supply tank  12  to tires  14 , and is biased to allow air to flow from the direction of the tires to the supply tank only when the pneumatic pressure in second deflation pneumatic conduit section  74   b  is greater than predetermined amount or fixed differential X over the target pressure. Turning to  FIG. 3B , in which third embodiment tire inflation system  70  is shown in a deflation mode and the direction of air flow is generally indicated by arrows D, when the pneumatic pressure is greater than the target pressure plus fixed differential X psi, air flows through second check valve  40  and through third deflation conduit section  74   c  to supply override valve  84 . 
     Supply override valve  84  is shown in  FIGS. 3A and 3B  in an energized state. Supply override valve  84  monitors the pneumatic pressure in first pneumatic conduit section  15 , and thus the pressure that is available from supply tank  12 . More particularly, a supply monitoring pneumatic conduit  90  extends between and is fluidly connected to supply override valve  84  and first pneumatic conduit section  15 . The connection of supply override valve  84  to first pneumatic conduit section  15  enables the supply override valve to detect the pneumatic pressure in the first pneumatic conduit section and thus supply tank  12 . This detection prevents deflation of tires  14  if the pneumatic pressure in supply tank  12  is below a minimum desired pressure level to increase the likelihood that the air pressure in the tires remains above a minimum recommended pressure, as will be described in greater detail below. 
     For example, if the minimum desired pressure level of supply tank  12  is 115 psi, supply override valve  84  is able to detect the pressure level of the supply tank through the connection of supply monitoring pneumatic conduit  90  to first pneumatic conduit section  15 . Supply override valve  84  preferably is a spring-biased pilot valve, so that when the valve detects a pressure level from first pneumatic conduit section  15  that is below 115 psi, the valve remains closed ( FIG. 3B ), thereby preventing exhaustion of air from second check valve  40 , which in turn prevents deflation of tires  14 . When supply override valve  84  detects a pressure level from first pneumatic conduit section  15  that is at or above 115 psi, the valve actuates and thus opens. When supply override valve  84  is open, air flows through the supply override valve  84 , through first deflation conduit section  74   a  to second pneumatic conduit section  16  and to supply valve  18 . Supply valve  18  then exhausts air until the pressure in second pneumatic conduit section  16  drops below a level of the target pressure plus fixed differential X psi, which then causes second check valve  40  to close and prevent further deflation. During deflation, if the pneumatic pressure in supply tank  12  drops below the minimum tank pressure, supply override valve  84  closes to prevent further deflation. 
     The use of supply override valve  84  thus prevents deflation of tires  14  when the pneumatic pressure in supply tank  12  is below a minimum pressure level. This prevention of deflation is desirable because if the pressure level in supply tank  12  becomes low due to air consumption from braking, it is possible that the supply tank may not be able to provide enough air to enable tires  14  to be inflated to the target pressure. If supply tank  12  does not have such sufficient air pressure, it is possible that tire inflation system  70  may actually undesirably remove or deflate air from tires  14 , which in turn would undesirably reduce the pressure in the tires to a level that is below the desired operating pressure. By limiting the amount of deflation that can occur, supply override valve  84  increases the likelihood that the air pressure in tires  14  remains above a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer. 
     In this manner, third embodiment tire inflation system  70  provides a constant-pressure system that includes discrete deflation circuit  72 . Discrete deflation circuit  72  accommodates an increased tire pressure due to operating conditions by enabling deflation of tires  14  to be controlled, employing fixed differential deflation pressure X to prevent deflation of the tires below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer. In addition, third embodiment tire inflation system  70  provides monitoring of the supply pressure to prevent exhaustion of air from tires  14  when the pneumatic pressure in supply tank  12  is low, thereby increasing the likelihood that the air pressure in the tires will remain above a minimum recommended pressure. Moreover, by being a constant-pressure system and using mechanical components that are mechanically and/or pneumatically actuated, rather than components that are electrically actuated and rely on the electrical system of the trailer, third embodiment tire inflation system  70  is more reliable, more economical, and is easier to install and use than the electrically-actuated and electrically-controlled systems of the prior art. 
     It is to be understood that deflation circuit  72  of third embodiment tire inflation system  70  has been described with reference to the use of separate check valves  22 ,  40 , tee fittings  34 ,  36 , conduit sections  16 ,  17 ,  19 ,  20 ,  74   a ,  74   b ,  74   c , and supply override valve  84  for the purposes of clear illustration of the invention. Preferably, check valves  22 ,  40  and/or supply override valve  84  are incorporated into a single or integrated valve body with corresponding passages in the valve body, thereby eliminating one or more of tee fittings  34 ,  36  and conduit sections  16 ,  17 ,  19 ,  20 ,  74   a ,  74   b ,  74   c , without affecting the overall concept or operation of the invention. In addition, as described above, check valve  40  is biased to allow air to flow from the direction of tires  14  to supply tank  12  when the pneumatic pressure in second deflation pneumatic conduit section  74   b  is at least fixed differential X greater than the target pressure. 
     Preferably, rather than employing supply valve  18  in combination with separate first check valve  22  and second check valve  40 , the use of fixed differential X by deflation circuit  72  is accomplished through the use of a relieving regulator with a built-in hysteresis for the supply valve. Such a construction eliminates check valves  22 ,  40  and associated tee fittings  34 ,  36  and conduit sections  17 ,  19 ,  74   a ,  74   b ,  74   c , without affecting the overall concept or operation of the invention. Preferred relieving regulators with a built-in hysteresis include first exemplary relieving regulator  200  and second exemplary relieving regulator  222 , which are shown in  FIGS. 7 and 8 , respectively, and are described above. 
     With reference now to  FIG. 4A , a fourth exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention is indicated generally at  100 .  FIG. 4A  shows tire inflation system  100  in an inflation mode, and the direction of air flow is generally indicated by arrows I. Fourth embodiment tire inflation system with discrete deflation circuit  100  is generally similar in structure and operation to first, second and third embodiments tire inflation system  10 ,  50 ,  70 , respectively, with the exception that the fourth embodiment tire inflation system employs a deflation circuit  102  that only allows deflation when the vehicle is parked. As a result, only the differences between fourth embodiment tire inflation system  100  and first embodiment tire inflation system  10  will be described below. 
     Fourth embodiment tire inflation system  100  retains air pressure in tires  14  during operating conditions by preventing deflation until the vehicle is parked, thereby reducing the likelihood that the vehicle will be operated with tires at a pressure that is too low. More particularly, fourth embodiment tire inflation system  100  employs deflation circuit  102  that includes a deflation pilot valve  104 , which only allows deflation of tires  14  to occur when the vehicle is parked. 
     Deflation circuit  102  is pneumatically connected to and includes a portion of pneumatic conduit C. By way of example, a preferred configuration is similar to deflation circuit  24  of first embodiment tire inflation system  10  ( FIG. 1A ), in which deflation circuit  102  of fourth embodiment tire inflation system  100  includes first and second tee fittings  34  and  36 , which are spaced apart from one another and are fluidly connected to pneumatic conduit C. First tee fitting  34  is fluidly connected to and extends between second pneumatic conduit section  16  and third pneumatic conduit section  17 , while second tee fitting is fluidly connected to and extends between fourth pneumatic conduit section  19  and fifth pneumatic conduit section  20 . First check valve  22  is disposed between first and second tee fittings  34  and  36 , and is fluidly connected to third pneumatic conduit section  17  and fourth pneumatic conduit section  19 . First check valve  22  enables air to flow in the direction from supply tank  12  to tires  14 , but prevents air from flowing in the opposite direction, that is, from the tires to the supply tank. 
     Deflation circuit  102  further includes a deflation pneumatic conduit  106 , which in turn includes a first deflation conduit section  106   a  and a second deflation conduit section  106   b . First deflation conduit section  106   a  includes a first end  108  and a second end  110 . First end  108  of first deflation conduit section  106   a  is fluidly connected to first tee fitting  34 , which provides fluid communication between second pneumatic conduit section  16  and the first deflation conduit section. Second deflation conduit section  106   b  includes a first end  112  and a second end  114 . First end  112  of second deflation conduit section  106   b  is fluidly connected to second tee fitting  36 , which provides fluid communication between fifth pneumatic conduit section  20  and the second deflation conduit section. Because fourth embodiment tire inflation system  100  does not include optional tire isolation system  130 , fifth pneumatic conduit section  20  extends directly to wheel valve  28 , eliminating sixth pneumatic conduit section  21  ( FIG. 1A ), which is employed in first embodiment tire inflation system  10 . 
     Second end  110  of first deflation conduit section  106   a  is fluidly connected to deflation pilot valve  104 , and second end  114  of second deflation conduit section  106   b  is also fluidly connected to the deflation pilot valve. In this manner, deflation pilot valve  104  is fluidly connected to and extends between first deflation conduit section  106   a  and second deflation conduit section  106   b . It is to be understood that deflation pilot valve  104  is shown in  FIG. 4A  in an energized state. 
     Turning to  FIG. 4B , in which fourth embodiment tire inflation system  100  is shown in a deflation mode and the direction of air flow is generally indicated by arrows D, deflation pilot valve  104  enables deflation of tires  14  when the vehicle is parked. More particularly, a parking brake conduit or circuit  116  extends between and is fluidly connected to pilot valve  104  and a parking brake  118  of the vehicle. For trailers of tractor-trailer heavy-duty vehicle applications, parking brake  118  is also referred to in the art as an emergency/supply. The connection of deflation pilot valve  104  to parking brake  118  enables the deflation pilot valve to allow deflation of tires  14  only when the vehicle is parked, thereby preventing deflation of the tires below any minimum recommended guidelines while the vehicle is traveling over-the-road. 
     For example, deflation pilot valve  104  preferably is a spring-biased pilot valve that is biased to an open position. As shown in  FIG. 4B , when the vehicle is parked, there is little or no air pressure on parking brake  118 , which enables deflation pilot valve  104  to remain open. When deflation pilot valve  104  is open, air flows through the deflation pilot valve, through first deflation conduit section  106   a  to second pneumatic conduit section  16  and to supply valve  18 . Supply valve  18  then exhausts air until the pressure in second pneumatic conduit section  16  drops to the target pressure, at which point the supply valve closes. In contrast, as shown in  FIG. 4A , when the vehicle is traveling over-the-road, air pressure is applied to parking brake  118  to release the parking brake. Based upon the connection of deflation pilot valve  104  to parking brake  118  by parking brake conduit  116 , this air pressure overcomes the bias of the deflation pilot valve, moving the valve to a closed position, which in turn prevents deflation of tires  14  during vehicle operation. 
     The use of deflation pilot valve  104  thus prevents deflation of tires  14  when the vehicle is operating over-the-road, and in turn only allows deflation when the vehicle is parked. Because the minimum recommended tire pressure for a specific vehicle load is set by NHTSA and/or the tire manufacturer based on a cold non-operating pressure, and tires  14  are not able to be deflated until the vehicle is parked, the likelihood of operating the vehicle with the tires below the minimum recommended tire pressure thus is reduced. 
     In this manner, fourth embodiment tire inflation system  100  provides a constant-pressure system that includes discrete deflation circuit  102 . Discrete deflation circuit  102  accommodates an increased tire pressure due to operating conditions by enabling deflation of tires  14  to be controlled, employing monitoring of vehicle parking brake  118  to prevent deflation of the tires while the vehicle is operating, thereby reducing the likelihood that the vehicle will be operated with tires at a pressure that is below a recommended inflation level. In addition, by being a constant-pressure system and using mechanical components that are mechanically and/or pneumatically actuated, rather than components that are electrically actuated and rely on the electrical system of the trailer, fourth embodiment tire inflation system  100  is more reliable, more economical, and is easier to install and use than the electrically-actuated and electrically-controlled systems of the prior art. 
     It is to be understood that, while deflation circuit  102  has been described with reference to the use of check valve  22 , tee fittings  34 ,  36 , deflation pilot valve  104 , and conduit sections  16 ,  106   a ,  106   b , the valves may alternatively be incorporated into a single or integrated valve body with corresponding passages in the valve body, thereby eliminating one or more of the tee fittings and conduit sections, without affecting the overall concept or operation of the invention. In addition, as an alternative to monitoring vehicle parking brake  118  to prevent deflation of tires  14  while the vehicle is operating, fourth embodiment tire inflation system  100  may employ other monitoring means. For example, deflation circuit  102  may be connected to the ignition circuit of a tractor of the vehicle to detect or determine whether the vehicle is prepared for operation. In such a case, if the ignition power of the vehicle is detected, thereby indicating that the vehicle is prepared for operation, deflation of tires  14  would be prevented. Also, deflation circuit  102  may be connected to a sensor that detects motion of a wheel of the vehicle, and if the wheel is moving, deflation of tires  14  is prevented. 
     Turning now to  FIG. 5 , an optional aspect or feature of the tire inflation system of the present invention, a tire isolation system, is indicated generally at  130 . Tire isolation system  130  is particularly useful in first, second and third embodiments tire inflation system  10  ( FIGS. 1A and 1B ),  50  ( FIGS. 2A and 2B ) and  70  ( FIGS. 3A and 3B ), respectively. 
     More particularly, as described above, when a vehicle has been parked for an extended period of time, the pneumatic pressure in supply tank  12  may drop or bleed down due to small air leaks that are typical in any pneumatic system. In addition, certain prior art pneumatically-controlled, constant-pressure tire inflation systems include a wheel valve that is capable of deflation, which keeps the inflation path from supply tank  12  to tires  14  open. As a result, when the pneumatic pressure in supply tank  12  drops, the pneumatic pressure in tires  14  also drops, which may be a drop of up to about 25 psi. Then, when the vehicle is started up to prepare for over-the-road travel, tires  14  must be re-inflated up to or near the target pressure, which may involve adding about 25 psi to each one of eight or more tires. This re-inflation process typically takes a great deal of time and places repeated demands on tire inflation system  10 ,  50 ,  70 , which may reduce the life of the system. In addition, if the vehicle operator does not wait for tires  14  to be re-inflated to the target pressure before operating the vehicle, the tires in turn may be operated in an under-inflated condition until the target pressure is reached, which reduces the life of the tires. 
     To minimize pressure loss and the need to provide significant re-inflation of tires  14 , tire isolation system  130  is an optional feature that isolates the tires from supply tank  12  when the vehicle is parked. Tire isolation system  130  includes isolation pilot valve  26 . As described above, when supply valve  18  is in an open position, pressurized air flows through the supply valve to second pneumatic conduit section  16 . In tire isolation system  130 , tee fittings  34 ,  36  ( FIG. 1A ), first check valve  22 , and third, fourth and fifth pneumatic conduit sections  17 ,  19  and  20 , respectively, are optional components. As a result, isolation pilot valve  26  is shown in  FIG. 5  as being fluidly connected to second pneumatic conduit section  16 , so that air flows from supply valve  18  through the second pneumatic conduit section to the isolation pilot valve. It is to be understood that isolation pilot valve  26  is shown in  FIGS. 1A-1B ,  2 A- 2 B, and  3 A- 3 B in an energized state. 
     While the operation of isolation pilot valve  26  will be described in detail below, when the isolation pilot valve is in an open position, air flows through the isolation pilot valve and proceeds to wheel valve  28  and through sixth pneumatic conduit section  21 . Air then flows through wheel valve  28  through seventh pneumatic conduit section  30  to tire valve  32  and into tire  14 . Isolation pilot valve  26  thus is disposed between and interconnects second pneumatic conduit section  16  and sixth pneumatic conduit section  21 , and its actuation affects air flow between supply tank  12  and tires  14 , so that the isolation pilot valve enables isolation of the tires when the vehicle is parked. 
     More particularly, a parking brake conduit  132  extends between and is fluidly connected to isolation pilot valve  26  and a parking brake  134  of the vehicle. For trailers of tractor-trailer heavy-duty vehicle applications, parking brake  134  is also referred to in the art as an emergency/supply. The connection of isolation pilot valve  26  to parking brake  134  enables the isolation pilot valve to isolate tires  14  when the vehicle is parked. 
     More specifically, isolation pilot valve  26  preferably is a spring-biased pilot valve, which is biased to a position that obstructs or blocks the flow of air coming from second pneumatic conduit section  16  and exhausts or vents to atmosphere  138  the flow of air coming from sixth pneumatic conduit section  21 . As a result, when the vehicle is parked, there is little or no air pressure on parking brake  134 , which enables isolation pilot valve  26  to obstruct or block the flow of air coming from second pneumatic conduit section  16  and exhaust to atmosphere  138  the flow of air coming from sixth pneumatic conduit section  21 , thereby interrupting fluid communication between supply tank  12  and tires  14 . This interruption of fluid communication between supply tank  12  and tires  14  and isolates the tires from the supply tank, which in turn minimizes the pressure loss of the tires when the vehicle is parked. For example, as described above, in the prior art, supply tank  12 , and thus tires  14 , may experience a pressure drop of up to 25 psi or more when the vehicle is parked for an extended period of time. With the use of tire isolation system  130 , including isolation pilot valve  26 , such a pressure drop in tires may be reduced to less than 1 psi. 
     When the vehicle travels over-the-road, air pressure is applied to parking brake  118  to release the parking brake. Based upon the connection of isolation pilot valve  26  to parking brake  134  by parking brake conduit  132 , this air pressure overcomes the bias of the isolation pilot valve, moving the valve to an open position. This opening of isolation pilot valve  26  enables air to flow between second pneumatic conduit section  16  and sixth pneumatic conduit section  21  during vehicle operation. 
     Optionally, isolation pilot valve  26  of tire isolation system  130  also includes detection of the pneumatic pressure in first pneumatic conduit section  15  and thus supply tank  12  to enable the isolation pilot valve to isolate tires  14  if the pneumatic pressure in the supply tank is below a minimum desired pressure level. Such an option provides isolation of tires  14  in the event that the pneumatic pressure in supply tank  12  is below a desired level, in which case isolation of the tires is necessary to minimize the pressure loss in tires  14  due to depletion of supply tank  12  when the vehicle is parked. In addition, isolation pilot valve  26  may optionally be a quick-release valve or may incorporate quick-release features to ensure that, upon isolation of tires  14 , sixth pneumatic conduit section  21  is exhausted as quickly as possible, thereby limiting the amount of pneumatic pressure of the tires lost by the exhaustion or venting process. 
     Moreover, as an alternative to monitoring vehicle parking brake  134  to isolate tires  14 , tire isolation system  130  may employ other monitoring means. For example, isolation pilot valve  26  may be connected to the ignition circuit of a tractor of the vehicle to detect or determine whether the vehicle is prepared for operation. In such a case, when the ignition power of the vehicle is not detected, thereby indicating that the vehicle is not prepared for operation, valve  26  would isolate tires  14  as described above. Also, isolation pilot valve  26  may be connected to a sensor that detects motion of a wheel of the vehicle, and if the wheel is not moving, the valve would isolate tires  14 . 
     Tire isolation system  130  thus is an optional feature that is particularly useful in first, second and third embodiments tire inflation system  10 ,  50  and  70 , respectively, to minimize pressure loss when the vehicle is parked, thereby minimizing the need to provide significant re-inflation of tires  14 . Minimizing the need to provide significant re-inflation of tires  14  in turn significantly reduces the time required to inflate the tires upon start-up of the vehicle, and also reduces undesirable demands on tire inflation system  10 ,  50 ,  70 , thereby increasing the life of the system. Tire isolation system  130  also increases the life of tires  14  by reducing the possibility that the tires will be operated before being re-inflated to the target pressure. Moreover, by using mechanical components that are mechanically and/or pneumatically actuated, rather than components that are electrically actuated and rely on the electrical system of the trailer, tire isolation system  130  is reliable, economical, and is easy to install and use. 
     With reference now to  FIG. 6A , a fifth exemplary embodiment of the tire inflation system with discrete deflation circuit of the present invention is indicated generally at  150 .  FIG. 6A  shows tire inflation system  150  in an inflation mode, and the direction of air flow is generally indicated by arrows I. Fifth embodiment tire inflation system with discrete deflation circuit  150  is generally similar in structure and operation to first, second, third, and fourth embodiments tire inflation system  10 ,  50 ,  70 ,  100 , respectively, with the exception that the fifth embodiment tire inflation system employs a deflation circuit  152  that only allows deflation when the vehicle is parked, similar to the fourth embodiment tire inflation system shown in  FIGS. 4A and 4B , and also incorporates a tire isolation system  154 , similar to tire isolation system  130  shown in  FIG. 5  and described above. As a result, only the differences between fifth embodiment tire inflation system  150  and fourth embodiment tire inflation system  100 , and the differences between tire isolation system  154  of the fifth embodiment tire inflation system and tire isolation system  130 , will be described below. 
     Deflation circuit  152  ensures deflation of tires  14  only when the vehicle is parked and the pneumatic pressure of supply tank  12  exceeds a minimum threshold, and also employs tire isolation system  154  to isolate the tires from the supply tank when the vehicle is parked. More particularly, in fifth embodiment tire inflation system  150 , inflation of tires  14  proceeds with air flowing from supply tank  12 , through first pneumatic conduit section  15  to supply valve  18 , and through the supply valve when the supply valve has been actuated, as described above. When supply valve  18  has been actuated, air flows into second pneumatic conduit section  16 . 
     By way of example, in a preferred configuration, first tee fitting  34  is fluidly connected to and extends between second pneumatic conduit section  16  and to third pneumatic conduit section  17 . Third pneumatic conduit section  17  is fluidly connected to and extends between first tee fitting  34  and first check valve  22 . First check valve  22  is fluidly connected to and extends between third pneumatic conduit section  17  and fourth pneumatic conduit section  19 , and enables air to flow in the direction from supply tank  12  to tires  14 , but prevents air from flowing in the opposite direction, that is, from the tires to the supply tank. Fourth pneumatic conduit section  19  is fluidly connected to and extends between first check valve  22  and second tee fitting  36 , which in turn is fluidly connected to and extends between the fourth pneumatic conduit section  19  and fifth pneumatic conduit section  20 . 
     During inflation, air thus flows through second pneumatic conduit section  16 , third pneumatic conduit section  17 , check valve  22 , fourth pneumatic conduit section  19 , and fifth pneumatic conduit section  20  to isolation pilot valve  172 , which is fluidly connected to the fifth pneumatic conduit section. 
     While the operation of isolation pilot valve  172  will be described in greater detail below, once air flows through the isolation pilot valve, it proceeds through a first portion  21   a  of sixth pneumatic conduit section  21 , which extends between and is fluidly connected to the isolation pilot valve and an optional quick release valve  174 . Optional quick release valve  174  provides more rapid actuation of isolation pilot valve  172 , as known in the art. The air then flows through a second portion  21   b  of sixth pneumatic conduit section  21 , which extends between and is fluidly connected to optional quick release valve  174  and mechanically-operated wheel valve  28 . After flowing through wheel valve  28 , air flows to tire valve  32  and into tire  14  through seventh pneumatic conduit section  30 . 
     Deflation circuit  152  includes a first deflation pneumatic conduit  176 , which extends between and is fluidly connected to first tee fitting  34  and a deflation pilot valve  178 . Deflation circuit  152  also includes a second deflation pneumatic conduit  184 , which extends between and is fluidly connected to deflation pilot valve  178  and second tee fitting  36 . It is to be understood that isolation pilot valve  172  and deflation pilot valve  178  are shown in  FIG. 6A  in an energized state. 
     Turning to  FIG. 6B , in which fifth embodiment tire inflation system  150  is shown in a deflation mode and the direction of air flow is indicated by arrows D, deflation pilot valve  178  enables deflation of tires  14  when the vehicle is parked. More particularly, a parking brake conduit or circuit  180  extends between and is fluidly connected to deflation pilot valve  178  and a parking brake  182  of the vehicle. For trailers of tractor-trailer heavy-duty vehicle applications, parking brake  182  is also referred to in the art as an emergency/supply. In a manner similar to that as described above for fourth embodiment tire inflation system  100  ( FIGS. 4A and 4B ), the connection of deflation pilot valve  178  to parking brake  182  enables the deflation pilot valve to allow deflation of tires  14  only when the vehicle is parked, thereby preventing deflation of the tires while the vehicle is traveling over-the-road. 
     Deflation pilot valve  178  preferable is a spring-biased pilot valve that is biased to an open position. As shown in  FIG. 6B , when the vehicle is parked, there is little or no air pressure on parking brake  182 , which enables deflation pilot valve  178  to remain open. When deflation pilot valve  178  is open, air flows through the deflation pilot valve, through first deflation conduit  176  to second pneumatic conduit section  16  and to supply valve  18 . Supply valve  18  then exhausts air until the pressure in second pneumatic conduit section  16  drops to the target pressure, at which point the supply valve closes. In contrast, as shown in  FIG. 6A , when the vehicle is traveling over-the-road, air pressure is applied to parking brake  182  to release the parking brake. Based upon the connection of deflation pilot valve  178  to parking brake  182  by parking brake conduit  180 , this air pressure overcomes the bias of the deflation pilot valve, moving the valve to a closed position, which in turn prevents deflation of tires  14  during vehicle operation. 
     The use of deflation pilot valve  178  thus prevents deflation of tires  14  when the vehicle is traveling, and in turn only allows deflation when the vehicle is parked. Because the minimum recommended tire pressure for a specific vehicle load is set by NHTSA and/or the tire manufacturer based on a cold non-operating pressure, and tires  14  are not able to be deflated until the vehicle is parked, the likelihood of operating the vehicle with the tires below the minimum recommended tire pressure thus is reduced. 
     As shown in  FIGS. 6A and 6B , fifth embodiment tire inflation system  150  also includes tire isolation system  154  to minimize pressure loss in tires  14  due to bleeding down of supply tank  12  when the vehicle is parked for an extended period of time. Minimizing the pressure loss in tires  14  reduces the time required to re-inflate the tires upon actuation of the vehicle, and also desirably minimizes the demands on tire inflation system  150 . Minimizing the pressure loss in tires  14  also increases the life of tires  14  by reducing the possibility that the tires will be operated before being re-inflated to the target pressure. 
     Tire isolation system  154  includes isolation pilot valve  172 , which is disposed between and interconnects fifth pneumatic conduit section  20  and first portion  21   a  of sixth pneumatic conduit section  21 . This location of isolation pilot valve  172  affects air flow between supply tank  12  and tires  14 , so that the isolation pilot valve enables isolation of the tires when the vehicle is parked. More particularly, a parking brake conduit  186  extends between and is fluidly connected to isolation pilot valve  172  and parking brake  182 . It is to be understood that isolation pilot valve  172  is shown in  FIGS. 6A and 6B  in an energized state. 
     Isolation pilot valve  172  preferably is a spring-biased pilot valve, which is biased to a position that obstructs or blocks the flow of air coming from fifth pneumatic conduit section  20  and exhausts or vents to atmosphere  190  the flow of air coming from sixth pneumatic conduit section  21 . As a result, when the vehicle is parked, there is little or no air pressure on parking brake  182 , which enables isolation pilot valve  172  to obstruct or block the flow of air coming from fifth pneumatic conduit section  20  and exhaust to atmosphere  190  the flow of air coming from sixth pneumatic conduit section  21 , thereby interrupting fluid communication between supply tank  12  and tires  14 . This interruption of fluid communication between supply tank  12  and tires  14  isolates the tires from the supply tank, which in turn minimizes the pressure loss of the tires when the vehicle is parked. 
     When the vehicle travels over-the-road, air pressure is applied to parking brake  182  to release the parking brake. Based upon the connection of isolation pilot valve  172  to parking brake  182  by parking brake conduit  186 , this air pressure overcomes the bias of the isolation pilot valve, moving the valve to an open position. This opening of isolation pilot valve  172  enables air to flow between fifth pneumatic conduit section  20  and sixth pneumatic conduit section  21  during vehicle operation. 
     Preferably, tire isolation system  154  includes a supply pressure monitoring conduit  192  that extends between and is fluidly connected to isolation pilot valve  172  and supply tank  12 . Supply pressure monitoring conduit  192  enables isolation pilot valve  172  to detect the pneumatic pressure in supply tank  12  to in turn enable the isolation pilot valve to isolate tires  14  if the pneumatic pressure in the supply tank is below a minimum desired pressure level. Supply pressure monitoring conduit  192  thus provides isolation of tires  14  in the event that the pneumatic pressure in supply tank  12  is below a desired level, in which case isolation of the tires is necessary to minimize the pressure loss in tires  14  due to depletion of supply tank  12  when the vehicle is parked. In addition, when the pressure in supply tank  12  is above a minimum desired pressure, the air pressure in supply pressure monitoring conduit  192  overcomes the bias of isolation pilot valve  172 , moving the valve to an open position. This opening of isolation pilot valve  172  enables air to flow between fifth pneumatic conduit section  20  and sixth pneumatic conduit section  21 , thereby enabling air to flow even when the vehicle is parked. 
     In this manner, fifth embodiment tire inflation system  150  provides a constant-pressure system that includes discrete deflation circuit  152 . Discrete deflation circuit  152  accommodates an increased tire pressure due to operating conditions by enabling deflation of tires  14  to be controlled, employing monitoring of vehicle parking brake  182  to prevent deflation of the tires while the vehicle is traveling, thereby reducing the likelihood that the vehicle will be operated with tires at a pressure that is too low. In addition, by being a constant-pressure system and using mechanical components that are mechanically and/or pneumatically actuated, rather than components that are electrically actuated and rely on the electrical system of the trailer, fifth embodiment tire inflation system  150  is more reliable, more economical, and is easier to install and use than the electrically-actuated and electrically-controlled systems of the prior art. 
     It is to be understood that, while deflation circuit  152  has been described with reference to check valve  22 , tee fittings  34 ,  36 , deflation pilot valve  178 , and conduit sections  16 ,  17 ,  19 ,  176 ,  184 , the valves may alternatively be incorporated into a single or integrated valve body with corresponding passages in the valve body, thereby eliminating one or more of the tee fittings and conduit sections, without affecting the overall concept or operation of the invention. In addition, as an alternative to monitoring vehicle parking brake  182  to prevent deflation of tires  14  while the vehicle is traveling, fifth embodiment tire inflation system  150  may employ other monitoring means. For example, deflation circuit  152  may be connected to the ignition circuit of a tractor of the vehicle to detect or determine whether the vehicle is prepared for operation. In such a case, if the ignition power of the vehicle is detected, thereby indicating that the vehicle is prepared for operation, deflation of tires  14  would be prevented. Also, deflation circuit  152  may be connected to a sensor that detects motion of a wheel of the vehicle, and if the wheel is moving, deflation of tires  14  is prevented. 
     Fifth embodiment tire inflation system  150  also includes tire isolation system  154 , which minimizes pressure loss when the vehicle is parked, thereby minimizing the need to provide significant re-inflation of tires  14 . Minimizing the need to provide significant re-inflation of tires  14  in turn significantly reduces the time required to inflate the tires upon start-up of the vehicle, and reduces undesirable demands on tire inflation system  150 , thereby increasing the life of the system. Tire isolation system  154  also increases the life of tires  14  by reducing the possibility that the tires will be operated before being re-inflated to the target pressure. 
     As an alternative to monitoring vehicle parking brake  182  to prevent deflation of tires  14  while the vehicle is traveling, tire isolation system  154  of fifth embodiment tire inflation system  150  may employ other monitoring means. For example, isolation pilot valve  172  may be connected to the ignition circuit of a tractor of the vehicle to detect or determine whether the vehicle is prepared for operation. In such a case, when the ignition power of the vehicle is not detected, thereby indicating that the vehicle is not prepared for operation, valve  172  would isolate tires  14  as described above. Also, isolation pilot valve  172  may be connected to a sensor that detects motion of a wheel of the vehicle, and if the wheel is not moving, the valve would isolate tires  14 . 
     It is to be understood that tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150  fluidly connects supply tank  12  to a plurality of vehicle tires  14 , and one tire has been shown herein for the purpose of convenience. Discrete deflation circuit  24 ,  52 ,  72 ,  102 ,  152  fluidly connects to and communicates with the plurality of vehicle tires  14 , thereby enabling control of deflation of multiple tires along or through a single common pneumatic circuit. In this manner, tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150 , with each respective discrete deflation circuit  24 ,  52 ,  72 ,  102 ,  152 , provides an efficient and economical system. 
     The above-described structure and function of tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ;  150  thus overcome the disadvantages of prior art tire inflation systems. More particularly, discrete deflation circuit  24 ,  52 ,  72 ,  102 ,  152  of tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150 , respectively, enables control of deflation of tires  14  based on specific predetermined conditions to accommodate an increased tire pressure based on operating conditions. This control prevents deflation based on a cold-tire target pressure setting when the tires increase to a higher operating pressure, thereby reducing the likelihood that the vehicle may be operated with tires  14  being below a level that is recommended by NHTSA or the tire manufacturer, which in turn optimizes tire performance. 
     More specifically, deflation circuit  24  of first embodiment tire inflation system  10  employs fixed differential deflation pressure X to prevent deflation of tires  14  below a minimum predetermined pressure, such as a minimum recommended pressure for a specific vehicle load as set by NHTSA and/or the tire manufacturer. Deflation circuit  52  of second embodiment tire inflation system  50  employs variable deflation pressure Y to prevent deflation of tires  14  below a minimum predetermined pressure. Deflation circuit  72  of third embodiment tire inflation system  70  employs fixed differential deflation pressure X to prevent deflation of the tires below a minimum predetermined pressure, and also monitors the supply pressure to prevent exhaustion of air when the pressure in supply tank  12  is below a predetermined level, thereby desirably reducing the demands placed on the supply tank and minimizing the time required to re-inflate tires  14 . Deflation circuit  102  of fourth embodiment tire inflation system  100  employs monitoring of the vehicle parking brake to prevent deflation of the tires while the vehicle is traveling over-the-road, which reduces the likelihood that the vehicle will be operated with tires at a pressure that is too low. Deflation circuit  152  of fifth embodiment tire inflation system  150  employs monitoring of the vehicle parking brake to prevent deflation of the tires while the vehicle is operating, and also includes tire isolation system  154 . 
     Tire isolation system  154  of fifth embodiment tire inflation system  150 , and optional tire isolation system  130  for use with first, second and third embodiments tire inflation system  10 ,  50 ,  70 , respectively, isolates tires  14  from supply tank  12  when the vehicle is parked. This isolation minimizes the pressure loss of tires  14  while the vehicle is parked, which in turn minimizes the amount of time needed to re-inflate the tires when the vehicle is activated, and desirably reduces the demand on each tire inflation system  10 ,  50 ,  70 ,  150  for re-inflation of the tires. Tire isolation system  130 ,  154  also increases the life of tires  14  by reducing the possibility that the tires will be operated before being re-inflated to the target pressure. 
     Tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150  preferably employs mechanical components that are mechanically and/or pneumatically actuated, rather than electronically-operated solenoid valves, electronic controllers, and other electronic components, which are expensive and often complex to install and configure. As a result, tire inflation system  10 ,  50 ,  70 ,  100 ,  150  is simple, economical and easy to install. In addition, by being a mechanically and pneumatically actuated system, tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150  is reliable, since it does not require the use of the electrical system of the trailer, which may be unreliable or even non-functional at times. 
     Moreover, by not exhausting when inflation of tires  14  is complete, tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150  is a constant-pressure system. Such a constant-pressure system  10 ,  50 ,  70 ,  100 ,  150  does not require expensive and complex electronic controls to determine when it is necessary to trigger or commence inflation. For this additional reason, tire inflation system  10 ,  50 ,  70 ,  100 ,  150  is simple, economical and easy to install, and by not employing electrical components, does not require the use of the electrical system of the trailer and thus is reliable. In addition, as a constant-pressure system, tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150  continuously monitors tire pressure and dynamically responds to pressure changes, thereby actively or quickly responding to reduced tire pressure conditions, such as in the case of an air leak. 
     An additional feature of tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150  is the ability to optionally locate deflation circuits  24 ,  52 ,  72 ,  102 ,  152 , respectively, near supply valve  18 , which enables the valves of the deflation circuit to be in an enclosure with the supply valve. Such an enclosure protects the valves, and in turn protects any valve ports that may exhaust to atmosphere. Enclosing and thus protecting the valve ports keeps them clean and open, in contrast to prior art tire inflation systems, which often employ exhaust valves that are adjacent tires  14  and thus cannot be enclosed. Such exhaust valves of prior art systems are exposed to the elements and often encounter problems with contamination, which impairs operation of the valves and reduces the efficiency of the system. By optionally enclosing and protecting valves of deflation circuits  24 ,  52 ,  72 ,  102 ,  152 , optimum valve operation is maintained, thereby maintaining the efficiency of tire inflation system of the present invention  10 ,  50 ,  70 ,  100 ,  150 , respectively. 
     Another feature that may optionally be included in certain embodiments of the invention, such as second embodiment tire inflation system  50 , is an option to include mechanical means, such as a spring or threaded drive, on supply valve  18  and/or valve  64  of deflation circuit  52  to adjust the regulator pressure of each valve simultaneously with a check of atmospheric pressure. This adjustment enables second embodiment tire inflation system  50  to make adjustments based on a comparison to atmospheric pressure, which improves the accuracy and efficiency of the system. 
     The present invention also includes a method of providing a tire inflation system with a deflation circuit that is discrete or separate from the inflation circuit, and a method of deflating a tire using a deflation circuit that is separate from an inflation circuit, both of which desirably enable control of the conditions under which deflation occurs. The present invention also includes a method of providing a tire inflation system with a tire isolation system when the vehicle is parked, and a method of isolating a tire when the vehicle is parked. Each method includes steps in accordance with the description that is presented above and shown in  FIGS. 1A-6B . 
     It is to be understood that the structure of the above-described tire inflation system with discrete deflation circuit of the present invention  10 ,  50 ,  70 ,  100 ,  150 , and tire isolation system  130 ,  154 , may be altered or rearranged, or certain components omitted or added, without affecting the overall concept or operation of the invention. For example, valves in addition to or other than those shown and described may be employed, including solenoid valves, and the location and arrangement of components may be adjusted based on specific design requirements. In addition, components such as optional tire isolation system  130 ,  154  may be omitted, or may be employed in tire inflation systems having configurations other than those shown herein. It is to be further understood that the present invention finds application in types of tire inflation systems for heavy-duty vehicles, other than those shown and described herein and which are known to those skilled in the art, without affecting the concept or operation of the invention. Moreover, gases other than air that may be compressed and follow the principles of fluid flow, including nitrogen, carbon dioxide, and the like, may be employed without affecting the concept or operation of the invention. 
     Accordingly, the improved tire inflation system with discrete deflation circuit is simplified, provides an effective, safe, inexpensive, and efficient structure which achieves all the enumerated objectives, provides for eliminating difficulties encountered with prior art tire inflation systems, and solves problems and obtains new results in the art. 
     In the foregoing description, certain terms have been used for brevity, clarity and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the present invention has been described with reference to exemplary embodiments. It shall be understood that this illustration is by way of example and not by way of limitation, as the scope of the invention is not limited to the exact details shown or described. Potential modifications and alterations will occur to others upon a reading and understanding of this disclosure, and it is understood that the invention includes all such modifications and alterations and equivalents thereof. 
     Having now described the features, discoveries and principles of the invention, the manner in which the improved tire inflation system with discrete deflation circuit is constructed, arranged and used, the characteristics of the construction and arrangement, and the advantageous, new and useful results obtained; the new and useful structures, devices, elements, arrangements, parts and combinations are set forth in the appended claims.