EP2577015A1

EP2577015A1 - Cooling system for piston of internal combustion engine

Info

Publication number: EP2577015A1
Application number: EP11732511.8A
Authority: EP
Inventors: Takashi Koyama; Naoki Umeda; Masao Ishida; Satoshi Nagata; Kenichi Harashina; Hideaki Nakasone; Ichiro Tsukada; Akinori Toyoshima; Masaya Hirano
Original assignee: Teikoku Piston Ring Co Ltd; Art Metal Manufacturing Co Ltd; Toyota Motor Corp
Current assignee: Art Metal Manufacturing Co Ltd; Toyota Motor Corp; TPR Co Ltd
Priority date: 2010-06-07
Filing date: 2011-06-06
Publication date: 2013-04-10
Anticipated expiration: 2031-06-06
Also published as: CN102933812A; JP2011256742A; EP2577015B1; WO2011154806A1; EP2577015B8; US20130139767A1

Abstract

A cooling system for a piston of an internal combustion engine includes a cooling channel (34) designed as an oil passage embedded in the piston and arranged adjacent to a top ring groove, and an oil supply portion (8) that supplies oil to the cooling channel. An amount of oil supplied from the oil supply portion to the cooling channel is made larger when an amount of heat generated in a combustion chamber is large than when the amount of heat generated in the combustion chamber is small.

Description

COOLING SYSTEM FOR PISTON OF INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to the cooling of a piston in an internal combustion engine, and more particularly, the cooling of a piston in which a cooling channel is formed. 2. Description of Related Art

[0002] The piston of an internal combustion engine is fitted with an annular piston ring having a cut (an abutment). The piston ring may have two end faces opposite each other via an abutment and provided with elastic resin pieces respectively (e.g., see Japanese Patent Application Publication No. 2010-031789 (JP-A-2010-031789)).

[0003] Generally, some of the heat generated during combustion of fuel in the combustion chamber of a cylinder is transferred to the piston ring via the piston. Thus, if the amount of the heat generated in the combustion chamber increases, the amount of thermal expansion of the piston ring also increases. As a result, the width of the abutment (a gap); decreases, and the amount of compression loss and the amount of blow-by gas are reduced.

[0004] However, if the amount of the heat generated in the combustion chamber increases further, the amount of thermal expansion of the piston ring further increases. Therefore, the opposite end faces of the abutment may bump against each other. If the opposite end faces of the abutment bump against each other, the stress applied to the piston ring may increase and thereby cause the contact load between the piston ring and a cylinder bore wall surface to increase. These problems are remarkable when it comes to the piston ring closest to the combustion chamber (a top ring).

[0005] One possible approach to mitigate this problem is to widen the gap of the abutment. However, if the amount of the heat generated in the combustion chamber or when the temperature of the piston is low, the amount of compression loss and the amount of blow-by gas may increase due to the increased width of the abutment gap. SUMMARY OF THE INVENTION

[0006] The invention reduces the variation width of a temperature of the top ring in a system for cooling the piston of an internal combustion engine.

[0007] The inventor has found out that the temperature of the top ring can be adjusted with the aid of a cooling channel provided in a piston. That is, as a result of strenuous experiments and verifications, the inventor has found out that the variation width of the temperature of a piston ring during the operation of the internal combustion engine is reduced by arranging the cooling channel adjacent to a top ring groove of the piston in which the top ring is fitted and adjusting the amount of oil supplied to the cooling channel in accordance with the amount of heat generated in a combustion chamber.

[0008] Thus, according to one aspect of the invention, a cooling system for a piston of an internal combustion engine is equipped with a piston that includes a top ring groove provided in an outer peripheral face of the piston and fitted with a top ring and a cooling channel designed as an oil passage provided in the piston and located adjacent to the top ring groove, an oil supply portion that supplies oil to the cooling channel, and a control portion that sets an amount of oil supplied from the oil supply portion to the cooling channel larger when an amount of heat generated in a combustion chamber is large than when the amount of heat generated in the combustion chamber is small.

[0009] The heat generated in the combustion chamber is transferred to a top face of the piston. The heat transferred to the top face of the piston is transferred in the piston mainly from the top face of the piston toward the top ring groove, and is discharged from the top ring groove to a cylinder bore wall surface via the top ring.

[0010] When the amount of the heat generated in the combustion chamber is large, the ratio of the amount of the heat transferred from the top ring to the cylinder bore wall surface to the amount of the heat transferred from the piston to the top ring is small. Thus, when the amount of the heat generated in the combustion chamber is large, the amount of rise in the temperature of the top ring is large.

[0011] In contrast, when the amount of the heat generated in the combustion chamber is small, the ratio of the amount of the heat transferred from the top ring to the cylinder bore wall surface to the amount of the heat transferred from the piston to the top ring is large. Thus, when the amount of the heat generated in the combustion chamber is small, the amount of rise in the temperature of the top ring is small.

[0012] As described hitherto, the temperature of the top ring greatly changes in accordance with the amount of the heat generated in the combustion chamber. When the temperature of the top ring greatly changes, the size of the abutment gap also greatly changes correspondingly. Thus, in the case where the top ring is formed such that the abutment gap assumes a suitable size when the temperature of the top ring is low, opposed end faces of the abutment bump against each other when the temperature of the top ring becomes high. In contrast, when the top ring is formed such that the abutment gap assumes a suitable size when the temperature of the top ring is high, the abutment gap becomes excessively wide when the temperature of the top ring becomes low.

[0013] In contrast, in the case where the cooling channel is arranged adjacent to the top ring groove, especially in the case where the cooling channel is arranged on a transfer path from the top face of the piston to the top ring groove, the heat transferred from the top face of the piston to the top ring groove is absorbed by the oil in the cooling channel.

[0014] Accordingly, in the case where the amount of the oil flowing through the cooling channel is made large when the amount of the heat generated in the combustion chamber is large, the amount of that heat traveling from the top face of the piston toward the top ring groove which is absorbed by the oil in the cooling channel becomes large. Thus, the amount of the heat transferred from the top face of the piston to the top ring groove can be prevented from becoming excessively large. As a result, the temperature of the top ring can be prevented from becoming excessively high when the amount of the heat generated in the combustion chamber is large.

[0015] However, when the amount of the oil flowing through the cooling channel is made small when the amount of the heat generated in the combustion chamber is small, the amount of that heat traveling from the top face of the piston toward the top ring groove which is absorbed by the oil in the cooling channel becomes small. Thus, the amount of the heat transferred from the top face of the piston to the top ring groove can be prevented from becoming excessively small. As a result, the temperature of the top ring can be prevented from becoming excessively low when the amount of the heat generated in the combustion chamber is small.

[0016] It should be noted that most of the heat transferred from the combustion chamber to the piston may be discharged to the cylinder bore wall surface when the amount of the heat generated in the combustion chamber is extremely small (e.g., when the internal combustion engine is operated at low load and low rotational speed). In such a case, the temperatures of the piston and the top ring may fall after temporarily rising.

[0017] In this view, the control portion according to the aspect of the invention may set the amount of the oil supplied from the oil supply portion to the cooling channel to zero (stop the oil supply portion) when the amount of the heat generated in the combustion chamber is equal to or smaller than a predetermined lower limit. "The lower limit" mentioned herein is a value at which the temperature of the top ring is considered to become lower than a presupposed temperature range (a temperature range where the abutment gap of the top ring assumes a supposed size), and is determined in advance through an adaptation processing with the aid of an experiment or the like.

[0018] When the oil supply portion is stopped, the amount of the heat discharged from the piston to oil is substantially zero. Further, when the oil supply portion is stopped, .the interior of the cooling channel is filled with air. The air in the cooling channel functions as a heat insulation layer for reducing or shutting off the heat transferred from the top face of the piston to the top ring groove. Thus, the amount of the heat discharged from the piston to the cylinder bore wall surface decreases. [0019] As described hitherto, when the amount of the heat discharged from the piston to oil and the amount of the heat discharged from the piston to the cylinder bore wall surface are reduced, the temperature of the piston (particularly a region around the top ring groove) is restrained from falling. When the temperature of the region around the top ring is restrained from falling, the temperature of the top ring is also restrained from falling correspondingly.

[0020] As described above, when the amount of the heat supplied from the oil supply portion to the cooling channel is adjusted, the temperature of the top ring is held equal to a substantially constant temperature (hereinafter referred to as "a suitable temperature"). As a result, during the operation of the internal combustion engine, the size of the abutment gap can be held substantially constant.

[0021] When the size of the abutment gap of the top ring is held substantially constant during the operation of the internal combustion engine, the top ring can be designed such that the abutment gap assumes a desired size at the aforementioned suitable temperature. As a result, the amount of compression loss and the amount of blow-by gas can also be minimized regardless of the amount of the heat generated in the combustion chamber.

[0022] It should be noted herein that the amount of the heat generated in the combustion chamber is correlated with the amount of fuel injection. Thus, the control portion may adjust the amount of the oil supplied from the oil supply portion to the cooling channel using the amount of fuel injection as a parameter. Further, since the amount of fuel injection is determined using an engine load and an engine speed as parameters, the control portion may adjust the amount of the oil supplied from the oil supply portion to the cooling channel using the engine speed and the engine load as parameters.

[0023] In the meantime, the pressure in a space surrounded by the top ring, the piston, and the cylinder bore (hereinafter referred to as "a first space") changes substantially in synchronization with changes in the pressure in the combustion chamber. In contrast, the pressure in a space surrounded by the top ring, the second ring, the piston, and the cylinder bore (hereinafter referred to as "a second space") changes with a delay from changes in the pressure in the combustion chamber. The time lag in this case increases as the flow rate of the blow-by gas flowing from the abutment gap of the top ring into the second space decreases. That is, the aforementioned time lag increases as the abutment gap of the top ring decreases.

[0024] Accordingly, when the abutment gap of the top ring is made as narrow as possible, the aforementioned time lag becomes long. Thus, the pressure in the second space may become higher than the pressure in the first space. In such a case, since the top ring floats up in the top ring groove, the sealability of the top ring may also deteriorate.

[0025] It should be noted that the phenomenon in which the pressure in the second space becomes higher than the pressure in the first space is likely to be caused when the amount of the heat generated in the combustion chamber is large. This is considered to result from the fact that the outer diameter of a second land of the piston (a region between the top ring groove and the second ring groove) increases to reduce the volume of the second space when the amount of the heat generated in the combustion chamber is large.

[0026] Thus, the cooling channel according to the aspect of the invention may be so formed as to be located adjacent to the second land as well as the top ring groove. According to this construction, when the amount of the heat generated in the combustion chamber is large, the heat of the second land is absorbed by the oil in the cooling channel. As a result, the temperature of the second land is restrained from rising.

[0027] When the temperature of the second land is restrained from rising, the outer diameter of the second land is restrained from increasing (the second land is restrained from thermally expanding). As a result, the decrease in the volume of the second space is alleviated. When the decrease in the volume of the second space is alleviated, the phenomenon in which the pressure in -the second space becomes higher than the pressure in the first space is unlikely to be caused.

[0028] Further, the control portion according to the aspect of the invention may control the oil supply portion such that the amount of the oil supplied from the oil supply portion to the cooling channel becomes larger when the internal combustion engine is being warmed up than when the internal combustion engine includes been warmed up, for an equivalent engine load and an equivalent engine speed.

[0029] The temperature difference between the piston and the cylinder bore is large when the internal combustion engine is being warmed up. This is because the piston is directly warmed by the heat generated in the combustion chamber but the cylinder bore is indirectly warmed receiving the heat discharged from the piston. Furthermore, since the cylinder bore is larger in thermal capacity than the piston, the speed of rise in the temperature of the cylinder bore is lower than the speed of rise in the temperature of the piston.

[0030] When the temperature difference between the piston and the cylinder bore is large, the piston is expanded (the outer diameter of the piston is increased) whereas the cylinder bore is hardly expanded (the inner diameter of the cylinder bore is hardly increased). Thus, the clearance between the piston and the cylinder bore and the clearance between the piston ring and the cylinder bore are small. As a result, the piston, the piston ring, the cylinder bore, and the like may be abraded, and the degree of friction therebetween may be increased.

[0031] In this view, in the case where the amount of the oil supplied from the oil supply portion to the cooling channel is made larger when the internal combustion engine is being warmed up than when the internal combustion engine includes been warmed up, the thermal expansion of the piston is alleviated. As a result, the aforementioned problem can be prevented from being caused.

[0032] Further, the control portion according to the aspect of the invention may continue to operate the oil supply portion when the temperature of coolant for the internal combustion engine is equal to or higher than a predetermined upper-limit coolant temperature or when the temperature of oil (the oil temperature) is equal to or higher than a predetermined upper-limit oil temperature. In other words, the oil supply portion may be prohibited from stopping when the coolant temperature is equal to or higher than the upper-limit coolant temperature or when the oil temperature is equal to or higher than the upper-limit oil temperature. "The upper-limit coolant temperature" mentioned herein and "the upper-limit oil temperature" mentioned herein are obtained by subtracting a predetermined margin from a temperature at which the internal combustion engine is considered to be overheated and a temperature at which an oil film is considered to be broken respectively. When the oil supply portion is thus controlled, the internal combustion engine can be prevented from being overheated, and the oil film can be prevented from being broken.

[0033] According to the above aspect of the invention, in the system for cooling the piston of the internal combustion engine, the variation width of the temperature of the top ring can be reduced. Thus, the abutment gap of the top ring can be held equal to a preferable clearance, and the amount of compression loss and the amount of blow-by gas can be made as small as possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 shows the overall structure of a cooling system for a piston according to the first embodiment of the invention;

FIG. 2 is a cross-sectional view of the piston according to the first embodiment of the invention;

FIG. 3 shows the correlation between a temperature difference ΔΤ, an engine load Q, and an engine speed Ne;

FIG. 4 is a view schematically showing a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne in the first embodiment of the invention;

FIG. 5 is a schematic view of the heat transfer path in the piston;

FIG. 6 is a cross-sectional view of a piston according to the second embodiment of the invention;

FIG. 7 is an enlarged view of the gap between the piston and a cylinder bore wall surface;

FIG. 8 shows changes in the pressure Pvl in a first space and changes in the pressure Pv2 in a second space;

FIG. 9 is a schematic view of a phenomenon in which a top ring floats up;

FIG. 10 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne in the third embodiment of the invention;

FIG. 11 is shows the changes in a coolant temperature over time;

FIG. 12 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne in when an internal combustion engine is steadily operated while being warmed up;

FIG. 13 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne when the internal combustion engine is operated under intermediate load and at intermediate rotational speed while being warmed up;

FIG. 14 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne when the internal combustion engine is operated under high load and at high rotational speed while being warmed up;

FIG. 15 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne when the coolant temperature is lower than that in the example shown in FIG. 12;

FIG. 16 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne when the coolant temperature is lower than that in the example shown in FIG. 13; and

FIG. 17 is a schematic view of a map prescribing a relationship among the amount of oil injection, the engine load Q, and the engine speed Ne when the coolant temperature is lower than that in the example shown in FIG. 14. DETAILED DESCRIPTION OF EMBODIMENTS

[0035] Example embodiments of the invention will be described below with reference to the drawings. The dimensions, materials, shapes, relative arrangement and the like of components described in the example embodiments of the invention are not intended to limit the technical scope of the invention thereto unless otherwise specified.

[0036] The first embodiment of the invention will be described on the basis of FIGS. 1 to 5. FIG. 1 shows the overall structure of an internal combustion engine to which the invention is applied. FIG. 2 is a cross-sectional view of a piston according to the first embodiment of the invention.

[0037] An internal combustion engine 1 may be a compression-ignition internal combustion engine (a diesel engine) having a plurality of cylinders 2. It should be noted that only one of the plurality of the cylinders 2 is shown in FIG. 1. A piston 3 is slidably fitted in each cylinder 2 of the internal combustion engine 1 so that the piston may reciprocate in the axial direction of the cylinder. The piston 3 is coupled to a crankshaft (not shown) via a connecting rod 4.

[0038] A generally cylindrically combustion chamber 30 is formed in the top face of the piston 3. In addition, three annular grooves 31, 32, and 33 are formed in an outer peripheral face of the piston 3. The annular groove 31 is located closest to a top dead center (at the highest position in FIG. 2), and is fitted with a top ring 5 (the annular groove 31 will be referred to hereinafter as the "top ring groove 31"). The annular, groove 32 is located directly below the top ring groove 31 and it fitted with a second ring 6 (the annular groove 32 will be referred to hereinafter as the "second ring groove 32"). The annular groove 33 is located closest to a bottom dead center (at the lowest position in FIG. 2), and is fitted with an oil ring 7 (hereinafter, the annular groove 33 will be referred to as the "oil ring groove 33"). It should be noted that the top ring 5, the second ring 6, and the oil ring 7 are annular members equipped with abutments.

[0039] The top ring groove 31 is formed in the outer peripheral face of an abrasion-resistant loop 300 that is cast in the piston 3. The abrasion-resistant loop 300 is an annular member formed of a material harder and more resistant to abrasion than the piston 3 (e.g., a Ni-Cr-Cu cast iron material).

[0040] A hollow abrasion-resistant loop 310 is cast in an inside of the abrasion-resistant loop 300. The hollow abrasion-resistant loop 310 is an annular member that is U-shaped in cross-section and has an opening portion on an outer periphery side thereof. The outer periphery side of the hollow abrasion-resistant loop 310 abuts on an inner peripheral face of the abrasion-resistant loop 300. That is, the opening portion of the hollow abrasion-resistant loop 310, which is U-shaped in cross-section, is closed up by the inner peripheral face of the abrasion-resistant loop 300. An annular space 34 surrounded by the hollow abrasion-resistant loop 310 and the abrasion-resistant loop 300 functions as a passage for oil supplied from a later-described oil jet 8 (the space 34 will be referred to hereinafter as "a cooling channel 34").

[0041] Communication passages 35 and 36 through which an opening portion formed through a bottom face of the piston 3 communicates with the cooling channel 34 are formed through the piston 3. The communication passage 35 as one of the communication passages 35 and 36 functions as a passage through which the oil injected from the oil jet 8 is introduced into the cooling channel 34 (the communication passage 35 will be referred to hereinafter as "an introduction passage 35"). The communication passage 36 as the other of the communication passages 35 and 36 functions as a discharge passage from which the oil flowing out from the cooling channel 34 is discharged (the communication passage 36 will be referred to hereinafter as "a discharge passage 36").

[0042] The internal combustion engine 1 is equipped with the oil jet 8, which injects oil from a bottom dead center side to a top dead center side in the cylinder 2. It should be noted that the oil jet 8 is so arranged as to be located below the piston 3 when the piston 3 is located at the bottom dead center. Furthermore, the oil jet 8 is arranged and formed such that the oil injected from the oil jet 8 is oriented toward the introduction passage 35.

[0043] The oil jet 8 communicates with an oil pan 10 via a supply passage 9. The supply passage 9 is provided at a midway position thereof with an oil pump 11 that sucks up the oil in the oil pan 10. A flow rate adjusting valve 12 is arranged in the supply passage 9 between the oil jet 8 and the oil pump 11. The flow rate adjusting valve 12 is a valve that adjusts the amount of the oil flowing in the supply passage 9. The amount of the oil injected from the oil jet 8 (the amount of oil injection) is increased or reduced through the adjustment of the flow rate of the oil in the supply passage 9 by the flow rate adjusting valve 12. It should be noted that the oil jet 8 functions as the oil supply portion of the invention.

[0044] It should be noted that an electrically operated valve mechanism whose ratio between an open-valve time and a closed-valve time is subjected to duty control or an electrically operated valve mechanism whose opening degree can be changed continuously or stepwise can be employed as the flow rate adjusting valve 12. Further, the flow rate adjusting valve 12 may be a valve mechanism including a check valve that opens when the pressure of oil is equal to or higher than a certain value and a pressure adjusting valve that adjusts the pressure of the oil in the supply passage d.

[0045] The supply passage 9 is provided with a return passage 13 that bypasses the oil pump 11. This return passage 13 is a passage for returning a surplus amount of oil from that region of the supply passage 9 which is located downstream of the oil pump 11 to that region of the supply passage 9 which is located upstream of the oil pump 11. A one-way valve (a check valve) 14 that allows only the flow of oil from that region of the supply passage 9 which is located downstream of the oil pump 11 toward that region of the supply passage 9 which is located upstream of the oil pump 11 is arranged in the return passage 13.

[0046] The internal combustion engine 1 thus constructed is accompanied by an ECU 15. The ECU 15 is an electronic control unit equipped with a CPU, a ROM, a RAM, a backup RAM, and the like. Output signals of various sensors sucincludes a coolant temperature sensor 16, a crank position sensor 18, an accelerator position sensor 19, an oil temperature sensor 20, and the like are input to the ECU 15.

[0047] The coolant temperature sensor 16 is a sensor that outputs an electric signal correlated with a temperature of the coolant circulating through the internal combustion engine 1. The crank position sensor 18 is a sensor that outputs an electric signal correlated with a rotational position of a crankshaft. The accelerator position sensor 19 is a sensor that outputs an electric signal correlated with a depression amount of an accelerator pedal (an engine load). The oil temperature sensor 20 is a sensor that outputs an electric signal correlated with a temperature of the oil circulating through the internal combustion engine 1 (an oil temperature).

[0048] On the basis of the output signals of the aforementioned various sensors, the ECU 15 performs the control of the amount of the oil supplied from the oil jet 8 to the cooling channel 34 (which will be referred to hereinafter as "oil jet control) as well as known types of control such as fuel injection control and the like. A method of performing oil jet control will be described hereinafter. It should be noted that the control portion according to the invention is realized through the performance of oil jet control by the ECU 15.

[0049] Oil jet control according to this embodiment of the invention is designed to adjust the amount of oil injection from the oil jet 8 such that the temperature of the top ring 5 becomes substantially constant. That is, oil jet control according to this embodiment of the invention is designed to adjust the amount of oil injection from the oil jet 8 such that the abutment gap of the top ring 5 becomes substantially constant.

[0050] The temperature of the top ring 5 changes in accordance with the amount of the heat generated in the combustion chamber 30. For example, when the amount of the heat generated in the combustion chamber 30 is large, the amount of the rise in the temperature of the piston 3 is large. Therefore, the amount of the rise in the temperature of the top ring 5 is also large correspondingly.

[0051] When the temperature of the top ring 5 becomes high, the top ring 5 thermally expands to narrow the abutment gap. When the temperature of the top ring 5 further rises, opposed end faces of the abutment gap bump against each other to generate a force acting to increase the outer diameter of the top ring 5.

[0052] In this case, when the temperature of an inner wall surface of the cylinder 2 (a cylinder bore wall surface) is high, the aforementioned force is counterbalanced due to an increase in the inner diameter of the cylinder 2. However, when the temperature of the cylinder bore wall surface is lower than the temperature of the piston 3 and the difference between the temperatures is large, the top ring 5 and the cylinder 2 are tightened. Thus, the stress applied to the top ring 5 may become excessively large, or the contact load between the top ring 5 and the cylinder bore wall surface may become excessively large.

[0053] Thus, the size of the abutment gap needs to be determined such that the opposed end faces of the abutment do not press each other when the temperature of the top ring 5 is high and the temperature of the cylinder bore wall surface is low. However, in the case where the size of the abutment gap is determined according to this method, when the temperature of the top ring 5 is low, the abutment gap may become excessively wide to cause a compression loss or an increase in the amount of blow-by gas.

[0054] Thus, in the oil jet control according to this embodiment of the invention, the ECU 15 so adjusts the amount of oil injection as to suppress the rise in the temperature of the top ring 5 when the amount of the heat generated in the combustion chamber 30 is large (when the difference in temperature between the piston 3 and the cylinder bore wall surface is large) and suppress the fall in the temperature of the top ring 5 or promote the rise in the temperature of the top ring 5 when the amount of the heat generated in the combustion chamber 30 is small (when the difference in temperature between the piston 3 and the cylinder bore wall surface is small).

[0055] The amount of the heat generated in the combustion chamber 30 changes in accordance with the amount of the fuel burned in the combustion chamber 30, namely, the amount of fuel injection. In principle, the amount of fuel injection is determined using a load Q of the internal combustion engine 1 (an engine load) and a rotational speed Ne of the internal combustion engine 1 (an engine speed) as parameters. Thus, in this embodiment of the invention, an example in which the amount of oil injection is adjusted using the engine load Q and the engine speed Ne as parameters will be described.

[0056] FIG. 3 is a view showing a relationship among a temperature difference ΔΤ, the engine load Q, and the engine speed Ne. "The temperature difference ΔΤ" mentioned herein is a difference between the temperature of the piston 3 (preferably the temperature of the top ring groove 31) and the temperature of the cylinder bore wall surface.

[0057] In FIG. 3, when the engine load Q and the engine speed Ne are low, the amount of the heat generated in the combustion chamber 30 is smaller than when the engine load Q and the engine speed Ne are high. Thus, the temperature difference ΔΤ is small, and the abutment gap of the top ring 5 is wide.

[0058] In contrast, if the engine load Q and the engine speed Ne are both high, the amount of the heat generated in the combustion chamber 30 is larger than when the engine load Q and the engine speed Ne are low. Thus, the temperature difference ΔΤ is large, and the abutment gap of the top ring 5 is narrow.

[0059] In this view, the ECU 15 controls the flow rate adjusting valve 12 such that the amount of oil injection from the oil jet 8 becomes larger when the amount of the heat generated in the combustion chamber 30 is large than when the amount of the heat generated in the combustion chamber 30 is small. In other words, the ECU 15 controls the flow rate adjusting valve 12 such that the amount of oil injection from the oil jet 8 becomes larger when the temperature difference ΔΤ is large than when the temperature difference ΔΤ is small.

[0060] More specifically, the ECU 15 may control the flow rate adjusting valve 12 according to a map shown in FIG. 4. The map shown in FIG. 4 is a map determining a relationship among the engine load Q, the engine speed Ne, and the amount of oil injection.

[0061] In FIG. 4, when the engine load Q and the engine speed Ne are high (a region A in FIG. 4), the amount of oil injection is set to a maxim amount. When the engine load Q and the engine speed Ne are low (a region C in FIG. 4), the amount of oil injection is set equal to zero (the oil jet 8 is stopped). However, oil may be injected for the purpose of lubricating a space between the piston 3 and the cylinder bore wall surface or lubricating a space between the piston 3 and the connecting rod 4. Further, when the engine load Q and the engine speed Ne are in a region between the region A and the region C (a region B in FIG. 4), the amount of oil injection is made smaller than the aforementioned maximum amount. It should be noted that the region C in FIG. 4 is a region where the amount of the heat generated in the combustion chamber 30 is equal to or smaller than a lower limit. "The lower limit" mentioned herein is a value at which the temperature of the top ring 5 may be lower than a later-described suitable temperature.

[0062] It should be noted herein that part of the heat generated in the combustion chamber 30 is transferred from the top face of the piston 3 toward the top ring groove 31 and discharged from the top ring groove 31 to the cylinder bore wall surface. More specifically, as shown in FIG. 5, the heat transferred from inside the combustion chamber 30 to the piston 3 is mainly transferred from an upper edge portion 30a of the combustion chamber 30 in the piston 3 toward the top ring groove 31 (the abrasion-resistant loop 300) (see an arrow in FIG. 5). In this view, when the cooling channel 34 is arranged inside the top ring groove 31, this cooling channel 34 is located on a path of the heat. In other words, it is preferable that the cooling channel 34 be arranged on the path of the aforementioned heat.

[0063] Thus, in the case where the amount of oil injection is set to the maximum amount when the engine load Q and the engine speed Ne are high, most of the heat traveling from the upper edge portion 30a toward the top ring groove 31 is absorbed by the oil in the cooling channel 34. As a result, the temperatures of the piston 3 and the top ring groove 31 are restrained from rising, and the temperature of the top ring 5 is also restrained from rising correspondingly.

[0064] In the case where the temperature of the top ring 5 is restrained from rising when the engine load Q and the engine speed Ne are high, the opposed end faces of the abutment of the top ring 5 can be prevented from bumping against each other. Therefore, the contact load between the top ring 5 and the cylinder bore wall surface can be prevented from becoming excessively large.

[0065] However, if the amount of oil injection is set to zero when the engine load Q and the engine speed Ne are low, the interior of the cooling channel 34 is filled with air. The air in the cooling channel 34 functions as a heat insulating layer that shuts off the heat traveling from the upper edge portion 30a toward the top ring groove 31. Thus, the amount of the heat discharged from the piston 3 to the cylinder bore wall surface decreases. As a result, the temperatures of the piston 3 and the top ring groove 31 are restrained from falling, and the temperature of the top ring 5 is also restrained from falling correspondingly.

[0066] In the case where the temperature of the top ring 5 is restrained from falling when the engine load Q and the engine speed Ne are low, the abutment gap of the top ring 5 can be prevented from becoming excessively wide. Thus, an increase in the amount of compression loss and an increase in the amount of blow-by gas can be avoided. Further, when the temperature of the top ring groove 31 is restrained from falling, the temperature of the atmosphere in a gap (a crevice) between a top land of the piston 3 and the cylinder bore wall surface is held high. When the temperature of the atmosphere in the crevice is high, the temperature of the gas flowing through the abutment gap of the top ring 5 is also higher than when the temperature of the atmosphere in the crevice is low. As a result, the mass of the gas flowing through the abutment gap of the top ring 5 further decreases.

[0067] In the case where the amount of oil injection is made smaller than the maximum amount when the engine load Q and the engine speed Ne are in an intermediate load/intermediate rotational speed range, the amount of the heat absorbed by oil from the piston 3 is prevented from becoming much larger than the amount of the heat traveling from the upper edge portion 30a toward the top ring groove 31. As a result, the piston 3 and the top ring groove 31 are restrained from being overcooled, and the top ring 5 is also restrained from being overcooled. It should be noted that the amount of oil injection in the aforementioned region B of FIG. 4 may be a fixed amount, but may also be an amount that is changed in accordance with the engine load Q and the engine speed Ne. The amount of oil injection in this case may be made larger when the engine load Q is high than when the engine load Q is low, and may be made larger when the engine speed Ne is high than when the engine speed Ne is low. Further, the amount of oil injection is increased as an amount of fuel injection increases.

[0068] The temperature of the top ring 5 can be held substantially constant (at the suitable temperature) regardless of the operation state of the internal combustion engine 1, through the performance of oil jet control by the ECU 15 as described above. "The suitable temperature" mentioned herein is a temperature at which the abutment gap is the narrowest within such a range that the opposed end faces of the abutment of the top ring 5 do not bump against each other. It should be noted that the top ring 5 is designed such that the abutment gap has a desired size at the aforementioned suitable temperature.

[0069] In consequence, the cooling system for the piston according to this embodiment of the invention makes it possible to prevent the abutment gap of the top ring 5 from becoming excessively narrow to cause the opposed end faces of the abutment to bump against each other when the amount of the heat generated in the combustion chamber 30 is large, and to prevent the abutment gap of the top ring 5 from becoming excessively wide to cause an increase in the amount of compression loss or an increase in the amount of blow-by gas when the amount of the heat generated in the combustion chamber 30 is small.

[0070] It should be noted that although the example in which the oil jet 8 is stopped when the engine load Q and the engine speed Ne are low has been described in this embodiment of the invention, the oil jet 8 may be prohibited from being stopped when an output signal of the coolant temperature sensor 16 (a coolant temperature) indicates a temperature equal to or higher than an upper-limit coolant temperature or when an output signal of the oil temperature sensor 20 (an oil temperature) indicates a temperature equal to or higher than an upper-limit oil temperature. "The upper-limit coolant temperature" mentioned herein and "the upper-limit oil temperature" mentioned herein are obtained by subtracting a predetermined margin from a temperature at which the internal combustion engine may be overheated or a temperature at which an oil film may be broken respectively. When the oil jet 8 is thus prohibited from being stopped, the internal combustion engine 1 can be prevented from being overheated, and the oil film can be prevented from being broken. [0071] Next, the second embodiment of the invention will be described on the basis of FIGS. 6 to 9. Only the structural details of the second embodiment that differ from those of the first embodiment of the invention will be described below.

[0072] The difference between the first embodiment of the invention and the second embodiment of the invention lies in the structure of the cooling channel. In the first embodiment of the invention, the cooling channel is arranged to cool the top ring groove in a concentrated manner. However, in the second embodiment, the cooling channel is arranged to cool a second land 37 as well as the top ring groove.

[0073] FIG. 6 is a cross-sectional view of the piston 3 according to the second embodiment of the invention. In FIG. 6, components identical to those of the first embodiment (see FIG. 2) are denoted by the same reference symbols. The abrasion-resistant loop 300 according to this embodiment extends further in the axial direction of the cylinder than that of the first embodiment. More specifically, the abrasion-resistant loop 300 has a width ranging from the top land of the piston 3 to a third land thereof.

[0074] A second ring groove 32, as well as the top ring groove 31, is formed in the abrasion-resistant loop 300. Accordingly, the abrasion-resistant loop 300 located between the top ring groove 31 and the second ring groove 32 serves also as the second land 37.

[0075] The hollow abrasion-resistant loop 310, which is substantially equal in width to the abrasion-resistant loop 300 of the piston 3, is cast in the inside of the abrasion-resistant loop 300. The hollow abrasion-resistant loop 310 is an annular member with a U-shaped cross-section as in the first embodiment of the invention. The opening portion of the hollow abrasion-resistant loop 310 is closed up by the inner peripheral face of the abrasion-resistant loop 300. The annular space 34 surrounded by the hollow abrasion-resistant loop 310 and the abrasion-resistant loop 300 functions as a cooling channel.

[0076] In the case where the same oil jet control as in the first embodiment of the invention is performed for the piston 3 thus constructed, when the amount of the heat generated in the combustion chamber 30 is large (when the engine load Q and the engine speed Ne are high), the temperature of the top ring 5 is restrained from rising, and the temperature of the second land 37 is also restrained from rising.

[0077] It should be noted herein that FIG. 7 is an enlarged view of a gap between the piston 3 and a cylinder bore wall surface. In FIG. 7, VI denotes a space (a first space) surrounded by the top ring 5, the piston 3 (the top land), and the cylinder bore wall surface. In FIG. 7, V2 denotes a space (a second space) surrounded by the top ring 5, the piston 3 (the second land 37), the second ring 6, and the cylinder bore wall surface.

[0078] As shown in FIG. 8, a pressure Pvl in the first space VI changes substantially in synchronization with the pressure in the combustion chamber 30 (see a solid line in FIG. 8). In contrast, a pressure Pv2 in the second space V2 changes with a delay from the pressure in the combustion chamber 30 (see the single-dash chain line in FIG. 8). The time lag in this case increases as the abutment gap of the top ring 5 decreases. Thus, as described in the first embodiment of the invention, when the abutment gap of the top ring 5 is made as narrow as possible, there may be a period in which the pressure Pv2 in the second space V2 is higher than the pressure Pvl in the first space VI (see a hatched region in FIG 8).

[0079] Especially when the engine load Q and the engine speed Ne are high, in other words, when the difference in temperature between the second land 37 and the cylinder bore wall surface is large, the volume of the second space V2 is reduced, so that the pressure Pv2 in the second space V2 is likely to become higher than the pressure Pvl in the first space VI.

[0080] When the pressure Pv2 in the second space V2 becomes higher than the pressure Pvl in the first space VI, a phenomenon in which the top ring 5 floats up to the top dead center side in the direction of the axis of the cylinder in the top ring groove 31 occurs as shown in FIG. 9. When the top ring 5 thus floats up, blow-by gas may leak from the gap between the top ring 5 and the top ring groove 31.

[0081] In contrast, in the case where the temperature of the second land 37 is restrained from rising when the engine load Q and the engine speed Ne are high, the outer diameter of the second land 37 is restrained from increasing, or the outer diameter of the second land 37 is reduced. In this case, the volume of the second space V2 is restrained from decreasing, of the volume of the second space V2 is increased. As a result, the pressure Pv2 in the second space V2 is unlikely to rise.

[0082] Further, when the temperature of the second land 37 is restrained from rising, the volume of the gas present in the second space V2 is also restrained from increasing. As a result, the pressure Pv2 in the second space V2 is more unlikely to rise. Furthennore, the second ring groove 32 and the second ring 6 are cooled in no small measure by the oil in the cooling channel 34. Therefore, the abutment gap of the second ring 6 increases. When the abutment gap of the second ring 6 increases, the gas in the second space V2 is discharged from the abutment gap. As a result, the pressure Pv2 in the second space V2 can be more reliably prevented from becoming higher than the pressure Pv l in the first space VI .

[0083] According to the embodiment of the invention described above, the pressure Pv2 in the second space V2 is prevented from becoming higher than the pressure Pvl in the first space VI . Therefore, in addition to an effect equivalent to that of the first embodiment of the invention, the phenomenon in which the top ring 5 floats up can also be suppressed. As a result, the amount of blow-by gas can be restrained from increasing due to a deterioration in the sealability of the top ring 5.

[0084] Next, the third embodiment of the invention will be described on the basis of FIG. 10. In this case, the constructional details different from those of the first embodiment of the invention will be described, and the constructional details identical to those of the first embodiment of the invention will not be described.

r

[0085] The difference between the first embodiment of the invention and this embodiment of the invention consists in that the method of performing oil jet control is changed depending on whether the internal combustion engine 1 is being warmed up or has been warmed up. The piston 3 is smaller in thermal capacity than the cylinder block. Furthermore, while the cylinder bore wall surface indirectly receives the heat of the combustion chamber 30 via the piston 3 and a piston ring, the piston 3 directly receives the heat of the combustion chamber 30. Thus, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface is likely to be larger when the internal combustion engine 1 is being warmed up than when the internal combustion engine 1 has been warmed up.

[0086] When the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface becomes large, the difference between the amount of the increase in the outer diameter of the top ring 5 and the amount of the increase in the inner diameter of the cylinder 2 increases. As a result, the stress applied to the top ring 5 may become excessively large, or the contact load between the top ring 5 and the cylinder bore wall surface may become excessively large.

[0087] The ECU 15 controls the oil jet 8 so that the oil injection amount as the internal combustion engine 1 is being warmed up is greater than that when the internal combustion engine 1 has been warmed up. FIG. 10 is a schematic view of a map shows the correlation between the oil injection amount, the engine load Q, and the engine speed Ne. In FIG. 10, the borders between the regions A, B, and C when the internal combustion engine 1 is being warmed up are shown with solid lines. In FIG. 10, the borders between the regions A, B, and C when the internal combustion engine 1 has been warmed up are shown with single-dash chain lines.

[0088] As shown in FIG. 10, the respective borders shift toward a low load/speed side when the internal combustion engine 1 is being warmed up than when the internal combustion engine 1 has been warmed up. Therefore, the oil injection amount from the oil jet 8 is greater when the internal combustion engine 1 is being warmed up than when the internal combustion engine 1 has been warmed up.

[0089] As a result, when the internal combustion engine 1 is being warmed up, the difference between the amount of the increase in the outer diameter of the top ring 5 and the amount of the increase in the inner diameter of the cylinder 2 is prevented from becoming larger than when the internal combustion engine 1 has been warmed up. Thus, even when the internal combustion engine 1 is being warmed up, the application of excessive stress to the top ring 5 may be prevented, and the contact load between the top ring 5 and the cylinder bore wall surface can be prevented from becoming excessively large, while minimizing the increases in compression loss and the amount of blow-by gas.

[0090] Next, the fourth embodiment of the invention will be described on the basis of FIGS. 11 to 17. In following description, only the structural details the fourth embodiment that differ from those of the third embodiment of will be described.

[0091] In the third embodiment of the invention, the method of performing oil jet control when the internal combustion engine 1 is steadily operated while being warmed up has been described. However, in the fourth embodiment of the invention, a method of performing oil jet control when the internal combustion engine 1 is transiently operated during warm up will be described.

[0092] When the internal combustion engine 1 is transiently operated after cold start, there occurs a situation in which the temperature of the cylinder bore wall surface (the cylinder block) hardly rises although the temperature of the piston 3 rapidly rises. In such a case, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface may become larger. When the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface becomes large, the difference between the amount of the increase in the outer diameter of the top ring 5 and the amount of the increase in the inner diameter of the cylinder 2 increases. As a result, the stress applied to the top ring 5 may become excessively large, or the contact load between the top ring 5 and the cylinder bore wall surface may become excessively large. This problem becomes more remarkable as the rate of the rise in the temperature of the piston 3 (the speed at which the temperature rises) increases and as the temperature of the cylinder bore wall surface falls.

[0093] Thus, in this embodiment of the invention, the method of performing oil jet control is changed using the rate of the rise in the temperature of the piston 3 and the temperature of the cylinder bore wall surface as parameters. For example, the ECU 15 controls the flow rate adjusting valve 12 such that the amount of oil injection becomes larger when the rate of the rise in the temperature of the piston 3 is high and the temperature of the cylinder bore wall surface is low than when the rate of the rise in the temperature of the piston 3 is low and the temperature of the cylinder bore wall surface is high. The engine load Q and the engine speed Ne remain equivalent regardless of whether "the rate of the rise in the temperature of the piston 3 is high and the temperature of the cylinder bore wall surface is low" as mentioned herein or "the rate of the rise in the temperature of the piston 3 is low and the temperature of the cylinder bore wall surface is high" as mentioned herein.

[0094] The rate of the rise in the temperature of the piston 3 is correlated with the rate of the rise in the coolant temperature. Thus, the amount of change in the coolant temperature per a certain time can be used as the rate of the rise in the temperature of the piston 3. Further, the temperature of the cylinder bore wall surface is substantially equal to the temperature of the coolant flowing through the cylinder block. Thus, the output signal of the coolant temperature sensor 16 (the coolant temperature) can be used as the temperature of the cylinder bore wall surface.

[0095] FIG. 11 shows the changes in the coolant temperature over time. The two-dash chain line XI in FIG. 11 shows changes in the coolant temperature when the internal combustion engine 1 is steadily operated while being warmed up. The single-dash chain line X2 in FIG 11 indicates changes in the coolant temperature when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up. In FIG. 11, changes in the coolant temperature when the internal combustion engine 1 is operated at high load/speed while being warmed up are shown with a solid line X3. In addition, thwO shows the coolant temperature during the performance of oil jet control. In FIG. 11, ΔΡΙ , ΔΡ2, and ΔΡ3 indicate the amounts of change in the coolant temperature (the rates of increase in the temperature) for a predetermined time t for each of XI, X2, and X3.

[0096] As indicated by the two-dash chain line XI in FIG. 11 , when the internal combustion engine 1 is steadily operated while being warmed up, the ECU 15 controls the amount of oil injection in accordance with the map shown in FIG. 12. It should be noted that the map shown in FIG. 12 is equivalent to the map described in the third embodiment of the invention (see FIG. 10), and that the borders A, B, and C are shifted toward the low load/speed side relative to when the internal combustion engine 1 has been warmed up.

[0097] Then, the rate ΔΡ2 of temperature increase when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up is higher than the rate ΔΡ1 temperature increase when the internal combustion engine 1 is idling while being warmed up. Thus, when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up, the temperature difference ΔΤ between the piston 3 and the cylinder block is greater than that when the internal combustion engine 1 is idling while being warmed up.

[0098] Thus, as indicated by the single-dash chain line X2 in FIG. 11, when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up, the ECU 15 controls the amount of oil injection in accordance with the map shown in FIG. 13. The solid lines in FIG. 13 indicate borders between the regions A, B, and C when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up. The chain lines in FIG. 13 indicate the borders between the regions A, B, and C when the internal combustion engine 1 is steadily operated while being warmed up.

[0099] In FIG. 13, the borders between the regions A, B, and C when the internal combustion engine 1 is operated at intermediate while being warmed up is shifted toward the low load/speed side relative to the borders between the regions A, B, and C when the internal combustion engine 1 is steadily operated while being warmed up. Thus, more oil is injected when the internal combustion engine 1 is operated at intermediate load/ speed during warm-up than when the internal combustion engine 1 is steadily operated during warm-up.

[0100] Further, the rate ΔΡ3 at which temperature increases when the internal combustion engine 1 is operated at high load/ speed during warm up is higher than the rate ΔΡ2 at which temperature increases when the internal combustion engine 1 is operated at intermediate load/speed during warm up. Thus, f the internal combustion engine 1 is operated at high load/speed during warm up, the temperature difference ΔΤ between the piston 3 and the cylinder block is expected to become larger than when the internal combustion engine 1 is operated at intermediate load/ speed during warm up.

[0101] Thus, as indicated by the solid line X3 in FIG. 11, when the internal combustion engine 1 is operated at high load/speed during warm up, the ECU 15 controls the amount of oil injected in accordance with to a map shown in FIG. 14. The solid line in FIG. 14 indicates the border between the regions A and B when the internal combustion engine 1 is operated at high load/speed during warm up. The chain line in FIG. 14 indicates the border between the regions A and B when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up.

[0102] In FIG. 14, the border between the regions A and B when the internal combustion engine 1 is operated at high load/speed during warm up is shifted toward the low load/speed side relative to the border between the regions A and B when the internal combustion engine 1 is operated at intermediate load/speed while being warmed up. Furthermore, in the map shown in FIG. 14, a region for stopping the oil jet 8 (a region corresponding to the region C in each of FIGS. 12 and 13) is eliminated. That is, even when the internal combustion engine 1 makes a transition from a high load/high rotational speed operation range to a low load/low rotational speed operation range, a small amount of oil is injected from the oil jet 8.

[0103] Thus, the amount of oil injection in the case where the internal combustion engine 1 is operated at high load/high rotational speed while being warmed up is larger than the amount of oil injection in the case where the internal combustion engine 1 is operated at intermediate load/intermediate rotational speed while being warmed up. As a result, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface can be restrained from increasing.

[0104] It should be noted that the temperature difference ΔΤ between the piston

3 and the cylinder bore wall surface may become larger when the coolant temperature (the temperature of the cylinder bore wall surface) during the performance of oil jet control is lower than the aforementioned value thwO than when the coolant temperature is equal to thwO. It is thus desirable that the amount of oil injection be made larger when the coolant temperature is lower than thwO than when the coolant temperature is equal to thwO.

[0105] For example, when the internal combustion engine 1 is steadily operated while being warmed up, the ECU 15 controls the amount of oil injection according to a map shown in FIG. 15. It should be noted that solid lines in FIG. 15 indicate the borders between the regions A, B, and C in the case where the coolant temperature is lower than thwO respectively, and that single-dash chain lines in FIG. 15 indicate the borders between the regions A, B, and C in the case where the coolant temperature is equal to thwO respectively (which are equivalent to the borders between the regions A, B, and C in FIG. 12 respectively).

[0106] In FIG. 15, the borders between the regions A, B, and C in the case where the coolant temperature is lower than thwO shift more to the low load/low rotational speed side than the borders between the regions A, B, and C in the case where the coolant temperature is equal to thwO respectively. Thus, the amount of oil injection in the case where the internal combustion engine 1 is steadily operated while being warmed up increases as the coolant temperature falls. As a result, even when the coolant temperature (the temperature of the cylinder bore wall surface) becomes low, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface is restrained from increasing.

[0107] Further, when the internal combustion engine 1 is operated at intermediate load/intermediate rotational speed while being warmed up, the ECU 15 controls the amount of oil injection according to a map shown in FIG. 16. It should be noted that a solid line in FIG. 16 indicates the border between the regions A and B in the case where the coolant temperature is lower than thwO, and that the single-dash chain line in FIG. 16 indicate the border between the regions A and B in the case where the coolant temperature is equal to thwO (which is equivalent to the border between the regions A and B in FIG. 13).

[0108] In FIG. 16, the border between the regions A and B in the case where the coolant temperature is lower than thwO shifts more toward the low load/low rotational speed side than the border between the regions A and B in the case where the coolant temperature is equal to thwO. Furthermore, in the map shown in FIG. 16, a region for stopping the oil jet 8 (a region corresponding to the region C in FIG. 13) is eliminated. Thus, the amount of oil injection in the case where the internal combustion engine 1 is operated at intermediate load/intermediate rotational speed while being warmed up increases as the coolant temperature falls. As a result, even when the coolant temperature (the temperature of the cylinder bore wall surface) becomes low, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface is restrained from increasing.

[0109] Furthermore, when the internal combustion engine 1 is operated at high load/high rotational speed while being warmed up, the ECU 15 controls the amount of oil injection according to a map shown in FIG. 17. In the map shown in FIG. 17, a region in which a small amount of oil is injected from the oil jet 8 (a region corresponding to the region B in FIG. 14) is eliminated. That is, the amount of oil injection is set equal to a maximum amount in all operation ranges of the internal combustion engine 1. Thus, even in the case where the internal combustion engine 1 is operated at high load/high rotational speed when the internal combustion engine 1 is being warmed up and the temperature of the cylinder bore wall surface is low, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface can be restrained from increasing.

[0110] According to the embodiment of the invention described above, even in the case where the rate ΔΡ of the rise in the temperature of the piston 3 becomes high when the internal combustion engine 1 is being warmed up, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface can be restrained from increasing. As a result, the amount of the increase in the outer diameter of the top ring 5 can be held small. Thus, the stress applied to the top ring 5 can be prevented from - becoming excessively large, and the contact load between the top ring 5 and the cylinder bore wall surface can be prevented from becoming excessively large.

[0111] It should be noted that although the example in which a changeover among the maps is made using the coolant temperature during the performance of oil jet control and the rate ΔΡ of the rise in the coolant temperature as parameters has been described in this embodiment of the invention, a function expression covering the aforementioned relationships shown in FIGS. 12 to 17 may be used. That is, the amount of oil injection may be determined using a function expression whose arguments are the coolant temperature, the rate of the rise in the temperature, the engine load Q, and the engine speed Ne.

[0112] Further, in this embodiment of the invention, the example in which the rate of the rise in the coolant temperature is used as the rate of the rise in the temperature of the piston 3 has been described. However, some time lag may be produced until changes in the temperature of the piston 3 are reflected by the coolant temperature.

[0113] Thus, it is also appropriate to calculate an amount of the heat transferred from the combustion chamber 30 to the piston 3 using the amount of fuel injection as a parameter, and make a changeover among the maps using a result of the calculation and the temperature of the cylinder bore wall surface (the coolant temperature) as parameters. In this case, an amount Hq of the heat transferred from the combustion chamber 30 to the piston 3 per a certain time tinj may be calculated on the basis of an expression shown below.

Hq = Hinjx/(∑Finj)dt÷tinj

In the aforementioned expression, Hinj represents a small amount (J/g) of heat generation of fuel, and∑Finj represents a sum of amounts Finj of fuel injection within the certain time tinj.

[0114] The ECU 15 may control the flow rate adjusting valve 12 such that the amount of oil injection becomes larger when the amount Hq of the heat calculated according to the aforementioned expression is large and the coolant temperature (the temperature of the cylinder bore wall surface) is low than when the amount Hq of the heat is small and the coolant temperature (the temperature of the cylinder bore wall surface) is high.

[0115] According to this method, oil jet control can be performed in accordance with the actual temperature of the piston 3. [0116] It should be noted that the ECU 15 may simultaneously calculate an amount of oil injection on the basis of the rate ΔΡ of the rise in the coolant temperature and calculate an amount of oil injection on the basis of the amount Hq of the heat transferred from the combustion chamber 30 to the piston 3, and may control the flow rate adjusting valve 12 in accordance with the larger one of two results of the calculation. According to this method, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface can be more reliably restrained from increasing.

[0117] Next, the fifth embodiment of the invention will be described. In this case, the constructional details different from those of the first embodiment of the invention will be described, and the constructional details identical to those of the first embodiment of the invention will not be described.

[0118] In this embodiment of the invention, an example in which oil jet control is performed when the cylinder bore wall surface is overcooled as in a case where there is a malfunction in the cooling system of the internal combustion engine 1, especially a case where a thermostat valve seizes in an open-valve state will be described.

[0119] In the case where the thermostat valve seizes in an open-valve state, even when the coolant temperature is lower than a valve-opening temperature (or a valve-closing temperature) of the thermostat valve, the coolant flows through a radiator. Thus, the coolant temperature may further fall. When the coolant temperature thus falls, the cylinder bore wall surface is overcooled.

[0120] In the case where the cylinder bore wall surface is overcooled, even when the amount of the heat generated in the combustion chamber 30 is small, the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface may increase. When the temperature difference ΔΤ between the piston 3 and the cylinder bore wall surface increases, the amount of increase in the outer diameter of the top ring 5 may become excessively large with respect to the amount of increase in the inner diameter of the cylinder 2. As a result, even when the amount of the heat generated in the combustion chamber 30 is small, the stress applied to the top ring 5 may become excessively large, or the contact load between the top ring 5 and the cylinder bore wall surface may become excessively large.

[0121] In this view, during oil jet control according to this embodiment of the invention, the ECU 15 sets the amount of oil injection to the maximum amount regardless of the operation state (the engine load Q and the engine speed Ne) of the internal combustion engine 1 as in the case of the aforementioned map of FIG. 17 when there is a malfunction in the cooling system.

[0122] In this case, as a method of detecting a malfunction in the cooling system, it is possible to use a method in which it is determined that there is a malfunction in the cooling system when the rate of fall in the coolant temperature (the speed at which the temperature falls) is higher than a predetermined upper-limit rate of fall or when the amount of fall in the coolant temperature is larger than a predetermined upper-limit amount of fall. "The upper-limit rate of fall" mentioned herein is a rate of fall in the case where there is a malfunction in the thermostat valve in the open-valve state or a value obtained by subtracting a predetermined margin from this rate of fall. Further, "the upper-limit amount of fall" may be an amount of fall in temperature in the case where the thermostat valve seizes in the open-valve state or a value obtained by subtracting a predetermined margin from this amount of fall in temperature.

[0123] According to this embodiment of the invention, when there is a malfunction in the cooling system, the temperature difference between the piston 3 and the cylinder bore wall surface can be prevented from increasing. As a result, the outer diameter of the top ring 5 is restrained from increasing. Thus, when the amount of the heat generated in the combustion chamber 30 is small, the stress applied to the top ring 5 can be prevented from becoming excessively large, and the contact load between the top ring 5 and the cylinder bore wall surface can be prevented from becoming excessively large.

[0124] It should be noted that at least two or all of the first to fifth embodiments of the invention can be combined with one another. As a result, the abutment gap of the top ring 5 can be held substantially constant in various cases, for example, a case where the internal combustion engine 1 is being warmed up, a case where the internal combustion engine 1 has been warmed up, a case where there is a malfunction in the cooling system of the internal combustion engine 1, and the like.

[0125] Further, in each of the first to fifth embodiments of the invention, the cooling channel 34 is constructed of the hollow abrasion-resistant loop. However, any construction may be adopted as long as the cooling channel 34 is arranged adjacent to the top ring 5 (and the second land 37).

Claims

CLAIMS:

1. A cooling system for a piston of an internal combustion engine, comprising: a piston that includes a top ring groove provided in an outer peripheral face of the piston and fitted with a top ring, and a cooling channel designed as an oil passage embedded in the piston and located adjacent to the top ring groove;

an oil supply portion that supplies oil to the cooling channel; and

a control portion that increases an amount of oil supplied from the oil supply portion to the cooling channel as an amount of heat generated in a combustion chamber increases.

2. The cooling system for the piston of the internal combustion engine according to claim 1, wherein the control portion stops supplying oil from the oil supply portion to the cooling channel when the amount of heat generated in the combustion chamber is equal to or smaller than a predetermined lower limit.

3. The cooling system for the piston of the internal combustion engine according to claim 1, wherein the control portion increases the amount of oil supplied from the oil supply portion to the cooling channel as an amount of fuel injection increases.

4. The cooling system for the piston of the internal combustion engine according to claim 1, wherein the control portion increases the amount of oil supplied from the oil supply portion to the cooling channel as an engine speed and an engine load increase.

5. The cooling system for the piston of the internal combustion engine according to any one of claims 1 to 4, wherein the cooling channel is so formed as to be located adjacent to the top ring groove and a second land.

6. The cooling system for the piston of the internal combustion engine according to claim 5, wherein the piston is further equipped with a second ring groove located directly below the top ring groove, and

the second land is provided between the top ring groove and the second ring groove.

7. The cooling system for the piston of the internal combustion engine according to any one of claims 1 to 6, wherein the control portion makes the amount of oil supplied from the oil supply portion to the cooling channel larger when the internal combustion engine is being warmed up than when the internal combustion engine includes been warmed up, for an equivalent engine load and an equivalent engine speed.

8. The cooling system for the piston of the internal combustion engine according to any one of claims 1 to 7, wherein the control portion prohibits the oil supply portion from being stopped when a temperature of coolant is equal to or higher than a predetermined upper-limit coolant temperature or when a temperature of oil is equal to or higher than a predetermined upper-limit oil temperature.

9. The cooling system for the piston of the internal combustion engine according to any one of claims 1 to 8, wherein the oil supply portion is an oil jet.

10. The cooling system for the piston of the internal combustion engine according to any one of claims 1 to 9, wherein the top ring groove and the cooling channel are annular.

11. A method of controlling a cooling system for a piston of an internal combustion engine, comprising:

providing a piston that includes a top ring groove provided in an outer peripheral face of the piston and fitted with a top ring, and a cooling channel designed as an oil passage embedded in the piston and located adjacent to the top ring groove, and an oil supply portion that supplies oil to the cooling channel; and

making an amount of oil supplied from the oil supply portion to the cooling channel larger when an amount of heat generated in a combustion chamber is large than when the amount of heat generated in the combustion chamber

12. The method of controlling the cooling system for the piston of the internal combustion engine according to claim 11, wherein the amount of oil supplied from the oil supply portion to the cooling channel is made larger when the internal combustion engine is being warmed up than when the internal combustion engine includes been warmed up, for an equivalent engine load and an equivalent engine speed.