Patent Publication Number: US-10309290-B2

Title: Piston cooling device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Embodiments of the present invention relates to a piston cooling device, and particularly relates to a piston cooling device for keeping a temperature of a piston disposed in a cylinder of an internal combustion engine, at an appropriate temperature. 
     Background Art 
     Patent Literature 1 discloses a cooling device for cooling a piston disposed in a cylinder of an internal combustion engine. The piston described in Patent Literature 1 includes a cooling cavity inside the piston. The cooling cavity communicates with two inlet/outlet holes provided at a bottom surface side of the piston. 
     At the bottom surface side of the piston, two oil jets are provided. One of the oil jets is provided to face one of the inlet/outlet holes when the piston is located in the vicinity of a top dead center. The other oil jet is provided to face the other inlet/outlet hole when the piston is located in the vicinity of the bottom dead center. 
     According to the above described configuration, a flow of oil can be made in the cooling cavity by supplying oil from the one of the inlet/outlet holes when the piston is located in the vicinity of the top dead center. Further, when the piston is located in the vicinity of the bottom dead center, a flow of the oil can be formed in the cooling cavity by supplying the oil from the other inlet/outlet hole. Consequently, according to the above described conventional cooling device, the piston can be properly cooled from the inside of the piston during an operation of the internal combustion engine. 
     LIST OF RELATED ART 
     Following is a list of patent literatures which the applicant has noticed as related arts of the present invention. 
     [Patent Literature 1] 
     Japanese Patent Laid-Open No. 2003-301744 A 
     [Patent Literature 2] 
     Japanese Patent Laid-Open No. 2008-163936 A 
     PROBLEM TO BE SOLVED BY EMBODIMENTS OF THE INVENTION 
     In an internal combustion engine, in addition to the oil jet as described above, a jet or the like for spraying oil toward the bottom surface of the piston may be used. When the jet that sprays oil toward the cooling cavity is used together with another oil jet, it is necessary to control the both the jets so as to spray just enough amount of oil in order to keep a proper temperature of the piston. 
     However, the cooling device described in Patent Literature 1 includes only the jets that supply oil to the cooling cavity of the piston, and does not provide a solution to the case where a jet of this kind and another oil jet having a different purpose are used in combination. 
     Embodiments of the present invention have been made to solve the problem as described above, and it is an object of the embodiments of the present invention to provide a piston cooling device that uses an oil jet for supplying oil to a cooling cavity of a piston and an oil jet having a different purpose in combination, and can properly keep a temperature of the piston stably for a long period of time. 
     SUMMARY 
     To achieve the above mentioned purpose, a first aspect of an embodiment of the present invention is a piston cooling device that is installed in an internal combustion engine. The piston cooling device comprises: a cooling cavity that is provided inside the piston, and includes an inlet/outlet hole that opens on a bottom surface of the piston; a first oil jet that sprays oil toward the inlet/outlet hole; and a second oil jet that sprays oil toward a part different from the inlet/outlet hole, wherein the first oil jet sprays oil in preference to the second oil jet. 
     A second aspect of an embodiment of the present invention is a piston cooling device according to the first aspect discussed above, wherein the piston cooling device further comprises an oil pump that generates an oil pressure that is necessary to spray oil, and wherein when a discharge oil amount of the oil pump is smaller as compared with an oil amount necessary to cause oil to be sprayed from both of the first oil jet and the second oil jet, the discharge oil amount is consumed preferentially in oil spray from the first oil jet. 
     A third aspect of an embodiment of the present invention is a piston cooling device according to the second aspect discussed above, wherein the first oil jet and the second oil jet communicate with the oil pump via a common oil pressure path, and a valve opening pressure of the first oil jet is a lower pressure as compared with a valve opening pressure of the second oil jet. 
     A fourth aspect of an embodiment of the present invention is a piston cooling device according to the third aspect discussed above; wherein the oil pump is a two stage type oil pump having a function of switching a generated oil pressure to two stages of a low pressure side and a high pressure side, the valve opening pressure of the first oil jet is equal to or lower than the generated oil pressure of the low pressure side, and the valve opening pressure of the second oil jet is higher than the generated oil pressure of the low pressure side, and is equal to or lower than a generated oil pressure of the high pressure side. 
     A fifth and sixth aspect of an embodiment of the present invention is a piston cooling device according to the third or fourth aspect discussed above; wherein the internal combustion engine comprises a plurality of cylinders, the first oil jet and the second oil jet are disposed in each of the cylinders, and the first oil jet and the second oil jet that belong to each of the plurality of cylinders communicate with the common oil pressure path. 
     A seventh aspect of an embodiment of the present invention is a piston cooling device according to the second aspect discussed above, wherein the first oil jet and the second oil jet communicate with the oil pump via a common oil pressure path. The piston cooing device further comprises: a control mechanism that controls an amount of oil that is sprayed from the second oil jet; and a control device that controls the control mechanism so that the amount of the oil that is sprayed from the second oil jet decreases, when an oil pressure that is supplied to the first oil jet is lower as compared with a pressure that is necessary to cause oil to be sprayed from the first oil jet. 
     A eighth aspect of an embodiment of the present invention is a piston cooling device according to the second aspect discussed above, wherein the piston cooling device further comprises: an oil pump that supplies an oil pressure to the first oil jet by being driven by drive torque of the internal combustion engine; an electric oil pump that is driven by electric power and supplies an oil pressure to the second oil jet; and a control device that causes oil that is supplied from the electric oil pump to be sprayed from the second oil jet under a condition in which oil spray by the second oil jet is required. 
     A ninth aspect of an embodiment of the present invention is a piston cooling device according to the first aspect discussed above, wherein an amount of oil that is sprayed from the second oil jet is preferentially decreased under an condition of excessive cooling ability in which excessive cooling occurs to the piston when oil is sprayed from both of the first oil jet and the second oil jet. 
     A tenth aspect of an embodiment of the present invention is a piston cooling device according to the ninth aspect discussed above, wherein the piston cooling device further comprises: a control mechanism that controls the amount of the oil that is sprayed from the second oil jet; and a control device that controls the control mechanism so that the amount of the oil that is sprayed from the second oil jet decreases under the condition of excessive cooling ability. 
     An eleventh aspect of an embodiment of the present invention is a piston cooling device according to the first aspect discussed above, wherein the piston is made of steel. 
     ADVANTAGES OF EMBODIMENTS OF THE PRESENT INVENTION 
     In the first aspect of an embodiment of the invention, the cooling cavity is provided inside the piston, and therefore easily receives a temperature from a piston top surface. Consequently, deposit is easily generated inside the cooling cavity. In the present invention, oil spray from the first oil jet is performed in preference to oil spray from the second oil jet, during an operation of the internal combustion engine. When oil is sprayed from the first oil jet, a flow of the oil is kept in the cooling cavity, and generation of deposit is suppressed. Consequently, according to the present invention, the cooling efficiency by the cooling cavity is hardly impaired, and the temperature of the piston can be properly kept stably for a long period of time. 
     According to the second aspect of an embodiment of the invention, oil can be preferentially sprayed from the first oil jet, when oil cannot be sprayed from both of the oil jets due to a constraint of the discharge oil amount of the oil pump. 
     According to the third aspect of an embodiment of the invention, in the process of the pressure of the oil pressure path increasing, oil spray from the first oil jet is started first. Consequently, according to the present invention, under the situation where the discharge oil amount of the oil pump is insufficient to cause oil to be sprayed from both of the oil jets, the discharge oil amount can be consumed preferentially in oil spray from the first oil jet. 
     According to the fourth aspect of an embodiment of the invention, when the generated oil pressure of the oil pump is at the low pressure side, oil is sprayed from the first oil jet, whereas oil is not sprayed from the second oil jet. Further, when the generated oil pressure of the oil pump is at the high pressure side, oil is sprayed from both of the oil jets. Consequently, according to the present invention, by switching the state of the oil pump, a state with priority given to the first oil jet, and a state of spray from both of the oil jets can be switched reliably. 
     According to the fifth or sixth aspect of an embodiment of the invention, the generated oil pressure of the oil pump is supplied to the first oil jet and the second oil jet that are disposed in each of the plurality of cylinders in common. When the valve opening pressure of the first oil jet and the valve opening pressure of the second oil jet are the same in this case, such a variation occurs that valves of both of the oil jets open in some of the cylinders, whereas none of valves of the oil jets opens in other cylinders, under the situation where the generated oil pressure is close to the valve opening pressure. In the present invention, the valves of the second oil jets do not open in all of the cylinders, unless the valves of the first oil jets open in all of the cylinders, due to the difference of the valve opening pressures. Consequently, according to the present invention, variations among the cylinders concerning piston cooling can be suppressed. 
     According to the seventh aspect of an embodiment of the invention, when the oil pressure that is supplied to the first oil jet is insufficient, the amount of oil that is sprayed from the second oil jet is decreased. Since the first oil jet and the second oil jet communicate with the oil pump via the common oil pressure path, if the amount of oil that is sprayed from the second oil jet decreases, the oil pressure that is supplied to the first oil jet increases. Consequently, according to the present invention, under the environment of an insufficient oil pressure, the first oil jet can be preferentially caused to spray oil. 
     According to the eighth aspect of an embodiment of the invention, the oil pressure is supplied from the mechanical type of electric pump to the first oil jet. Since the oil pressure is supplied to the second oil jet from the electric pump, the oil pressure that is generated by the mechanical type electric pump is not consumed in the second oil jet, but is supplied to the first oil jet. Consequently, according to the present invention, even at the low load time in which the ability of the oil pump is low, a sufficient oil amount can be provided to the first oil jet, and oil can be preferentially sprayed from the first oil jet. 
     According to the ninth aspect of an embodiment of the invention, under the condition of excessive cooling ability, the oil amount from the second oil jet is preferentially decreased. As a result, the situation where the first oil jet sprays oil in preference to the second oil jet is created. According to the situation, accumulation of deposit in the cooling cavity can be effectively inhibited while avoiding excessive cooling of the piston. 
     According to the tenth aspect of an embodiment of the invention, by using the control mechanism that controls the oil amount from the second oil jet, and the control device that controls the control mechanism, the state that does not cause excessive cooling and does not accumulate deposit in the cooling cavity can be effectively created under establishment of the condition of excessive cooling ability. 
     According to the eleventh aspect of an embodiment of the invention, the steel piston can be properly cooled. The steel piston easily has a high temperature as compared with an aluminum piston, and deposit is easily generated in the cooling cavity. According to the present invention, oil can be preferentially supplied into the cooling cavity, and therefore even if the piston is made of steel, accumulation of deposit can be effectively prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the configuration of a first embodiment of the present invention; 
         FIG. 2  is a diagram showing properties of aluminum and iron in a comparative manner; 
         FIG. 3  is a diagram showing a temperature of an aluminum piston and a temperature of a steel piston in a comparative manner; 
         FIG. 4  is a flowchart of a routine executed in the first embodiment of the present invention; 
         FIG. 5  is a diagram showing the configuration of a second embodiment of the present invention; 
         FIG. 6  is a flowchart of a routine executed in the second embodiment of the present invention; 
         FIG. 7  is a diagram showing the configuration of a third embodiment of the present invention; 
         FIG. 8  is a timing chart for explaining an operation of the third embodiment of the present invention; 
         FIG. 9  is a timing chart for explaining an ununiformity arising between cylinders under a situation in which the opening pressure of two oil jets are identical; 
         FIG. 10  is a timing chart for explaining operations with which the ununiformity between cylinders are eliminated in the third embodiment of the present invention; and 
         FIG. 11  is a timing chart for explaining an operation performed by a fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       12  Piston 
       18  Ring-shaped cavity 
       20 ,  22  Inlet/outlet holes 
       26  Cooling cavity 
       30  First oil jet 
       34 ,  72  Second oil jet 
       40  Oil Jet 
       42  Main gallery 
       54  Electronic controlled unit (ECU) 
       70  Electric oil pump 
     DETAILED DESCRIPTION 
     First Embodiment 
     [Configuration of First Embodiment] 
       FIG. 1  is a diagram for explaining a configuration of a piston cooling device according to a first embodiment of the present invention.  FIG. 1  illustrates a cylinder liner  10  disposed in one cylinder of an internal combustion engine, and a piston  12  disposed therein. Although the internal combustion engine has a plurality of cylinders,  FIG. 1  illustrates only one the cylinders for convenience. 
     On a top surface of the piston  12 , a cavity  16  that opens to a combustion chamber  14  is provided. In the piston  12 , a ring-shaped cavity  18  is formed to surround the cavity  16 . The ring-shaped cavity  18  is provided inside the piston  12 . Inside the piston  12 , two inlet/outlet holes  20  and  22  that are disposed to face each other in a diameter direction of the piston  12  are further provided. The inlet/outlet holes  20  and  22  open to a bottom surface of the piston  12 , extend in an axial direction of the piston  12  to communicate with the ring-shaped cavity  18 . Hereinafter, the ring-shaped cavity  18  and the inlet/outlet holes  20  and  22  are generically called “a cooling cavity”  26 . 
     The piston  12  includes a pin boss  28  on a bottom surface side. A connecting rod that connects to a crankshaft (both are not illustrated) is connected to the pin boss  28 . A first oil jet  30  is disposed at the bottom surface side, that is, at a crankcase side of the piston  12 . The first oil jet  30  contains a check valve that opens at a certain valve opening pressure. When the first oil jet  30  receives supply of an oil pressure that exceeds the valve opening pressure, the first oil jet  30  sprays oil from an injection hole  32  thereof. The injection hole  32  is provided so that the axis of oil sprayed from the injection hole  32  becomes parallel with a direction of a reciprocating motion of the piston  12 , and heads for an opening portion of the one inlet/outlet hole  20 . Consequently, the oil that is sprayed from the first oil jet  30  is sprayed toward the opening portion of the inlet/outlet hole  20  in a substantially entire process of the reciprocating motion of the piston  12 . 
     On the bottom surface side of the piston  12 , a second oil jet  34  is also disposed. The second oil jet  34  has a function of spraying oil by receiving supply of an oil pressure that exceeds a certain valve opening pressure similarly to the first oil jet  30 . In the present embodiment, the valve opening pressure of the first oil jet  30  and the valve opening pressure of the second oil jet  34  are the same. An injection hole  36  of the second oil jet  34  is provided to spray oil toward the bottom surface of the piston  12 , more specifically, toward a periphery of the pin boss  28 , instead of the opening portions of the inlet/outlet holes  20  and  22 . 
     A configuration illustrated in  FIG. 1  includes an oil pressure circuit for supplying oil to the first oil jet  30  and the second oil jet  34 . The oil pressure circuit includes an oil pump  40  that pumps up oil from an oil pan  38  of the internal combustion engine. The oil pump  40  is a mechanical type pump that is driven by drive torque of the internal combustion engine. 
     The outlet port of the oil pump  40  communicates with a main gallery  42 . To the main gallery  42 , an oil pressure sensor  44  for detecting an oil pressure inside the main gallery  42  is attached. Further, an oil passage  46  for supplying oil to respective parts of the internal combustion engine, an oil passage  48  that leads to the first oil jet  30 , and an oil passage  50  that leads to the second oil jet  34  communicate with the main gallery  42 . 
     An oil control valve  52  is incorporated into the oil passage  50  that leads to the second oil jet  34 . The oil control valve  52  is an electronically controlled valve mechanism that changes an opening degree by receiving an instruction from an outside. According to the oil control valve  52 , an amount of oil that flows to the second oil jet  34  from the main gallery  42  can be controlled. 
     An electronic controlled unit (ECU)  54  is connected to the oil control valve  52 . The oil pressure sensor  44  is connected to the ECU  54 . Further, various sensors that the internal combustion engine is equipped with are connected to the ECU  54 . More specifically, sensors as follows are connected to the ECU  54 :
         an engine rotation speed sensor  56  that detects an engine rotation speed;   an air flow meter  58  that detects an intake air amount;   an air-fuel ratio sensor  60  that detects an exhaust air-fuel ratio;   a water temperature sensor  62  that detects a cooling water temperature;   an oil amount sensor incorporated in the first oil jet  30 ; and   an oil amount sensor incorporated in the second oil jet  34 .       

     In the present embodiment, the piston  12  of the internal combustion engine is made of steel.  FIG. 2  is a diagram showing properties of aluminum and iron in a comparative manner. Further,  FIG. 3  is a diagram showing a temperature track of an aluminum piston and a temperature track of a steel piston in a comparative manner. Note that the temperatures belong to the tracks  64  and  66  illustrated in  FIG. 3  are ones that are obtained when a load of the internal combustion engine is changed under a fixed engine rotation speed. 
     As illustrated in  FIG. 2 , a heat conductivity of iron is much lower as compared with the heat conductivity of aluminum. Consequently, as illustrated in  FIG. 3 , the temperature  66  of the steel piston easily becomes higher as compared with the temperature  64  of the aluminum piston. Here, a broken line shown by being assigned with reference numeral  68  in  FIG. 3  represents a temperature at which carbonization of oil starts. As illustrated in  FIG. 3 , in the steel piston, the temperature  66  thereof more easily reaches the carbonization start temperature  68  as compared with the aluminum piston. Consequently, in the internal combustion engine using the steel piston  12  as in the present embodiment, it is especially important to cool the piston  12  properly so that carbonization of oil does not occur. 
     [Operation of Piston Cooling Device of First Embodiment] 
     During an operation of the internal combustion engine, large combustion heat is generated inside the combustion chamber  14 . The top surface of the piston  12  is exposed to the combustion heat and has a high temperature. The ring-shaped cavity  18  is provided at a position close to the top surface of the piston  12 . Consequently, an inside of the ring-shaped cavity  18  more easily has a high temperature as compared with the bottom surface of the piston  12 . On the other hand, cooling the ring-shaped cavity  18  properly with oil can reduce the top surface temperature of the piston  12  more efficiently than cooling the bottom surface of the piston  12  with oil. 
     (Condition of Normal Operation) 
     The piston cooling device of the present embodiment sprays oil toward the piston  12  from both the first oil jet  30  and the second oil jet  34  under the condition of normal operation. The oil that is sprayed from the first oil jet  30  enters into the cooling cavity  26  from the inlet/outlet hole  20 , and flows in the ring-shaped cavity  18  to flow out from the inlet/outlet hole  22 . On the other hand, the oil that is sprayed from the second oil jet  34  is sprayed to the bottom surface of the piston  12 , in particular, the periphery of the pin boss. According to this manner, oil can be continued to flow in the cooling cavity  26 , and the piston  12  can be also cooled from the bottom surface. As a result, overheating of the piston  12  can be properly prevented. 
     (Condition of Excessive Cooling Ability) 
     The amount of heat received by the piston  12  from the combustion chamber  14  varies in accordance with a load state of the internal combustion engine. For example, in a low load state of an idle operation or the like, the heat amount is inevitably small. When oil is sprayed from both of the first oil jet  30  and the second oil jet  34  under the situation like this, the piston  12  may be excessively cooled. Hereinafter, a condition under which excessive cooling like this occurs will be referred to as “a condition of excessive cooling ability”. 
     Under the condition of excessive cooling ability, stopping oil cooling of the piston  12  is conceivable, for example, in order to avoid excessive cooling of the piston  12 . However, the inside of the ring-shaped cavity  18  is close to the top surface of the piston  12 , and easily has a high temperature even under the condition like this. If oil cooling of the piston  12  is stopped under the cooling ability excess condition, the temperature in the ring-shaped cavity  18  may reach the carbonization start temperature  68  (refer to  FIG. 3 ). 
     Consequently, under the condition of excessive cooling ability, the present embodiment closes the oil control valve  52  so as to stop only oil spraying from the second oil jet  34 . According to the procedure, the cooling ability to the entire piston  12  is reduced as compared with the cooling ability under the condition of normal operation. As a result, excessive cooling of the piston  12  can be properly avoided. Further, since spraying from the first oil jet  30  is continued, oil continues to flow in the cooling cavity  26 . As a result, the temperature in the ring-shaped cavity  18  is lowered, generation of deposit by carbonization of oil is avoided, and even if carbonization of oil occurs, carbide of the oil can be washed away, so that accumulation of deposit can be prevented. 
     (Condition of Insufficient Oil Pressure) 
     In the present embodiment, a generated oil pressure of the oil pump  40  varies in accordance with the state of the internal combustion engine. For example, when the internal combustion engine is in a low load state of an idle operation or the like, the generated oil pressure becomes relatively low. On the other hand, when the internal combustion engine is in a high load state of an acceleration operation or the like, the oil pressure becomes high. Further, the generated oil pressure of the oil pump  40  is also influenced by viscosity of the oil. Consequently, under a situation where the oil temperature is high and the oil viscosity is low, the generated oil pressure of the oil pump  40  becomes relatively low. 
     As illustrated in  FIG. 1 , in the piston cooling device of the present embodiment, the first oil jet  30  and the second oil jet  34  both receive supply of the oil pressure from the oil pump  40  via the main gallery  42 . Consequently, under a low load state or a high oil temperature state where the generated oil pressure of the oil pump  40  is low, there arises a situation where oil cannot be sprayed from both the first oil jet  30  and the second oil jet  34 . 
     Under such a situation, practically, the valve of either the first or the second oil jet opens so as to start oil spray just from the jet, at a time point at which the oil pressure of the main gallery  42  reaches the valve opening pressure of the first oil jet  30  and the second oil jet  34 . When either one of the jets starts spraying oil, the oil pressure of the main gallery  42  is reduced, and the other jet is brought into a state where its valve is not able to open. In a case where the second oil jet  34  starts spraying oil precedently, oil is not sprayed from the first oil jet  30  thereafter. Hereinafter, such a condition in which the discharge oil amount from the oil pump  40  is insufficient for making both of the oil jets spray oil will be referred to as “a condition of insufficient oil pressure”. 
     In the present embodiment, under the condition of insufficient oil pressure, the oil control valve  52  is closed to cut off oil pressure supply to the second oil jet  34 . When the oil pressure supply to the second oil jet  34  is cut off, oil spray from the first oil jet  30  is reliably started at the time point at which the oil pressure of the main gallery  42  reaches the valve opening pressure. 
     As described above, oil cooling by the cooling cavity  26  can cool the piston  12  more efficiently as compared with oil cooling of the bottom surface of the piston  12 . Further, the inside of the cooling cavity  26 , in particular, the inside of the ring-shaped cavity  18  more easily has a high temperature as compared with the bottom surface of the piston  12 , and is in an environment where deposit is readily generated. Consequently, under the environment where oil can be sprayed from only either one of the jets, priority is desirably given to spray from the first oil jet  30 . According to the piston cooling device of the present embodiment, the request can be satisfied under the condition of insufficient oil pressure. 
     (Control Flow in the First Embodiment) 
       FIG. 4  is a flowchart of a routine executed by the ECU  54  in the first embodiment of the present invention. The routine illustrated in  FIG. 4  is repeatedly started up at predetermined periods after start-up of the internal combustion engine. In the routine, an operating state of the internal combustion engine is detected first (step  100 ). More specifically, in the present step, information necessary for estimation of the temperature of the piston  12  and detection of the oil temperature is detected from various sensors which are installed in the internal combustion engine. 
     Next, a temperature of the piston  12  is estimated (step  102 ). The temperature of the piston  12  can be calculated based on a heat input amount Qin to the piston  12  and a heat release amount Qout from the piston  12 . Further, the heat input amount Qin can be calculated by a known method based on the engine rotation speed, the fuel injection amount, the amount of gas flowing in the combustion chamber  14  and the like. On the other hand, the heat release amount Qout can be calculated by a known method based on an amount of oil sprayed toward the piston  12  and an oil temperature or the like. The temperature of the piston  12  may be estimated in accordance with a map that is set in advance with the engine rotation speed, the intake air amount and the like as parameters. 
     Next, it is determined whether or not the temperature of the piston  12  is lower than a determination temperature of the condition of excessive cooling ability (step  104 ). Processing of the present step is executed to determine whether or not the condition of excessive cooling ability is established. Note that whether or not the condition of excessive cooling ability is established may be determined based on the engine rotation speed, the engine load, the cooling water temperature, the intake air temperature and the like. 
     When it is determined that the temperature of the piston  12  is lower than the determination temperature in step  104  described above, it can be determined that the condition of excessive cooling ability is established. That is, in this case, it can be determined that the piston  12  reaches the state of being excessively cooled if oil is continued to be sprayed from both the first oil jet  30  and the second oil jet  34 . In this case, the oil control valve  52  is closed so that oil spray from the second oil jet  34  is stopped (step  106 ). 
     When it is determined that the temperature of the piston  12  is not lower than the determination temperature in the processing of step  104  described above, it can be determined that the condition of excessive cooling ability is not established. In this case, it is determined whether or not the pressure of the main gallery  42  is lower than a determination pressure of the condition of insufficient oil pressure (step  108 ). Processing of the present step is executed to determine whether the condition of insufficient oil pressure is established or not. It should be noted that the determination about the condition of insufficient oil pressure may be performed based on a discharge oil amount (capacity) of the oil pump  40 . 
     When it is determined that the oil pressure of the main gallery  42  is lower than the determination oil pressure in step  108  described above, it can be determined that the condition of insufficient oil pressure is established. That is, in this case, it can be determined that the discharge oil amount of the oil pump  40  is insufficient to spray oil from both of the first oil jet  30  and the second oil jet  34 . In this case, in order to give priority to oil spray from the first oil jet  30 , the processing in step  106  described above is executed. 
     When it is determined that the oil pressure of the main gallery  42  is not lower than the determination oil pressure in step  108  described above, it can be determined that the condition of excessive cooling ability and the condition of insufficient oil pressure are not established. In this case, the oil control valve  52  is opened to cause oil to be sprayed from the second oil jet  34  in addition to the first oil jet  30  (step  110 ). 
     According to the above processing, oil can be sprayed from the first oil jet  30  in preference to oil spray from the second oil jet  34  under the condition of excessive cooling ability as well as under the condition of insufficient oil pressure. Consequently, according to the piston cooling device of the present embodiment, the piston  12  can be kept at a proper temperature even under those conditions, and generation or accumulation of deposit in the cooling cavity  26  can be effectively inhibited. Therefore, according to the device, the ability to cool the piston to a proper temperature stably can be kept over a long period of time. 
     [Modification Example of First Embodiment] 
     In the above described first embodiment, the steel piston  12  is used, but the material of the piston  12  is not limited to iron. That is, the present invention may be applied to pistons that are formed from materials other than iron, such as a piston of aluminum. 
     In the above described first embodiment, the second oil jet  34  which sprays oil to a part different from the inlet/outlet holes  20  and  22  of the cooling cavity  26  sprays oil to the bottom surface of the piston  12 . The present invention is not limited to this. That is, for example, the second oil jet  34  may inject oil toward the part other than the piston  12 . Further, the number of jets that spray oil to the parts different from the inlet/outlet holes  20  and  22  is not limited to one, but may be two or more. 
     In the first embodiment described above, spray from the second oil jet  34  is stopped when priority should be given to oil spray from the first oil jet  30 . However, the present invention is not limited to this. For example, when priority is given to injection from the first oil jet  30  under the condition of excessive cooling ability, the oil spray amount from the second oil jet  34  may be decreased to an amount with which excessive cooling of the piston  12  does not occur. Further, when priority is given to injection from the first oil jet  30  under the condition of insufficient oil pressure, the spray amount from the second oil jet  34  may be decreased so that the oil amount that is supplied to the first oil jet  30  becomes a sufficient amount. 
     In the above described first embodiment, the first oil jet  30  always sprays oil during an operation of the internal combustion engine. The present invention is not limited to this. At a cold time in which cooling of the piston  12  is not required at all or the like, spray from the first oil jet  30  can be also stopped as well as spray from the second oil jet  34 . 
     It should be noted that the main gallery  42  in the above described first embodiment corresponds to “a common oil pressure path” in the above described third aspect of the invention. Further, the oil control valve  52  and the ECU  54  correspond to “a control mechanism” and “a control device” in the above described seventh or tenth aspect of the invention. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 .  FIG. 5  is a diagram for explaining a configuration of a piston cooling device of the second embodiment of the present invention. A configuration illustrated in  FIG. 5  is similar to the configuration illustrated in  FIG. 1 , except for a point that the second oil jet  34  receives supply of an oil pressure from an electric oil pump  70  instead of the main gallery  42 . Hereinafter, elements in  FIG. 5  that are the same as or correspond to the elements illustrated in  FIG. 1  will be assigned with common reference numerals, and explanation thereof will be omitted or simplified. 
     According to the configuration of the present embodiment, oil spray from the second oil jet  34  does not influence the oil pressure of the main gallery  42 . Consequently, unlike the case of the first embodiment, the condition of insufficient oil pressure is not established in the present embodiment. That is, in the piston cooling device of the present embodiment, a state where a generated oil pressure of the oil pump  40  is preferentially given to the first oil jet  30  is structurally ensured. Thus, according to the device, even in a condition in which an oil temperature is high and an oil viscosity is low, oil can be properly sprayed from the first oil jet  30 . 
     Further, according to the configuration of the present embodiment, even under the situation where the condition of insufficient oil pressure is established in the first embodiment, oil can be sprayed from both of the first oil jet  30  and the second oil jet  34  by operating the electric oil pump  70 . Consequently, according to the device of the present embodiment, a cooling ability that is more excellent as compared with the device of the first embodiment can be exhibited, under the situation like this. 
       FIG. 6  is a flowchart of a routine executed by the ECU  54  in the present embodiment. The flowchart illustrated in  FIG. 6  is similar to the routine illustrated in  FIG. 4  except for a point that step  108  is omitted. When stop of the second oil jet  34  is instructed in step  106  in the present embodiment, a stop instruction is issued to the electric oil pump  70 . When the electric oil pump  70  is stopped, oil spray from the second oil jet  34  is also stopped. Consequently, according to the device of the present embodiment, excessive cooling can be properly prevented from occurring to the piston  12  while oil is caused to flow in the cooling cavity  26 , under the condition of excessive cooling ability. 
     When oil spray from the second oil jet  34  is instructed in step  110  in the routine illustrated in  FIG. 6 , an operation instruction is issued to the electric oil pump  70 . The electric oil pump  70  can generate an oil pressure exceeding the valve opening pressure of the second oil jet  34 . Consequently, when the oil pump  70  is operated, the second oil jet  34  sprays oil toward the bottom surface of the piston  12 . Thus, according to the device of the present embodiment, the piston  12  can be properly cooled by using both of the first oil jet  30  and the second oil jet  34  under the condition of normal operation. 
     As described above, the piston cooling device of the present embodiment can cause the oil to flow in the cooling cavity  26  similarly to the case of the first embodiment. Further, according to the device, the piston  12  can be cooled more properly than in the case of the first embodiment. Consequently, by the device of the present embodiment, the temperature of the piston  12  can be also properly kept stably for a long period of time. 
     [Modification Example of Second Embodiment] 
     Note that in the present embodiment, the piston  12  may be made of aluminum similarly to the case of the first embodiment. Further, the second oil jet  34  may spray oil toward a part other than the piston  12 . Further, the number of second oil jets  34  may be two or more. Further, under the condition of excessive cooling ability, the oil spray amount from the second oil jet  34  may be decreased. Subsequently, oil from the first oil jet  30  may be stopped in accordance with necessity. 
     Note that in the above descried second embodiment, the ECU  54  corresponds to a “control device” in the above described seventh aspect of the invention. Further, the electric oil pump  70  and the ECU  54  correspond to a “control mechanism” and a “control device” in the above described tenth aspect of the invention. 
     Third Embodiment 
     Next, with reference to  FIGS. 7 to 10 , the third embodiment of the present invention will be described.  FIG. 7  is a diagram for explaining a configuration of a pump cooling device of the third embodiment. Hereinafter, in  FIG. 7 , elements that are the same as or correspond to the elements illustrated in  FIG. 1  will be assigned with the common reference numerals and explanation thereof will be omitted or simplified. 
     The piston cooling device of the present embodiment includes a second oil jet  72 . The second oil jet  72  directly communicates with the main gallery  42  without passing through the oil control valve  52  (refer to  FIG. 1 ). The second oil jet  72  is given a valve opening pressure P 2  that is higher than a valve opening pressure P 1  of the first oil jet  30 . The piston cooling device of the present embodiment does not require an electronic control unit, unlike the case of the first or the second embodiment. 
     [Operation of Third Embodiment] 
     (Operation in Single Cylinder) 
       FIG. 8  is a timing chart for explaining an operation of the piston cooling device illustrated in  FIG. 7  and an operation of a device of a comparative example in a comparative manner. Here, “the device of the comparative example” refers to a device in which the second oil jet  72  is replaced with the second oil jet  34  in the configuration illustrated in  FIG. 7 . A valve opening pressure of the second oil jet  34  used in the comparative example is set at P 1  as in the case of the first oil jet  30 . 
     The uppermost chart in  FIG. 8  illustrates an engine rotation speed which increases at a fixed rate after a time point t 0 . Since the oil pump  40  is a mechanical type pump, a discharge oil amount from the oil pump  40  increases as the engine rotation speed increases. 
     In the second through the lowermost charts in  FIG. 8 , waveforms  74 ,  76 ,  78  and  80  illustrated by broken lines commonly express an operation of the device of the comparative example. On the other hand, waveforms  82 ,  84 ,  86  and  88  that are illustrated by solid lines in these charts commonly express the operation of the device of the present embodiment. 
     The second chart in  FIG. 8  illustrates a change of the oil pressure in the main gallery  42 . A third chart illustrates a valve opening state of the first oil jet  30 . Further, a fourth chart illustrates a valve opening state of the second oil jet  72  or  34 . 
     The waveform  74  illustrated in the second chart in  FIG. 8  expresses that the oil pressure in the comparative example reaches the valve opening pressure P 1  at a time point t 1 , then temporarily reduces, and thereafter reaches the valve opening pressure P 1  again at a time point t 2 . In the comparative example, the valve opening pressure of the first oil jet  30  and the valve opening pressure of the second oil jet  34  are both P 1 . Accordingly, as the waveform  78  in the fourth chart illustrates, the valve of the second oil jet  34  may open at the time point t 1  in the device of the comparative example. 
     When the valve of the second oil jet  34  opens, the oil pressure of the main gallery  42  temporarily drops to be a value which is lower than the valve opening pressure P 1  at the time point (see, the waveform  74 ). At this time, the valve of the first oil jet  30  cannot open and keeps a closed state. When the oil pressure reaches to the valve opening pressure P 1  again in the time point t 2 , the first oil jet  30  shifts to the valve opening state at this point of the time (see, the waveform  76  in the third chart). 
     The lowermost chart in  FIG. 8  illustrates a temperature change of the piston  12 . When the valve of the second oil jet  34  opens at the time point t 1 , and the valve of the first oil jet  30  opens at the time point t 2 , the temperature of the piston  12  shows a change along the waveform  80 . Cooling efficiency by the second oil jet  34  is not as high as cooling efficiency by the first oil jet  30 . Thus, in the case of the comparative example, after the time point t 1 , the temperature of the piston  12  temporarily lowers slightly, but thereafter increases again. Subsequently, after oil cooling of the cooling cavity  26  is started at the time point t 2 , the temperature increases continuously until the influence of the oil cooing is exerted onto the piston top surface. Accordingly, as shown by the waveform  80 , the temperature of the piston  12  in the comparative example easily becomes a high temperature until the temperature converges to a stable value. 
     In the device of the present embodiment, the valve opening pressure of the second oil jet  72  is set at P 2  that is higher than P 1 . Consequently, in the device, the valve of the second oil jet  72  does not open at the time point t 1  (see, the waveform  86  in the fourth chart), and the valve of the first oil jet  30  reliably opens at the time point (see, the waveform  84  in the third chart). 
     The waveform  82  in the second chart shows that the oil pressure in the present embodiment temporarily drops at the time point t 1 , and thereafter reaches the valve opening pressure P 2  at a time point t 3 . Further, the waveform  86  in the fourth chart illustrates that the valve of the second oil jet  72  in the present embodiment shifts to a valve opening state at the time point t 3  receiving upon the oil pressure increase. 
     As described above, in the present embodiment, oil spray by the first oil jet  30  is reliably started at the time point t 1 . In this case, the temperature of the piston  12  drops greatly at the time point t 1 , and thereafter, keeps a substantially stable value, as illustrated by the waveform  88  in the lowermost chart. Subsequently, when spray from the second oil jet  72  is started at the time point t 3 , the temperature of the piston  12  further drops to converge to a stable value. 
     As described above, the piston cooling device of the present embodiment cam make the first oil jet  30  spray oil in preference to the second oil jet  72  under the environment where the oil pressure is low. The condition of excessive cooling ability is apt to be established when the internal combustion engine operates with a light load. During the light load operation of the internal combustion engine, the discharge oil amount of the oil pump  40  is small, and the generated oil pressure often becomes a low pressure. The device of the present embodiment causes only the first oil jet  30  to spray oil preferentially under the environment like this. Consequently, according to the device, the piston  12  can be kept at a proper temperature without generating deposit in the cooling cavity  26  under the environment where the condition of excessive cooling ability is established. 
     Further, according to the device of the present embodiment, under the condition of insufficient oil pressure, oil spray from the second oil jet  72  is inevitably stopped, and the situation where only the first oil jet  30  sprays oil is created. Consequently, according to the device, the piston  12  can be kept at a proper temperature without generating deposit in the cooling cavity  26 , even under the condition of insufficient oil pressure. 
     (Operation in a Plurality of Cylinders) 
     The configuration illustrated in  FIG. 7  is provided in each of a plurality of cylinders included by the internal combustion engine. The first oil jets  30  and the second oil jets  72  of the respective cylinders all communicate with the main gallery  42 . 
       FIG. 9  is a timing chart for explaining operations that occur to cylinder # 1  and cylinder # 2  in a device of a comparative example. A waveform  90  in a third chart in  FIG. 9  shows that a valve of the first oil jet  30  of cylinder # 1  opens at the time point t 1 . Further, a waveform  91  in a fourth chart shows that a valve of the second oil jet  34  of cylinder # 1  opens at the time point t 2 . A waveform  92  in a lowermost chart shows that a valve of the first oil jet  30  of cylinder # 2  opens at the time point t 3 . 
     In the device of the comparative example, the first oil jet  30  and the second oil jet  34  that are equal in valve opening pressure are included in each of a plurality of cylinders. Valves of these oil jets open at the same valve opening pressure P 1 , and therefore there is no order concerning which oil jet opens under the environment where the oil pressure of the main gallery  42  is in the vicinity of P 1 . That is, a valve opening sequence illustrated in  FIG. 9  is only one of sequences that are likely to be realized, and there is no reproducibility of the sequence. A variation between cylinders occurs in combinations of the oil jets that are in a valve opening state, until all the valves of the oil jets open after first valve opening occurs. As a result, according to the device of the comparative example, various variations occur to the cooling abilities of the piston  12  in the respective cylinders. 
       FIG. 10  is a timing chart for explaining operations that occurs to cylinder # 1  and cylinder # 2  in the device of the present embodiment. A waveform  93  in a fourth chart in  FIG. 10  shows that the second oil jet  72  of cylinder # 1  stably keeps closed state under a situation where the oil pressure repeatedly reaches the valve opening pressure P 1 . 
     In the device of the present embodiment, the valve opening pressure of the second oil jet  72  is set at a value higher than P 1  as described above. Under the setting, the valves of the second oil jet  72  does not open in any of the cylinders until the valves of the first oil jets  30  open in all of the cylinders. In this way, in the device of the present embodiment, all of the first oil jets  30  can be brought into valve opening states in preference to all of the second oil jets  72 . Consequently, according to the device, the variation in cooling ability in the respective cylinders can be restrained more significantly as compared with the case of the comparative example. 
     [Modification Example of Third Embodiment] 
     In the present embodiment, the piston  12  may be made of aluminum as in the case of the first embodiment. Further, the second oil jet  34  may spray oil toward a part other than the piston  12 . Further, the number of the second oil jets  34  may be two or more. The oil from the first oil jet  30  may be stopped in accordance with necessity. 
     Fourth Embodiment 
     Next, a fourth embodiment of the present invention will be described with reference to  FIG. 11  together with  FIG. 7 . A piston cooling device of the present embodiment can be realized by incorporating three features as follows in a configuration illustrated in  FIG. 7 .
     (1) A two stage type pump is adopted as the oil pump  40 . Note that “the two stage type pump” refers to a pump having a function of switching a generated oil pressure selectively to two kinds that are a low pressure side (PL) and a high pressure side (PH).   (2) The valve opening pressure P 1  of the first oil jet  30  is set at a value of equal to or lower than the generated oil pressure PL of the low pressure side.   (3) The valve opening pressure P 2  of the second oil jet  72  is set at a value that is higher than the generated oil pressure PL of the low pressure side and is equal to or lower than the generated oil pressure PH of the high pressure side.   

     The two stage type oil pump  40  inevitably generates the oil pressure PL of the low pressure side at the time of the light load operation in which a drive force from the internal combustion engine is insufficient to generate the oil pressure PH of the high pressure side. Further, the oil pump  40  generates the oil pressure PL of the low pressure side under the situation where generation of PH is unnecessary even under the situation where the drive force sufficient to generate the oil pressure PH of the high pressure side can be obtained. According to the configuration of the present embodiment, an amount of useless work can be reduced by properly switching the state of the oil pump  40 . 
     Further, according to the configuration of the present embodiment, only the first oil jet  30  is brought into a valve opening state under the situation where the oil pump  40  generates the oil pressure PL of the low pressure side. Further, when the oil pump  40  generates the oil pressure PH of the high pressure side, both of the first oil jet  30  and the second oil jet  72  are brought into a valve opening state. Consequently, according to the device, a combination of jets that spray oil can be intentionally switched by switching the state of the oil pump  40 . 
       FIG. 11  is a timing chart for explaining the operation of the piston cooling device of the present embodiment and an operation of a device of a comparative example in a comparative manner. Here, “the device of the comparative example” refers to a device in which the second oil jet  72  is replaced with the second oil jet  34  in the configuration of the present embodiment. 
     A second chart in  FIG. 11  shows a generated oil pressure of the two stage type oil pump  40 . Here, at the time point t 2 , the generated oil pressure is switched to the value PH of the high pressure side from the value PL of the low pressure side. 
     In a third chart through a lowermost chart in  FIG. 11 , waveforms illustrated by broken lines express the operation of the device of the comparative example. On the other hand, waveforms illustrated by solid lines in these charts express the operation of the device of the present embodiment. 
     The third chart in  FIG. 11  illustrates a change of the oil pressure of the main gallery  42 . Here, the oil pressure reaches the valve opening pressure P 1  at the time point t 1 . Thereafter, the oil pressure temporary reduces, then reaches the valve opening pressure P 2  at the time point t 2  with switching of the generated oil pressure. 
     In the device of the comparative example, the valve opening pressure of the second oil jet  34  is P 1  similarly to the valve opening pressure of the first oil jet  30 . Consequently, as the waveform  94  in a fifth chart shows, the valve of the second oil jet  34  may open at the time point t 1  in the device of the comparative example. In this case, the valve of the first oil jet  30  does not open until the time point t 2  as the waveform  95  in a fourth chart shows. As a result, as the waveform  96  in the lowermost chart shows, the temperature of the piston  12  temporarily rises to a high temperature before converging to a stable value. 
     In the device of the present embodiment, the valve opening pressure of the second oil jet  72  is P 2 , and therefore, the valve of the second oil jet  72  does not open at the time point t 1  (see, the waveform  97  in the fifth chart). As a result, in the device, the valve of the first oil jet  30  reliably opens at the time point t 1  (see, the waveform  98  in the fourth chart). 
     As above, in the device of the present embodiment, only the first oil jet  30  reliably sprays oil while the oil pump  40  generates the oil pressure PL of the low pressure side. When the generated oil pressure is switched to the value PH of the high pressure side, oil also starts to be sprayed from the second oil jet  72  in addition to the first oil jet  30 . 
     Consequently, according to the device of the present embodiment, the piston  12  can be continued to be cooled at a proper temperature stably for a long period of time without generating deposit in the cooling cavity  26 , as in the case of the third embodiment. Further, according to the device of the present embodiment, the combination of the jets that spray oil can be accurately switched by switching the state of the oil pump  40 . Thus, according to the device, the cooling ability to the piston  12  can be controlled more intentionally as compared with the case of the third embodiment. 
     [Commonness with Third Embodiment] 
     The device of the present embodiment is similar to the device in the third embodiment in the point that variations in the cooling abilities in a plurality of cylinders can be suppressed. Further, the modification examples explained about the third embodiment are also applicable to the device of the present embodiment.