Patent Publication Number: US-2015075369-A1

Title: Oil-free air compressor for rail vehicles with air ventilation

Description:
CROSS REFERENCE TO APPLICATIONS 
     This application incorporates by reference U.S. patent application Ser. No. 13/350,980, filed Jan. 16, 2012 entitled “Oil-Free Air Compressor for Rail Vehicles”, which claims the benefit of U.S. Provisional Patent Application No. 61/437,333, filed Jan. 28, 2011, and entitled “Oil-Fee Air Compressor for Rail Vehicles”, the disclosures of which are incorporated herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates to the field of air compressors adapted for use on rail vehicles for the purpose of supplying compressed air to pneumatic units associated with the rail vehicle and, in particular, to an oil-free air compressor on a rail vehicle with air ventilation; the oil-free air compressor is used for supplying compressed air to various pneumatic units associated with the rail vehicle. 
     2. Description of Related Art 
     Normally, a pneumatic system is provided for a rail vehicle by which the brakes of the rail vehicle are operated. An air compressor is used to supply compressed air to one or more pneumatic units associated with the rail vehicle involved in the operation of the brakes. The air compressor usually consists of a driving unit, such as an electric motor, and of a compressor unit, which typically consists of several piston-cylinder arrangements that are driven by a crankshaft. The crankshaft is driven by the driving unit and includes connecting rods to convert the rotating movement of the driving unit into linear movement for each piston to supply compressed air to the downstream units. Screw-type air compressors are also generally known in the field for this purpose and are also included within the scope of the present invention. Furthermore, air compressor units for use on rail vehicles may have a single-stage or a multi-stage construction with at least one low-pressure stage and one high-pressure stage. 
     The air compressors used in the rail vehicle field may be subjected to continuous operation or to frequent on-and-off operation. In either mode of operation, friction during operation of the compressor leads to high heat development. As a result, in the past, air compressors that were predominantly used in the rail vehicle field used oil lubrication to ensure sufficient cooling during operation. However, oil lubrication carries a risk that the lubricating oil, usually situated in the housing of the compressor unit in the case of a piston air compressor, can penetrate past the piston-cylinder interface and into the pneumatic system, which may result in oil fouling the pneumatically operated brake units on the rail vehicle. Furthermore, condensate, which occurs during the required air drying of a pneumatic system, will typically contain some oil that has to be collected for environmental protection reasons. This condensate is typically stored in heatable containers and has to be drained and disposed of at regular intervals. This collection process leads to increased maintenance and disposal expenditures as well as to high oil consumption. In addition to the foregoing difficulties, emulsion formations in the oil circuit of these oil-lubricated compressor units can occur if the oil-lubricated compressor units are used infrequently or for limited periods of time as during cold weather operation. 
     Recently, dry-running air compressors have found increased usage in the rail vehicle field. A dry-running air compressor operates without lubricating oil situated in the housing and is said to be “oil-free”. In the case of oil-free air compressors, the lubrication on the piston travel path is replaced by a particularly low-friction dynamic sealing arrangement. All rotating components are normally disposed in roller bearings. The encapsulated roller bearings are provided with a temperature-stable long-lived grease filling. In the valve area, slidably guided components are largely avoided. Because of these measures, oil lubrication is not required in the air compressor unit. The risk of fouling by oil of the compressed air can therefore also be excluded. As a result of the elimination of an oil circuit, the oil-free air compressor can have a relatively light construction. In the rail vehicle field, there is a current trend toward lighter construction, and light carrier structures are also increasingly used for frame constructions. However, such light carrier structures frequently have a number of unfavorable natural frequencies that are close to the rotational speed of the air compressor of the pneumatic system which is arranged thereon. Therefore, it is difficult to sufficiently observe the required specifications concerning permissible structure-born noise levels. 
     U.S. Pat. No. 6,776,587 to Hartl et al. and U.S. Pat. No. 7,059,841 to Meyer et al. are patents directed to oil-free air compressor technology. The Meyer et al. patent discloses an arrangement of an oil-free compressor apparatus on a rail vehicle for supplying compressed air to pneumatic units assigned to the rail vehicle. The arrangement includes an oil-free air compressor and a cooler unit connected with the air compressor. The arrangement also includes a rail vehicle having a floor with at least one opening. The air compressor is fastened on at least one side to the vehicle floor such that a main axis of rotation of the air compressor is arranged essentially vertical with respect to the vehicle floor. The Hartl et al. patent discloses a piston arrangement for a dual-stage piston air compressor that includes a crankshaft and several piston-cylinders. The arrangement allows two or more low-pressure stages and at least one high-pressure stage to be formed. The arrangement allows the two or more low-pressure cylinders to be arranged in relation to the high-pressure stage in such a way that said two or more low-pressure cylinders are in phase or are offset by less than a predetermined amount and compress in a position which is offset by another predetermined amount in relation to one or more of the high-pressure cylinders. 
     United States Patent Application Publication No. 2007/0292289 to Hartl et al. discloses a compressor piston including a piston and a cylinder, a connecting rod connecting the piston to a crankshaft in a crankcase by a roller bearing, an air inlet line, and an air outlet line in a cylinder head. A tube connection between the air inlet line and the crankcase transports cooling air from the inlet line to the crankcase. The tube connection is exterior of the cylinder. An air inlet valve is connected to the tube connection which opens when the pressure in the crankcase is less than the pressure in the air inlet line, and an air outlet valve is connected to the crankcase which opens when the pressure in the crankcase exceeds a predetermined value. 
     Further, United States Patent Application Publication No. 2009/0016908 to Hartl et al. discloses a multi-cylinder dry-running piston compressor for generating compressed air. The piston compressor includes a crankcase having an interior and a crankshaft rotatably mounted in the crankcase. Also included are two connecting rods mounted on the crankshaft and configured to run counter to one another. Further included are two cylinders mounted in the crankcase and a piston arranged at an end of each of the connecting rods and configured to run in a respective one of the two cylinders. 
     SUMMARY OF THE INVENTION 
     In one embodiment, an oil-free compressor for a rail vehicle includes a compressor housing comprising at least a first housing portion and a second housing portion, a first piston cylinder supported in a first opening in the compressor housing, a second piston cylinder supported in a second opening in the compressor housing and fluidly connected to the first piston cylinder, a multi-piece crankshaft assembly supported by the compressor housing and linked to the pistons of the first and second piston cylinders by respective connecting rods, and an air plenum in fluid communication with the compressor housing interior to provide a volume of air to the compressor housing interior. 
     The first housing portion and the second housing portion may form respective halves of the compressor housing and may be secured together with mechanical fasteners. The first piston cylinder may be larger than the second piston cylinder. The crankshaft assembly may comprise a crankshaft center section and two end sections. The end sections may contain counterweights. Opposing ends of the crankshaft center section may be secured within respective cavities in the end sections. The crankshaft center section may comprise a first arm section offset from a second arm section and each of the arm sections may define a circumferential recess for receiving a bearing associated with the respective connecting rods. The end sections may be mounted to the crankshaft center section to secure the bearings associated with the respective connecting rods. 
     The oil-free compressor may include having the air plenum in fluid communication with the first piston cylinder. The oil-free compressor may further comprise an air intake valve, such as a check valve or reed valve, in the compressor housing enabling air to be drawn into the compressor housing interior from the air plenum. Moreover, the oil-free compressor may further comprise an air discharge valve, such as a check valve or reed valve, in the compressor housing enabling air to be discharged from the compressor housing interior. 
     In another embodiment, the oil-free compressor for a rail vehicle includes a multi-piece compressor housing, a first piston cylinder supported in a first opening in the compressor housing, a second piston cylinder supported in a second opening in the compressor housing and fluidly connected to the first piston cylinder, and a multi-piece crankshaft assembly supported by the compressor housing and linked to the pistons of the first and second piston cylinders by respective connecting rods. The connecting rods may connect to a wrist pin associated with each of the pistons, and the wrist pins are respectively supported by a dry lubricant bushing to the associated piston. The oil-free compressor may further comprise an air plenum in fluid communication with the compressor housing interior to provide a volume of air to the compressor housing interior. 
     The compressor housing may comprise at least a first housing portion and a second housing portion. The first housing portion and the second housing portion may form respective halves of the compressor housing and may be secured together with mechanical fasteners. The first piston cylinder may be larger than the second piston cylinder. The crankshaft assembly may comprise a crankshaft center section and two end sections. The end sections may contain counterweights. Opposing ends of the crankshaft center section may be secured within respective cavities in the end sections. The crankshaft center section may comprise a first arm section offset from a second arm section and each of the arm sections may define a circumferential recess for receiving a bearing associated with the respective connecting rods. The end sections may be mounted to the crankshaft center section to secure the bearing associated with the respective connecting rods. The dry lubricant bushing may be coated with PEAK or comprise a PEAK liner. 
     The oil-free compressor may include having the air plenum in fluid communication with the first piston cylinder. The oil-free compressor may further comprise an air intake valve, such as a check valve or reed valve, in the compressor housing enabling air to be drawn into the compressor housing interior from the air plenum. Moreover, the oil-free compressor may further comprise an air discharge valve, such as a check valve or reed valve, in the compressor housing enabling air to be discharged from the compressor housing interior. 
     Further details and advantages will become apparent upon reviewing the detailed description set forth herein in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an oil-free air compressor for railway vehicles shown in association with a drive motor and cooling fan. 
         FIG. 2  is a first perspective and isolation view of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 3  is a second perspective and isolation view of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 4  is a third perspective and isolation view of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 5  is a cross-sectional view taken along lines  5 - 5  in  FIG. 4 . 
         FIG. 6  is a longitudinal cross-sectional view of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 7  is an exploded perspective and isolation view of a piston of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 8  is a cross-sectional view of an assembled piston of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 9  is an exploded perspective view of a multi-component compressor housing of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 10  is a perspective view of a multi-component crankshaft assembly of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 11  is a longitudinal cross-sectional view of the multi-component crankshaft assembly of  FIG. 10 . 
         FIG. 12  is an exploded perspective view of another embodiment of the multi-component crankshaft assembly for a three-cylinder embodiment of the oil-free air compressor shown in  FIG. 1 . 
         FIG. 13  is a cross-sectional view of the multi-component crankshaft according to another embodiment. 
         FIG. 14  is a perspective view of an embodiment of an oil-free air compressor for railway vehicles with air ventilation. 
         FIG. 15  is a cross-section view taken along lines  15 - 15  in  FIG. 14 . 
         FIG. 16  is a bottom view of a portion of the housing of the oil-free air compressor shown in  FIGS. 14-15 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
     For purposes of the description hereinafter, spatial orientation terms, as used, shall relate to the referenced embodiment as it is oriented in the accompanying drawing figures or otherwise described in the following detailed description. However, it is to be understood that the embodiments described hereinafter may assume many alternative variations and configurations. It is also to be understood that the specific components, devices, and features illustrated in the accompanying drawing figures and described herein are simply exemplary and should not be considered as limiting. 
     Referring to  FIGS. 1-6 , an air compressor  2  according to one embodiment is shown. As shown, the air compressor  2  is a multi-cylinder air compressor  2  comprising at least a first piston-cylinder  10  and a second piston-cylinder  100 . The respective first and second piston-cylinders  10 ,  100  (hereinafter referred to as “first piston cylinder  10 ” and “second piston cylinder  100 ”) are supported by a compressor housing or crankcase  170  and are each driven by a crankshaft assembly  240  disposed within the compressor housing  170  and rotationally supported by the compressor housing  170 . The foregoing components of the air compressor  2  are described in detail herein. 
     As shown in cross-section in  FIG. 5 , the first and second piston cylinders  10 ,  100  are of substantially identical construction with the first piston cylinder  10  operating as the first cylinder and the second piston cylinder  100  operating as the second cylinder in the multi-cylinder air compressor  2 . The first piston cylinder  10  is generally larger than the second piston cylinder  100  and has an overall larger diameter than the second piston cylinder  100 . The first piston cylinder  10  comprises a cylindrical housing  12  that has a first end  14  adapted to be inserted into a corresponding opening, as described herein, in the compressor housing  170 , and a second end  16 . The cylindrical housing  12  is formed with a flange  18  located proximal of the first end  14  for interfacing with the exterior of the compressor housing  170 . Heat-dissipating fins  19  may be provided about the cylindrical housing  12 , and the cylindrical housing  12  may be formed of any suitable material providing sufficient strength and heat-dissipating characteristics such as aluminum. 
     A cylinder head  20  is secured to the second end  16  of the cylindrical housing  12 . The cylinder head  20  generally comprises a valve plate  22  and an air connecting unit  24 , with the air connecting unit  24  securing the valve plate  22  on the second end  16  of the cylindrical housing  12  via mechanical fasteners  26 . An additional mechanical fastener  27  secures the valve plate  22  to the air connecting unit  24 . The air connecting unit  24  comprises an air inlet port  28 . An air intake line  30  extends from the air inlet port  28  and is connected to the compressor housing  170  as described herein. The air connecting unit  24  further comprises an air outlet port  32 . An air connecting line  34  extends from the air outlet port  32  to fluidly couple, either directly or indirectly, to an air inlet port provided on the second piston cylinder  100  as described herein. Additionally, the valve plate  22  comprises a conventional reed valve assembly (not shown) for permitting airflow into the cylindrical housing  12  via the air intake line  30  and the air inlet port  28  and to be expelled from the cylindrical housing  12  via the air outlet port  32  and the air connecting line  34 , to provide pressurized air to the second piston cylinder  100 . The air connecting unit  24 , the air intake line  30 , and the air connecting line  34  may be formed of any suitable material providing sufficient strength and heat transfer characteristics such as aluminum. The cylindrical housing  12  defines an interior surface  36 . 
     Referring additionally to  FIGS. 7-8 , the first piston cylinder  10  further comprises a piston  40  that is reciprocally operable within the cylindrical housing  12 . The piston  40  comprises a first end  42  and a second end  44 , and is made of any suitable material providing sufficient strength and heat transfer characteristics such as aluminum. One or more wear bands or rings  46  is provided about the body of the piston  40  proximal of the first end  42  of the piston  40 . The wear bands or rings  46  are desirably non-metallic to interface with the interior surface  36  of the cylindrical housing  12  and may be made of a Torlon® polyamide-imide. A pair of piston rings  48  is provided about the first end  42  of the piston  40  and which also interfaces with the interior surface  36  of the cylindrical housing  12 . The piston rings  48  are desirably also of non-metallic construction, such as Teflon® (e.g., PTFE), to form a generally fluid-tight seal with the interior surface  36  of the cylindrical housing  12 . The body of the piston  40  defines an axial cavity or recess  50  and a transverse cavity or bore  52 , which is generally orthogonal to the axial cavity or recess  50 . The transverse bore  52  supports a wrist pin  54  that extends transversely through the body of the piston  40 . The wrist pin  54  may be a solid wrist pin or, as illustrated, a cylindrical-shaped wrist pin  54 . The wrist pin  54  is held in place within the transverse bore  52  by mechanical fasteners  55  that extend into second end  44  of the piston  40  to engage the wrist pin  54 . The wrist pin  54  is provided to interface or link with a connecting rod associated with the crankshaft assembly  240 , as described further herein. The wrist pin  54  may be made of any suitable material providing sufficient strength and heat transfer characteristics such as aluminum. 
     Known wrist pin assemblies are generally solid shaft wrist pins where a needle bearing is fitted. These wrist pins are precision-ground and act as an inner race for the needle bearing. These wrist pins must have a cross-sectional area large enough to withstand bending stresses at their centers, and their surfaces must be hard enough to withstand the loading of the needle rollers of the bearing. The needle bearing requires high temperature grease and high temperature seals to contain the grease in a bearing cavity. These prior art wrist pins can slide within the needle bearing and, therefore, the ends of the wrist pins must be fastened to the piston with fasteners, and shock absorbing non-metallic bushings that are located between the wrist pin ends and the piston wrist pin bore. 
     The wrist pin  54 , described previously, is supported in the transverse bore  52  by an oil-free assembly that is comprised by a pair of dry lubricant bushings  56  that are press-fitted into the transverse bore  52 . The dry lubricant bushings  56  typically comprise a metal case with a polymer liner. Dry bushings are usually plain composite bushes that are able to run with marginal or no lubrication and have a low coefficient of friction. Dry bushings can include polymer dry bushings and alloy bushings. This oil-free assembly allows the transmission of compression and suction forces from a center portion  58  of the wrist pin  54  to the opposing ends  60 ,  62  of the wrist pin  54 , thus reducing the bending moment of the wrist pin  54  and allowing the wrist pin  54  to have a uniform cross-section of homogeneous material with no additional components thereby reducing weight. The dry lubricant bushings  56  also provide bearing support transmitted directly through the piston  40  instead of the load being transmitted directly through the connecting rod associated with the crankshaft assembly  240 , as described further herein. Consequently, the load due to compression is supported by greater bearing area and greater bearing capacity. In addition, the dry lubricant bushings  56  self-lubricate as the dry lubricant bushings  56  are coated with PEAK material or comprise a PEAK liner. In operation, the self-lubricating, dry lubricant bushings  56  lubricate the sliding joint made between the dry lubricant bushings  56  and the wrist pin  54 . The dry lubricant bushings  56  and the wrist pin  54  described previously eliminate the need for a “thick” wrist pin as required in the prior art because compression loading shifts from the center portion  58  of the wrist pin  54  to the two ends  60 ,  62  of the wrist pin  54 . Since the wrist pin  54  does not have to withstand bending stresses at its center portion  58 , the surface of the wrist pin  54  need not be hard enough to withstand the loading of a needle bearing, as described herein in connection with the crankshaft assembly  240 . Additionally, there is no requirement for high temperature grease and high temperature seals to contain the grease in a bearing cavity. Further, the wrist pin cannot slide within the needle bearing since the wrist pin  54  is press-fitted in the hoop of the connecting rod. Therefore, the ends  60 ,  62  of the wrist pin  54  can be free to float without any fasteners. The shock absorbing non-metallic bushings required in the prior art wrist pins discussed previously are also eliminated. These characteristics are also present in the wrist pin discussed herein in connection with the second piston cylinder  100 . 
     In operation, the piston  40  operates in a reciprocating movement which is generated via the crankshaft assembly  240 . Air within the compressor housing  170  is drawn into the cylinder housing  12  via the air intake line  30  and the air inlet port  28  as a result of the downward movement of the piston  40  and is compressed during the upward movement of the piston  40 . The reed valve associated with the valve plate  22  has a portion that is opened during the downward movement of the piston  40 , drawing air into the cylinder housing  12  from the air intake line  30  and the air inlet port  28 , and closes during the upward movement. Further, the reed valve (not shown) has another portion that closes during the downward movement of the piston  40  and opens in the upward movement of the piston  40  whereby the air in the cylinder housing  12  is compressed and is guided out of the cylinder housing  12  via the air outlet port  32  and the air connecting line  34  and is fed to the air inlet port, discussed herein, associated with the second piston cylinder  100 . 
     As noted previously, the second piston cylinder  100  has substantially identical construction to the first piston cylinder  10 , as now described hereinafter. The first piston cylinder  10  is generally larger than the second piston cylinder  100  and has an overall larger diameter than the second piston cylinder  100 . The second piston cylinder  100  comprises a cylindrical housing  112  that has a first end  114  adapted to be inserted into a corresponding opening, as described herein, in the compressor housing  170 , and a second end  116 . The cylindrical housing  112  is formed with a flange  118  located proximal of the first end  114  for interfacing with the exterior of the compressor housing  170 . Heat-dissipating fins  119  may be provided about the cylindrical housing  112 , and the cylindrical housing  112  may be formed of any suitable material providing sufficient strength and heat-dissipating characteristics such as aluminum. 
     A cylinder head  120  is secured to the second end  116  of the cylindrical housing  112 . The cylinder head  120  generally comprises a valve plate  122  and an air connecting unit  124 , with the air connecting unit  124  securing the valve plate  122  on the second end  116  of the cylindrical housing  112  via mechanical fasteners  126 . An additional mechanical fastener  127  secures the valve plate  122  to the air connecting unit  124 . The air connecting unit  124  comprises an air inlet port  128  which is fluidly connected (directly or indirectly) to the air connecting line  34  that extends from the air outlet port  32  associated with the air connecting unit  24  of the first piston cylinder  10 . As shown in  FIG. 1 , an air manifold  300  may be provided as an intermediary device in the air connecting line  34  that extends from the air outlet port  32  associated with the air connecting unit  24  of the first piston cylinder  10  to the air inlet port  128  on the air connecting unit of the second piston cylinder  100 . The air connecting unit  124  further comprises an air outlet port  132  which is connected via an air connecting line  134  to a downstream requirement or apparatus, such as an outlet air manifold  302 . Additionally, the valve plate  122  comprises a conventional reed valve assembly (not shown) for permitting airflow into the cylindrical housing  112  via the air connecting line  34  and the air inlet port  128  and to be expelled from the cylindrical housing  112  via the air outlet port  132  and the air connecting line  134 , to provide pressurized air via the air connecting line  134  to a downstream requirement, such as the outlet air manifold  302 . The air connecting unit  124  and the air connecting line  134  may be formed of any suitable material providing sufficient strength and heat transfer characteristics such as aluminum. The cylindrical housing  112  defines an interior surface  136 . 
     With continued reference to  FIGS. 1-8 , the second piston cylinder  100  also comprises a piston  140  that is reciprocally operable within the cylindrical housing  112 . The piston  140  comprises a first end  142  and a second end  144 . One or more wear bands or rings  146  are provided about the body of the piston  140  proximal of the first end  142  of the piston  140 . The wear bands or rings  146  are desirably non-metallic to interface with the interior surface  136  of the cylindrical housing  112 , and may be made of a Torlon® polyamide-imide. A pair of piston rings  148  is provided about the first end  142  of the piston  140  and which also interfaces with the interior surface  136  of the cylindrical housing  112 . The piston rings  148  are desirably of non-metallic construction, such as Teflon® (e.g., PTFE), to form a generally fluid-tight seal with the interior surface  136  of the cylindrical housing  112 . The body of the piston  140  defines an axial cavity or recess  150  and a transverse cavity or bore  152 , which is generally orthogonal to the axial cavity or recess  150 . The transverse bore  152  supports a wrist pin  154  that extends transversely through the body of the piston  140 . The wrist pin  154  may be a solid wrist pin or, as illustrated, a cylindrical-shaped wrist pin  154 . The wrist pin  154  is held in place within the transverse bore  152  by mechanical fasteners  155  that extend into second end  144  of the piston  140  to engage the wrist pin  154 . The wrist pin  154  is provided to interface or link with a connecting rod associated with the crankshaft assembly  240 , as described further herein. The wrist pin  154  may be made of any suitable material providing sufficient strength and heat transfer characteristics such as aluminum. 
     In a similar manner to the wrist pin  54 , the wrist pin  154  is also supported within the transverse bore  152  by an oil-free assembly that is comprised of a pair of dry lubricant bushings  156  which are press-fitted in the transverse bore  152 . The dry lubricant bushings  156  typically comprise a metal case with polymer liner. This oil-free assembly allows the transmission of compression and suction forces from a center portion  158  of the wrist pin  154  to the ends  160 ,  162  of the wrist pin  154  thus reducing the bending moment of the wrist pin  154  and allowing the wrist pin  154  to have a uniform cross-section of homogeneous material with no additional components thereby reducing weight. The dry lubricant bushings  156  also provide bearing support transmitted directly through the piston  140  instead of the load being transmitted directly through the connecting rod. Consequently, the load, due to compression, is supported by greater bearing area and greater bearing capacity. In addition, the dry lubricant bushings  156  self-lubricate as the dry lubricant bushings  156  are coated with PEAK material or include a PEAK liner. In operation, the self-lubricating, dry lubricant bushings  156  lubricate the sliding joint made between the dry lubricant bushings  156  and the wrist pin  154 . The various advantages described previously with respect to the wrist pin  54  are likewise applicable to the wrist pin  154 . 
     In operation, the piston  140  operates in a reciprocating movement which is generated via the crankshaft assembly  240 . Air is drawn into the cylinder housing  112  via the air connecting line  130  and the air inlet port  128  as a result of the downward movement of the piston  140  and is compressed during the upward movement of the piston  140 . The reed valve assembly (not shown) associated with the valve plate  122  has a portion that is opened during the downward movement of the piston  140 , drawing air into the cylinder housing  112  from the air connecting line  130  and the air inlet port  128  and closes during the upward movement. Further, the reed valve (not shown) includes another portion that is closed during the downward movement of the piston  140  and opens in the upward movement of the piston  140  whereby the air in the cylinder housing  112  is compressed and is guided out of the cylinder housing  112  via the air connecting line  134  and is fed via the air connecting line  134  to a downstream requirement such as the outlet air manifold  302 . 
     Referring additionally to  FIG. 9 , the compressor housing or crankcase  170  is desirably a compound structure comprising at least a first housing portion  172  and a second housing portion  174 . The first and second housing portions  172 ,  174  are each generally rectangular shaped structures that are adapted to be joined together to form the overall compressor housing  170 . For this purpose, the first and second housing portions  172 ,  174  have respective lateral flanges  176 ,  178  that are adapted to be joined together using conventional mechanical fasteners  177 , such as bolt and nut combinations. Locating bushings  179  may be provided on the lateral flanges  176 ,  178  to properly align corresponding openings in the lateral flanges  176 ,  178  to accept the mechanical fasteners  177 . The first housing portion  172  defines an opening  180  sized to accept the first end  14  of the cylindrical housing  12  of the first piston cylinder  10 . Similarly, the second housing portion  174  defines an opening  182  sized to accept the first end  114  of the cylindrical housing  112  of the second piston cylinder  100 . Mounting elements  184  may be welded or otherwise secured at locations about the respective openings  180 ,  182 . The mounting elements  184  may be mounting pegs or bolts that are adapted to engage openings (not shown) in the respective flanges  18 ,  118  on the cylindrical housings  12 ,  112  of the first and second piston cylinders  10 ,  100  to secure the piston cylinders  10 ,  100  in place within the openings  180 ,  182  with conventional nuts or like fastening components. 
     As shown in  FIG. 4 , the first housing portion  172  further comprises opposing lateral walls  186 . The air intake line  30  is placed in fluid communication with an air intake port or opening  188  and may be defined in the first housing portion  172  in one of the opposing lateral walls  186  and is secured via mechanical fasteners to the lateral wall  186  of the first housing portion  172  to place the first piston cylinder  10  in fluid communication with the interior of the compressor housing  170 . As an alternative, the air intake port or opening  188  may be provided in the same wall of the first housing portion  170  supporting the first piston cylinder  10  and this modification is also shown in  FIGS. 2-3  and in cross-section in  FIG. 6 .  FIG. 9  shows both locations for air intake port  188 , and when not in use, the unused air intake port  188  is covered by a cover plate  189 . The second housing portion  174  further includes an air intake port  190  for providing air intake generally to the interior of the assembled compressor housing  170 . The air intake port  190  may be adapted to interface or connect to an air inlet line  192  connected to a filtering apparatus  304  for filtering air entering the compressor housing  170 , as shown in  FIG. 1 . 
     The first housing portion  172  and second housing portion  174 , when assembled as described previously, form the compressor housing  170 . When the first piston cylinder  10  and second piston cylinder  100  are secured in the respective openings  180 ,  182  in the first housing portion  172  and second housing portion  174 , the respective first and second piston cylinders  10 ,  100  extend outward from opposing longitudinal walls  194  of the compressor housing  170 . Two end walls  196  of the compressor housing  170  are defined by assembly of the first and second housing portions  172 ,  174  and these end walls  196  define respective axial openings  198 ,  200  in the compressor housing  170 . 
     In summary, the compressor housing  170  as depicted is made up of at least two separate “halves” in the form of housing portions  172 ,  174  that are assembled together and machined as one. The two halves are located with respect to each other by the locating bushings  179  and held together by mechanical fasteners  177 . Benefits of the split compressor housing  170  relate to manufacturing and assembly costs, for example. Because the compressor housing  170  is in at least two major parts, the tooling required to cast the compressor housing  170  may be smaller and, as a result, more foundries are capable of manufacturing this component. This manufacturing advantage can lead to cost savings over a large one-piece housing that requires large tooling and equipment to cast. As known in the art, a one-piece compressor crankcase must be large because the crankshaft has to be assembled before it is placed into the crankcase, and an opening must be provided in the crankcase that is large enough to allow the assembled crankshaft to pass therethrough. Installing an assembled crankshaft through an opening in a one-piece crankcase that is just large enough to accommodate the crankshaft is time consuming and difficult. Typically, the crankshaft has to be carefully threaded into the crankcase while continually repositioning the connecting rods to avoid contact with the inside of the crankcase. A single piece crankshaft can weigh over 80 pounds and maneuvering it is very difficult. The presently disclosed compressor housing  170  allows the crankshaft assembly  240  to be assembled and held stationary while the at least two housing portions  172 ,  174  are placed on either side of the crankshaft assembly  240  and secured. This assembly step eliminates the need to manipulate a heavy crankshaft as in the prior art. By providing a compound compressor housing  170 , overall, the compressor housing  170  may be made smaller, lighter, easier to cast and machine, and easier to assemble. The first and second housing portions  172 ,  174  forming the compressor housing  170  may be formed of any suitable material providing sufficient strength and heat-dissipating characteristics such as aluminum. 
     The first axial opening  198  in the compressor housing  170  supports a first crankshaft mounting element  202 , which generally encloses the first axial opening  198  and is supported to the end wall  196  of the compressor housing  170  via mechanical fasteners  203 . The first crankshaft mounting element  202  comprises an annular portion  204  that is seated within a receiving annular portion  206  formed by the assembly of the first housing portion  172  and second housing portion  174 . The annular portion  204  of the first crankshaft mounting element  202  supports a first main crankshaft bearing  208  which, in turn, supports one end of the crankshaft assembly  240 . The first main crankshaft bearing  208  is sealed in place by a first shaft seal  210  adapted to seat against the crankshaft assembly  240 , and a second shaft seal  212  disposed interiorly within the annular portion  204  of the first crankshaft mounting element  202 . The first crankshaft mounting element  202  also supports an external mounting cage  214  for mounting the air compressor  2  in association with a drive component such as a drive motor  306 . 
     The second axial opening  200  in the compressor housing  170  supports a second crankshaft mounting element  222 , which generally encloses the second axial opening  200  and is supported to the opposing end wall  196  of the compressor housing  170  via mechanical fasteners  223 . The second crankshaft mounting element  222  comprises an annular portion  224  that is seated within a receiving annular portion  226  defined by the assembly of the first housing portion  172  and second housing portion  174 . The annular portion  224  of the second crankshaft mounting element  222  supports a second main crankshaft bearing  228  which, in turn, supports the other end of the crankshaft assembly  240 . The second main crankshaft bearing  228  is sealed in place by a first shaft seal  230  adapted to seat against the crankshaft assembly  240 , and a second shaft seal  232  disposed interiorly within the annular portion  224  of the second crankshaft mounting element  222 . The respective first and second crankshaft mounting elements  202 ,  222  support the opposing ends of the crankshaft assembly  240  and enclose the first and second axial openings  198 ,  200  defined by the assembly of the first and second housing portions  172 ,  174  which form the compressor housing  170 . As shown in  FIGS. 1-4  and  9 , the first and second housing portions  172 ,  174  define several additional openings  234  to provide access to the interior of the compressor housing  170  or to provide other points of connection for additional air handling conduits to the compressor housing  170 . These additional openings  234  may be covered with additional covers  236  that are secured to the compressor housing  170  via appropriate mechanical fasteners. 
     Referring additionally to  FIGS. 10-12 , the crankshaft assembly  240  is a compound assembly comprised generally by a crankshaft center section  242  and two crankshaft end sections  244 ,  246 . The first crankshaft end section  244  is supported by the first main crankshaft bearing  208  in the first crankshaft mounting element  202 . As described previously, the first crankshaft mounting element  202  supports the external mounting cage  214  for mounting the air compressor  2  in association with a drive component such as the drive motor  306  shown in  FIG. 1 . Thus, the first crankshaft end section  244  is positioned to interface with a drive motor to impart rotary motion to the crankshaft assembly  240 . The opposite crankshaft end section  246  is supported by the second main crankshaft bearing  228  in the second crankshaft mounting element  222  and this end section  246  is positioned to interface with a cooling air fan  308  associated with the air compressor  2 . Opposing ends  248  of the crankshaft center section  242  are secured within respective cavities  250  in the crankshaft end sections  244 ,  246  by a press-fit connection and like connections. 
     As shown in  FIGS. 10-11 , the crankshaft assembly  240  includes at least two connecting rods  252 ,  254  which link to the pistons  40 ,  140 , respectively, of the first and second piston cylinders  10 ,  100 . The connecting rods  252 ,  254  each comprise a first circular end flange  256  supported on the crankshaft center section  242  by respective spherical roller bearings  258  that are press-fit into respective circumferential recesses  260  defined adjacent the respective ends  248  of the crankshaft center section  242 . The spherical roller bearings  258  are held in place in the recesses  260  by the respective press-fit crankshaft end sections  244 ,  246 . Referring briefly to  FIG. 12 , while the foregoing discussion relates to an air compressor  2  having two compressing piston-cylinders provided by the first and second piston cylinders  10 ,  100 , additional piston-cylinders may be included in the air compressor  2 .  FIG. 12  shows that if one or more additional piston cylinders (not shown) are added to the air compressor  2 , an additional connecting rod  262  may be mounted on the crankshaft center section  242  adjacent the connecting rod  254  to provide motive forces for operating the additional piston cylinder (not shown). Spacers  264  of predetermined lengths may also be used to mount the respective connecting rods  252 ,  254 ,  262  to the crankshaft center section  242  as needed in this embodiment. 
     The connecting rods  252 ,  254  each comprise a second circular end flange  266  supported on the respective wrist pins  54 ,  154  associated with the pistons  40 ,  140  by respective needle bearings  268 . Shaft seals  270  are provided outboard on either side of each of the spherical roller bearings  258  and about the crankshaft center section  242  to seal the spherical roller bearings  258 . Likewise, shaft seals  272  are provided outboard on either side of each of the needle bearings  268  and about the respective wrist pins  54 ,  154  to seal the needle bearings  268 . Further, as shown in cross-section in  FIG. 11 , the crankshaft center section  242  generally comprises an offset construction defined by two opposed shaft portions or arm sections  274 ,  276  that terminate in ends  248 . Respective internal passages  278 ,  280  are defined in the shaft arm sections  274 ,  276  that are each sealed with a plug  282 . The crankshaft center section  242 , end sections  244 ,  246 , and connecting rods  252 ,  254 ,  262  may be formed of any suitable material providing sufficient strength such as steel. 
     The multi-piece crankshaft assembly  240  may be used to replace one-piece crankshafts which are large and heavy. Such single-piece crankshafts are cast or forged by large machinery that requires expensive tooling. Additionally, special machines are needed to machine and balance a one-piece crankshaft. With a one-piece crankshaft, the bearings for the connecting rods have to be sized so that they can be installed on the one-piece crankshaft, often over the bearing seat for the crankshaft main bearings. This means the bearings for the connecting rods have to be larger than necessary, thus adding more weight and bulk. Also, this prior art arrangement requires the addition of bolt-on counterweights which could become loose and cause compressor failure. 
     The multi-piece crankshaft assembly  240  described hereinabove is made up of a crankshaft center section  242  that is relatively small and can be made from a casting or forging. The two crankshaft end sections  244 ,  246  also contain counterweights as integral parts and require no fasteners. The foregoing components are small enough to be cast or forged without large equipment. Thus, specialized crankshaft manufacturing equipment is also unnecessary. Since the spherical roller bearings  258  associated with the connecting rods  252 ,  254 ,  262  do not have to pass over crankshaft main bearing seats or over crankshaft bends as in a one-piece crankshaft situation, they can be sized based on the loading of the pistons  40 ,  140  and, as a result, may be smaller. 
     The crankshaft center section  242  may be designed with the proper throw based on the intended application, including a motor end shaft arm section  274  with the same throw and appropriate end counterweight section  244  and a fan end shaft arm section  276  with the same throw and appropriate end counterweight section  246 . The spacers  264  are also used to hold the spherical roller bearings  258  and place them in the proper location in a multi-connecting rod arrangement as shown in  FIG. 12 . The crankshaft center section  242  is provided to hold the connecting rods  252 ,  254 ,  262  by securing the spherical roller bearings  258  in the proper location. As noted previously, for air compressors  2  of more than two piston cylinders, the spacers  264  hold the associated spherical roller bearings  258  in place by pressing onto the inner bearing race for each bearing  258 . The crankshaft center section  242  is also provided so that the opposing ends  248  are press-fit into the respective cavities  250  in the crankshaft end sections  244 ,  246 . The two crankshaft end sections  244 ,  246  contain the crankshaft center section  242  and press onto the inner race of the spherical roller bearings  258 , or onto the spacers  264  which press onto the inner races of the spherical roller bearings  258  in a multi-connecting rod arrangement as shown in  FIG. 12 . The interface between the spherical roller bearings  258  and the crankshaft center section  242  does not have to be a press-fit interface because the crankshaft end sections  244 ,  246  or the spacers  264  are sufficient to hold the inner races from spinning To enable easy disassembly of the crankshaft assembly  240  for replacing the connecting rod bearings  268  at overhaul, holes may be drilled into the crankshaft center section  242  to intersect with internal passages  278 ,  280  and are defined in the shaft arm sections  274 ,  276  so that a hydraulic pump may be attached to push-off the two crankshaft end sections  244 ,  246  from the center section  242 . 
     Moreover, as shown in  FIG. 13 , in another embodiment the crankshaft center section  242  comprises an offset construction defined by two opposed and separate shaft portions or aim sections  274 ,  276  that terminate in ends  248 . Respective internal passages  278 ,  280 , which are not shown  FIG. 13  but may be in the form shown in  FIG. 11  discussed previously, may be defined in the shaft aim sections  274 ,  276  and be sealed with respective plugs  282 . The crankshaft center section  242  in  FIG. 13  defines a pair of through holes  292  to accept mating ends  298  of the respective shaft portions or arm sections  274 ,  276 . The multi-component crankshaft center section  242  may be readily be used in place of the singular or unitary crankshaft center section  242  discussed previously. The multi-component crankshaft center section  242  facilitates easier manufacturing. The mating ends  298  may be secured in the through holes  292  via mechanical fastening or friction fit methods and like methods known in the mechanical arts. 
     Referring to  FIGS. 14-16 , another embodiment of the air compressor  2  is shown. The air compressor  2  shown in  FIGS. 14-16  is adapted to improve the exchange of air in the compressor housing or crankcase  170 , which aids in extending the longevity of the air compressor  2 . In the embodiments of the air compressor  2  described previously, cooling air flows are drawn into the crankcase  170  due to the suction strokes of the pistons  40 ,  140  (see  FIG. 6 ) in the first piston cylinder  10 . This method is effective at cooling the crankcase  170  but may have the effect of lowering the overall efficiency of the air compressor  2  due to the introduction of preheated suction air into the first piston cylinder  10 . In the modified embodiment shown in  FIGS. 14-16 , an arrangement and method is provided that brings cool air into the crankcase  170  and discharges heated air therefrom while having minimal effect on air compressor efficiency. 
     As shown in  FIGS. 14-16 , an air plenum  400  is disposed on the crankcase  170 , typically on the second housing portion  174  thereof. The air plenum  400  generally rectangular shaped (e.g., box-shaped) housing  402  that defines a hollow interior  404 , which provides a volume of air that can be drawn into crankcase  170 . An end wall  406  of the housing  402  defines an air inlet  408  which may be connected to an air filter or other apparatus (not shown) used to filter cool ambient air entering the air plenum housing  402  via the inlet  408  and thereby providing a volume of filtered air in the air plenum housing  402 . Other advantages of the air plenum  400  are that the air plenum  400  serves to depulse the intake air prior to entering the first piston cylinder  10  aiding in the induction of air and dampening the intake noise of the air compressor  2  contributing to overall noise reduction. The air plenum housing  402  is connected to the first piston cylinder  10  via the air intake line  30 . A sidewall  410  of the air plenum housing  402  defines an opening  412  to which the air intake line  30  is connected to place the air intake line  30  in fluid communication with the hollow interior  404 . 
     As shown in  FIG. 15 , the air plenum housing  402  encloses an air intake valve  414  situated in bottom opening  416  of the air plenum housing  402 . The air intake valve  414  extends through a corresponding opening  418  in the compressor housing or crankcase  170 . The air intake valve  414  may be a check valve or a reed-type valve adapted to allow cool air to be drawn into the crankcase  170  in response to the pistons  40 ,  140  moving toward top dead center. As the pistons  40 ,  140  move to top dead center, a vacuum develops in the crankcase  170  causing check valve plunger  420  (or an alternative reed) to open allowing air into the crankcase  170  from the air plenum housing  402 . The air intake valve  414  prevents return flow into the air plenum  402 . 
     As further shown in  FIGS. 15-16 , one or more air discharge valves  422  are provided in plate element  424  disposed in an opening in the bottom of the crankcase  170 . The discharge valves may be check valves or reed-type valves as shown and allow the heated crankcase air to be vented to atmosphere. As the pistons  40 ,  140  move to bottom dead center, the air intake valve  414  is closed and the pressure in the crankcase  170  increases. The increased pressure causes the air discharge valves  422  to open venting the crankcase  170 . 
     The two-valve method described above of bringing in cool air and discharging hot air takes advantage of the large air volumes displaced as the pistons  40 ,  140  stroke up and down in their respective cylinders  12 ,  112 . Since both pistons  40 ,  140  travel from bottom dead center to top dead center at the same time, a significant volume of air is displaced. This displaced air is constantly going from pressure to vacuum as the crankshaft assembly  240  spins. By placing the air intake valve  414  in the air plenum housing  402 , ideally connected to an air filtration element connected to the air inlet  408 , filtered air is drawn into the crankcase  170 . By placing the air discharge valves  422 , on the opposite side of the crankcase  170  from the air intake valve  414 , as shown in  FIGS. 15-16 , the cooling air will have to pass over the crankshaft assembly  240  to reach the air discharge valves  422 . As the air travels through the crankcase  170  it will remove heat from all radiating surfaces and the effects of gas blow-by and expel it from the crankcase  170 . Additional air intake valves  414  and air discharge valves  422  may be added if needed to maximize the cooling air flows. 
     While embodiments of an oil-free air compressor for a rail vehicle are provided in the foregoing description, those skilled in the art may make modifications and alterations to these embodiments without departing from the scope and spirit of the invention. Accordingly, the foregoing description is intended to be illustrative rather than restrictive. The invention described hereinabove is defined by the appended claims and all changes to the invention that fall within the meaning and the range of equivalency of the claims are to be embraced within their scope.