Patent Publication Number: US-9884522-B2

Title: Central tire inflation system and method

Description:
TECHNICAL FIELD 
     The present invention relates to a central tire inflation system (CTIS) and to a related method. The present invention also relates to a processor for controlling operation of a central tire inflation system (CTIS) to inflate at least one tire of a vehicle and to a related method. The present invention also relates to an electronic control unit (ECU); to a vehicle; and to related computer program products. 
     BACKGROUND 
     Central tire inflation systems (CTISs) were originally developed for military applications, in particular for military applications concerning off-road military wheeled trucks and trailers. However, CTISs are nowadays incorporated into non-military vehicles such as specialist construction equipment and some agricultural vehicles. 
     A CTIS typically comprises a compressed air source located on-board the vehicle and connected to one or more tires. Tire pressure can therefore be adjusted by operating the CTIS. The CTIS delivers compressed air to tire supply lines. In some examples, the supply lines are integrated into the vehicle axles. Various valves are provided in the CTIS to control flow of compressed air. 
     At least in certain embodiments, the present invention aims to solve, or at least mitigate, at least some problems that can be identified in the prior art, and/or to provide an improved CTIS and/or CTIS control strategy compared to the prior art. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention relate to a CTIS; to a vehicle; to a related method; and to a related computer program product. Aspects of the present invention also relate to a processor; to an ECU; to a CTIS; to a vehicle; to a related method; and to a related computer program product. 
     According to an aspect of the present invention, there is provided a central tire inflation system for a vehicle comprising:
         at least one pneumatic control valve for controlling inflation of a respective tire;   a first compressed air source for supplying compressed air to inflate the tire;   a second compressed air source for supplying compressed air to control said at least one pneumatic control valve; and   control means for controlling the supply of compressed air from the second compressed air source to generate a pneumatic control signal to control said at least one pneumatic control valve.       

     At least in certain embodiments, the first and second compressed air sources can have different operating parameters suitable for performing the respective functions of inflating the tire(s) and generating the pneumatic control signal. The second compressed air source can operate at a higher pressure than the first compressed air source. The at least one pneumatic control valve can have a higher activation pressure than the operating pressure of the first compressed air source. Thus, at least in certain embodiments, the operation of the first compressed air source will not interfere with operation of the at least one pneumatic control valve. Moreover, the first compressed air source can have a higher flow rate than the second compressed air source suitable for inflating the tire. 
     The pneumatic control valve is operable to change state in response to the pneumatic control signals. The pneumatic control signal comprises fluid pressure changes in a control line in communication with the pneumatic control valve. The first compressed air source can optionally also be connected to the control line. Thus, the control line can function as a supply line for supplying compressed air from the first and second compressed air sources to the tire. The pneumatic control valve can be disposed in said supply line. The pneumatic control valve can cycle between different states in dependence on said pneumatic control signal. For example, the pneumatic control valve can cycle between one or more of the following: a closed state (for normal running), an open state (for inflation), and a deflation/pressure measuring state. The first compressed air source can provide a substantially steady state supply of compressed air for inflating the tire. The second compressed air source can supply a control signal for controlling operation of the pneumatic control valve. The control signal can be a pneumatic control signal for controlling operation of the pneumatic control valve. 
     The first compressed air source can be distinct and/or separate from the second compressed air source, or may be connected. For example the first and second air sources may each comprise a dedicated compressor, or a single compressor could be used in combination with one or more reservoir vessels to provide the second air source. The first and second compressed air sources can each be in fluid communication with one or more compressed air supply lines. The first compressed air source can, for example, consist of a dedicated compressor without an associated reservoir such that air is supplied directly from the compressor to the tires of the vehicle, via associated valves. 
     The second compressed air source can comprise a reservoir operating at a higher pressure than the supply of air from the first compressed air source for inflating the tires of the vehicle. The use of a reservoir in the second compressed air source is advantageous as it is not necessary periodically to operate a compressor to generate the pneumatic control signal which would require additional compressor cycles, contributing to compressor deterioration over time. The second compressed air source can be configured to deliver compressed air at a lower pressure than the reservoir pressure, at or above a minimum activation pressure required to switch the pneumatic control valve, by means of a pressure reduction valve at the reservoir outlet. The switching pressure may be regulated to approximately 5 bar. 
     The vehicle can have more than one tire. For example, the vehicle can have four tires; two front tires mounted on a front axle and two rear tires mounted on a rear axle. The CTIS can be configured to inflate one or more tires at a time. In particular, the CTIS can be configured to simultaneously inflate all of the tires of the vehicle, or to simultaneously inflate two or more tires connected to a same axle of the vehicle. The inflation of each tire can be controlled by a respective pneumatic control valve. However, a pneumatic control valve could simultaneously control the supply of compressed air to two or more tires. 
     A staggered tire inflation strategy can be implemented to inflate the tires on each axle in sequence. The tires on the front axle could be inflated before the tires on the rear axle. Alternatively, the tires on the rear axle can be inflated before the tires on the front axle. At least in certain embodiments, the latter approach can help to preserve the dynamic stability or handling balance of the vehicle. In particular, by inflating the tires mounted to the rear axle and then inflating the tires mounted to the front axle the handling characteristics of the vehicle can, for example, be biased towards understeer. This approach is advantageous if the CTIS is to operate when the vehicle is moving. 
     The first compressed air source can be a first compressor associated with the CTIS. The first compressor can be a dedicated compressor for inflating one or more tires. The first compressed air source can be a high-flow compressed air source configured to deliver a higher flow rate than the second compressed air source. The second compressed air source can be a low-flow compressed air source configured to deliver a lower flow rate than the first compressed air source. The first compressed air source can be configured to deliver air at a high flow rate and a low pressure (HF/LP) to the tires of the vehicle. The second compressed air source can be configured to deliver air at a low flow rate and a high pressure (LF/HP). The characteristics of the air delivered by each of said first and second air sources can be controlled by respective flow control devices, such as a flow regulator. 
     The terms “high” and “low” are used herein as relative terms to define the characteristics of the air delivered by the first and second compressed air sources. The first compressed air source can be configured to deliver air at a lower pressure than the second compressed air source. Equally, the first compressed air source can be configured to deliver air at a higher flow rate than the second compressed air source. It will be appreciated that references herein to the delivery of low pressure air occurs at a pressure above the tire pressure in order to perform the required tire inflation function. 
     The dedicated compressor of the first air supply can be powered by an electric motor or from the vehicle engine. The dedicated compressor can deliver air at a maximum pressure output of at least 5 bar, preferably in the range of 5 to 12 bar, more preferably in the range of 8 to 10 bar. 
     The CTIS can comprise a valve block. The valve block can be configured for receiving incoming compressed air supplied by the first and/or second compressed air sources. The valve block can be configured to distribute said compressed air to the one or more tires, as required. The valve block can comprise one or more inlet valves, each for controlling flow of compressed air from a respective one of the first and second compressed air sources. The one or more inlet valves can control the supply of compressed air into a gallery provided in said valve block. A first inlet valve can be provided for controlling flow of compressed air from the first compressed air source. A second inlet valve can be provided for controlling flow of compressed air from the second compressed air source. The second inlet valve can function as the flow control means for generating the pneumatic control signals. The valve block can comprise one or more outlet valves, each for controlling flow of compressed air from the valve block to one or more of the tires of the vehicle. Each tire can have an associated outlet valve. Each tire can have an associated tire supply line extending from the valve block to the corresponding pneumatic control valve. The tire supply lines can be in fluid communication with said one or more compressed air supply lines via the valve block. 
     The control means is provided for controlling the supply of compressed air from the second compressed air source to generate said pneumatic control signal. The flow control means can comprise one or more of said inlet valves and/or one or more of said outlet valves. The one or more valves can each be an electromechanical valve, for example a solenoid valve. The valve can be an inlet valve for controlling the supply of compressed air from the second compressed air source to the valve block. The compressed air from the second compressed air source can be supplied to a gallery in the valve block. The valve can control the supply of compressed air into said gallery. The outlet valve can control the supply of compressed air from the gallery into one or more of said tire supply lines. 
     In use, the control means generates a pneumatic control signal to control said pneumatic control valve(s). In systems comprising a plurality of pneumatic control valves, the control means can be configured to control each pneumatic control valve independently. Alternatively, or in addition, the control means can be configured to generate a pneumatic control signal to control more than one of said plurality of pneumatic control valves simultaneously. In certain arrangements, the control means can be configured to generate a pneumatic control signal to control all of said plurality of pneumatic control valves simultaneously. The central tire inflation system can comprise a separate valve associated with each tire supply line to control the communication of the pneumatic control signal to the pneumatic control valve associated with that tire supply line. For example, the outlet valves of the valve block could be operated selectively to deliver the pneumatic control signal only to those tire lines containing a pneumatic control valve which is to be controlled. 
     Each tire supply line can be operative to supply compressed air from said first compressed air source and/or from said second compressed air source. In use, compressed air can be supplied to the tire supply line either from the first compressed air source or from the second compressed air source. Alternatively, compressed air can be supplied from both the first and second compressed air sources simultaneously. 
     The pneumatic control valve has a plurality of operating states. The pneumatic control valve can toggle between said operating states, or cycle through said operating states in dependence on the pneumatic control signal. For example, upon receipt of a pneumatic control signal, the pneumatic control valve can toggle to another operating state, or can cycle to the next operating state in a sequence. The pneumatic control signal can, for example, comprise a pulse of high pressure air. 
     An electronic control unit (ECU) can be provided to control operation of the CTIS. The ECU can control the first compressed air source and/or the second compressed air source. In addition, or alternatively, the ECU can be configured to control the valve block. The ECU can control the operation of said first inlet valve, said second inlet valve and said at least one outlet valve to control the supply of compressed air to the or each tire supply line. It will be understood that control by the ECU of the first and/or second compressed air source, dedicated compressor, valve block and/or any other component of the CTIS as described herein, is by generation of one or more signals which can be routed and delivered as appropriate by a vehicle communication network. The vehicle communication network can be part of a vehicle control system. The vehicle communication network can comprise a controller area network (CAN). The vehicle control system can comprise said ECU. 
     The CTIS can comprise an air dryer unit for drying compressed air supplied by said first compressed air source and/or said second compressed air source. The valve block can control the supply of air to the air dryer unit. The ECU can control a purge function of the air dryer unit. For example, the ECU can control the valve block to exhaust air through the air dryer unit to expel collected water. The ECU can initiate a purge cycle when the air dryer unit has reached a predefined saturation threshold. 
     In one arrangement the second compressed air source can be a compressed air source associated with the air suspension system for controlling the vehicle suspension. The second compressed air source can be an air suspension reservoir. The air suspension system can comprise a compressor, an air suspension reservoir, and one or more pneumatic actuators. At least in certain embodiments, the one or more pneumatic actuators can control the operating characteristics of the vehicle suspension, for example to adjust the ride height of the vehicle. The compressor can charge the reservoir with air at a pressure up to 25 bar, preferably in the region of 12 to 20 bar. In one embodiment the reservoir may be charged up to a pressure of 16 bar. A pressure regulator is preferably provided between the second compressed air source and the pneumatic control valve to reduce the air pressure to within a working pressure range of the pneumatic control valve. Using the air suspension reservoir to function as the second compressed air source is advantageous since the relatively low volumes of high pressure air required to generate the pneumatic control signal can be taken from the air suspension reservoir without affecting the vehicle ride height or vehicle stability, and avoids the need for a further reservoir and/or reservoir. 
     In an alternative arrangement the first compressed air source can comprise a first compressor. In a first operating mode, the first compressor can supply air directly to the tires of the vehicle, without passing through any intervening reservoir. The first compressor can also be selectively connected to a reservoir associated with the second compressed air source. The compressor can be operated to charge said reservoir. In use, the reservoir can be charged to a pressure significantly above the tire pressure, for example, for a tire pressure in the range of 1.5 to 3 bar the reservoir pressure of the second air supply may be in the range of 5 to 10 bar, or greater. Advantageously, by providing the second compressed air source at a higher pressure, each pulse of air provided therefrom to operate the pneumatic control valve can result in a small change in the reservoir pressure. In this manner it may not be required to run the compressor each time the pneumatic control valves are operated, thereby reducing compressor cycling and potentially increasing compressor life. Once the reservoir pressure falls below a lower threshold, providing the compressor is not supplying air to the tires of the vehicle, the first compressor can be operated to recharge the reservoir. For example, one or more control valves controlling the flow of air from the compressor may be configured to direct air from the compressor to the reservoir. Alternatively, whenever the compressor is run, prior to shutting off the compressor the reservoir can be recharged, thereby maintaining the reservoir in a charged condition without increasing the on/off cycles of the compressor. For example, the one or more control valves can be operated to switch from providing a fluid connection between the compressor and said tire(s) to establishing a fluid connection between the compressor and said reservoir. 
     The valve block can comprise a gallery. The first compressed air source and/or the second compressed air source can be in selective fluid communication with said gallery. The valve block can comprise a first inlet valve for controlling the flow of compressed air from the first compressed air source to the gallery. The valve block can comprise a second inlet valve for controlling flow of compressed air from the second compressed air source to the gallery. 
     An outlet valve can be provided to control the supply of compressed air to each tire supply line. In an embodiment, the valve block can have four outlet valves, each connected to a respective one of four tire supply lines, each for supplying compressed air to a respective one of four tires. The four tires can be a front left, front right, rear left and rear right tire of an vehicle. 
     Although described herein in respect to inflation of wheels on two axles of a vehicle, in another embodiment the CTIS may supply pressure only to tires on the driven axle of the vehicle. In such an embodiment the valve block can have two outlet valves, each connected to a respective one of two tire supply lines, each for supplying compressed air to a respective one of two tires disposed on the driven axle of a two wheel drive vehicle. The two tires may be a front left and front right tire (front wheel drive) or may be a rear left and rear right tire (rear wheel drive). 
     The valve block can comprise one or more exhaust valves for exhausting compressed air from the valve block to atmosphere. 
     The inlet valve(s) and/or the outlet valve(s) and/or the exhaust valve(s) can each be a solenoid valve. A solenoid valve can be a normally-closed type valve. A solenoid valve can be a normally-open type valve. The valve block can also comprise a safety valve. The safety valve can be a solenoid, normally-open type valve. 
     Each pneumatic control valve can be operably coupled to a respective one of the tire supply lines of the vehicle. The pneumatic control valve can be mounted to a wheel, optionally removably. In an arrangement, the pneumatic control valve can be a latching valve. The pneumatic control valve can cycle between one or more of the following: a closed state (for normal running), an open state (for inflation), and a deflation/pressure measuring state. The pneumatic control valve can cycle between said states in dependence on the control signal supplied by the second compressed air source. 
     The ECU can control operation of one or more of the following: the first inlet valve; the second inlet valve; the outlet valve(s); the exhaust valve(s); and/or the safety valve(s). 
     According to another aspect of the present invention, there is provided a method of operating a CTIS to inflate a tire of a vehicle, the method comprising the steps of:
         using a first compressed air source, providing a first supply of compressed air for inflating the tire;   using a second compressed air source, generating a pneumatic control signal to control operation of a pneumatic control valve for controlling inflation of the tire.       

     According to another aspect of the present invention, there is provided an ECU for controlling a CTIS, the ECU comprising a processor as described herein. 
     According to another aspect of the present invention, there is provided a CTIS comprising a processor as described herein, or an ECU as described herein. 
     According to another aspect of the present invention, there is provided a vehicle comprising a CTIS as described herein. 
     According to another aspect of the present invention, there is provided a computer program product comprising a computer readable storage medium including computer readable program code, wherein the computer readable program code when executed causes a CTIS to implement a method as described herein. 
     As used throughout the application, the singular form of “a”, “an” and “the” may include plural referents unless the context clearly dictates otherwise. 
     For the avoidance of doubt, references herein to a central tire inflation system (CTIS) are to an apparatus for controlling the pressure of one or more tires. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, in which: 
         FIG. 1A  is a schematic representation of a central tire inflation system (CTIS) according to an embodiment of the invention; 
         FIG. 1B  is a schematic representation of a valve arrangement of the CTIS shown in  FIG. 1A ; 
         FIG. 1C  is a vehicle system boundary diagram representing the CTIS shown in  FIGS. 1A and 1B ; 
         FIG. 2  is a high level block diagram illustrating the operation of the CTIS described herein; 
         FIG. 3  is a block diagram illustrating a target tire pressure setting strategy implemented by the CTIS described herein; 
         FIG. 4  is a block diagram illustrating a target tire pressure maintaining strategy implemented by the CTIS described herein; 
         FIG. 5  is a block diagram illustrating in detail a tire deflation strategy implemented by the CTIS described herein; 
         FIG. 6  is a block diagram illustrating in detail a tire inflation strategy implemented by the CTIS described herein; 
         FIG. 7  is a block diagram illustrating in detail an alternative tire deflation strategy implemented by the CTIS described herein; 
         FIG. 8  is a table showing the status of the valves of the CTIS described herein through a tire deflation cycle involving simultaneous deflation of all four tires; 
         FIG. 9  is a table showing the status of the valves of the CTIS described herein through a tire deflation cycle involving one tire at a time; 
         FIG. 10  is a table showing the status of the valves of the CTIS described herein through a tire inflation cycle which switches the tire pressures from those relating to an off road driving mode to those relating to an on road driving mode; 
         FIG. 11  is a table showing the status of the valves of the CTIS described herein through a tire inflation cycle involving one tire at a time; 
         FIG. 12  is a table showing an estimated annual valve usage for the valves of the CTIS described herein; 
         FIG. 13  is a series of graphs showing: a) the pressure measured by a tire pressure monitoring sensor; and b) the status (open=1; closed=0) of the valves of the CTIS described herein, through part of a tire deflation cycle concerning the rear axle; 
         FIG. 14  is a series of graphs showing: a) the pressure measured by a tire pressure monitoring sensor; and b) the status (open=1; closed=0) of the valves of the CTIS described herein, through another part of the tire deflation cycle referred to in  FIG. 13 ; 
         FIG. 15  is a group of graphs showing: a) the pressure measured by a tire pressure monitoring sensor; and b) the status (open=1; closed=0) of the valves of the CTIS described herein, through part of a tire inflation cycle concerning the rear axle; and 
         FIG. 16  is a group of graphs showing: a) the pressure measured by a tire pressure monitoring sensor; and b) the status (open=1; closed=0) of the valves of the CTIS described herein, through another part of the tire inflation cycle referred to in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description and in the drawings, reference letters are used to collectively or un-specifically identify equivalent or essentially equivalent components. Where necessary, a specific component in a collection of equivalent or essentially equivalent components is identified by suffixing a reference letters in subscript format. 
     A central tire inflation system (CTIS)  1  in accordance with an embodiment of the present invention will now be described with reference to the accompanying Figures. As shown schematically in  FIG. 1A , the CTIS  1  is installed in a vehicle VH having four wheels W each having a tire T mounted on a wheel hub (not shown). The wheels W (and the tires T) are identified herein based on their relative position on the vehicle VH, namely: front left (FL), front right (FR), rear left (RL) and rear right (RR). This nomenclature is employed to identify the components of the CTIS  1  associated with the respective tires T. The front tires T FR , T FL  are mounted on a front axle and the rear wheels T RR , T RL  are mounted on a rear axle of the vehicle. 
     The CTIS  1  comprises four pneumatic control valves PCV fixedly mounted to the wheel hubs and arranged to control the supply of compressed air to and from a respective tire cavity. The pneumatic control valves PCV are pneumatically operated in response to changes in the pressure in the associated tire supply line TSL. Specifically, the pneumatic control valves PCV are operable to cycle sequentially (i.e. to toggle) between an open state and a closed state in response to the application of a pressure exceeding a valve activation pressure. The pneumatic control valves PCV are stable in both the open and closed state via a latching mechanism, i.e. they can each be considered as a pressure actuated bi-stable valve. Herein the application of air at a pressure and time sufficient to switch the valve from one state to its other state, i.e. from open to closed or from closed to open, is referred to as “toggling” the valve, and the application of said air in this manner is referred to as a high pressure (pneumatic) control signal. A suitable pneumatic control valve PCV for this application is available in the form of a pneumatic latching valve from Norgren Limited of PO Box 22, Eastern Avenue, Lichfield, Staffordshire, WS13 6SB, United Kingdom. UK Patent application GB 1313622.1 filed on 30 Jul. 2013, the description of which is appended hereto, and the contents of which are incorporated herein by reference in their entirety, describes such pneumatic latching valves. It will be appreciated that each pneumatic control valve could have additional operating states which are cycled through sequentially in dependence on said pneumatic control signal. 
     The CTIS  1  further comprises a valve block  3  for controlling the supply of compressed air to each of the pneumatic control valves PCV. The valve block  3  is fluidly coupled to a first compressed air source  5  and a second compressed air source  7 . The first compressed air source  5  provides air at a high flow rate and low pressure (HF/LP); and the second compressed air source  7  is operable to provide air at a higher pressure. As described herein, the first and second compressed air sources  5 ,  7  are distinct from each other. In the present embodiment, the first compressed air source  5  comprises a first compressor  9 ; and the second compressed air source  7  comprises a second compressor  11  and a reservoir  13 . 
     The activation pressure of each said pneumatic control valve PCV is below the pressure of the second compressed air source  7 . In this manner the second compressed air source  7  can be applied for a short duration to switch one or more selected pneumatic control valves PCV from a closed state to an open state, and vice versa. 
     An ECU  15  is provided to control operation of the CTIS  1 . Specifically, the ECU  15  is configured to control operation of the valve block  3  and the first compressor  9 . The second compressor  11  forms part of the vehicle air suspension and could be controlled indirectly by the ECU  15 . 
     A tire supply line TSL is provided to supply compressed air from the valve block  3  to each tire T. Specifically, the CTIS  1  comprises a front left tire supply line TSL FL , a front right tire supply line TSL FR , a rear left tire supply line TSL RL  and a rear right tire supply line TSL RR . The pneumatic control valves PCV are provided at the ends of the tire supply lines TSL to control the supply of compressed air to the respective tires T. A section of each tire supply line TSL extends along the respective vehicle axles to supply compressed air to the pneumatic control valves PCV mounted in each wheel hub. A rotary air coupling (RAC) is provided in each tire supply line TSL to provide a fluid coupling to supply compressed air from the valve block  3  to the section of each tire supply line TSL disposed in the vehicle axle. 
     The valve block  3  will now be described in more detail with reference to  FIG. 1B . The valve block  3  comprises first and second inlet valves V INC , V INSS . The first inlet valve V INC  operatively controls the supply of compressed air from the first compressed air source  5  which is connected to the valve block  3  by a first supply line  17 . The second inlet valve V INSS  operatively controls the supply of compressed air from the second compressed air source  7  which is connected to the valve block  3  by a second supply line  19 . 
     The valve block  3  comprises four outlet valves (collectively referenced as V O ) for controlling the supply of compressed air to the respective tire supply lines TSL. In particular, the valve block  3  comprises: a front left outlet valve V FLO  for controlling the supply of compressed air to the front left tire supply line TSL FL ; a rear left outlet valve V RLO  for controlling the supply of compressed air to the rear left tire supply line TSL RL ; a front right outlet valve V FRO  for controlling the supply of compressed air to the front right tire supply line TSL FR ; and a rear right outlet valve V RRO  for controlling the supply of compressed air to the rear right air supply line TSL RR . The outlet valves V O  are operable independently of each other to enable the selective supply of compressed air to one or more of the tire supply lines TSL. 
     The valve block  3  also comprises first and second exhaust valves E 1 , E 2  coupled to an exhaust line  21 . The exhaust line  21  terminates with an exhaust outlet  23  which is open to atmosphere to vent exhaust air from the CTIS  1 . The first and second exhaust valves E 1 , E 2  are operable to control the flow of exhaust air to the exhaust line  21 , for example during tire deflation. A safety valve V SAFE  is also provided in the valve block  3 . The safety valve V SAFE  is operable to vent to atmosphere any excess air which might accumulate in the valve block  3  due, for example, to malfunction of any of the components of the CTIS  1 . 
     The inlet valves V INC , V INSS , the outlet valves V O  and the exhaust valves E 1 , E 2  are solenoid valves having a normally-closed configuration (illustrated by a filled symbol in  FIG. 1B ). The inlet valves V INC , V INSS , the outlet valves V O  and the exhaust valves E 1 , E 2  are operable independently of each other and are actuated by control signals received from the ECU  15 . The safety valve V SAFE  is also a solenoid valve but has a normally-open configuration (illustrated by an open symbol in  FIG. 1B ). The safety valve V SAFE  is closed by receiving a control signal from the ECU  15 . 
     The first compressor  9  is a dedicated compressor for the CTIS  1 . The second compressor  11  forms part of an air suspension system (not shown). The first and second compressors  9 ,  11  are controlled, directly or indirectly, by the ECU  15 . The first compressor  9  comprises an electric motor  27  and has an operating pressure of approximately 9 bar. A dryer unit  29  is coupled to the compressor  9  partway through a first compressed air supply line  31  connecting the first compressor  9  to the valve block  3 . The air suspension system comprises pneumatic actuators (not shown) which connect the chassis (not shown) of the vehicle VH to the wheels W. The pneumatic actuators are operable to maintain the vehicle at a target ride height and optionally also to request ride height changes (for example to improve off-road performance or ease of access/loading). Compressed air at a pressure of up to 20 bar is stored in the reservoir  13 . 
     A fluid gallery  33  is provided in the valve block  3 . The gallery  33  is open to each of: the outlet valves V O , the inlet valves V INC , V INSS , the exhaust valves E 1 , E 2 , and the safety valve V SAFE . The valve block  3  houses a pressure sensor  35  arranged to measure the pressure in a gallery  33 . The gallery  33  can be selectively placed in fluid communication with one or more of the tire supply lines TSL by opening one or more of the outlet valves V O . In the present embodiment, the pressure sensor  35  measures the pressure in the individual tire supply lines TSL by placing the tire supply line TSL in fluid communication with the gallery  33 . By opening the pneumatic control valve PCV associated with that tire supply line TSL, the pressure sensor  35  can measure the air pressure in the tire cavity (hereinafter referred to, for simplicity, as the tire pressure). 
     The gallery  33  receives compressed air from each of said first and second compressed air sources  9 ,  11  via the respective first and second inlet valves V INC , V INSS . The first inlet valve V INC  is operable to control the supply of compressed air from the first compressed air source  5  to the gallery  33 . The second inlet valve V INSS  is operable to control the supply of compressed air from the second compressed air source  7  to the gallery  33 . In use, the first compressed air source  5  is the primary source of compressed air for inflating the tires T. One or more of the tire supply lines TSL can be placed in communication with the first compressed air source  5  by opening the first inlet valve V INC  and the appropriate outlet valves V O . The second compressed air source  7  provides a higher pressure supply which is controlled by the second inlet valve V INSS  to generate a pneumatic control signal as described above for controlling operation of one or more of said pneumatic control valves PCV to tires for which the respective outlet valves V O  are open, i.e. to toggle the valves from one stable state to the other stable state. Specifically, the second inlet valve V INSS  is operated to generate the pneumatic control signal to cycle the pneumatic control valve(s) PCV in communication with the gallery  33 . The pneumatic control valve(s) PCV cycle through their respective operating states in response to the pneumatic control signal and, therefore, can be operated to control the supply of compressed air to and from the respective tires T. By opening the pneumatic control valve(s) PCV, one or more of the tires T can be placed in communication with the respective tire supply lines TSL. The valve block  3  can be operated to place the tire supply lines TSL in communication with the first compressed air source  5  to inflate one or more of said tires T; or in communication with the exhaust line  21  to deflate one or more of said tires T. Furthermore, the valve block  3  can be operated to measure the pressure of the air in the tires T. 
     To measure an individual tire pressure of a wheel having a closed PCV, the valve block  3  is operated to close the first and second inlet valves V INC , V INSS , and the exhaust valves E 1 , E 2 . The outlet valve V O  corresponding to the tire supply line TSL for the particular tire is opened to place the tire supply line TSL in fluid communication with the gallery  33 . A pneumatic control signal is then generated by operating the second inlet valve V INSS  to open the pneumatic control valve PCV for that tire T. The tire T is thereby placed in communication with the gallery  33  via the corresponding tire supply line TSL. The pressure sensor  35  then measures the air pressure in the gallery  33  to determine the tire pressure for that particular tire T. Once the pressure has been measured, if no further action is required in relation to that tire a pneumatic control signal may be generated by operating the second inlet valve V INSS  to close the pneumatic control valve PCV. 
     The CTIS  1  according to the present embodiment uses a single pressure sensor  35  for measuring the pressure in the gallery  33 . It will be appreciated that more than one pressure sensor  35  could be provided. For example, a pressure sensor  35  could be provided in communication with each tire supply line TSL. Equally, a separate valve block  3  could be provided for each tire T or for each axle. 
     The ECU  15  is programmed to control the overall operation of the CTIS  1 . The ECU  15  is configured to determine an inflation time or a deflation time. The inflation time is the period of time over which compressed air must be supplied from the first compressed air source  5  to the one or more tire(s) T to reach the target tire pressure. The inflation time is a function of one or more of the following: the pressure differential between the target tire pressure and the current tire pressure (the current tire pressure being the tire pressure prior to inflation); the operating characteristics of the first compressed air source (e.g. nominal pressure and flow rate); the number of tires T which are being inflated at any given time (if more than one tire T can be inflated simultaneously); and the volume and/or temperature of the tire cavities. It will be appreciated that is not necessarily equivalent to a compressor run time as the compressed air generated by the first compressor  9  is buffered in the first reservoir. 
     The deflation time is the period of time over which compressed air must be vented from the one or more tire(s) T through the first and second exhaust valves E 1 , E 2  to reach the target tire pressure. The deflation time is also a function of the number of tires T being simultaneously deflated, of the current tire pressure (i.e. the tire pressure before deflation), of the target tire pressure and/or the pressure differential to be achieved by tire deflation and/or of the volume and/or temperature of the tire cavities. 
     In the present embodiment, the ECU  15  retrieves the inflation time and/or the deflation time from a look-up table stored in a memory device accessible to the ECU  15 . The look-up table can take the form of a double entry table indexed according to the current tire pressure and the target tire pressure. Based on the current tire pressure and the target tire pressure, the ECU  15  can retrieve from the look-up table a value corresponding to, or representative of, the predetermined tire inflation time for a given flow rate and air supply air pressure. Alternatively the look-up table may give a volume of air required and the ECU calculates the inflation time based on measured or estimated pressures and flow rates and the retrieved volumetric air requirement. Other methods may be useful. 
     The ECU  15  controls tire inflation and/or deflation by opening and closing, as appropriate, the various valves V O , V INC , V INSS , E 1 , E 2  of the valve block  3 . Tire inflation and deflation can thus potentially be performed one tire T at a time, or according to any combination of tires T simultaneously. In the present embodiment, however, the ECU  15  is programmed to simultaneously deflate all the tires T, or in pairs, and to inflate the tires T one at a time or, simultaneously in pairs. If deflated/inflated in pairs, the pairs of tires T are selected according to their location at the front or rear of the vehicle VH. In this event, the tires T are said to be deflated/inflated by the CTIS  1  ‘per axle’. 
     In the present embodiment, the ECU  15  uses an algorithm to refer the current and target tire pressures to a nominal tire temperature of 25° C. and to the case of tire inflation of an individual tire T. Alternatively, different look-up tables each corresponding to a tire temperature and/or to the case of tire inflation for two or more tires T could be used. Compressed air losses in the CTIS  1  may affect the period of time taken for the CTIS  1  to achieve a predetermined pressure. The values stored in the look-up table could be dynamically updated to take into account the effects of said losses. The values could, for example, be updated via one or more self-learning algorithms. 
     The ECU  15  is in addition configured to provide information relating to the status and/or operation of the CTIS  1  to a vehicle user via a human-machine interface (HMI)  37 . A dashboard (not shown) of the vehicle VH is in addition equipped with a visual output, for example a tire operation dial  39 , to provide a user with information as to whether compressed air is being supplied to, or exhausted from, the tire cavities. 
       FIG. 1C  illustrates the relationship between the main mechanical components of the CTIS  1  described herein (which incorporates the valve block  3  illustrated in  FIG. 1B ) and a vehicle control system  41 . The vehicle control system  41  comprises the ECU  15 . The ECU  15  is programmed to implement the control strategies and procedures described herein. In this embodiment, the ECU  15  receives from a tire pressure monitoring system (TPMS)  43 , via a vehicle controller area network (CAN)  45 , real-time information relating to the current tire pressures for the four tires T of the vehicle VH. The pressure sensor communicates directly with the ECU  15  via electric signals representative of the pressure measured by the pressure sensor  35  in the gallery  33  of the valve block  3 . As described earlier, the pneumatic control valves PCV and the various valves V, I, E of the valve block  3  can be configured such that the pressure sensor  35  measures a pressure which is representative of the pressure inside each of the tires T. The TPMS  43  also monitors the temperature inside the tires T so that the tire pressures can be referred to a nominal temperature of 25 degrees C. using appropriate algorithms, for example a thermocouple or other thermal sensor may be located in or through the hub of each wheel. 
     The ECU  15  implements tire inflation and/or deflation strategies as described herein on the basis of the relationship between the target tire pressures and the current tire pressures as measured by the TPMS  43  and/or pressure sensor  35 . To do this, the ECU  15  is required to control the various mechanical components of the CTIS  1 . As seen in  FIG. 1C , the ECU  15  is configured to control the outlet valves V O  via a pulse width modulation (PWM) control network  47 . Via the same PWM control network  47 , the ECU  15  also controls the purging function of the dryer unit  29 , the electric motor  27  which drives the compressor  9 , the first and second inlet valves V INC , V INSS , the exhaust valves E 1 , E 2  and the safety valve V SAFE . As described herein, the target tire pressures can be manually selected by the driver of the vehicle via the human-to-machine interface (HMI)  37  or they can be automatically selected by the vehicle control system  41  on the basis of other information including vehicle driving modes. The HMI  37  communicates with the ECU  15  via the vehicle CAN  45 . As an alternative to PWM control full cycle on/off solenoid valves could be used. 
     A first block diagram  100  illustrating the operating modes of the CTIS  1  is show in  FIG. 2 . The CTIS  1  is initially switched off (STEP  101 ). In the present embodiment, the CTIS  1  can be switched off by a vehicle user command and/or by switching off the engine of the vehicle VH (CONDITION  103 ). When the engine is started (CONDITION  105 ) the CTIS  1  carries out an initial system check (STEP  107 ). If the initial system check detects a preliminary fault (CONDITION  109 ), e.g. compressor malfunction, the CTIS  1  enters a System Fault Mode (STEP  111 ) and the vehicle user is informed accordingly. If the initial check is successful (CONDITION  113 ) the CTIS  1  is ready to be switched on. When the CTIS  1  is then switched on by a user command (CONDITION  115 ), the CTIS  1  performs a system start-up check (STEP  117 ) to check that the components of the CTIS  1  are correctly working together and therefore capable of delivering compressed air to the tires T. If a fault is detected (CONDITION  119 ) the CTIS  1  enters the System Fault Mode (STEP  111 ) and the vehicle user is informed accordingly. If the check is successful (CONDITION  121 ) the CTIS  1  enters a Maintain Pressure Mode (STEP  123 ). 
     In the Maintain Pressure Mode (STEP  123 ), the CTIS  1  operates to ensure that a target tire pressure substantially equals a current tire pressure (CONDITION  125 ) for each of the tires T or for any selected tire combination (e.g. for the rear axle tires). If for any of the tires T the target tire pressure is greater than the current tire pressure (CONDITION  127 ), the CTIS  1  switches to an Inflate Mode (STEP  129 ). On the basis of the pressure difference between the target and current tire pressures, the ECU  15  retrieves an inflation time from a look-up table stored in a memory in the ECU  15 . 
     When the inflation time has elapsed (CONDITION  131 ) for each tire T or for each combination of tires being simultaneously inflated, the CTIS  1  returns to the Maintain Pressure Mode (STEP  123 ). 
     If the current tire pressure is greater than the target tire pressure (CONDITION  133 ), the CTIS  1  enters a Deflate Mode (STEP  135 ). In the deflate mode, compressed air is vented from one or more tires T simultaneously and routed via one or more of the tire supply lines TSL to the valve block  3  and, from there, to atmosphere through the exhaust valves E 1 , E 2 , the exhaust outlet  23  and exhaust port  25 . On the basis of the pressure difference between the target and current tire pressures, the ECU  15  retrieves the deflation time from the look-up table stored in a memory in the ECU  15 . When the current/actual tire pressure reaches or substantially matches the target tire pressure (CONDITION  137 ) the CTIS  1  returns to the Maintain Pressure Mode (STEP  123 ). 
     If a fault develops during the Inflate Mode (STEP  129 ), or during the Deflate Mode (STEP  135 ), the CTIS  1  enters the System Fault Mode (STEP  111 ). 
     The target tire pressures can vary between different tires T and for all the tires T dependent on the vehicle driving mode. This will now be described with reference to a second block diagram  200  shown  FIG. 3 . The second block diagram  200  illustrates the four main vehicle driving modes, namely: an Economy Mode (Step  201 ); an On Road Mode (STEP  203 ); an Off Road Mode (STEP  205 ) and a Recovery Mode (STEP  207 ) (listed in decreasing order of target tire pressures). The Economy Mode (STEP  201 ) requires a target tire pressure of 2.8 bar for the front tires T FL , T FR  and 3.0 bar for the rear tires T RR , T RL . The On Road Mode (STEP  203 ) requires a target tire pressure of 2.3 bar for the front tires T FL , T FR  and 2.5 bar for the rear tires T RR , T RL . The Off Road Mode (STEP  205 ) requires a target tire pressure of 1.5 bar both for the front and rear tires T FL , T FR , T RR , T RL . The Recovery Mode (STEP  207 ) requires a target tire pressure of 1.2 bar both for the front and rear tires T FL , T FR , T RR , T RL . It will be apparent to the skilled person that these tire pressures are exemplary only and that actual tire pressures for different conditions will be dependent upon the specific tires and/or other factors. 
     When the vehicle switches from the Off Road Mode (STEP  205 ) to the On Road Mode (STEP  203 ), it is appropriate to inflate the tires to achieve the target tire pressure for the On Road Mode. The transition from the Off Road Mode (STEP  205 ) to the On Road Mode ( 203 ) can, for example, be performed automatically when the ECU  15  determines that the vehicle speed is greater than a reference threshold speed; or in dependence on a driver request. The ECU  15  enters an interim tire inflation mode (STEP  209 ). The ECU  15  is configured to inflate the tires T in two, distinct stages. The tire inflation procedure is performed ‘per axle’. In a first phase (STEP  211 ), the ECU  15  inflates both the front and rear tires T FL , T FR , T RR , T RL  from the current tire pressures to an intermediate target tire pressure of, for example 2.0 bar. In a second phase (STEP  213 ), the ECU  15  inflates both the front and rear tires T FL , T FR , T RR , T RL  from the intermediate target tire pressure to the target tire pressures of 2.3 bar for the front tires T FL , T FR  and 2.5 bar for the rear tires T RR , T RL , as specified in the On Road Mode (STEP  203 ). In the described embodiment, the intermediate target tire pressure has been set to 2.0 bar to satisfy a local legal requirement. However, other selection criteria for selecting the intermediate target tire pressure could be used, in any case the intermediate pressure will be a pressure at which it is appropriate for the vehicle to be driven on a road. It will be appreciated that the tires T could be inflated in more than two stages. 
     In the present embodiment, the first phase (STEP  211 ) performed by the CTIS  1  requires less than one minute to inflate the tires T from their current tire pressure in the Off Road Mode (STEP  205 ) to the intermediate target tire pressure of 2.0 bar and the second phase (STEP  213 ) takes longer than one minute to successively inflate the tires T from the intermediate target tire pressure of 2.0 bar to the target tire pressures specified in the On Road Mode  51 . The first phase (STEP  211 ) can be performed when the vehicle/vehicle VH is still off road, or when the vehicle VH is about to enter the road, followed by the second phase (STEP  213 ) which is performed subsequently, for example when the vehicle is on the road. 
     A first recovery Mode (STEP  207 ) is activated to recover the vehicle VH, for example when a vehicle belly-out event occurs. A vehicle belly-out event refers to a situation in which the vehicle body is partially or completely supported by the ground under the vehicle instead of being supported by the tires T. The Recovery Mode (STEP  207 ) can be activated automatically by the ECU  15 ; or by the driver of the vehicle. When the Recovery Mode (STEP  207 ) is activated, the vehicle verifies whether an adjustable ride height of the vehicle is in the highest position setting and if not the ECU controls the suspension to raise the vehicle and the CTIS  1  operates to reduce the pressure of the front tires T FL , T FR  and the rear tires T RR , T RL . to, for example, 1.2 bar. The Recovery Mode (STEP  207 ) is only activated when the vehicle VH is in the Off Road Mode  205 . By decreasing the tire pressure a larger surface area of the tire will come into contact with the ground that will increase traction between the wheel and the surface, thereby assisting in reducing wheel slip and moving the vehicle, and increasing the ride height will lift the vehicle body slightly to raise it of the ground. 
     A second recovery mode is activated to recover the vehicle VH, for example when a vehicle becomes bogged down in sand or mud, i.e. the tires are spinning and have dug into the surface, but prior to the vehicle bellying out. The second recovery mode can be activated automatically by the ECU  15 ; or by the driver of the vehicle. When the second recovery mode is activated, the CTIS  1  operates to reduce the pressure of the front tires T FL , T FR  and the rear tires T RR , T RL . to, for example, 1.2 bar. The second recovery mode is only activated when the vehicle VH is in the Off Road Mode  205 . By decreasing the tire pressure a larger surface area of the tire will come into contact with the ground that will increase traction between the wheel and the surface, thereby assisting in reducing wheel slip and moving the vehicle. 
     The Economy Mode (STEP  201 ) can be selectively activated by the user, or automatically by the ECU  15 , for example based on vehicle speed. When the Economy Mode (STEP  201 ) is activated, the CTIS  1  operates to increase the pressure of the front tires T FL , T FR  to, for example, 2.8 bar; and to the rear tires T RR , T RL  to, for example, 3.0 bar. The increase in tire pressure can be performed in a single phase or multiple phases, for example to maintain the pressure differential between the front and rear tire pressures within a predefined margin. The Economy Mode (STEP  201 ) can be activated only when the vehicle is in the On Road Mode (STEP  203 ). 
     In the described embodiment, the Off Road Mode (STEP  205 ) needs to be manually requested by the driver. When the driver requests the Off Road Mode (STEP  205 ), the ECU  15  is programmed to deflate the tires T to the target tire pressure defined in the Off Road Mode (STEP  205 ). In the described embodiment, the tire deflation from the On Road Mode (STEP  203 ) to the Off Road Mode (STEP  205 ) lasts approximately 2.5 minutes (i.e. it is comparatively slower than the tire inflation procedure). The tire deflation from the On Road Mode to the Off Road mode can be performed on all four tires T at the same time. 
     The target tire pressures are set depending on the vehicle driving mode, as illustrated in  FIG. 3 . The actual or current tire pressure can be measured by the pressure sensor  35  coupled to the gallery  33  disposed in the valve block  3 . Alternatively and/or additionally, the actual or current tire pressure is measured by a tire pressure monitoring system (TPMS)  43  provided on the vehicle VH (as shown in  FIG. 1C ). The TPMS  43  is responsible for providing the vehicle control system  9  with continuous information on the current tire pressures. The pressure sensor  35  is used to check the tire pressures after tire inflation and/or deflation. The TPMS  43  communicates wirelessly with a sensor provided in each tire T. 
     Depending on the relationship between the target and current tire pressures, the ECU  15  may be configured to implement inflation and deflation strategies to try to maintain the current tire pressure as close as possible to, or equal to, the target tire pressures. A third block diagram  300  in  FIG. 4  illustrates in more detail the strategy for maintaining the target tire pressures. Alternatively, or in addition, the present control strategy may be used for increasing and decreasing pressure in response to driver commands for different operating modes without the need to continuously attempt to maintain the pressure. 
     The TPMS  43  operates to measure the pressure of each tire T (STEP  301 ). A comparison is made between the measured (actual) tire pressure and a target tire pressure (STEP  303 ). If the target tire pressures for one or more of the tires T exceed the current tire pressure and the difference between the target tire pressures and the current tire pressures is greater than a predetermined reference pressure difference threshold (CONDITION  305 ), the ECU  15  is programmed to implement an Inflate Per Axle strategy (STEP  307 ). On the basis of the pressure difference between the target and current tire pressures, the ECU  15  first retrieves from memory the inflation time required to inflate each of the tires T FL , T FR , T RR , T RL  as shown in  FIG. 4 . Then, the ECU  15  controls the valve block  3  to inflate the tires of the rear axle T RR , T RL . Once the rear tires T RR , T RL  have been inflated, the ECU  15  controls the valve block  3  to inflate the tires of the front axle T FL , T FR . If there is a pressure difference discrepancy between right T FR , T RR  and left tires T FL , T RL  the corresponding right V FR , V RR  and left V FL , V RL  outlet valves can be opened for different time periods to compensate for such a discrepancy. 
     By using this staggered tire inflation strategy, the dynamic stability or handling balance of the vehicle  2  can be preserved during the tire inflation procedure. By inflating the tires of the rear axle T RR , T RL  followed by the tires of the front axle T FL , T FR , the handling characteristics of the vehicle VH can be biased towards understeer (rather than oversteer). In a driving experience ideally the user does not experience a change in driving dynamics, i.e. the manner in which the vehicle responds to the same input commands (e.g. steering wheel angle and throttle position). However driving dynamic is related to tire pressures and changing these dynamically while the vehicle is moving may affect driving dynamics. In order to minimize any perceived change to the driver the change is biased towards understeer during the change as an experience of understeer is less likely to cause the driver to experience a feeling of loss of composure of the vehicle than a bias towards overseer. 
     If the current tire pressure exceeds the target tire pressure and the difference between the current tire pressure and the target tire pressure is less than a predetermined reference pressure difference threshold (CONDITION  309 ), the CTIS  1  enters into a Deflate All Tires at Once mode (STEP  311 ). A deflate all tires at once strategy is described below with reference to  FIG. 5 . 
     If the current tire pressure exceeds the target tire pressure and the difference between the current tire pressure and the target tire pressure is greater than a predetermined reference pressure difference threshold and the vehicle operating mode is set on On Road Mode  51  (CONDITION  313 ), the ECU  15  is programmed to enter the CTIS  1  into a Deflate Per Axle strategy (STEP  315 ). A reverse staggered deflation logic is then applied by the CTIS  1  compared to the staggered Inflation Per Axle (STEP  307 ) described above. The front axle tires T FL , T FR  are first deflated to the required target pressure and the rear axle tires T RR , T RL  are deflated subsequently until the required target pressure is reached. This is done in order to maintain dynamic vehicle stability as explained above to bias any change in vehicle handling towards understeer rather than oversteer. 
     A fourth block diagram  400  shown in  FIG. 5  illustrates in more detail the Deflate All Tires at Once strategy (STEP  311 ) illustrated in  FIG. 4 . When the CTIS  1  enters the Deflate All Tires at Once strategy (STEP  311 ), the ECU  15  outputs a first signal S 1  to start a Deflate strategy (MODE  401 ). The outlet valves V O  are opened (STEP  403 ) and the second inlet valve V INSS  is toggled (i.e. opened and closed) so that the second compressed air source  7  generates a pneumatic control signal to open the pneumatic control valves PCV. The exhaust valves E 1 , E 2  are then opened (STEP  407 ) and tire deflation commences. After a predetermined time period (retrieved by the ECU  15  from the look-up table stored in memory), the outlet valves V O  are closed (STEP  409 ). The outlet valves V O  may be closed on an individually timed basis to achieve the desired opening time of each outlet valve V O . The ECU  15  produces a signal which causes the CTIS  1  to enter a Measure procedure (STEP  411 ). 
     In the Measure procedure (MODE  411 ), the exhaust valves E 1 , E 2  are closed (STEP  415 ) and the outlet valves V O  are opened individually, i.e. one at a time (STEP  413 ). The first one of the outlet valves V O  may be opened substantially at the same time as the exhaust valves are closed, or alternatively the exhaust valves may be closed prior to opening any of the outlet valves. The pressure sensor  35  measures the pressure in the gallery  33 , which is representative of the tire pressure of the tire T being measured (STEP  417 ). If, for any of the tires T, the measured tire pressure is greater than the target pressure (CONDITION  419 ), the exhaust valves E 1 , E 2  are opened (STEP  421 ). Since the pneumatic control valve PCV corresponding to the tire being measured is still in the open position, compressed air is vented from the CTIS  1  and the relevant tire T is deflated further until measured tire pressure matches the target pressure. The procedure is repeated for the other three tires T, until the tire pressure of each of the four tires T matches the respective target pressure. Optionally tires on one axel may be measured/further deflated before moving on to the other axle. Before a tire pressure measurement for another tire T is taken, however, the outlet valve V O  relating to the tire being measured is closed. The target tire pressure for the front tires T FL , T FR  and the rear tires T RR , T RL  tires can be the same (as in the case of the Off Road Mode  205 ) or can be different (as in the case of the Economy Mode  203 ). When the target tire pressure has been reached for all of the tires T, a strategy is implemented to close the pneumatic control valves PCV (MODE  423 ) is initiated by the ECU  15 . 
     To close the pneumatic control valves PCV, a pneumatic control signal is generated by the second compressed air source  7  to cause the pneumatic control valves PCV to close for each tire T. This is achieved by toggling the LF/HP inlet valve V INSS  (STEP  425 ). The exhaust valves E 1 , E 2  are then also toggled, i.e. opened and then subsequently closed, to vent the air still present in the relevant tire supply line TSL to restore atmospheric pressure in this tire supply line TSL (STEP  427 ). The pressure sensor  35  can then measure a pressure in the gallery  33  representative of the pressure in this tire supply line TSL to check that the pneumatic control valve PCV associated with each tire has been successfully closed (STEP  429 ). If the latching is successful, the pressure sensor  35  measures a pressure equal to, or approximately equal to, atmospheric pressure. If latching is not successful (CONDITION  431 ), the latching procedure is repeated. If the latching has been successful, the relevant outlet valve V O  is closed to isolate the respective tire supply line TSL. The procedure is later repeated for each of the remaining outlet valves V O  of the valve block  3  (STEP  433 ), after the tire pressure of each of the tires T has been measured according to the Measure procedure (MODE  411 ). 
     It will be appreciated that the second inlet valve V INSS  can be controlled to generate a pneumatic control signal for actuating a single pneumatic control valve PCV or for actuating a plurality of said pneumatic control valves PCV simultaneously. The outlet valve(s) V O  can be opened or closed to communicate the pneumatic control signal along the appropriate tire supply lines TSL to control said one or more pneumatic control valves PCV. By controlling several pneumatic control valves PCV at the same time (for example to open or close them in unison), a time saving may be achieved. 
     When all the tires have reached the target tire pressure and all the pneumatic control valves PCV have been latched, the exhaust valves E 1 , E 2  are opened (STEP  435 ). 
     A fifth block diagram  500  is shown in  FIG. 6  to illustrate the Inflate Per Axle strategy (STEP  307 ) illustrated in  FIG. 4 . The ECU  15  generates a second signal S 2  to initiate the Inflate mode (STEP  501 ) to inflate the tires T on each axle in sequence. As outlined herein, the tires T on the rear axle are inflated first, and then the tires T on the front axle are inflated. The first inlet valve V INC  is opened and the compressor  9  is activated (STEP  505 ). The airflow from the compressor  9  is estimated based on the current/voltage drawn by the compressor  9  (which indicates the work done compressing the air) optionally together with pressure/temperature information (which approximates the density of air drawn into the compressor  9 ). A performance table for the compressor  9  is typically available from the supplier to enable the airflow to be estimated. As the tire pressure rises, the compressor  9  will be working against a higher pressure so the flow rate of air delivered to the tire supply line will change dynamically. 
     The ECU  15  accesses the stored look-up table to determine the inflation time for each outlet valve (the front outlet valves V FL , V FR  or rear outlet valves V RL , V RR ). The relevant outlet valves V O  (the front outlet valves V FL , V FR ; or the rear outlet valves V RL , V RR ) are then opened (STEP  507 ). The second inlet valve V INSS  is controlled to toggle the pneumatic control valves PCV for the tires T to an open position (if they are not already in an open state). It will be appreciated that the second inlet valve V INSS  can be controlled to generate a pneumatic control signal for controlling more than one pneumatic control valve PCV simultaneously when the corresponding outlet valves V O  are open. The outlet valves V O  are closed when the compressed air has been delivered to achieve the target pressure (STEP  509 ). At this stage, the first inlet valve V INC  is also closed, and the dryer unit  29  may optionally be purged from water removed from the delivered compressed air (STEP  511 ). 
     When all the tires T have been inflated, i.e. when the Inflate mode (STEP  501 ) has been repeated for the other axle (CONDITION  513 ), the exhaust valves E 1 , E 2  are opened to purge excess compressed air from the valve block  3  (STEP  513 ) so that the Measure mode (STEP  515 ) can be performed. The pneumatic control valves PCV for all of the tires T are then closed (STEP  517 ). These procedures are unchanged from those described herein with reference to  FIG. 5  for each of the tires T in anticlockwise sequence. When the target tire pressure for all the tires T has been reached and all the pneumatic control valves PCV have been closed (CONDITION  519 ), the exhaust valves E 1 , E 2  are closed (STEP  521 ). If the target tire pressure has not been reached or any of the pneumatic control valves PCV remain open, the appropriate procedures (STEP  515  and STEP  517 ) are repeated. 
     A sixth block diagram  600  is shown in  FIG. 7  to illustrate in more detail the Deflate Per Axle strategy (STEP  315 ). When the entry conditions are satisfied, the ECU  15  causes the CTIS  1  to enter the Deflate Per Axle strategy (STEP  315 ). The ECU  15  produces a third signal S 3  to cause the CTIS  1  to start the Deflate procedure (STEP  601 ). All the outlet valves V O  associated with one of the axles (the front outlet valves V FL , V FR  or rear outlet valves V RL , V RR ) are initially opened (STEP  603 ). The second inlet valve V INSS  is toggled (STEP  605 ) to open the pneumatic control valves PCV in the relevant axle. The exhaust valves E 1 , E 2  are then opened (STEP  607 ) so that the deflation of the tires T connected to the relevant axle commences. A check (STEP  609 ) is then performed to ensure that only the appropriate outlet valves V O  associated with each axle are open (the front outlet valves V FL , V FR  or the rear outlet valves V RL , V RR ). 
     When the outlet valves (the front outlet valves V FL , V FR  or rear outlet valves V RL , V RR ) have been opened for the desired time period to reach the target amount of compressed air removed from the tires T, the outlet valves V O  are closed (STEP  611 ). The procedure is repeated if the tires connected to the other axle need also to be deflated (CONDITION  613 ). 
     When all the tires T have been deflated, the Measure procedure (STEP  615 ) and the procedure to close the pneumatic control valves PCV (STEP  617 ) are carried out. These procedures are the same as those described herein with reference to  FIG. 5 . When the target tire pressure for all the tires T has been reached and all the pneumatic control valves PCV have been closed (CONDITION  619 ), the exhaust valves E 1 , E 2  are closed (STEP  621 ). It will be appreciated that the deflation procedure can be performed with a single toggle (open/closed) of the appropriate pneumatic control valve(s) PCV. The deflation of the tire(s) T is thereafter controlled by the outlet valves V O  in the valve block  3 . It is not necessary to toggle the pneumatic control valve(s) PCV between axle deflations. Rather, the pneumatic control valve(s) PCV remain open following deflation for measurement (STEP  614 ) before being closed together. 
     As illustrated by  FIG. 7 , the CTIS  1  can also deflate all four tires T of the vehicle VH by first deflating two tires T of one of the axles (in the described embodiment, the tires mounted to the front axle T FL , T FR  are deflated first, as described above), and then the other two tires T of the other one of the axles. This tire deflation strategy is also referred to as per axle deflation. Alternatively, the CTIS  1  can deflate the four tires T one tire T at a time, i.e. the tires T can be individually deflated, measured and closed via the procedure for closing the pneumatic control valves PCV. 
     As illustrated by  FIG. 6 , the CTIS  1  can inflate all four tires T of the vehicle VH by first inflating the two tires T of one of the axles (in the present embodiment, the tires mounted to the rear axle T RL , T RR  are inflated first), and then the other two tires T of the other one of the axles. Alternatively, the CTIS  1  can inflate all four tires T individually. The CTIS  1  is not designed to inflate all four tires T simultaneously due to the flow rate and maximum pressure limitations of the compressor  9 . However, it will be understood that it is in principle possible to adapt the CTIS  1  to simultaneously inflate all the tires T, by using a compressor  9  with an adequate flow-rate and pressure rating. 
     The valve usage per unit time will now be considered. A complete inflation or deflation event (i.e. including tire inflation or tire deflation, tire pressure measurement and latching of the pneumatic control valve PCV), which may involve some or all of the tires T, is referred to herein as a complete tire inflation or deflation cycle. This is in contrast to tire inflation or deflation (operations) which, as described above, only relates to the presence of compressed air flow (into or out from the tires T). It will be appreciated that, in practice, the CTIS  1  may be required to change the tire pressure of only some of the tires T. A first table  710  shown in  FIG. 8  summarizes valve usage for a complete tire deflation cycle involving simultaneous deflation of all four of the tires T. 
     The first table  710  refers to deflation of all the tires to 1.5 bar, as described in  FIG. 3  in connection with the passage of the vehicle from the On Road mode  203  to the Off Road mode  205 . A first column  711  of the first table  710  lists the inlet valves V INC , V INSS , the outlet valves V O  and the exhaust valves V of the CTIS valve block  3 . A second column  712  counts, for each of the valves listed in the first column  711 , the respective valve cycles in the Deflate procedure  401  illustrated in  FIG. 5 . For each of the tires T, a third  713 , fourth  714 , fifth  715  and sixth  716  column of the first table  710  list the respective valve cycles during the Measure procedure  411  and Close Pneumatic control valve procedure  423  of  FIG. 5 . A seventh column  717  lists the total of the valve cycles for each of the valves listed in the first column  711 . If after a measurement there is a need to deflate further a tire, the exhaust valves E 1 , E 2  undergo the same number of cycles as described in the last two rows  718  of the first table  710  plus one cycle for each of the exhaust valves. 
     A second table  720  shown in  FIG. 9  summarizes similar valve usage information for a deflation procedure wherein the tires T are deflated individually (this procedure is not illustrated in  FIGS. 4 to 7 , and is used in connection with the passage of the vehicle from the Economy Mode  201  to the On Road Mode  203  of  FIG. 3 ). In this procedure, all of the outlet valves V O  of the valve block  3  are first opened together as illustrated by step  403  of  FIG. 5 . This allows the CTIS  1  to be prepared for deflation. Then, each of the tires T FL , T FR , T RR , T RL  is individually deflated until the target tire pressures of the On Road Mode  203  illustrated in  FIG. 3  are reached. For this reason, the valve cycle count for each of the corresponding outlet valves V FL , V FR , V RR , V RL  in a second column  721  of the second table  720  is different compared to the second column  712  of the first table  710 , with the outlet valves V FL , V FR , V RR , V RL  now counting two valve cycles each instead of one. Likewise, if after a measurement there is a need to deflate further each tire T FL , T FR , T RR , T RL , the deflation procedure is individually repeated, and the exhaust valves E 1 , E 2  undergo the same number of cycles as described in the last two rows  721  of the second table  720  plus one cycle for each of the exhaust valves E 1 , E 2 . 
     A third table  730  shown in  FIG. 10  summarizes similar valve usage information for a tire inflation cycle where the tires T are inflated from the target pressures of the Off Road Mode  205  to the target pressures of the On Road Mode  203  of  FIG. 3  via the Interim tire inflation step  209 , also shown in  FIG. 3 . The third table  730  is divided in five main table sections  731 ,  732 ,  733 ,  734 ,  735 . The first table section  731  relates to the inflation of the tires of the rear axle T RR , T RL  from 1.5 bar to 2.0 bar according to Inflate procedure  501 , Measure procedure  515  and Close Pneumatic control valve procedure  517  of  FIG. 6 . The columns in this first table section  731  list the valve cycle counts for each valve V O , V INC , V INSS , E 1 , E 2  of the valve block  3  and for each procedure. The second table section  732  relates to the inflation of the tires of the front axle T FL , T FR  from 1.5 bar to 2.0 bar also according to Inflate procedure  501 , Measure procedure  515  and Close Pneumatic control valve procedure  517  of  FIG. 6 . Likewise, the columns in the second table section  732  list the valve cycle counts for each valve V O , V INC , V INSS , E 1 , E 2  of the valve block  3  and for each procedure. The third table section  733  relates to the inflation of the tires of the rear axle T RR , T RL  from 2.0 bar to 2.5 bar. However, in this sequence the rear tires T RR , T RL  are inflated, measured and then the corresponding pneumatic control valves PCV RR , PCV RL  closed individually instead of being closed per axle as was the case for the previous two table sections  731 ,  732 . The columns in the third table section  733  list the valve cycle counts for each valve V O , V INC , V INSS , E 1 , E 2  of the valve block  3  and for each individual procedure. The fourth table section  144  of the third table  140  relates to the inflation of the tires of the front axle T FL , T FR  from 2.0 bar to 2.3 bar. Likewise, in this sequence the front tires T FL , T FR  are inflated, measured and the corresponding pneumatic control valves PCV FL , PCV FR  closed individually. The columns in the fourth table section  734  of the third table  730  list the valve cycle counts for each valve V O , V INC , V INSS , E 1 , E 2  of the valve block  3  and for each individual procedure. The fifth table section  735  summarizes the total valve cycles for these valves V O , V INC , V INSS , E 1 , E 2 . 
     A fourth table  740  is shown in  FIG. 11  and summarizes similar valve usage information for a tire inflation cycle wherein the tires T are inflated from the target pressures of the On Road Mode  203  to the target pressures of the Economy Mode  201 . The fourth table  740  is divided in three table sections  741 ,  742 ,  743 . The first table section  741  of the fourth table  740  is equivalent to the third table section  733  of the third table  730 , and the second table section  742  of the fourth table  740  is equivalent to the fourth table section  734  of the third table  730 . The third table section  743  of the fourth table  740  represents the total valve cycles in the inflation procedure for the same valves V O , V INC , V INSS , E 1 , E 2  of the valve block  3 . 
     A fifth table  750  is shown in  FIG. 12  to summarize, again for each of the valves V O , V INC , V INSS , E 1 , E 2  of the valve block  3  (listed in the first columns of the first, second, third and fourth tables  710 ,  720 ,  730 ,  740  described above), the number of valve cycles estimated in a time period of one year assuming different vehicle operation conditions (off road and on road) and different usage types (sand, wet grass, mud and snow for the off road vehicle operation condition and economy (ECO), high speed, gross vehicle weight (GVW) and towing for the on road vehicle operating condition). Estimated valve cycles in one year are also provided in  FIG. 12  for a tire pressure maintenance condition (in the row before the last in the fifth table  750  shown in  FIG. 12 ). The tire pressure maintenance condition relates to the pressure maintaining strategy illustrated in  FIG. 4 . As can be seen from the fifth table  750 , the valves V O , V INC , V INSS , E 1 , E 2  of the valve block  3  are estimated to collectively undergo 31,590 cycles in the unit time of one year. 
       FIGS. 13 to 16  show for parts of a per axle deflation cycle and a per axle inflation cycle, the pressure measured in the gallery  33  by the pressure sensor  35  alongside the status of the various valves V, I, E of the valve block  3  illustrated in  FIG. 1B . 
     A first graph  800  in  FIG. 13  shows the pressure measured by the pressure sensor  35  during a first part of a tire deflation cycle identified over a time period from 151.5 sec (designated time t 0 ) to 153.5 sec. At time t 0 , the pressure measured by the pressure sensor  35  is substantially equal to atmospheric pressure (Patm). This represents a stand-by condition for the CTIS  1 . At this time, the pressure at the pressure sensor  35  is representative of the pressure in the relevant tire supply lines TSL. At time t 0 , the measured pressure is equal to Patm since the exhaust valves E 1 , E 2  are open, as illustrated by an eighth graph  835  in  FIG. 13 . At time t 1  (152 sec), the rear outlet valves V RL , V RRO  are opened in preparation for deflating the tires of the rear axle T RL , T RR  of the vehicle VH, as shown in a fourth and fifth graph  815 ,  820  in  FIG. 13 . The front outlet valves V FLO , V FRO  remain closed throughout the time period represented in  FIG. 13 , as illustrated by a second and third graph  805 ,  810  in  FIG. 13 . At time t 2  (approximately 152.2 sec), the exhaust valves E 1 , E 2  are closed thereby sealing the tire supply lines TSL from atmosphere, as shown by the eighth graph  835 . At time t 2 , the inlet valve V INSS  is instead opened to allow compressed air from the second compressed air source  7  into the tire supply lines TSL, as shown by a sixth graph  825  in  FIG. 13 . At time t 2 , therefore, the pressure sensor  35  detects a pressure increase in the gallery  33  of the valve block  3  as shown in the first graph  800 . The incoming compressed air from the second compressed air source  7  toggles the pneumatic control valves located on the rear wheels PCV RL , PCV RR  to an open state. This is possible because the rear outlet valves V RLO , V RRO  are open at time t 2 . The instant at which the rear pneumatic control valves PCV RL , PCV RR  are opened is time t 3  (approximately 152.4 sec) in  FIG. 13 . At time t 3 , the pressure sensor  35  detects a dip in the pressure measured in the tire supply lines TSL. For a short time period between time t 3  and time t 4  (approximately 152.6 sec), pressure within the tires, the gallery and the tire supply lines equalizes resulting in a pressure fluctuation being seen at the pressure sensor. At time t 5 , the exhaust valves E 1 , E 2  are opened and the second inlet valve V INSS  is closed. These events initiate tire deflation. From time t 5  onwards, compressed air flows from inside the rear tire cavities through the rear tire supply lines TSL RL , TSL RR  out to atmosphere. Thereafter, the pressure sensor  35  senses a pressure just above atmospheric pressure Patm due to the incoming flow of compressed air from the rear tire cavities, as shown by the first graph  800  in  FIG. 13 . It will be appreciated that during the tire deflation procedure, as shown by a seventh graph  830  in  FIG. 13 , the first compressed air source  5  does not operate. Accordingly, the first inlet valve V INC  remains closed for the time period illustrated in  FIG. 13 . 
     In the described tire deflation procedure, compressed air flows from the rear tire cavities out to atmosphere for a different deflation time for each of the rear tires T RL , T RR . This deflation occurs over a time period of approximately 100 seconds for the rear right tire T RR . In particular, the deflation of the rear right tire T RR  commences at time t 5  (152.5 sec) when the exhaust valves E 1 , E 2  open, as shown in  FIG. 13 . The deflation of the rear right tire T RR  ends at time t 6  (255.2 sec) when the rear right outlet valve V RRO  is closed, as shown in  FIG. 14 . As explained above, the deflation period is retrieved by the ECU  15  on the basis of a look-up table stored in a memory. The look-up table correlates pressure decrease with deflation time for the tire deflation procedure, and pressure increase with inflation time for the tire inflation procedure. The rear left outlet valve V RLO  is closed at an intermediate time which is not shown in  FIGS. 13 and 14 .  FIG. 14  only relates to the rear axle tire measurement and closure procedures starting from time t 8 . 
     Referring now to  FIG. 14 , a first graph  840  of  FIG. 14  shows the pressure measured by the pressure sensor  35  during a part of a tire deflation cycle identified between a time period extending from 255 sec to 259 sec. As shown by the first graph  840  in  FIG. 14 , the pressure measured by the pressure sensor  35  is substantially equal to the atmospheric pressure (Patm) since the rear right tire T RR  is deflated until time t 6  (approximately 255.3 sec) and the exhaust valves E 1 , E 2  are open, as illustrated by an eighth graph  875  in  FIG. 14 . At time t 6 , the rear right outlet valve V RRO  is closed and, therefore, both the rear tires T RL , T RR  are now ready to undergo the tire pressure measurement and pneumatic control valve closure procedures. For the rear left tire T RL , this procedure begins at time t 7  (just after 255.5 sec), as shown in  FIG. 14 , with the opening of the rear left outlet valve T RL  as shown by a fourth graph  855  in  FIG. 14 . At time t 8  (approximately 255.7 sec), the exhaust valves E 1 , E 2  are closed so that the pressure sensor  35  can now detect a tire pressure representative of the tire pressure in the rear left tire T RL . As soon as the exhaust valves E 1 , E 2  are closed, the pressure sensed by the pressure sensor  35  starts to increase until it reaches the pressure representative of the tire pressure in the rear left tire T RL . The ECU  15  causes the pressure sensor  35  to take a reading at time t 9  (approximately 256.3 sec). Once the reading has been taken, the second inlet valve V INSS  is opened at t 10  (256.5 sec) and closed at t 11  (256.7 sec). This operation, as explained above, toggles the rear left pneumatic control valve PCV RL  to a closed state and causes the pressure sensor  35  to detect pressures of up to about 9 bar, as shown by the first graph  840  of  FIG. 14 . When the rear left pneumatic control valve PCV RL  is latched and the exhaust valves E 1 , E 2  are opened at time t 11  (256.7 sec), the pressure sensed by the pressure sensor  35  is atmospheric pressure Patm again, which, as explained above, corresponds to a stand-by condition. The first compressed air source  5  is not involved in this procedure, as shown by a seventh graph  870  in  FIG. 14  showing the first (i.e. HF/LP) inlet valve V INC  permanently closed. The front outlet valves V FLO , V FRO  are also not involved in the procedure, as shown by a second and third graph  845 ,  850  in  FIG. 14 . The tire pressure measurement and pneumatic control valve closure procedures are then repeated for the rear right tire T RR  by nearly simultaneously closing the rear left outlet valve V RLO  (as shown by the fourth graph  855  of  FIG. 14  at around time t 12  (approximately 257.1 sec) and opening the rear right outlet valve V RRO , as shown by a fifth graph  860  at t 13  (257.3 sec). At time t 14  (257.5 sec), the exhaust valves E 1 , E 2  are closed and the pressure sensed by the pressure sensor  35  increases as compressed air from the rear right deflated tire cavity populates the corresponding tire supply line TSL RR . The ECU  15  causes a reading to be taken from the pressure sensor  35  at time t 15  (258 sec), as shown in the first graph  840  in  FIG. 14 , when the pressure measured by the pressure sensor  35  is representative of the tire pressure in the rear right tire cavity. Between time t 16  and time t 17  the second inlet valve V INSS  (i.e. the LF/HP inlet valve) is opened and closed, which causes the rear right pneumatic control valve PCV RR  to be closed. At time t 17  (258.4 sec), the exhaust valves E 1 , E 2  are opened to allow the compressed air in the tire supply line TSL to be vented to atmosphere. At time t 18  (approximately 258.8 sec), the pressure sensor  35  detects a pressure equal to Patm, and therefore the system can return to stand-by at time t 18 . The rear tires T RL , T RR  have now been deflated to the target pressure of approximately 2.5 bar sensed, which corresponds to 1.5 bar gauge. 
     Referring now to  FIG. 15 , a first graph  880  in  FIG. 15  shows the pressure measured by the pressure sensor  35  during a preparatory part of a tire inflation cycle from an initial time t′ 0  (303.5 sec). At t′ 1  (304.3 sec), the ECU  15  causes the first compressed air source  5  to deliver compressed air by opening the inlet valve V INC , as shown by a seventh graph  910  in  FIG. 15 . Pressure starts to build up in the gallery  33  of the valve block  3 . At time t′ 2  (304.5 sec), the rear outlet valves V RLO , V RRO  are opened by the ECU  15 , as shown by a fourth and a fifth graph  895 ,  900  in  FIG. 15 . Compressed air from the compressor  9  can therefore flow into the tire supply lines TSL RL , TSL RR  relating to the rear axle of the vehicle VH. A dip in the measured pressure is sensed by the pressure sensor  35  at t′ 3  (304.6 sec) as a consequence of this flow of compressed air into the rear tire supply lines TSL RL , TSL RR . Subsequently, the pressure sensed by the pressure sensor  35  increases again due to additional compressed air input to the rear tire supply lines TSL RL , TSL RR  by the compressor  9 . The system is now ready for inflating the rear tires T RL , T RR . The front outlet valves V FLO , V FRO  are not involved in this procedure (i.e. these valves remain closed), as shown by a second and a third graph  885 ,  890  in  FIG. 15 . Likewise, the second (denoted LF/HP in  FIG. 15 ) inlet valve V INSS  and the exhaust valves E 1 , E 2  are not involved, as shown by respectively a sixth and an eighth graph  905 ,  915  in  FIG. 15 . 
     Inflation is achieved by opening and closing the second inlet valve V INSS  (i.e. the LF/HP valve) so as to toggle the rear left and rear right pneumatic control valves PCV RL , PCV RR  to an open state (this is not shown in  FIG. 15 or 16 ). Compressed air can thus flow from the first compressed air source  5  to the relevant tire cavities for a predetermined time period. The pressure sensor  35  senses the pressure of the compressed air supplied from the first compressed air source  5 . As described herein, the target tire pressure is achieved by controlling the inflation time. The tire inflation ends for the rear left tire T RL  at a time not shown in  FIGS. 15 and 16 . 
     Referring now to  FIG. 16 , the tire inflation ends for the rear right tire T RR  at time t′ 4  (323.7 sec) shown in  FIG. 16 , when the rear right outlet valve V RRO  is closed as shown by a fifth graph  940  in  FIG. 16 . The system is then reconfigured to measure the pressure in the rear left tire cavity. This procedure starts in  FIG. 16  at time t′ 5  (232.8 sec), when the first inlet valve V INC  (denoted HF/LP valve in  FIG. 16 ) is closed (thereby cutting out flow of compressed air from the compressor  9 ) as shown by a seventh graph  950  in  FIG. 16 , and the exhaust valves E 1 , E 2  are opened to vent out excess compressed air from the relevant tire supply lines TSL RR , TSL RL  as shown by an eighth graph  955  in  FIG. 16 . At time t′ 6  (323.9 sec), the rear left outlet valve V RLO  is opened to put the rear left tire supply line TSL RL  in communication with the inflated rear left tire cavity, as shown by a fourth graph  935  in  FIG. 16 . Soon after time t′ 7  (234.2 sec), the exhaust valves E 1 , E 2  are closed to prevent air from the rear left tire T RL  from venting out thereby deflating the previously inflated rear left tire T RL . The rear left tire supply line TSL RL  fills with compressed air. The pressure in the rear left tire supply line TSL RL  is now representative of the pressure in the rear left tire T RL  which can thus be estimated via the pressure measured by the pressure sensor  35  in the gallery  33  of the valve block  3  at time t′ 8  (324.7 sec), as shown by a first graph  920  in  FIG. 16 . If the pressure achieved is correct, the second inlet valve V INSS  (denoted LF/HP in  FIG. 16 ) can be opened and closed to isolate the rear left tire cavity. This happens in  FIG. 16  between time t′ 9  (324.9 sec) and time t′ 10  (325.2 sec), as shown by a sixth graph  945  in  FIG. 16 . This procedure closes the rear left pneumatic control valve PCV RL  to isolate the rear left tire cavity. At time t′ 10 , the exhaust valves E 1 , E 2  are also opened to vent out excess air from the rear left tire supply line TSL RL . Since the target pressure (2.5 bar) has been reached, the procedure can be terminated for the rear left tire T RL . This happens, in  FIG. 16 , at t′ 11  (325.5 sec) when the rear left outlet valve V RLO  is closed. The procedure is then repeated for the rear right tire T RR . The front outlet valves V FLO , V FRO  are not involved in this procedure (i.e. they remain permanently closed), as shown by a second and a third graph  925 ,  930  in  FIG. 16 . At time t′ 12  (325.7 sec), the rear right outlet valve V RRO  is opened and at time t′ 13  (325.9 sec) the exhaust valves E 1 , E 2  are closed to isolate the rear right tire supply line TSL RR  so that the rear right tire pressure can be measured. The measurement is taken by the pressure sensor  35  at time t′ 14  (326.4 sec). The second inlet valve V INSS  is then opened and closed at times t′ 15  (326.7 sec) and t′ 16  (329.9 sec) respectively to operate the rear right pneumatic control valve PCV RL  to seal the rear right tire cavity. At time t′ 16 , the exhaust valves E 1 , E 2  are also opened to vent excess compressed air from the rear right tire supply line TSL RR . At time t′ 17  (237.3 sec) approximately, the rear right outlet valve V RR  and the exhaust valves E 1 , E 2  are closed to terminate the procedure. 
     An additional control strategy implemented by the CTIS  1  described herein is described below. This control strategy relates to a target tire pressure over-shooting strategy. 
     Over the life of the vehicle VH, the various joints in the compressed air paths between the compressed air sources and the tire cavities in the CTIS  1  may develop minor leaks due to normal, ageing wear. As a consequence, during a tire inflation or deflation operation the CTIS  1  may, respectively, undershoot or overshoot the target tire pressures. This is due to the presence of such leaks, or losses, whereby during a tire inflation operation some of compressed air destined to the tire cavities is instead vented from the CTIS  1  to atmosphere due to one or more leaks. During a tire deflation operation, an additional quantity of compressed air may vent from the CTIS  1  to atmosphere due to these potential leaks. 
     Since the CTIS  1  described herein is based on an open loop system architecture (i.e. there is no feedback means to feedback to the ECU  15  the amount of compressed air being actually or instantaneously transferred to the tire cavities as the tires are inflated or being actually or instantaneously vented to atmosphere as the tires are deflated), a tire inflation or deflation operation is terminated after a given/predetermined tire inflation or deflation time has elapsed. As described earlier, these given/predetermined times are retrieved by the ECU  15  from the look-up table. Subsequently, the tire pressures are measured for each of the tires T involved in the inflation or deflation procedures to check whether the tire inflation or deflation has delivered the target tire pressures. In this open loop system, the ECU  15  is only programmed to check that the inflation or deflation time is equivalent to the inflation or deflation time retrieved from memory and to successively verify the tire pressure and close the pneumatic control valves PCV by activating respectively the pressure measurement procedure and the pneumatic control valve closure procedure as described herein. These events terminate the overall tire inflation or deflation cycle/procedure. 
     In both scenarios (tire inflation or deflation), it is undesirable to have to inflate the tires further in a subsequent step to reach the target tire pressures, since this would require at least another inflation or deflation cycle (i.e. inclusive of the tire pressure measurement and pneumatic control valve closure operations). A further tire inflation cycle is especially undesirable since it could require a further activation of the compressor  9  via the electric motor  27  with related noise. Further, any additional inflation or deflation cycle/procedure equates to additional energy consumed, potential additional noise emitted by the CTIS  1 , additional time required to reach the target pressures, additional valve cycles for one or more of the various valves of the CTIS  1  and, in particular, the pneumatic control valves PCV. Once the tire pressures have been measured after inflation or deflation the system is required to cycle through the pneumatic valve closure procedure before the valves can be brought back to the tire inflation configuration. Then the pneumatic control valves PCV are required to cycle again through the pressure check/measurement configuration before the pneumatic control valves PCV can be closed/latched to terminate the inflation or deflation cycles/procedures as earlier described. An additional problem is that if the required additional inflation time is particularly short, initialization of the compressor  9  and the air dryer unit  29  may be problematic and the CTIS  1  may consequently generate a system fault as illustrated by System Fault status  25  illustrated in  FIG. 2 . 
     In the described embodiment, the ECU  15  of the CTIS  1  is programmed to provide compensation to try to address the aforementioned problem. In particular, the ECU  15  is programmed to ensure that at the end of the inflation or deflation operations (identified by flow of compressed air to or from one or more tires T) the tire pressures are each greater than the target tire pressures. In other words, the ECU  15  is programmed so as always to include a safety overshooting or compensation margin at the end of the inflation or deflation operations/times. As a consequence, the ECU  15  is programmed to overshoot the target tire pressure in the case of tire inflation and to undershoot the target tire pressure in the case of tire deflation. The overshooting tire pressure (or compensation tire pressure) corresponds to the target tire pressure plus a margin. 
     In an example, the ECU  15  retrieves from the look-up table an inflation time equal to 60 sec and adds a safety margin of 6 sec to overshoot the target tire pressure thereby inflating above the target tire pressure. In another example, the ECU  15  retrieves from memory a deflation time of 100 sec and subtracts a safety margin of 15 sec to undershoot the target tire pressure thereby under-deflating the relevant tire T. Once the target tire pressure have been overshot or undershot by the CTIS  1  so as to avoid further one or more full tire inflation/deflation cycles, the ECU  15  is programmed to adjust the target tire pressure during the following Measure procedure  515  (examples of which as shown in  FIGS. 5 to 7 ) to adjust the tire pressure to the target pressure. During the Measure procedure  515 , the CTIS  1  can adjust the tire pressure as required by simply opening the exhaust valves E 1 , E 2  until the target tire pressure has been achieved. The CTIS  1  is therefore no longer required to undertake one or more additional inflation cycles. 
     Criteria can be developed to estimate, or calculate, the safety margin (also referred to herein as the over-shooting margin or the compensation margin). In particular, it would be preferable to be able to estimate or calculate safety margins which allow the system to overshoot (in the case of tire inflation) or undershoot (in the case of tire deflation) the target tire pressure as little as possible while at the same time resulting into inflation above or slightly above the target tire pressure at the end of the inflation or deflation operation/time. In some embodiments, the over-shooting margin is established on the basis of a worst-case scenario which is associated to a maximum of compressed air loss through the joints. In this case, the safety margin equals to the additional inflation time, or to the subtracted deflation time, as the case may be, which is required to compensate for the maximum of compressed air loss associated with the worst-case scenario. As it will be understood, a variety of worst-case scenarios could be hypothesized and, accordingly, a range of maximum losses could be used. For example, the worst case scenario may be estimated based on the possible number of points at which a leak may occur and the estimated maximum leak at each location. Empirical testing may be used to obtain data from which to estimate the maximum leak at each location. In order to determine the condition of the seals the safety margin may use a worst case scenario of the expected air leakage after a predetermined usage, for example after 100,000 miles. If, using this valve, prior to the vehicle reaching 100,000 miles usage, after the first part of the inflation the pressure is below the target pressure, this may be indicative of significant seal deterioration outside of expected parameters and a notification may be issued to the driver, for example through a vehicle HMI. 
     The compensation margin could be calculated as the additional inflation time, or to the subtracted deflation time, as the case may be, which is required to compensate for the maximum loss of compressed air loss through one or more of the rotary air couplings VH. 
     It is also predicted that the aforementioned compressed air losses may depend on vehicle age or mileage. Accordingly, the over-shooting margin could be dependent on, or be a function of, vehicle age or mileage. 
     It is also conceivable to use the CTIS  1  to measure the air loss, or air loss rates, through the joints at one or more times throughout the life of the vehicle. In this case, the CTIS  1  is configured to convert a measured discrepancy between the target tire pressure and the current tire pressure for one or more of the tires following a tire inflation or deflation procedure into an estimated air loss or air loss rate. In particular, the CTIS  1  could be configured to individually test the rotary air couplings RAC in this manner to estimate the air loss, or air loss rate, through each of the rotary air couplings RAC at one or more times. The resulting compensation margins could then be applied to the tires individually, or for each axle, if appropriate. Alternatively, the outlet valves V O  may be opened, the PCV valves closed, the exhaust valves E 1 , E 2  closed, and pressure applied by opening and closing the first inlet valve V INC  to pressurize the air-path to the tire. The pressure sensed at the pressure sensor  35  can then be observed and any reduction can be assumed to be as a result of system leakage losses between the valve block  3  and the pneumatic control valves PCV. The outlet valves V O  may be individually opened to estimate the losses in the air path to each tire T. 
     It is also conceivable to use combinations of the above described criteria or methodologies for the calculation or estimation of the compensation margin or margins, or to refine the above described criteria or methodologies in various manners which are however not described herein in detail.