Abstract:
A tire inflation system comprising pneumatically controlled wheel valves that are situated in the wheels of a motor vehicle. An electrically actuated pilot valve that is fixed to the vehicle controls the respective wheel valves. To produce a small, low-cost, reliable tire inflation system, a first and a second compressor are provided. The first compressor supplies a first compressed air system with medium-pressure compressed air and the second compressor supplies a second compressed air system with high-pressure compressed air. The wheel valves are situated in the first compressed air system and the pilot valves are situated in the second compressed air system.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a tire inflation system, comprising a compressed-air generating system, a compressed-air reservoir, a valve block and pneumatically driven wheel valves in the wheels of a motor vehicle, with an electrically operated pilot control valve which is fixed to the vehicle in each case driving at least one wheel valve. 
     DE 40 09 687 A1 discloses a tire inflation system whose pressure generator is connected via axle valves to wheel valves which are not described in any more detail. In this case, the former control the latter by means of a pressure surge in the supply line leading to the wheel valve. Air let out from the tires is fed to the pressure generator, and from this to a pressure reservoir. This arrangement admittedly has the advantage of requiring only a single rotating transmission means into each of the wheels, although accurate and, in particular, operationally reliable, operation of the wheel valves is therefore impossible. 
     The system described in Austrian Utility Model AT 5548 U1 overcomes this defect at the expense of having to pass a pressure line and a control line via a rotating means for introducing them into the respective wheel. The wheel valves are pneumatically controlled valves fed from a common compressed-air source. Because the pressure in the tires is relatively low for a compressed-air system (2 to a maximum of 5 bar) and the volumes of air to be conveyed are relatively large, large valve cross sections are required. In addition, the wheel valves must be designed for the minimum system pressure of 2 bar, and their operation is uncertain with the low pressure differences that occur. In addition, high switching rates are desirable. This necessitates very large valves and actuators, which cannot be accommodated in the wheel of a motor vehicle. 
     DE 103 38 162 discloses a compressed-air generating system which supplies a plurality of load circuits (a compressed-air braking system and a pneumatic suspension system) with different pressure levels. The graduated pressure levels are created by means of pressure-limiting valves. However, this requires a compressor which is designed for the maximum pressure, and is therefore large, thus incurring high losses. 
     The object of the invention is therefore to propose a tire inflation system which is sufficiently small that it can be accommodated in a wheel and works quickly and reliably. In addition, the system is intended to be as simple and cheap as possible. This means pressure generators that are as simple as possible, short lines and, if possible, interaction with other compressed-air loads. 
     SUMMARY OF THE INVENTION 
     The foregoing object is obtained by providing a compressed-air generating system which has a first compressor and a second compressor, the first compressor provides compressed air at a medium pressure level to a first compressed-air system and the second compressor provides compressed air at a high pressure level to a second compressed-air system, with the wheel valves being connected to the first compressed-air system, and the pilot control valves being connected to the second compressed-air system. Since the pilot control valves are operated at a high and largely constant pressure level, the actuators for the wheel valves are small and operate reliably and quickly. They require only a very small amount of compressed air. The use of two compressors allows them to be designed specifically for the requirements and economically; the first compressor for the lower pressure level and higher feed rates, and the second compressor for the higher pressure level and lower feed rates. Because of the reservoir, the compressors also do not need to be designed for load peaks. 
     In a development of the invention, the second compressed-air system also supplies other loads, in particular pneumatic suspension bellows for pneumatic wheel suspension or level control. This results in better utilization of the second compressor, with pneumatic wheel suspension and the tire inflation system complementing one another well because of the low feed rates. In this case, at least one of the pneumatic suspension bellows can also be used as a pressure reservoir. 
     The induction side of the first compressor preferably is or can be connected to the surrounding area, and its pressure side is connected via a first non-return valve to a pressure reservoir, and the second compressor can be connected both to the first compressed-air system and to the second compressed-air system. The first compressor therefore has to feed only when the pressure reservoir is empty, and the second can carry out various functions. For this purpose, it can be connected by line in various ways to the two systems. 
     In a first advantageous embodiment, the induction side of the second compressor has a first valve, and its pressure side has a second valve and two bypass lines, with the first valve making the connection to the first compressed-air system, and the second valve making the connection to the second compressed-air system. Its suction side can be selectively connected via the first valve either to the first compressed-air system or via a bypass line, and the second valve can be connected to the second compressed-air system. In the latter case, air flowing back from the high-pressure system (the pneumatic suspension) is fed into the pressure reservoir. From its pressure side, the second compressor can selectively either feed the second compressed-air system via the second valve, or can feed the first compressed-air system via the second bypass line and the first valve. Furthermore, the first compressed-air system and the second compressed-air system can be connected to one another via a third valve. 
     In a second advantageous embodiment, both the induction side and the pressure side of the second compressor can be connected via a second valve to the second compressed-air system, with the induction side also being connected via a second non-return valve to the first compressed-air system, and with the non-return valve opening for flow toward the compressor. In this case, the second valve is a so-called 4/2 valve (4 connections and two positions). This arrangement also allows feeding to the first or second compressed-air system and reception of compressed air flowing back from the second compressed-air system. 
     In order to allow compressed air flowing back from the first compressed-air system to be supplied to the pressure reservoir as well, a fourth solenoid valve and possibly (if the air pressure for the front wheels and for the rear wheels is intended to be controlled independently of one another) a fifth solenoid valve are provided in the first compressed-air system, which solenoid valve releases the path to the wheel valve(s) in its first position or, in its second position, supplies air flowing back therefrom via at least a third non-return valve for further use. This may be the regeneration of an air dryer. Because the return flow from the tire inflation system (when the tire pressure is reduced) once again involves a relatively large volume flow at a relatively low pressure, it is advantageous to operate the fourth and if appropriate fifth solenoid valves pneumatically, for which purpose a sixth valve is operated as a pilot control valve with compressed air from the second compressed-air system (higher pressure). 
     It is also within the scope of the invention for the control valves of the pneumatic suspension system and the electrically operated pilot control valves, which are fixed to the vehicle, for the wheel valves to be arranged in the vicinity of the wheels, so that the second compressed-air system has to feed only one (high-pressure) compressed-air line to each wheel. As a continuation of this idea, the electrically operated pilot control valves can be connected for flow purposes to the respective pneumatic suspension bellows such that they take the control air for the wheel valves therefrom. This results in a minimum number of compressed-air lines having to be installed in the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described and explained in the following text with reference to figures, in which: 
         FIG. 1  shows a schematic diagram of a first embodiment of a system according to the invention; 
         FIG. 2  shows a schematic diagram of a second embodiment of a system according to the invention; 
         FIG. 3  shows detail III in  FIG. 2 , in a first position; 
         FIG. 4  shows detail III in  FIG. 2 , in a second position; 
         FIG. 5  shows detail III in  FIG. 2 , in a third position; and 
         FIG. 6  shows detail III in  FIG. 2 , in a fourth position. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , the wheels of a motor vehicle are annotated  1 . 1  to  1 . 4 , wheel valves fitted to or in them are annotated  2 . 1  to  2 . 4 , and associated pilot control valves are annotated  3 . 1  to  3 . 4 . Wheel valves  2 . 1  to  2 . 4  and pilot control valves  3 . 1  to  3 . 4  are part of a tire inflation system. Pneumatic suspension bellows  6 . 1  to  6 . 4  and pneumatic suspension valves  7 . 1  to  7 . 4  are part of a pneumatic suspension system or a pneumatic level control system. The tire inflation system and pneumatic suspension system are connected to two compressed-air systems at different pressure levels. 
     The first compressed-air system produces a pressure of between 2 and 5 bar in the lines  8 . 1  and  8 . 2 , which is passed via two-channel rotating introduction means  4 . 1  to  4 . 4  and the wheel valves  2 . 1  to  2 . 4  providing inflation air to the wheels, to be more precise to their tires. The second compressed-air system is at a pressure of, for example, 16 to 20 bar and comprises a pressure line  9 , supplying the pilot control valves  3 . 1  to  3 . 4  and the pneumatic suspension valves  7 . 1  to  7 . 4  via lines  9 ,  9 . 1 ,  9 . 2  and the branches  9 . 3 . The pilot control valves  3 . 1  to  3 . 4  control air from the lines  9 . 1 ,  9 . 2 , via the same two-channel rotating introduction means  4 . 1  to  4 . 2  to the pneumatically controlled wheel valves  2 . 1  to  2 . 4 . 
     The valve which are not operated pneumatically are controlled by means of electrical signals; the associated control center and the control lines leading to the valves are not shown. A dashed line  40  in  FIG. 1  also indicates that the control air can be supplied at a high pressure level to the pilot control valve  3 . 1  from the pneumatic suspension bellows  6 . 1  as well. 
     A first compressor  11 , which is driven by an electric motor  10 , and a second compressor  21 , which is driven by an electric motor  20 , are provided in order to supply the two compressed-air systems. The compressors are preferably piston-type compressors with internal non-return valves that are not illustrated; their feed direction is indicated by a small triangle in the figures. The first compressor  11  is in the form of a low-pressure compressor and its induction side is connected to the atmosphere. Its pressure side passes via an air dryer  12  and a first non-return valve  13  to a pressure reservoir  15  and from there to a line  17 , which is fitted with a pressure sensor  16  and is itself part of the first compressed-air system. The pressure side of the first compressor  11  can be connected via a line  18  with a shut-off valve  19  to the atmosphere. 
     The second compressor  21  is designed such that it provides a feed with optimum efficiency from the pressure level of the first compressed-air system to the compressed-air level of the second compressed-air system which, however, can also cover different pressure ranges, although with sub-optimum efficiency. For this purpose, it is connected in a particular manner to the first compressed-air system  8  and to the second compressed-air system  9 . There is a first valve  22 . 1  on the induction side of the second compressor  21  and a second valve  22 . 2  on its pressure side, as well as a third and a fourth non-return valve  23 . 1 ,  23 . 2  and a first and second bypass line  24 . 1  and  24 . 2 . In this case, the two valves  21 . 1  and  21 . 2  are so-called three/two-way valves (3/2 valves) which connect three connections to one another in two different ways. 
     The first valve  22 . 1  connects the line  17  that belongs to the first compressed-air system selectively either via the non-return valve  23 . 1  to the induction side of the second compressor  21  or via the first bypass line  24 . 1  to its pressure side. The first bypass line  24 . 1  allows compressed air that has been let out of the second compressed-air system  9  to be fed back into the line  17  of the first compressed-air system. The second valve  22 . 2  connects the second compressed-air system  9  selectively either to the pressure side of the second compressor  21  or to the second bypass line  24 . 2 , which makes the connection to the induction side of the second compressor  21  via a fourth non-return valve  23 . 2 , so that compressed air which has been let out of the second compressed-air system  9  is compressed again and can be supplied via the first bypass line  24 . 2  and the first valve  22 . 1  to the line  17  of the first compressed-air system. 
     The second compressed-air system  9  is therefore connected to the second valve  22 . 2  on the opposite side to the compressor  21 , and the pressure there is measured by a second pressure sensor  26 . The line  9 , which belongs to the second compressed-air system, can be connected via a third valve  27 , a connecting line  28 , the second non-return valve  14  and the first non-return valve  13  to the pressure reservoir  15 . Furthermore, the second compressed-air system  9  feeds a sixth valve  33 , which acts as a pilot control valve for a fourth and a fifth valve  32 . 1  and  32 . 2 . The two latter valves are therefore operated pneumatically and selectively make the connection between the first compressed-air system  8 . 1 ,  8 . 2  and either the line  17  leading to the pressure reservoir  15  or via fifth and sixth non-return valves  34 . 1 ,  34 . 2  to the connecting line  28  and thus to the pressure reservoir  15 . A further high-pressure reservoir  35 , which is accessible via a further valve  36 , can be provided in the second compressed-air system  9 . 1 . 
     In the embodiment shown in  FIG. 2 , analogous elements have reference symbols increased by 100. The numbers following the decimal point are omitted if possible. Wheels  101 . 1  to  101 . 4 , wheel valves  102 . 1  to  102 . 4  and rotating introduction means  104 . 1  to  104 . 2  are the same as in  FIG. 1  for all four wheels. The first compressed-air system is in this case formed by the lines  108 . 1  and  108 . 2 , and the second compressed-air system is formed by the lines  109 . 1  and  109 . 2 , in which the pilot control valves  103 . 1  and  103 . 2  are arranged. The second compressed-air system also includes the supply line  109 . 5  with the non-return valve  109 . 4  to the two pilot control valves  103 . 1  and  103 . 2 , as well as a separate line  109 . 3 , which leads to the pneumatic suspension valves  107 . 1  to  107 . 4  and also to the pneumatic suspension bellows  106 . 1  to  106 . 4 . 
     Once again, two compressors  111 ,  121  are provided in order to produce the compressed air for the two compressed-air systems. As in  FIG. 1 , the first compressor feeds a pressure reservoir  115  and a line  117 , which is itself part of the first compressed-air system and is connected via a fourth and fifth valve  132 . 1 ,  132 . 2  to the lines  108 . 1 ,  108 . 2 . The valves  132 . 1 ,  132 . 2  selectively make the connection between the lines  108 . 1 ,  108 . 2  of the first compressed-air system and either the line  117  or, via non-return valves  134 . 1 ,  134 . 2  of a connecting line  128  via a second non-return valve  114  and a first non-return valve  113  to the pressure reservoir  115 . 
     In this case, the second compressor  121  can be connected via a first valve  122  both to the lines  109 . 3  and  109 . 5  in the second compressed-air system and to the line  117  in the first compressed-air system. This valve  122  is a valve with 4 connections and two positions (a 4/2 valve). Furthermore, a bypass line  124  is provided, having a non-return valve  123  and connecting the first valve  122  to the pressure reservoir  115 . The particularly simple inclusion of the second compressor  121  in  FIG. 2  nevertheless unexpectedly offers a large number of options for the widely differing range of operating states. 
     In  FIG. 3 , the pressure reservoir  115  is replenished without any action on the tire inflation or pneumatic suspension. For this purpose, the first compressor  111  is switched on first of all, followed by the second compressor  121 , so that both compressors  111 ,  121  then replenish the pressure reservoir in parallel; to be precise the first compressor  111  via the non-return valve  113  and the second compressor  121  via the non-return valve  123  and the first valve  122  in the position shown, and then via the line  117 . When a specific pressure level is reached in the pressure reservoir  115 , the non-return valve  113  is closed and the second compressor  121  increases the pressure level in the pressure reservoir  115  further by further compressing the air, which has been initially compressed by the compressor  111 , via the non-return valve  123 . The two-stage compression process thus produced results in a feed performance with better efficiency, allowing a higher final pressure to be achieved in the pressure reservoir  115 . 
     If the pressure level in the pneumatic suspension bellows is reduced as shown in  FIG. 4 , the air flows from these bellows via the line  109 . 3 , the valve  122 , the bypass line  124 , the second compressor  121  and the line  117  into the pressure reservoir  115 . During this process, the two non-return valves  113 ,  123  are closed. The recompression of air from the pneumatic suspension bellows results in a significant improvement in efficiency and allows the pressure in the pressure reservoir  115  to be raised to a higher level. 
     In  FIG. 5 , the motor vehicle has been raised by inflating the pneumatic suspension bellows. For this purpose, compressed air is passed from the pressure reservoir  115  via the valve  122 , whose position has now been reversed, and the bypass line  124  to the second compressor  121  and from there to the pressure line  109 . 3 , which is part of the second compressed-air system and leads to the pneumatic suspension valves. 
     As shown in  FIG. 6 , the two compressors  111 ,  121  can each carry out the function of the other, after a fashion, in the event of a defect. If the first compressor  111  is defective, the second compressor  121  can suck in air through the compressor  111  and can pass compressed air to the second compressed-air system  109 . If the second compressor  121  is defective, the first compressor  111  can still replenish the pressure reservoir  115 , even if only very slowly.