CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority of Korean Patent Application Number 10-2013-0113910 filed Sep. 25, 2013, the entire contents of which application is incorporated herein for all purposes by this reference. 
       BACKGROUND OF INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a turbocharger in which a turbine is rotated by an exhaust gas, a compressor operated by the turbine increases the power of an engine by compressing intake air, and the quality of the exhaust gas is improved. 
         [0004]    2. Description of Related Art 
         [0005]    Turbochargers have been generally used for internal combustion engines, particularly, for the engine of heavy trucks or ships. Recently, those turbochargers are used for vehicles in order to achieve large power from small engines. Further, the turbochargers improve the quality of an exhaust gas by making fuel more completely burn. General function and design of the turbochargers are known in the art. 
         [0006]    The turbochargers include a turbine rotated by an exhaust gas and a compressor operated to compress intake air by the turbine, in which the turbine and the compressor are connected by a shaft. On the other hand, rotation reaction delay time (turbo lag) of the turbine, the shaft, and the compressor is generated in the turbochargers and studies for reducing the turbo lag have been conducted. Further, studies for effectively lubricating and cooling the periphery of the shaft while the shaft rotates at a high speed have been conducted. 
         [0007]    The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
       SUMMARY OF INVENTION 
       [0008]    The present invention has been made in an effort to provide a turbocharger having advantages of having improved durability and performance by cooling and lubricating a shaft, which is equipped with a turbine and a compressor at both sides and rotates in a sensor housing, through a bearing. 
         [0009]    Various aspects of the present invention provides a turbocharger including: a shaft having a central hole longitudinally formed therein, a supply hole for supplying oil to the central hole, and an exhaust hole for discharging the oil from the central hole to an outside of the shaft; a turbine disposed at a side of the shaft and rotated by an exhaust gas; a compressor disposed at the other side of the shaft and compressing intake air; a bearing rotatably supporting the shaft; and a center housing mounted with the bearing. 
         [0010]    The supply hole and the exhaust hole may be disposed with a predetermined distance therebetween in a longitudinal direction of the shaft. The supply hole may be formed close to the compressor and the discharge hole may be formed close to the turbine. The supply hole and the exhaust hole each may be formed at a predetermined angle in the rotational direction of the shaft. The central hole may be formed in an area corresponding to the bearing. The oil may cool and lubricate the periphery of the bearing and the shaft while circulating an oil line formed in the center housing, the supply hole, the central hole, and the exhaust hole. 
         [0011]    In accordance with various aspects of the present invention, by forming the shaft having the turbine and the compressor at both sides and disposed in the center housing to be rotatable by the bearing as a hollow shaft with a central hole, and by reducing rotational inertia moment, turbo lag can be reduced. Further, as the bearing and the shaft are lubricated and cooled by circulating oil through the central hole at the center of the shaft, the performance and the durability can be improved. 
         [0012]    The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view showing a turbocharger. 
           [0014]      FIG. 2  is a cross-sectional view showing an exemplary turbocharger according to the present invention. 
           [0015]      FIG. 3  is a partial exploded cross-sectional view of an exemplary turbocharger according to the present invention. 
           [0016]      FIG. 4  is a partial exploded perspective view of an exemplary turbocharger according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0018]      FIG. 1  is a cross-sectional view showing a turbocharger. Referring to  FIG. 1 , a turbocharger includes a turbine housing  100 , a center housing  110 , a compressor housing  120 , a compressor  130 , a shaft  140 , a turbine  150 , and a bearing  160 . 
         [0019]    The turbine  150  is fitted on one end of the shaft  140  and the compressor  130  is fitted on the other end of the shaft  140 . The bearing  160  is disposed on the shaft  140 , between the compressor  130  and the turbine  150 , and the shaft is disposed rotatably in the center housing  110  by the bearing  160 . 
         [0020]    The turbine housing  100  is combined with the center housing  110 , covering the turbine  150 , and the compressor housing  120  is combined with the center housing  110 , covering the compressor  130 . An exhaust gas supplied from an exhaust manifold to the turbine housing  100  rotates the turbine  150  and operates the compressor  130  through the shaft  140 . The compressor  130  takes air inside and pumps it to an intake manifold. 
         [0021]      FIG. 2  is a cross-sectional view showing a turbocharger according to various embodiments of the present invention. The configuration in  FIG. 2  which is different from that in  FIG. 1  is described in detail, while the detailed description of the same or similar configuration is not provided. 
         [0022]    As shown in the figure, the shaft  140 , which is a hollow type, has a central hole  200  inside, and a supply hole  210  for supplying oil to the central hole  200  and an exhaust hole  220  for discharging oil from the central hole  200  are formed in the shaft  140 . Accordingly, the oil supplied into the center housing  110  circulates through the bearing  160 , the supply hole  210 , the central hole  200 , and the exhaust hole  220 . In various embodiments of the present invention, channels or oil lines are formed in the center housing  110  and the bearing  160 . 
         [0023]    As described above, the oil lubricates the bearing  160  and the shaft  140  and cools the periphery of them while circulating through the bearing  160 , the supply hole  210 , the central hole  200 , and the exhaust hole  220 , such that the entire durability and performance are improved. Further, since the central hole  200  is formed at the center of the shaft  140 , the weight of the shaft  140  decreases, the rotational inertia moment reduces, and turbo lag reduces, such that reactivity of an engine can be improved. 
         [0024]      FIG. 3  is a partial exploded cross-sectional view of the turbocharger according to various embodiments of the present invention. Referring to  FIG. 3 , the central hole  200  is longitudinally formed in the shaft  140 , at the area corresponding to the bearing  160 . In more detail, the central hole  200  is formed at a predetermined length through the shaft  140  from the end portion where the turbine  150  is fitted. 
         [0025]    The exhaust hole  220  is formed toward the center of the central hole  200  from the outer circumference of the shaft  140 , the supply hole  210  is formed to the compressor  130  from the central hole  200 , and the exhaust hole  220  is formed at the end portion where the turbine  150  is fitted. Accordingly, the supply hole  210  and the exhaust hole  220  are formed with a predetermined gap or distance therebetween in the longitudinal direction of the shaft  140 . Further, in many cases, the diameter of the shaft  140  is large at the portion corresponding to the central hole  200  and small at the portion where the central hole  200  is not formed. 
         [0026]      FIG. 4  is a partial exploded perspective view of the turbocharger according to various embodiments of the present invention. Referring to  FIG. 4 , the supply hole  210  and the exhaust hole  220  are formed inward from the outer circumference of the shaft  140 . The supply hole  210  is formed at a predetermined angle in the rotational direction of the shaft  140  and the exhaust hole  220  is formed at a predetermined angle in the rotational direction of the shaft  140 . 
         [0027]    In various embodiments of the present invention, the diameter of the central hole formed in the shaft  140  is 0.5 to 0.25 times the diameter of the shaft  140 , the supply hole  210  and the exhaust hole  220  are formed in the area corresponding to the bearing  160 , the inner diameter of the supply hole  210  and the exhaust hole  20  is 1 to 3 mm, and the number of each of the supply hole  210  and the exhaust hole  220  may be two or more. 
         [0028]    For convenience in explanation and accurate definition in the appended claims, the terms “outer” or “inner”, “inside” or “outside”, and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
         [0029]    The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.
A turbocharger may include a shaft having a central hole longitudinally formed therein, a supply hole for supplying oil to the central hole, and an exhaust hole for discharging the oil from the central hole to the outside of the shaft. The turbocharger may also include a turbine disposed at a side of the shaft and rotated by an exhaust gas, a compressor disposed at the other side of the shaft and compressing intake air, a bearing rotatably supporting the shaft, and a center housing mounted with the bearing.