Patent Publication Number: US-2019178246-A1

Title: Oil pump

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application 2017-235279, filed on Dec. 7, 2017, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure relates to an oil pump, and in particular, to an oil pump having an inner rotor and an outer rotor. 
     BACKGROUND DISCUSSION 
     In the related art, an oil pump equipped with an inner rotor and an outer rotor has been known (see, e.g., JP 2008-308991A (Reference 1)). 
     Reference 1 discloses an oil pump equipped with an inner rotor having external teeth, an outer rotor having internal teeth that engage with the external teeth of the inner rotor, and a housing that accommodates the inner rotor and the outer rotor. The housing of the oil pump of Reference 1 is formed with an inlet port that guides oil into a pump chamber between the external teeth and the internal teeth, an outlet port which guides the oil to the outside of the pump chamber, and a discharge hole that guides bubbles contained in the oil to the outside of the pump chamber. In the oil pump of Reference 1, the discharge hole is configured to communicate with the pump chamber in a state where the pump chamber and the outlet port communicate with each other. 
     In the oil pump of Reference 1, the discharge hole, which guides the bubbles contained in the oil to the outside of the pump chamber, is configured to communicate with the pump chamber in a state where the pump chamber and the outlet port communicate with each other. For this reason, when the pump chamber and the outlet port communicate with each other, the bubbles separated from the oil are disturbed by the flow of the outflow oil, and as a result, the effect of removing the bubbles from the discharge hole deteriorates. Therefore, there is a need for an oil pump capable of effectively removing bubbles contained in oil. 
     Thus, a need exists for an oil pump which is not susceptible to the drawback mentioned above. 
     SUMMARY 
     An oil pump according to an aspect of this disclosure includes: an inner rotor including external teeth; an outer rotor including internal teeth that engage with the external teeth of the inner rotor; a housing configured to accommodate the inner rotor and the outer rotor; an inlet port formed in the housing and configured to guide oil into a pump chamber between the external teeth and the internal teeth; an outlet port formed in the housing and configured to guide the oil to the outside of the pump chamber; and a discharge hole formed in the housing and configured to guide bubbles contained in the oil to the outside of the pump chamber, in which the discharge hole is provided to communicate with the pump chamber earlier than a timing when the pump chamber and the outlet port communicate with each other, and the outlet port is provided to communicate with the pump chamber later than a timing when the pump chamber has a maximum volume. 
     In the oil pump according to the aspect of this disclosure, the pump chamber and the discharge hole communicate with each other before the pump chamber and the outlet port communicate with each other as described above, so that bubbles may be discharged from the discharge hole, and as a result, it is possible to inhibit the bubbles separated from the oil from being disturbed by a flow of the oil from the pump chamber toward the outlet port. In addition, since the pump chamber and the outlet port communicate with each other after the timing when the pump chamber has the maximum volume, the pressure in the pump chamber may be increased while the volume of the pump chamber is decreased from the state where the pump chamber has the maximum volume. Therefore, the bubbles may be discharged from the discharge hole by the increased pressure. Therefore, the bubbles may be more efficiently discharged from the discharge hole, it is possible to effectively remove the bubbles contained in the oil. Here, there are water, NO x  (nitrogen oxide), HC (hydrocarbon), and the like as substances that degrade an engine. It has been known that if water, NO x , and HC are contained in the oil, the oil is degraded due to chemical reactions between water, NO x , and HC. In the oil pump of this disclosure, it is possible to effectively remove water, NO x , and HC contained as bubbles in oil, and as a result, it is possible to effectively inhibit degradation of oil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein: 
         FIG. 1  is a view illustrating an oil pump according to a first embodiment; 
         FIG. 2  is a cross-sectional view taken along line II-II in  FIG. 1 ; 
         FIG. 3A  is a view illustrating a state where an opening of an inlet port is closed with respect to a pump chamber; 
         FIG. 3B  is a view illustrating a state where the pump chamber has a maximum volume; 
         FIG. 3C  is a view illustrating a state where the pump chamber and a discharge hole communicate with each other; 
         FIG. 3D  is a view illustrating a state where the pump chamber and an opening of an outlet port begin to communicate with each other; 
         FIG. 4  is a view illustrating a simulation result of pressure in the pump chamber of the oil pump according to the first embodiment; 
         FIG. 5  is an enlarged view of a portion from −20 degrees to 20 degrees in  FIG. 4 ; 
         FIG. 6  is a view illustrating an oil pump according to a second embodiment; 
         FIG. 7  is a cross-sectional view taken along line VII-VII in  FIG. 6 ; 
         FIG. 8A  is a view illustrating a state where bubbles are collected at an end of a groove portion; and 
         FIG. 8B  is a view illustrating a state where bubbles are introduced into the groove portion. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described with reference to the drawings. 
     First Embodiment 
     A configuration of an oil pump  100  according to a first embodiment will be described with reference to  FIGS. 1 to 5 . 
     The oil pump  100  according to the first embodiment is mounted in an automobile (not illustrated) having an engine. The oil pump  100  serves to pump oil (lubricant) in an oil pan and supply (pump) the oil to the periphery of a piston of the engine and to moving parts (sliding parts) such as a crank shaft. 
     (Configuration of Oil Pump) 
     As illustrated in  FIG. 1 , the oil pump  100  has a housing  10 , an inner rotor  11 , and an outer rotor  12 . The inner rotor  11  has external teeth  111 . The outer rotor  12  has internal teeth  121  that engage with the external teeth  111  of the inner rotor  11 . Pump chambers  13  are formed between the external teeth  111  of the inner rotor  11  and the internal teeth  121  of the outer rotor  12 . 
     The housing  10  accommodates the inner rotor  11  and the outer rotor  12  so that the inner rotor  11  and the outer rotor  12  are rotatable. The housing  10  has an inlet port  14  that guides oil into the pump chamber  13  between the external teeth  111  and the internal teeth  121 . In addition, the housing  10  has an outlet port  15  that guides the oil to the outside of the pump chamber  13 . As illustrated in  FIG. 2 , the housing  10  includes a first portion  10   a  and a second portion  10   b . The inner rotor  11  and the outer rotor  12  are accommodated between the first portion  10   a  and the second portion  10   b . The housing  10  is made of an aluminum alloy. 
     The inner rotor  11  is configured to be rotated by a rotating shaft  112 . The rotating shaft  112  is rotated by an operation of the engine. A rotation center of the inner rotor  11  is eccentric only by a predetermined degree with respect to a rotation center of the outer rotor  12 . When the inner rotor  11  is rotated in a direction of the arrow R (clockwise), the outer rotor  12  is rotated in the same direction. During the rotation, the external teeth  111  of the inner rotor  11  and the internal teeth  121  of the outer rotor  12  mesh with each other at a portion where a distance between the inner rotor  11  and the outer rotor  12  is short. In contrast, because the number of external teeth  111  of the inner rotor  11  is smaller by one than the number of internal teeth  121 , a gap (pump chamber  13 ) is formed between the external teeth  111  and the internal teeth  121  at a portion where the distance is long. In addition, the pump chamber  13  is expanded or contracted simultaneously with the rotation in the direction of the arrow R, so that a pumping function is created. Therefore, as a volume of the pump chamber  13  is increased, the oil is sucked into the pump chamber  13 . In addition, as the volume of the pump chamber  13  is decreased, the oil in the pump chamber  13  is ejected to the outside. 
     The inlet port  14  has an opening  141  at a portion where the pump chamber  13  is gradually expanded. The inlet port  14  is connected to the oil pan, so that the oil is supplied from the oil pan. The outlet port  15  has an opening  151  at a portion where the pump chamber  13  is gradually contracted. The outlet port  15  is connected to oil supply destinations of respective parts in the engine. As illustrated in  FIG. 2 , the inlet port  14  and the outlet port  15  are formed in a concave shape in an inner surface of the housing  10  (a surface opposite to a side at which the inner rotor  11  and the outer rotor  12  are rotatably fitted). In addition, the inlet port  14  and the outlet port  15  are formed in the housing  10  so as to have predetermined flow path shapes. 
     In the first embodiment, the housing  10  has a discharge hole  16  that guides bubbles contained in the oil to the outside of the pump chamber  13 . As illustrated in  FIG. 2 , the discharge hole  16  communicates with the outside of the housing  10 . Specifically, an opening  161  of the discharge hole  16  communicates with the gap between the rotating shaft  112  and the housing  10 . Therefore, the bubbles discharged from the discharge hole  16  are discharged to the outside of the housing  10 . 
     As illustrated in  FIG. 1 , the discharge hole  16  is formed to be opened at a position which is closer in a radial direction to the rotating shaft  112  than the opening  151  of the outlet port  15 . In addition, the discharge hole  16  is formed to be opened at the portion which is closer in the radial direction to the rotating shaft  112  than the opening  141  of the inlet port  14 . The opening  161  of the discharge hole  16  is formed to be connected to the pump chambers  13  in the vicinity of tooth bottoms of the external teeth  111  of the inner rotor  11 . 
     In the first embodiment, the inlet port  14  is provided to be closed earlier than the timing when the pump chamber  13  has a maximum volume. Specifically, the inlet port  14  is provided to be closed earlier than the timing when the pump chamber  13  has the maximum volume so that pressure in the pump chamber  13  becomes pressure that generates bubbles. For example, in a state where the pump chamber  13  has the maximum volume, the pressure in the pump chamber  13  is configured to be lower than vapor pressure of water at a room temperature (about 25° C.). In addition, the outlet port  15  is provided to communicate with the pump chamber  13  later than the timing when the pump chamber  13  has the maximum volume. 
     In the first embodiment, the discharge hole  16  is provided to communicate with the pump chamber  13  earlier than the timing when the pump chamber  13  and the outlet port  15  communicate with each other. Specifically, the discharge hole  16  is formed to be opened at a position, between the opening  141  of the inlet port  14  and the opening  151  of the outlet port  15 , which is closer to the opening  151  of the outlet port  15  than the opening  141  of the inlet port  14 . In addition, the discharge port  16  is provided to communicate with the pump chamber  13  later than the timing when the pump chamber  13  has the maximum volume. A region where the outlet port  15  and the pump chamber  13  communicate with each other and a region where the discharge hole  16  and the pump chamber  13  communicate with each other partially overlap with each other. That is, the pump chamber  13  communicates with the discharge hole  16  first. Thereafter, the pump chamber  13  communicates with the outlet port  15  in the state where the pump chamber  13  communicates with the discharge hole  16 . Further, the pump chamber  13  communicates with the outlet port  15  even after the communication between the pump chamber  13  and the discharge hole  16  is closed. 
     As illustrated in  FIG. 3A , the communication between the inlet port  14  and the pump chamber  13  is closed earlier than the timing when the pump chamber  13  (hatched area) has the maximum volume. That is, the opening  141  of the inlet port  14  is formed to be terminated at a position opposite in a rotation direction to the position at which the pump chamber  13  has the maximum volume. When the pump chamber  13  (hatched area) is rotated in the direction of the arrow R from the state in  FIG. 3A , the pump chamber  13  has the maximum volume in the state in  FIG. 3B . In this case, the pump chamber  13  does not communicate with any one of the inlet port  14 , the outlet port  15 , and the discharge hole  16 . 
     When the pump chamber  13  is rotated in the direction of the arrow R from the state in  FIG. 3B , the pump chamber  13  communicates with the discharge hole  16  in the state in  FIG. 3C . In this case, the pump chamber  13  does not communicate with the inlet port  14  and the outlet port  15 . That is, in the state in  FIG. 3C , the pump chamber  13  communicates with the discharge hole  16 , so that the bubbles are discharged from the discharge hole  16 . 
     When the pump chamber  13  is rotated in the direction of the arrow R from the state in  FIG. 3C , the pump chamber  13  communicates with the outlet port  15  in the state in  FIG. 3D . In this case, the pump chamber  13  does not communicate with the inlet port  14 . In addition, the pump chamber  13  communicates with the discharge hole  16 . Thereafter, when the pump chamber  13  is rotated in the direction of the arrow R, the communication between the pump chamber  13  and the discharge hole  16  is terminated. 
     (Explanation of Pressure in Pump Chamber of Oil Pump) 
       FIGS. 4 and 5  illustrate a relationship between a rotation angle (degree) of the inner rotor  11  and pressure (kPa) in the pump chamber  13  in the oil pump  100  of the first embodiment. A position at which the rotation angle of the inner rotor  11  is 0 degree is a closing timing position in the related art. In addition, the pressure in the pump chamber  13  is pressure relative to atmosphere. As illustrated in  FIG. 4 , the pressure in the pump chamber  13  is increased in a range in which the rotation angle of the inner rotor  11  varies from about 20 degrees to 40 degrees. In this case, the pressure in the pump chamber  13  is increased up to a maximum of 12 Mpa. That is, it is possible to eject the oil with high pressure by retarding the opening timing of the outlet port  15 . In addition, since the pressure in the pump chamber  13  is increased, the discharge hole  16  is provided in this region to make it possible to efficiently discharge the bubbles. 
     As illustrated in  FIG. 5 , when the suction timing is closed 10 degrees earlier, the pressure in the pump chamber  13  is decreased, so that absolute pressure in the pump chamber  13  becomes equal to or lower than 3 kPa which is lower than the atmospheric pressure. 3 kPa is pressure at which water boils at 25° C. Therefore, in the pump chamber  13 , water, which is an impurity, is educed as gas. 
     Effect of First Embodiment 
     The following effects may be obtained in the first embodiment. 
     In the oil pump  100  of the first embodiment, bubbles may be efficiently discharged from the discharge hole  16 , as a result, it is possible to effectively remove bubbles contained in oil. In addition, in the oil pump  100  of the present embodiment, it is possible to effectively remove water, NO x  (nitrogen oxide), and HC (hydrocarbon) contained in oil as bubbles, and as a result, it is possible to effectively inhibit degradation of oil. 
     In the first embodiment, the bubbles may be discharged from the discharge hole  16  immediately before the oil is ejected from the opening  151  of the outlet port  15  after the oil is sucked into the pump chamber  13  from the inlet port  14 . Therefore, it is possible to efficiently discharge the bubbles from the discharge hole  16  without causing turbulence of the oil in the pump chamber  13 . 
     In the first embodiment, the pump chamber  13  is further expanded after the oil is sucked into the pump chamber  13  from the inlet port  14 , and as a result, it is possible to decrease the pressure in the pump chamber  13 . Therefore, unnecessary substances such as water dissolved, as a liquid, in the oil may be educed (vaporized) as bubbles, and as a result, it is possible to easily discharge unnecessary substances such as water as bubbles from the discharge hole  16 . 
     In the first embodiment, unnecessary substances dissolved in the oil may be assuredly educed (vaporized) as bubbles, and as a result, it is possible to effectively remove unnecessary substances from the oil. 
     In the first embodiment, the bubbles, which are moved toward the inside of the pump chamber  13  due to an influence of centrifugal force because the bubbles have a smaller specific weight than the oil, may be easily discharged from the discharge hole  16  opened at the inside of the pump chamber  13 . 
     Second Embodiment 
     Next, a second embodiment will be described with reference to  FIG. 6  to  FIGS. 8A and 8B . In the second embodiment, a configuration in which groove portions  113  are formed in the inner rotor  11  unlike the first embodiment will be described as an example. In addition, in the drawings, constituent elements identical to the constituent elements in the first embodiment are denoted by the same reference numerals as the constituent elements in the first embodiment. 
     An oil pump  200  according to the second embodiment is mounted in an automobile (not illustrated) having an engine. The oil pump  200  serves to pump oil (lubricant) in an oil pan and supply (pump) the oil to the periphery of a piston of the engine and to moving parts (sliding parts) such as a crank shaft. 
     (Configuration of Oil Pump) 
     As illustrated in  FIG. 6 , the oil pump  200  has a housing  10 , an inner rotor  11 , and an outer rotor  12 . The inner rotor  11  has external teeth  111 . The outer rotor  12  has internal teeth  121  that engage with the external teeth  111  of the inner rotor  11 . Pump chambers  13  are formed between the external teeth  111  of the inner rotor  11  and the internal teeth  121  of the outer rotor  12 . 
     The housing  10  accommodates the inner rotor  11  and the outer rotor  12  so that the inner rotor  11  and the outer rotor  12  are rotatable. The housing  10  has an inlet port  14  that guides oil into the pump chamber  13  between the external teeth  111  and the internal teeth  121 . In addition, the housing  10  has an outlet port  15  that guides the oil to the outside of the pump chamber  13 . 
     The inner rotor  11  is configured to be rotated by a rotating shaft  112 . The inner rotor  11  is rotated in a direction of the arrow R (clockwise). The outer rotor  12 , which engages with the inner rotor  11 , is rotated together with the inner rotor  11 . 
     In the second embodiment, the inner rotor  11  has the groove portions  113  formed in tooth bottom portions  111   a  of the external teeth  111 . In addition, the groove portions  113  are formed in the tooth bottom portions  111   a  of the multiple external teeth  111 , respectively. As illustrated in  FIG. 7 , the groove portions  113  are provided in both end surfaces of the inner rotor  11  in the direction of the rotating shaft. In addition, the groove portion  113  is formed to have a larger cross-sectional area than a bubble. For example, the groove portion  113  has a depth and a width of several millimeters. 
     In the second embodiment, as illustrated in  FIG. 6 , the housing  10  has a discharge hole  16  that guides bubbles contained in the oil to the outside of the pump chamber  13 . The discharge hole  16  communicates with the outside of the housing  10 . Specifically, an opening  161  of the discharge hole  16  communicates with a gap between the rotating shaft  112  and the housing  10 . Therefore, the bubbles discharged from the discharge hole  16  are discharged to the outside of the housing  10 . 
     In the second embodiment, the outlet port  15  is provided to communicate with the pump chamber  13  later than the timing when the pump chamber  13  has a maximum volume. In addition, the discharge hole  16  is provided to communicate with the pump chamber  13  earlier than the timing when the pump chamber  13  and the outlet port  15  communicate with each other. In addition, the discharge hole  16  is formed to be opened at a position, in a radial direction, which overlaps with the groove portion  113  but does not overlap with the pump chamber  13 . That is, the pump chamber  13  and the discharge hole  16  are configured to communicate with each other through the groove portion  113 . 
     The groove portion  113  of the inner rotor  11  is formed in an arc shape when viewed in the rotating shaft. In addition, the groove portion  113  of the inner rotor  11  is formed such that both ends of the groove portion  113  are connected to the pump chamber  13 . As illustrated in  FIGS. 8A and 8B , as the inner rotor  11  rotates, the oil is moved by centrifugal force from the arc-shaped groove portion  113  into the pump chamber  13 . In this case, the oil flows out from one end of the groove portion  113 , so that attractive force is generated at the other end. Further, bubbles  20 , which are moved to the tooth bottom portion  111   a  of the inner rotor  11 , are drawn into the groove portion  113 . That is, the bubbles  20  are introduced into the arc-shaped groove portion  113  by a pump-priming effect. The groove portion  113  and the discharge hole  16  communicate with each other in a state where the bubbles  20  are collected in the groove portion  113 , so that the bubbles are discharged from the discharge hole  16 . 
     The other configurations of the second embodiment are identical to those of the first embodiment. 
     Effect of Second Embodiment 
     The following effects may be obtained in the second embodiment. 
     Similar to the first embodiment, in the second embodiment, it is possible to effectively remove the bubbles contained in the oil. 
     In the second embodiment, the bubbles  20 , which are moved toward the inside of the pump chamber  13  due to an influence of centrifugal force because the bubbles have a smaller specific weight than the oil, may be collected in the groove portion  113  provided in the tooth bottom portion  111   a  of the inner rotor  11 . Therefore, the groove portion  113  and the discharge hole  16  communicate with each other, so that the bubbles may be easily discharged. 
     In the second embodiment, the bubbles  20  may be smoothly guided into the groove portion  113 , so that the bubbles  20  may be effectively collected in the groove portion  113 . 
     In the second embodiment, the oil in the groove portion  113  is discharged into the pump chamber  13  from one end of the groove portion  113 , so that the bubbles may be guided to be sucked into the groove portion  113  from the other end of the groove portion  113 , and as a result, it is possible to more effectively collect the bubbles in the groove portion  113 . 
     In the second embodiment, the discharge hole  16  and the pump chamber  13  are not directly connected to each other, and as a result, it is possible to inhibit the oil in the pump chamber  13  from being discharged from the discharge hole  16 . 
     The other effects of the second embodiment are identical to those of the first embodiment. 
     Modified Example 
     Further, it should be considered that all of the disclosed embodiments are illustrative in all aspects but not limitative. The scope of the embodiments disclosed here are defined by the appended claims instead of the description of the embodiments and includes all alterations (modified examples) within the meanings and scope equivalent to the claims. 
     For example, in the first and second embodiments, an example in which the embodiment disclosed here is applied to the oil pump for supplying oil (lubricant) to the engine (internal combustion engine) has been described, but the embodiment disclosed here is not limited thereto. For example, the embodiment disclosed here may be applied to an oil pump for supplying an AT fluid (AT oil) to an automatic transmission (AT) that automatically switches a gear ratio in accordance with a rotational speed of an internal combustion engine. In addition, the embodiment disclosed here may be applied to an oil pump for supplying a lubricant to a sliding portion in a continuously variable transmission (CVT) capable of changing a gear ratio continuously without a stage unlike the AT (multistage transmission) that performs the gear shift operation by changing a combination of gears. In addition, the embodiment disclosed here may be applied to an oil pump for supplying power steering oil to a power steering device that performs steering (operates a steering device) in a vehicle. 
     In the first and second embodiments, the configuration in which the oil pump is rotationally operated by the operation of the engine has been described as an example, but the embodiment disclosed here is not limited thereto. In the embodiment disclosed here, the oil pump may be rotationally operated by an electric motor. 
     In the first and second embodiments, the configuration in which the outer rotor is driven by driving the inner rotor has been described as an example, but the embodiment disclosed here is not limited thereto. In the embodiment disclosed here, the inner rotor may be driven by driving the outer rotor. 
     In the first and second embodiments, the configuration in which the pump chamber and the outlet port communicate with each other in the state where the pump chamber and the discharge hole communicate with each other has been described as an example, but the embodiment disclosed here is not limited thereto. In the embodiment disclosed here, the pump chamber and the outlet port may communicate with each other after the pump chamber and the discharge hole are closed. Further, the pump chamber and the outlet port may communicate with each other at the same time when the pump chamber and the discharge hole is closed. 
     In the first and second embodiments, the configuration in which the inlet port is closed earlier than the timing when the pump chamber has the maximum volume has been described as an example, but the embodiment disclosed here is not limited thereto. In the present embodiment, the inlet port may be closed at the timing when the pump chamber has the maximum volume. 
     In the first and second embodiments, the configuration in which the discharge holes are provided at both sides of the housing in the direction of the rotating shaft has been described as an example, but the embodiment disclosed here is not limited thereto. In the present embodiment, the discharge hole may be provided at one side of the housing in the direction of the rotating shaft. 
     In the second embodiment, the configuration in which the groove portions are provided at both sides of the inner rotor in the direction of the rotating shaft has been described as an example, but the embodiment disclosed here is not limited thereto. In the present embodiment, the groove portion may be provided at one side of the inner rotor in the direction of the rotating shaft. 
     In the first and second embodiments, the example in which the oil pump is mounted in a vehicle such as an automobile having an engine has been described, but the embodiment disclosed here is not limited thereto. For example, the present embodiment may be applied to an oil pump mounted in facility equipment other than the vehicle having the internal combustion engine. In addition, a gasoline engine, a diesel engine, a gas engine, and the like may be applied as the internal combustion engine. 
     An oil pump according to an aspect of this disclosure includes: an inner rotor including external teeth; an outer rotor including internal teeth that engage with the external teeth of the inner rotor; a housing configured to accommodate the inner rotor and the outer rotor; an inlet port formed in the housing and configured to guide oil into a pump chamber between the external teeth and the internal teeth; an outlet port formed in the housing and configured to guide the oil to the outside of the pump chamber; and a discharge hole formed in the housing and configured to guide bubbles contained in the oil to the outside of the pump chamber, in which the discharge hole is provided to communicate with the pump chamber earlier than a timing when the pump chamber and the outlet port communicate with each other, and the outlet port is provided to communicate with the pump chamber later than a timing when the pump chamber has a maximum volume. 
     In the oil pump according to the aspect of this disclosure, the pump chamber and the discharge hole communicate with each other before the pump chamber and the outlet port communicate with each other as described above, so that bubbles may be discharged from the discharge hole, and as a result, it is possible to inhibit the bubbles separated from the oil from being disturbed by a flow of the oil from the pump chamber toward the outlet port. In addition, since the pump chamber and the outlet port communicate with each other after the timing when the pump chamber has the maximum volume, the pressure in the pump chamber may be increased while the volume of the pump chamber is decreased from the state where the pump chamber has the maximum volume. Therefore, the bubbles may be discharged from the discharge hole by the increased pressure. Therefore, the bubbles may be more efficiently discharged from the discharge hole, it is possible to effectively remove the bubbles contained in the oil. Here, there are water, NO x  (nitrogen oxide), HC (hydrocarbon), and the like as substances that degrade an engine. It has been known that if water, NO x , and HC are contained in the oil, the oil is degraded due to chemical reactions between water, NO x , and HC. In the oil pump of this disclosure, it is possible to effectively remove water, NO x , and HC contained as bubbles in oil, and as a result, it is possible to effectively inhibit degradation of oil. 
     In the oil pump according to the aspect of this disclosure, it is preferable that the discharge hole is formed to be opened at a position, between an opening of the inlet port and an opening of the outlet port, and closer to the opening of the outlet port than the opening of the inlet port. 
     With this configuration, the bubbles may be discharged from the discharge hole immediately before the oil is ejected from the opening of the outlet port after the oil is drawn into the pump chamber from the inlet port, and as a result, it is possible to efficiently discharge the bubbles from the discharge hole without causing turbulence of the oil in the pump chamber. 
     In the oil pump according to the aspect of this disclosure, it is preferable that the inlet port is provided to be closed earlier than the timing when the pump chamber has the maximum volume. 
     With this configuration, the pump chamber is further expanded after the oil is sucked into the pump chamber from the inlet port, and as a result, it is possible to decrease the pressure in the pump chamber. Therefore, unnecessary substances dissolved, as a liquid, in the oil may be educed (vaporized) as bubbles, and as a result, it is possible to easily discharge the unnecessary substances as bubbles from the discharge hole. 
     In the oil pump according to the aspect of this disclosure, it is preferable that the discharge hole is formed to be opened at a position closer to a rotating shaft of the inner rotor than the opening of the outlet port in a radial direction of the inner rotor. 
     With this configuration, the bubbles, which are moved toward the inside of the pump chamber due to an influence of centrifugal force because the bubbles have a smaller specific weight than the oil, may be easily discharged from the discharge hole opened at the inside of the pump chamber. 
     In the oil pump according to the aspect of this disclosure, it is preferable that the inner rotor has groove portions provided in tooth bottom portions of the external teeth. 
     With this configuration, the bubbles, which are moved toward the inside of the pump chamber due to an influence of centrifugal force because the bubbles have a smaller specific weight than the oil, may be collected in the groove portion provided in the tooth bottom portion of the inner rotor, and as a result, it is possible to easily discharge the bubbles as the groove portion and the discharge hole communicate with each other. 
     In this disclosure, the following configurations may be conceived regarding the oil pump according to one aspect. 
     That is, in the configuration in which the groove portions are provided in the inner rotor, a groove portion of the inner rotor may be formed in an arc shape when viewed in a direction of the rotating shaft of the inner rotor. 
     With this configuration, the bubbles may be smoothly guided into the groove portion, and as a result, it is possible to effectively collect the bubbles in the groove portion. 
     In the configuration in which the groove portions are provided in the inner rotor, the groove portion of the inner rotor may be formed such that both ends of the groove portion in a rotation direction of the inner rotor are connected to the pump chamber. 
     With this configuration, the oil in the groove portion is discharged into the pump chamber from one end of the groove portion, so that the bubbles may be guided to be sucked into the groove portion from the other end of the groove portion, and as a result, it is possible to more effectively collect the bubbles in the groove portion. 
     In the configuration in which the groove portions are provided in the inner rotor, the discharge hole may be opened at a position, in a radial direction of the inner rotor, which overlaps with the groove portion but does not overlap with the pump chamber. 
     With this configuration, the discharge hole and the pump chamber are not directly connected to each other, and as a result, it is possible to inhibit the oil in the pump chamber from being discharged from the discharge hole. 
     In the configuration in which the inlet port is provided to be closed earlier the timing when the pump chamber has the maximum volume, the inlet port is provided to be closed earlier than the timing when the pump chamber has the maximum volume so that the pressure in the pump chamber becomes a pressure that generates bubbles. 
     With this configuration, unnecessary substances dissolved in the oil may be assuredly educed (vaporized) as bubbles, and as a result, it is possible to effectively remove the unnecessary substances from the oil. 
     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.