阅读说明：本技术 内燃机 (Internal combustion engine ) 是由 堀田慎太郎 桥本晋 定金伸治 多田博 高宫二三郎 北条明 于 2019-06-04 设计创作，主要内容包括：本发明提供一种内燃机,抑制燃油经济性的恶化并降低在内燃机的运转中产生的噪音。内燃机(1)具有多个气缸(13)且变速器(80)相邻配置。将比通过进气门(21)的阀芯(21a)的中心而沿各气缸的周向延伸的假想圆筒面(VC)靠内侧的区域的燃烧室(15)的高度的平均值设为中央高度,并将比假想圆筒面靠外侧的区域的燃烧室的高度的平均值设为周边高度时,内燃机的燃烧室形成为,多个气缸中的位于最靠变速器侧的变速器侧气缸的中央高度比变速器侧气缸以外的气缸即通常气缸的中央高度高且变速器侧气缸的周边高度比通常气缸的周边高度低。(The invention provides an internal combustion engine, which restrains deterioration of fuel economy and reduces noise generated in operation of the internal combustion engine. The internal combustion engine (1) has a plurality of cylinders (13) and transmissions (80) arranged adjacent to each other. When the average of the heights of combustion chambers (15) in the region inside an imaginary cylindrical surface (VC) extending in the circumferential direction of each cylinder through the center of a valve element (21a) of an intake valve (21) is defined as the central height, and the average of the heights of combustion chambers in the region outside the imaginary cylindrical surface is defined as the peripheral height, the combustion chambers of an internal combustion engine are formed such that the central height of a transmission-side cylinder located closest to the transmission among a plurality of cylinders is higher than the central height of a normal cylinder, which is a cylinder other than the transmission-side cylinder, and the peripheral height of the transmission-side cylinder is lower than the peripheral height of the normal cylinder.)

1. An internal combustion engine having a plurality of cylinders and drive train constituent members other than the internal combustion engine disposed adjacent to each other,

when an average value of a height of a combustion chamber when a piston of a region inside a virtual cylindrical surface extending in a circumferential direction of each cylinder through a center of a valve element of an intake valve is at a top dead center is set as a center height, and an average value of a height of the combustion chamber when the piston of a region outside the virtual cylindrical surface is at a top dead center is set as a peripheral height, the combustion chamber is formed such that a central height of a component-side cylinder located closest to the power train component among the plurality of cylinders is higher than a central height of a normal cylinder which is at least one cylinder other than the component-side cylinder, and the peripheral height of the component-side cylinder is lower than a peripheral height of the normal cylinder.

2. The internal combustion engine according to claim 1,

the normal cylinder includes a cylinder farthest apart from the power train constituent member.

3. The internal combustion engine according to claim 2,

the combustion chamber is formed such that the center height of the cylinder on the power train component side of two adjacent cylinders is equal to or greater than the center height of the cylinder on the opposite side of the power train component side, and the peripheral height of the cylinder on the power train component side of the two adjacent cylinders is equal to or less than the peripheral height of the cylinder on the opposite side of the power train component side.

4. An internal combustion engine according to any one of claims 1 to 3,

the combustion chamber is at least partially defined by a cylinder head and a piston,

the shape of the cylinder head defining each combustion chamber is the same regardless of the central height and the peripheral height, and the shape of the piston defining each combustion chamber is formed in different shapes depending on the central height and the peripheral height.

5. The internal combustion engine according to claim 4,

a groove is provided in the center of the upper surface of the piston in a cross section extending in a direction in which the plurality of cylinders are arranged in parallel through the center of the piston,

the piston is formed such that, among the cylinders having a relatively high center height, the average depth of the groove of the piston is deeper than that of the cylinders having a relatively low center height.

6. The internal combustion engine according to claim 5,

the groove of the piston is formed to be deepest at the center of the piston and gradually shallower toward the radial outside of the piston, and is formed to be deeper at the center of the piston relative to the cylinder in which the center height is relatively low, among the cylinders in which the center height is relatively high.

7. The internal combustion engine according to claim 5 or 6,

the piston includes an inclined portion outside the groove, and an upper surface of the inclined portion is inclined toward a crankshaft side toward a radially outer side.

Technical Field

The present invention relates to internal combustion engines.

Background

Various methods for reducing noise generated during operation of an internal combustion engine have been studied (for example, patent documents 1 and 2). One of the noises generated during the operation of the internal combustion engine is a large vibration noise generated by a large end face vibration generated in a flywheel due to a large combustion excitation force in a cylinder closest to the flywheel. In patent document 1, in order to reduce such vibration noise, it is proposed to retard the ignition timing of the cylinder closest to the flywheel to reduce the combustion exciting force, thereby reducing the vibration noise.

[ Prior Art document ]

[ patent document ]

[ patent document 1 ] Japanese examined patent publication No. 7-058059

[ patent document 2 ] Japanese patent laid-open publication No. 2017-096245

Disclosure of Invention

Drawings

Fig. 1 is a schematic configuration diagram of an internal combustion engine.

Fig. 2 is a schematic cross-sectional view of the periphery of one cylinder of an engine main body of an internal combustion engine.

Fig. 3 is a side view, partially in section, of an engine body and a transmission disposed adjacent to the engine body.

FIG. 4 is a side view of the transmission housing.

Fig. 5 is a diagram showing a change in crank angle of the heat generation rate.

Fig. 6 is a plan view of the piston as viewed from the combustion chamber side.

Fig. 7 is a sectional view of the cylinder upper portion (near the cylinder head) of a normal cylinder.

Fig. 8 is a sectional view of the cylinder upper portion (the vicinity of the cylinder head) of the transmission-side cylinder.

Fig. 9 is a cross-sectional view schematically showing combustion of the air-fuel mixture in the combustion chamber of the transmission-side cylinder.

Fig. 10 is a diagram showing a relationship between a state of flame spread and an area of the outer peripheral surface of the flame.

Fig. 11 is a diagram showing a change in crank angle of the heat generation rate.

[ Mark Specification ]

1 internal combustion engine

10 Engine body

11 cylinder block

12 cylinder head

13 cylinder

14 piston

15 combustion chamber

17 air inlet

18 exhaust port

21 inlet valve

22 exhaust valve

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same reference numerals are given to the same components. In the present specification, a direction from the crankshaft toward the cylinder head is referred to as an upward direction, and a direction opposite thereto is referred to as a downward direction. However, the direction in which the engine main body is disposed is not necessarily limited to this direction.

< construction of internal Combustion Engine >

First, the structure of the internal combustion engine 1 according to the first embodiment will be described with reference to fig. 1 to 3. Fig. 1 is a schematic configuration diagram of an internal combustion engine 1. Fig. 2 is a schematic cross-sectional view of the periphery of one cylinder of the engine body 10 of the internal combustion engine 1. As shown in fig. 1, the internal combustion engine 1 includes an engine body 10, a fuel supply device 30, an intake system 40, an exhaust system 50, and a control device 60.

In the present embodiment, the internal combustion engine 1 is a V-type 6 cylinder, and the engine body 10 includes a cylinder block 11 in which a plurality of banks 11a and 11b are formed, and a cylinder head 12 provided in each of the banks 11a and 11 b. Each bank 11a has 3 cylinders 13, and a piston 14 reciprocating in the cylinder 13 is disposed in each cylinder 13. A combustion chamber 15 in which the mixture is burned is formed in the cylinder 13 between the piston 14 and the cylinder head 12. Thus, the combustion chamber 15 is delimited by the piston 14, the cylinder head 12, and the cylinder 13. In the cylinder head 12, an ignition plug 16 for igniting an air-fuel mixture in the combustion chamber 15 is provided near the center of each cylinder 13.

The internal combustion engine 1 of the present embodiment is a V-type 6-cylinder engine, but may be a straight-line type or a horizontally opposed type, or may be an internal combustion engine having a different number of cylinders other than 6 cylinders, such as 3 cylinders, 4 cylinders, 8 cylinders, and 10 cylinders.

The cylinder head 12 is provided with an intake port 17 and an exhaust port 18. The intake port 17 and the exhaust port 18 communicate with the combustion chamber 15 of each cylinder 13. An intake valve 21 is disposed between the combustion chamber 15 and the intake port 17, and the intake valve 21 opens and closes the intake port 17. Similarly, an exhaust valve 22 is disposed between the combustion chamber 15 and the exhaust port 18, and the exhaust valve 22 opens and closes the exhaust port 18. The spool of the intake valve 21 and the spool of the exhaust valve 22 also define the combustion chamber 15.

The fuel supply device 30 includes a fuel injection valve 31, a delivery pipe 32, a fuel supply pipe 33, a fuel pump 34, and a fuel tank 35. The fuel injection valve 31 is disposed in the cylinder head 12 so as to directly inject fuel into each cylinder 13. The fuel pressure-fed by the fuel pump 34 is supplied to the delivery pipe 32 via the fuel supply pipe 33, and is injected from the fuel injection valve 31 into each cylinder 13.

The intake system 40 includes an intake manifold 41, an intake pipe 42, an air cleaner 43, and a throttle valve 44. The intake port 17 of each cylinder 13 communicates with an air cleaner 43 via an intake manifold 41 and an intake pipe 42. The throttle valve 44 is provided in the intake pipe 42 and is driven to open and close by a throttle valve drive actuator 45. The intake port 17, the intake manifold 41, and the intake pipe 42 form an intake passage.

The exhaust system 50 includes an exhaust manifold 51, an exhaust pipe 52, and an exhaust purification catalyst 53. The exhaust port 18 of each cylinder 13 communicates with an exhaust purification catalyst 53 via an exhaust manifold 51 and an exhaust pipe 52. The exhaust purification catalyst 53 is, for example, a three-way catalyst or an NOx storage reduction catalyst, and purifies components in the exhaust gas such as NOx and unburned HC in the exhaust gas. The exhaust port 18, the exhaust manifold 51, the exhaust pipe 52, and the exhaust purification catalyst 53 form an exhaust passage.

The control device 60 includes an Electronic Control Unit (ECU) 61. The ECU61 includes a RAM (random access memory) 63, a ROM (read only memory) 64, a CPU (microprocessor) 65, an input port 66, and an output port 67, which are connected to each other via a bidirectional bus 62.

The intake pipe 42 is provided with a flow rate sensor (e.g., an airflow meter) 71 that detects the flow rate of intake air flowing in the intake pipe 42. The output of the flow sensor 71 is input to the input port 66 via the corresponding AD converter 68. A load sensor 73 that generates an output voltage proportional to the amount of depression of the accelerator pedal 72 is connected, and the output voltage of the load sensor 73 is input to the input port 66 via the corresponding AD converter 68. The crank angle sensor 74 generates an output pulse every time the crankshaft 23 of the engine body 10 rotates by, for example, 10 °. The output pulse is input to the input port 66, and the CPU65 calculates the engine speed based on the output pulse.

On the other hand, the output port 67 of the ECU61 is connected to each actuator that controls the operation of the internal combustion engine 1 via the corresponding drive circuit 69. In the example shown in fig. 1, the output port 67 is connected to the fuel injection valves 31, the fuel pump 34, and the throttle valve drive actuator 45. The ECU61 outputs control signals for controlling these actuators from the output port 67 to control the operation of the internal combustion engine 1.

Fig. 3 is a side view, partially in section, of the transmission disposed adjacent to the engine body 10 and the engine body 10. As shown in fig. 3, the engine body 10 includes a crankshaft 23 extending in the direction in which the cylinders 13 of the banks 11a and 11b are arranged.

The crankshaft 23 includes a plurality of crankshaft journals 24, and the crankshaft journals 24 are rotatably supported by crankshaft bearings 25 disposed in the cylinder block 11. The crankshaft 23 is provided with a connecting rod journal 26 connected to the piston 14 of each cylinder via a connecting rod (not shown). The crankshaft 23 is provided with the same number of connecting rod journals 26 as the number of cylinders 13. A connecting rod journal 26 on one end side of the crankshaft 23 is connected to the piston 14 of the first cylinder 13# 1. On the other hand, the connecting rod journal 26 on the other end side of the crankshaft 23 is connected to the piston 14 of the sixth cylinder 13# 6. In fig. 3, the bracket in the reference number indicating the connecting rod journal 26 indicates the number of the cylinder 13 in which the piston 14 connected to the connecting rod journal 26 is disposed. The connecting rod journal 26 is formed such that its axis is eccentric from the axis of the crank journal 24 (the axis of the crankshaft 23).

The transmission 80 is disposed adjacent to the engine body 10 of the internal combustion engine 1. The transmission 80 is disposed adjacent to the engine body 10 on the side surfaces of the fifth cylinder 13#5 and the sixth cylinder # 6. The sixth cylinder side end of the crankshaft 23 is connected to a transmission 80 via a flywheel 81. In the illustrated example, the transmission 80 is an automatic transmission provided with a torque converter 82, but may be another automatic transmission such as a manual transmission or a CVT. The transmission 80 is fixed to the engine body 10 by bolts or the like.

< mechanism of generating vibration noise >

In the drive train including the internal combustion engine 1 and the transmission 80 configured as described above, unpleasant noise and vibration (hereinafter, also referred to as "vibration noise") such as gurgling occur depending on the operating state (e.g., acceleration) of the internal combustion engine 1. The mechanism of occurrence of such vibration noise is considered to be mainly as follows.

A downward force (i.e., a direction from the combustion chamber 15 toward the crankshaft 23) is applied to the crankshaft 23 via the piston 14 and the connecting rod by combustion of the air-fuel mixture in each combustion chamber 15 of the internal combustion engine 1. As a result, bending stress is applied to the crankshaft 23. When bending stress is applied to the crankshaft 23, a downward force is applied to the crankshaft bearing 25 that supports the crankshaft 23.

When a downward force is applied to the transmission-side crankshaft bearing 25a among the plurality of crankshaft bearings 25, the transmission-side cylinder block 11 is deformed. Specifically, the cylinder block 11 is deformed so that an upper side (cylinder head 12 side) and a lower side (oil pan side) of the crankshaft 23 are separated from each other in a side surface on the transmission 80 side.

Fig. 4 is a side view of a housing (hereinafter, also referred to as "transmission housing") 83 of the transmission 80. The side surface shown in fig. 4 is a side surface of the transmission 80 on the side connected to the cylinder block 11. The state shown by the solid line in fig. 4 shows the shape of the transmission case 83 when the cylinder block 11 is not deformed, for example, when the internal combustion engine 1 is stopped.

On the other hand, the broken line in fig. 4 indicates the shape of the transmission case 83 when the cylinder block 11 is deformed due to the exciting force caused by the combustion of the air-fuel mixture. The cylinder block 11 is deformed so that the upper side and the lower side of the crankshaft 23 are separated from each other as described above, and accordingly, a region of the transmission case 83 coupled to the cylinder block 11 on the engine main body 10 side is deformed so as to extend and contract vertically. Since the region of the transmission housing 83 on the engine main body 10 side is deformed in a manner to telescope up and down as such, the above-described vibration noise of gurgling and grunting is generated.

< measures against vibration noise >

As described above, the vibration noise is generated by a large exciting force accompanying the combustion of the air-fuel mixture in the cylinder on the transmission side. In other words, if the exciting force accompanying the combustion of the air-fuel mixture in the cylinder on the transmission side is small, the vibration noise can be reduced. In the present embodiment, for example, if the exciting force generated along with the combustion of the air-fuel mixture in the fifth cylinder 13#5 and the sixth cylinder 13#6 is reduced, the vibration noise can be reduced.

As a method of reducing the exciting force associated with the combustion in the specific cylinder 13, it is conceivable to retard the ignition timing by the ignition plug 16 in the cylinder compared with the other cylinders. If the ignition timing is retarded, the proportion of the thermal energy generated by combustion that is converted into kinetic energy (rotational energy of the crankshaft 23) decreases, and therefore the exciting force applied to the crankshaft 23 decreases. Therefore, by retarding the ignition timing in the fifth cylinder 13#5 and the sixth cylinder 13#6 from the ignition timing in the other cylinders, the vibration noise can be reduced.

Fig. 5 is a diagram showing changes in crank angle in the amount of change in the amount of heat generation per unit crank angle (dQ/d θ. hereinafter, also referred to as "heat generation rate"). The lines in the figure represent the transition of the heat generation rate at different ignition timings. The line MBT indicates a change in the heat generation rate when the ignition by the spark plug 16 is performed in MBT, and the line MBT-2 indicates a change in the heat generation rate when the ignition is performed at a timing 2 degrees behind MBT. The lines MBT-4 to MBT-12 show the transition of the ignition performed from the time at which the delay angle is 4 degrees to the time at which the delay angle is 12 degrees compared to MBT, respectively.

The larger the maximum value of the heat generation rate is, the larger the exciting force accompanying combustion is. Therefore, the larger the maximum value of the heat generation rate is, the larger the vibration noise is. As is clear from fig. 5, the more retarded the ignition timing, the smaller the maximum value of the heat generation rate and, therefore, the smaller the vibration noise.

However, if the ignition timing is retarded, the proportion of the thermal energy generated accompanying combustion that is converted into kinetic energy decreases as described above, and therefore the thermal efficiency deteriorates. Therefore, if the ignition timing of the fifth cylinder 13#5 and the sixth cylinder 13#6 is retarded, the vibration noise can be reduced, but as a result, the fuel economy is deteriorated.

< shape of Combustion Chamber >

Therefore, in the internal combustion engine of the present embodiment, the combustion chamber 15 is formed such that the center height of the transmission-side cylinder located closest to the transmission side among the plurality of cylinders 13 is higher than the center height of the normal cylinder, which is at least one cylinder other than the transmission-side cylinder, and the peripheral height of the transmission-side cylinder is lower than the peripheral height of the normal cylinder. Here, the central height is an average value of the heights of the combustion chambers 15 when the piston 14 in a region inside an imaginary cylindrical surface VC extending in the circumferential direction of each cylinder 13 through the center of the valve element 21a of the intake valve 21 is at top dead center. The peripheral height is an average value of the heights of the combustion chambers 15 when the piston 14 is positioned at the top dead center in the region outside the virtual cylindrical surface VC.

Fig. 6 is a plan view of the piston 14 as viewed from the combustion chamber 15 side. Fig. 7 is a cross-sectional view of an upper portion (near the cylinder head) of the cylinder 13 from the first cylinder 13#1 to the fourth cylinder 13#4 (hereinafter, also referred to as a "normal cylinder" in the present embodiment). Fig. 7 a is a cross-sectional view of the piston 14 taken along the axis XP of the piston and extending in the direction in which the cylinders 13 of the same cylinder bank are arranged (hereinafter, also referred to as "arrangement direction cross-section"), and is a cross-sectional view taken along the line a-a in fig. 6. Fig. 7B is a cross-sectional view of a section extending perpendicular to the arrangement direction of the cylinders 13 through the axis XP of the piston 14 (hereinafter, also referred to as a "vertical cross-section"), and is a cross-sectional view taken along the line B-B of fig. 6 and the line B-B of fig. 7 a. Fig. 7(C) is a cross-sectional view of a cross-section extending perpendicular to the arrangement direction of the cylinders 13 through the center of the valve body 21a of one of the intake valves 21, and is a cross-sectional view viewed along the line C-C of fig. 6 and the line C-C of fig. 7 (a).

As is apparent from fig. 6 and 7, a flat portion 91, a central groove 92, a raised portion 93, an inclined portion 94, an intake recess 95, and an exhaust recess 96 are provided on the top surface of a piston (hereinafter, also referred to as a "normal piston") 14 of the first cylinder 13#1 to the fourth cylinder 13# 4.

The flat portion 91 is formed on the top surface of the piston 14 and extends perpendicularly to the axis XP of the piston 14. The flat portion 91 is provided near the outer periphery of the piston 14.

The central groove 92 is provided in the center of the top surface of the piston 14. As shown in fig. 6 and 7, the central groove 92 is formed such that the width in the arrangement direction of the cylinders 13 is larger than the width in the direction perpendicular to the arrangement direction of the cylinders 13 (hereinafter, also referred to as "arrangement perpendicular direction"). Further, the central groove 92 is formed so as to be deepest at the center of the piston 14 and become gradually shallower as it separates from the center of the piston 14 toward the radially outer side, when the height from the plane where the flat portion 91 is located to the lower side is referred to as the depth.

In particular, in the present embodiment, as shown in fig. 7(a), the bottom surface of the center groove 92 is formed to be entirely curved in the cross section in the arrangement direction. The bottom surface of the central groove 92 may be curved with the same radius of curvature over the entire arrangement direction cross section, or may be formed so that the radius of curvature gradually changes (e.g., increases) from the center toward the outer side in the radial direction. However, the bottom surface of the central groove 92 may extend linearly from the center toward both edges in the cross section in the arrangement direction, or may be partially curved.

In the present embodiment, the bottom surface of the center groove 92 is formed to extend linearly from the center toward both edges in a vertical cross section. However, the central groove 92 may be formed such that the bottom surface thereof is entirely or partially curved in a vertical cross section.

The raised portions 93 are provided on both sides of the central groove 92 in the arrangement direction of the cylinders 13. The raised portion 93 is formed to protrude upward from the flat portion 91. Therefore, a part of the bottom surface of the central groove 92 is located above the flat portion 91.

The inclined portion 94 is provided radially outward of the bulging portion 93 in the arrangement direction of the cylinders 13. The inclined portion 94 is configured to be inclined downward (toward the crankshaft 23) from the raised portion 93 toward the radial outside. The inclined portion 94 is formed such that the height of the outer periphery thereof becomes the same as the height of the flat portion 91.

The intake valve recess 95 is formed to be recessed downward from the flat portion 91. Further, the intake valve recess 95 is formed at a position opposing a part of the valve body 21a of the intake valve 21 when the piston 14 is at the top dead center. In the present embodiment, two intake valves 21 are provided for each cylinder 13, and therefore, two intake-valve concave grooves 95 are provided for each piston 14. The intake valve recess 95 has an inclined surface extending substantially perpendicularly to the axis XI of the intake valve 21, and the inclined surface is configured to extend to the edge on the side of the central groove 92 in the arrangement perpendicular direction. Therefore, the intake valve recess 95 is provided in the top face of the piston 14 so as to avoid interference of the intake valve 21 with the piston 14 when the piston 14 is at intake top dead center.

The exhaust valve recess 96 is formed to be recessed downward from the flat portion 91. The exhaust valve recess 96 is formed at a position facing a part of the valve element 22a of the exhaust valve 22 when the piston 14 is at the top dead center. In the present embodiment, two exhaust valves 22 are provided for each cylinder 13, and therefore two exhaust valve recesses 96 are provided for each piston 14. The exhaust valve recess 96 has an inclined surface extending substantially perpendicularly to the axis XE of the exhaust valve 22, and the inclined surface is configured to extend to the edge on the side of the central groove 92 in the arrangement perpendicular direction. Therefore, the exhaust valve recess 96 is provided in the top face of the piston 14 so as to avoid interference of the exhaust valve 22 with the piston 14 when the piston 14 is at the intake top dead center.

Fig. 8 is a cross-sectional view of an upper portion (near the cylinder head) of the cylinder 13 of the fifth cylinder 13#5 and the sixth cylinder 13#6 (hereinafter, also referred to as "transmission-side cylinder" in the present embodiment). Fig. 8(a) is a cross-sectional view similar to fig. 7(a) taken along the arrangement direction. Fig. 8(B) is a cross-sectional view similar to fig. 7(B) taken along the vertical direction, and is a cross-sectional view taken along line B-B of fig. 8 (a). Fig. 8(C) is a cross-sectional view similar to fig. 7(C) of a cross-section extending perpendicularly to the arrangement direction of the cylinders 13 through the center of the valve element of one of the intake valves 21, and is a cross-sectional view taken along line C-C of fig. 8 (a).

As is apparent from fig. 8, the piston 14' of the transmission-side cylinder (hereinafter also referred to as "transmission-side piston") is basically formed in the same manner as the piston (normal piston) 14 of the first cylinder 13#1 to the fourth cylinder 13#4 shown in fig. 6 and 7. Therefore, the flat portion 91, the central groove 92, the raised portion 93, the inclined portion 94, the intake recess 95, and the exhaust recess 96 are provided on the top surface of the transmission side piston 14'.

As shown in fig. 8, in the transmission-side piston 14', the central groove 92 is also formed so as to be deepest at the center of the piston 14 and gradually become shallower as it separates from the center of the piston 14 toward the radially outer side. However, as can be seen from a comparison between fig. 7 and 8, the center groove 92 of the transmission-side piston 14' is formed to have a depth from the plane in which the flat portion 91 is located at the center of the piston 14, which is deeper than the center groove 92 of the normal piston 14. The center groove 92 of the transmission-side piston 14' is formed to be deeper than the average depth of the center groove 92 of the normal piston 14.

In the transmission-side piston 14', the central groove 92 is also formed to be entirely curved in the cross section in the arrangement direction. The central groove 92 may be formed so as to be curved with the same radius of curvature over the entire arrangement direction cross section, or may be formed so that the radius of curvature gradually changes (for example, increases) from the center toward the outer side in the radial direction.

In the transmission-side piston 14', ridges 93 are also provided on both sides of the central groove 92 in the arrangement direction of the cylinders 13. The bulging portion 93 of the transmission-side piston 14' is formed so that the amount of projection upward from the flat portion 91 is larger than the bulging portion 93 of the normal piston 14. In the present embodiment, the bulging portion 93 of the transmission-side piston 14' is formed radially inward of the bulging portion 93 of the normal piston 14. Therefore, the radius of curvature of the central groove 92 of the transmission-side piston 14' is smaller on average than the radius of curvature of the central groove 92 of the normal piston 14. In particular, in the present embodiment, the radius of curvature of a certain portion of the center groove 92 of the transmission-side piston 14' is smaller than the radius of curvature of a corresponding portion of the center groove 92 of the normal piston 14.

In the transmission-side piston 14', the bottom surface of the center groove 92 may extend linearly from the center toward both edges in the cross section in the arrangement direction, or may be partially curved. In this case, the angle of the inclined surface of the central groove 92 of the transmission-side piston 14' with respect to the plane of the flat portion 91 is larger than the angle of the inclined surface of the central groove 92 of the normal piston 14 with respect to the plane of the flat portion 91.

The inclined portion 94 of the transmission-side piston 14' is also configured to be inclined downward from the bulging portion 93 toward the radially outer side. In the present embodiment, the inclination angle of the bulging portion 93 of the transmission-side piston 14' with respect to the plane in which the flat portion 91 is located is larger than the inclination angle of the bulging portion 93 of the normal piston 14.

On the other hand, in the present embodiment, the cylinder head 12 is formed so that the portion thereof covering the cylinders 13 has the same shape between the cylinders 13. Therefore, the shape of the cylinder head 12 that defines each combustion chamber 15 is the same regardless of the transmission-side cylinder or the normal cylinder. Therefore, in the present embodiment, only the shape of the top face of the piston 14 that defines the combustion chamber 15 differs in the transmission-side cylinder from the normal cylinder.

Here, as described above, a virtual circumferential surface extending in the circumferential direction of each cylinder through the center CI of the valve body 21a of the intake valve 21 when the piston 14 is at the top dead center is defined as the virtual circumferential surface VC. The average of the heights (vertical lengths) of the combustion chambers 15 when the pistons 14 are at top dead center in the region radially inward of the virtual cylindrical surface VC is referred to as the center height, and the average of the heights of the combustion chambers 15 when the pistons 14 are at top dead center in the region radially outward of the virtual cylindrical surface VC is referred to as the peripheral height. The volume of the combustion chamber 15 when the piston 14 is at top dead center in a region radially inward of the virtual cylindrical surface VC is referred to as a central volume, and the volume of the combustion chamber 15 when the piston 14 is at top dead center in a region radially outward of the virtual cylindrical surface VC is referred to as a peripheral volume.

In the present embodiment, the upper surface of the piston 14 is configured as described above, and thus the transmission-side cylinder is higher than the normal cylinder with respect to the center height of the combustion chamber 15 when the piston 14 is at the top dead center. Further, with respect to the height of the periphery of the combustion chamber 15 when the piston 14 is at the top dead center, the transmission-side cylinder is lower than the normal cylinder.

Further, the volume of the combustion chamber 15 when the piston 14 is at the top dead center is substantially the same in all the cylinders. However, by configuring the upper surface of the piston 14 as described above, the center volume of the transmission-side cylinder is larger than the center volume of the normal cylinder. The peripheral volume of the transmission-side cylinder is smaller than that of the normal cylinder.

< action, Effect >

Next, the operation and effect of the internal combustion engine 1 of the present embodiment will be described with reference to fig. 9 to 11. Fig. 9 is a cross-sectional view schematically showing combustion of the air-fuel mixture in the combustion chamber 15 of the transmission-side cylinder. Fig. 10 is a diagram showing a relationship between a state in which the flame spreads and an area of the outer peripheral surface of the flame. Fig. 11 is a diagram showing a change in crank angle of the heat generation rate. In fig. 10 and 11, the solid line indicates the relationship and transition of the transmission-side cylinder, and the broken line indicates the relationship and transition of the normal cylinder.

Fig. 9(a) shows the state in the combustion chamber 15 immediately after ignition of the air-fuel mixture by the ignition plug 16. As shown in fig. 9, immediately after ignition of the air-fuel mixture, the flame F generated by the ignition spreads concentrically. In particular, in the transmission-side cylinder, the central groove 92 of the piston 14 is deep and the height of the combustion chamber 15 is high within the virtual cylindrical surface VC, so that the surface of the flame F (the outer peripheral surface of the flame on which combustion spreads) FS can be delayed from coming into contact with the wall surface of the piston 14. Therefore, as shown in fig. 10, in the region where the ratio of the flame radius to the cylinder diameter is 0.2 to 0.5, the area of the flame propagation surface FS of the transmission-side cylinder is larger than that of the flame propagation surface FS of the normal cylinder. As a result, as shown in fig. 11, the transmission-side cylinder is faster than the normal cylinder with respect to the maximum rising speed of the heat generation rate (dQ/d θ) in the region where the heat generation rate rises with a crank angle of about 0 to 5 degrees.

Fig. 9(B) shows the state in the combustion chamber 15 after the combustion of the air-fuel mixture in the combustion chamber 15 has progressed to some extent. The lower surface of the flame is in contact with the upper surface of the piston 14 (the upper surface of the central groove 92), and therefore the flame propagation surface FS progresses in the radial direction of the combustion chamber 15 as indicated by the arrow in the drawing. Here, as described above, in the transmission-side cylinder, the height of the combustion chamber 15 is lower in the region outside the virtual cylindrical surface VC. Therefore, as can be seen from fig. 9(B), when the combustion of the air-fuel mixture progresses to a certain extent, the area of the flame advance surface FS of the transmission-side cylinder is smaller than that of the normal cylinder. This can also be seen from the following: in fig. 10, in the region where the ratio of the flame radius to the cylinder diameter is 0.6 or more, the area of the flame advance surface FS of the transmission-side cylinder is smaller than that of the flame advance surface FS of the normal cylinder.

In particular, the flame propagation surface FS at the time when the heat generation rate becomes a peak usually reaches the outer side of the virtual cylindrical surface VC. Therefore, in the transmission-side cylinder, the height of the combustion chamber 15 is low before and after the timing at which the heat generation rate becomes a peak, and thus the area of the flame advancing face FS is small. As a result, as shown in fig. 11, in the vicinity of the region where the heat generation rate becomes a peak, the heat generation rate hardly increases in the transmission-side cylinder, and as a result, the maximum value of the heat generation rate of the transmission-side cylinder is lower than the maximum value of the heat generation rate of the normal cylinder.

As described above, the larger the maximum value of the heat generation rate is, the larger the exciting force accompanying combustion is. In the present embodiment, since the maximum value of the heat generation rate can be reduced in the transmission-side cylinder, the exciting force associated with combustion can be reduced in the transmission-side cylinder, and as a result, vibration noise can be reduced.

As can be seen from fig. 11, in the transmission-side cylinder, although the maximum value of the heat generation rate is lower than that in the normal cylinder, the period during which the heat generation rate is increased is the same in the transmission-side cylinder as in the normal cylinder. Therefore, the period in which the heat generation rate increases does not shift to the retard side as in the case of retarding the ignition timing, and therefore, the thermal efficiency hardly changes in the transmission side cylinder and the normal cylinder. Therefore, in the present embodiment, it is possible to suppress deterioration of fuel economy as in the case where the ignition timing is retarded. As described above, according to the present embodiment, it is possible to reduce vibration noise generated during operation of the internal combustion engine while suppressing deterioration of fuel economy.

< modification example >

In the above embodiment, the combustion chambers 15 of the two cylinders 13 of the fifth cylinder 13#5 and the sixth cylinder 13#6 are formed in the same shape in which the center height is relatively high and the peripheral height is relatively low, and the combustion chambers 15 of the other cylinders are formed in the same shape in which the center height is relatively low and the peripheral height is relatively high.

However, the combustion chamber 15 of only the sixth cylinder 13#6 may be formed in the same shape in which the center height is relatively high and the peripheral height is relatively low as the transmission-side cylinder, and the combustion chambers 15 of the other cylinders may be formed in the same shape in which the center height is relatively low and the peripheral height is relatively high as the normal cylinder. Alternatively, the combustion chambers 15 of the three cylinders 13 of the fourth cylinder 13#4 to the sixth cylinder 13#6 may be formed in the same shape in which the center height is relatively high and the peripheral height is relatively low as the transmission-side cylinder, and the combustion chambers 15 of the other cylinders may be formed in the same shape in which the center height is relatively low and the peripheral height is relatively high.

In the above embodiment, the internal combustion engine 1 is a V-type 6-cylinder engine, but the internal combustion engine 1 may be a straight-line type or a horizontally opposed type as described above, or may be an internal combustion engine having a different number of cylinders other than 6 cylinders, such as 3 cylinders, 4 cylinders, 8 cylinders, and 10 cylinders. For example, in an internal combustion engine having 4 cylinders arranged in series, the combustion chamber 15 of the fourth cylinder 13#4 closest to the transmission side is formed in the same shape as the transmission side cylinder in which the center height is relatively high and the peripheral height is relatively low, and the combustion chambers 15 of the other cylinders are formed in the same shape as the normal cylinder in which the center height is relatively low and the peripheral height is relatively high. Alternatively, the combustion chambers 15 of the third cylinder 13#3 and the fourth cylinder #4 on the transmission side may be formed in the same shape in which the center height is relatively high and the peripheral height is relatively low as the transmission side cylinder, and the combustion chambers 15 of the other cylinders may be formed in the same shape in which the center height is relatively low and the peripheral height is relatively high as the normal cylinder.

In short, the internal combustion engine 1 may be formed as desired as long as the combustion chamber 15 of the transmission-side cylinder located closest to the transmission among the plurality of cylinders is formed so that the center height is relatively high and the peripheral height is relatively low, and the combustion chamber 15 of the normal cylinder including the cylinder farthest from the transmission among the plurality of cylinders is formed so that the center height is relatively low and the peripheral height is relatively high.

In the above embodiment, the center height and the peripheral height change in 2 steps (2 steps of the first to fourth cylinders 13#1 to 13#4, the fifth cylinder 13#5, and the sixth cylinder 13# 6). However, the central height and the peripheral height may be changed in multiple stages. For example, the combustion chambers 15 of the third cylinder 13#3 and the fourth cylinder 13#4 are formed to have a lower central height and a higher peripheral height than the combustion chambers 15 of the fifth cylinder 13#5 and the sixth cylinder 13#6, and the combustion chambers 15 of the first cylinder 13#1 and the second cylinder 13#2 are formed to have a lower central height and a higher peripheral height than the combustion chambers 15 of the third cylinder 13#3 and the fourth cylinder 13# 4.

Alternatively, the combustion chambers 15 of all the cylinders 13 may be formed so that the central height and the peripheral height are different from each other. In this case, each combustion chamber 15 is formed such that the height of the center of the combustion chamber 15 of the transmission-side cylinder of the adjacent two cylinders is higher than the height of the center of the combustion chamber 15 of the transmission-side cylinder 13, and the height of the periphery of the combustion chamber 15 of the transmission-side cylinder of the adjacent two cylinders is lower than the height of the periphery of the combustion chamber 15 of the transmission-side cylinder 13.

In short, the combustion chamber 15 may be formed arbitrarily as long as the height of the center of the combustion chamber 15 of the transmission-side cylinder 13 of the two adjacent cylinders is equal to or greater than the height of the center of the combustion chamber of the cylinder on the opposite side of the transmission side, and the height of the periphery of the combustion chamber of the transmission-side cylinder of the two adjacent cylinders is equal to or less than the height of the periphery of the combustion chamber 15 of the cylinder 13 on the opposite side of the transmission side.

In the above embodiment, the cylinder head 12 is formed so that the portion thereof covering the cylinders 13 has the same shape between the cylinders 13. However, the cylinder head 12 may be formed in a different shape between the cylinders 13 as long as the combustion chamber 15 of the transmission-side cylinder located closest to the transmission among the plurality of cylinders is formed so that the center height is relatively high and the peripheral height is relatively low, and the combustion chamber 15 of the normal cylinder including the cylinder located farthest from the transmission among the plurality of cylinders is formed so that the center height is relatively low and the peripheral height is relatively high.

In the above embodiment, the transmission 80 is disposed adjacent to the engine body 10 of the internal combustion engine 1. However, instead of the transmission 80, a power train constituent member of a vehicle mounted with the internal combustion engine 1, which is different from the transmission 80 constituting a part of the power train, may be disposed adjacent to the engine main body 10. In this case, the power train constituent members are preferably housed in a case such as a transmission case. Examples of such a power train component include an electric motor, a generator, a motor generator, a power split mechanism of a hybrid vehicle, and a reduction gear. In summary, the internal combustion engine 1 can be said to be an internal combustion engine in which drive train components other than the internal combustion engine 1 are disposed adjacent to each other.