Precombustion chamber gas engine

Abstract

A precombustion chamber gas engine includes a main-chamber forming portion forming a main combustion chamber, a precombustion-chamber forming portion forming a precombustion chamber communicating with the main combustion chamber via a plurality of nozzle holes, and an ignition device disposed in the precombustion chamber and having an ignition portion spaced from a main chamber central axis of the main combustion chamber at a predetermined distance. In a plan view, the precombustion chamber has a near-ignition region including the ignition portion and a far-ignition region opposite to the near-ignition region separated by a borderline passing through a precombustion chamber central axis of the precombustion chamber and perpendicular to a straight line passing through the precombustion chamber central axis and the ignition portion. The distance between the precombustion chamber central axis and a precombustion-chamber-side opening end, connected to the precombustion chamber, of a specific far nozzle hole which is at least one nozzle hole in the far-ignition region is shorter or longer than the distance between the precombustion chamber central axis and a precombustion-chamber-side opening end of a specific near nozzle hole which is at least one nozzle hole in the near-ignition region.

Claims

1. A precombustion chamber gas engine comprising: a main-chamber forming portion forming a main combustion chamber; precombustion-chamber forming portion forming a precombustion chamber communicating with the main combustion chamber via a plurality of nozzle holes; and an ignition device disposed at an upper portion of an interior of the precombustion chamber and having an ignition portion spaced from a main chamber central axis of the main combustion chamber at a predetermined distance, wherein, in a plan view, the precombustion chamber has a near-ignition region in which the ignition portion is disposed and a far-ignition region opposite to the near-ignition region, the near-ignition region and the far-ignition region being separated by a borderline passing through a precombustion chamber central axis of the precombustion chamber and perpendicular to a straight line passing through the precombustion chamber central axis and the ignition portion, wherein the plurality of nozzle holes includes a specific far nozzle hole which is at least one nozzle hole in the far-ignition region and a specific near nozzle hole which is at least one nozzle hole in the near-ignition region, and a nozzle hole length of the specific far nozzle hole is shorter than a nozzle hole length of the specific near nozzle hole, and wherein the precombustion chamber is formed so as to deviate the precombustion chamber central axis from the main chamber central axis so that a distance between the main chamber central axis and a precombustion-chamber-side opening end of the specific far nozzle hole is longer than a distance between the main chamber central axis and a precombustion-chamber-side opening end of the specific near nozzle hole in the precombustion-chamber forming portion.

2. The precombustion chamber gas engine according to claim 1, wherein the specific near nozzle hole includes a nearest nozzle hole closest to the ignition portion among the plurality of nozzle holes in the near-ignition region in a plan view, and wherein the specific far nozzle hole includes a farthest nozzle hole farthest from the ignition portion among the plurality of nozzle holes in the far-ignition region in a plan view.

3. The precombustion chamber gas engine according to claim 1, wherein the precombustion chamber is formed so as to deviate the precombustion chamber central axis from the main chamber central axis so that a distance between the main chamber central axis and a precombustion-chamber-side opening end of the specific far nozzle hole is longer than a distance between the main chamber central axis and a precombustion-chamber-side opening end of the specific near nozzle hole in the precombustion-chamber forming portion.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic cross-sectional view of a precombustion chamber gas engine according to an embodiment of the present invention, where the distance between the precombustion chamber central axis and a specific far nozzle hole is shorter than the distance between the precombustion chamber central axis and a specific near nozzle hole.

(2) FIG. 2 is a schematic cross-sectional view of a precombustion chamber gas engine taken along line aa in FIG. 1 and shows a cross-section of a precombustion-chamber forming portion of FIG. 1.

(3) FIG. 3 is a schematic cross-sectional view of a precombustion chamber gas engine according to an embodiment of the present invention, where the distance between the precombustion chamber central axis and a specific near nozzle hole is shorter than the distance between the precombustion chamber central axis and a specific far nozzle hole.

(4) FIG. 4 is a schematic cross-sectional view of a precombustion chamber gas engine taken along line aa in FIG. 3 and shows a cross-section of a precombustion-chamber forming portion of FIG. 3.

(5) FIG. 5 is a schematic cross-sectional view of a precombustion chamber gas engine according to an embodiment of the present invention, where the nozzle hole length of a specific far nozzle hole is shorter than the nozzle hole length of a specific near nozzle hole.

(6) FIG. 6 is a schematic cross-sectional view of a precombustion chamber gas engine taken along line aa in FIG. 5 and shows a cross-section of a precombustion-chamber forming portion according to an embodiment of the present invention.

(7) FIG. 7 is a schematic cross-sectional view of a precombustion chamber gas engine according to an embodiment of the present invention, which is a modified example of the shape of the precombustion chamber of FIG. 5.

(8) FIG. 8 is a schematic cross-sectional view of a precombustion chamber gas engine taken along line aa in FIG. 7 and shows a cross-section of a precombustion-chamber forming portion according to an embodiment of the present invention.

(9) FIG. 9 is a schematic cross-sectional view of a precombustion chamber gas engine according to an embodiment of the present invention, where a precombustion-chamber-side opening end of a specific far nozzle hole has a bell-mouth shape.

DETAILED DESCRIPTION

(10) Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention.

(11) For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function.

(12) For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function.

(13) Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved.

(14) On the other hand, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components.

(15) FIG. 1 is a schematic cross-sectional view of a precombustion chamber gas engine 1 according to an embodiment of the present invention, where the distance rf between the precombustion chamber central axis Cs and a specific far nozzle hole 43 is shorter than the distance between the precombustion chamber central axis Cs and a specific near nozzle hole 42. FIG. 2 is a schematic cross-sectional view of a precombustion chamber gas engine taken along line aa in FIG. 1 and shows a cross-section of a precombustion-chamber forming portion 3 of FIG. 1. FIG. 3 is a schematic cross-sectional view of a precombustion chamber gas engine 1 according to an embodiment of the present invention, where the distance rn between the precombustion chamber central axis Cs and a specific near nozzle hole 42 is shorter than the distance between the precombustion chamber central axis Cs and a specific far nozzle hole 43. FIG. 4 is a schematic cross-sectional view of a precombustion chamber gas engine 1 taken along line aa in FIG. 3 and shows a cross-section of a precombustion-chamber forming portion 3 of FIG. 3. FIG. 5 is a schematic cross-sectional view of a precombustion chamber gas engine 1 according to an embodiment of the present invention, where the nozzle hole length Lf of a specific far nozzle hole 43 is shorter than the nozzle hole length Ln of a specific near nozzle hole 42. FIG. 6 is a schematic cross-sectional view of a precombustion chamber gas engine 1 taken along line aa in FIG. 5 and shows a cross-section of a precombustion-chamber forming portion 3 according to an embodiment of the present invention. FIG. 7 is a schematic cross-sectional view of a precombustion chamber gas engine 1 according to an embodiment of the present invention, which is a modified example of the shape of the precombustion chamber 3r of FIG. 5. FIG. 8 is a schematic cross-sectional view of a precombustion chamber gas engine 1 taken along line aa in FIG. 7 and shows a cross-section of a precombustion-chamber forming portion 3 according to an embodiment of the present invention. FIG. 9 is a schematic cross-sectional view of a precombustion chamber gas engine 1 according to an embodiment of the present invention, where a precombustion-chamber-side opening end 41s of a specific far nozzle hole 43 has a bell-mouth shape.

(16) As shown in FIGS. 1 to 9, the precombustion chamber gas engine 1 includes a main-chamber forming portion 2 forming a main combustion chamber 2r, a precombustion-chamber forming portion 3 forming a precombustion chamber 3r, and an ignition device 5 disposed in the precombustion chamber 3r.

(17) More specifically, in the embodiments shown in FIGS. 1 to 9, as shown in FIGS. 1, 3, 5, and 7, the precombustion chamber gas engine 1 includes: a cylinder 13 including a cylinder liner 11 having therein a cylinder structure of cylindrical shape and a cylinder head 12 having therein a recessed structure capable of capping the top of the cylinder structure; a piston 14 reciprocably disposed in the cylinder 13; an intake port 15 and an exhaust port 16 connected around the cylinder head 12; an intake valve 17 for opening and closing the intake port 15; an exhaust valve 18 for opening and closing the exhaust port 16; and a precombustion chamber cap 19. A main combustion chamber 2r (main chamber) is defined between the cylinder 13 and the piston 14. A precombustion chamber 3r is formed by the precombustion chamber cap 19 disposed on the cylinder head 12 so as to be positioned above the main combustion chamber 2r (opposite to the piston 14). That is, in the embodiments shown in FIGS. 1 to 9, the cylinder 13 and the piston 14 form the main-chamber forming portion 2 and the precombustion chamber cap 19 forms the precombustion-chamber forming portion 3. Additionally, the precombustion chamber gas engine 1 further includes a precombustion-chamber-gas supply device 6 for supplying a precombustion chamber fuel gas to the precombustion chamber 3r not via the main combustion chamber 2r, and the supply of the precombustion chamber fuel gas to the precombustion chamber 3r is controlled by a precombustion-chamber-fuel-gas supply valve 61 (see FIGS. 1, 3, 5, and 7).

(18) Further, the ignition device 5 has an ignition portion 51 capable of igniting an air-fuel mixture, and is disposed on an engine so that the ignition portion 51 is spaced from the central axis of the main combustion chamber 2r (hereinafter, main chamber central axis Cm) at a predetermined distance D. For instance, the ignition device 5 may be an ignition plug of a spark ignition type. In this case, the ignition portion 51 is a portion including an electrode which generates a spark. The predetermined distance D may be larger than 10% of a maximum value of the inner diameter of the main combustion chamber 2r. As described later, in a case where the precombustion chamber 3r includes a small-diameter cylinder chamber 31r and a large-diameter cylinder chamber 33r, the ignition portion 51 may be disposed in the large-diameter cylinder chamber 33r (constant diameter cylinder portion 33c), and the predetermined distance D may be a maximum value of the inner diameter of the large-diameter cylinder chamber 33r.

(19) On the other hand, the precombustion-chamber forming portion 3 includes a plurality of nozzle holes 4 connecting the precombustion chamber 3r formed in the precombustion-chamber forming portion 3 to the outside. The main combustion chamber 2r communicates with the precombustion chamber 3r via the plurality of nozzle holes 4. More specifically, each of the nozzle holes 4 has a main-chamber-side opening end 41m connected to the main combustion chamber 2r on one side and a precombustion-chamber-side opening end 41s connected to the precombustion chamber 3r on the other side. The precombustion-chamber-side opening end 41s may be connected to the precombustion chamber 3r above a bottom portion 35 including a portion furthest from the top portion where the ignition portion 51 is disposed (see FIGS. 1, 3, 5, and 7). Further, as shown in FIG. 2, the plurality of nozzle holes 4 are arranged around the central axis of the precombustion chamber 3r (hereinafter, precombustion chamber central axis Cs) at an equal interval, and each of the nozzle holes 4 is formed so as to penetrate the precombustion-chamber forming portion 3 linearly along the radial direction of the precombustion chamber central axis Cs, in a plan view. For instance, the cross-sectional shape (shape in plan view) of the precombustion-chamber forming portion 3 (precombustion chamber cap 19) having the precombustion chamber central axis Cs as a normal is circular. In a case where the precombustion-chamber forming portion 3 has, for instance, six linear nozzle holes 4, the nozzle holes 4 are disposed along the radial direction and arranged at an equal interval of 60° around the precombustion chamber central axis Cs (see FIG. 2). Further, each of the nozzle holes 4 extends linearly at a predetermined angle θ of depression (described later) (see FIGS. 1, 3, 5, and 7). The angle θ of depression of the nozzle hole 4 is an angle between a line perpendicular to the main chamber central axis Cm and the central line Cp of the nozzle hole 4.

(20) In the embodiments shown in FIGS. 1 to 9, the precombustion chamber 3r includes a small-diameter-cylinder forming portion 31 forming a small-diameter cylinder chamber 31r having a predetermined inner diameter and connected to the plurality of nozzle holes 4 and a large-diameter-cylinder forming portion 33 forming a large-diameter cylinder chamber 33r which has an inner diameter larger than that of the small-diameter cylinder chamber 31r and in which the ignition portion 51 of the ignition device 5 is disposed. In other words, the precombustion-chamber forming portion 3 (precombustion chamber cap 19) includes a small-diameter-cylinder forming portion 31 and a large-diameter-cylinder forming portion 33. Further, the large-diameter-cylinder forming portion 33 includes a cylindrical enlarged diameter cylinder portion 33d connected to the small-diameter cylinder chamber 31r and having an inner diameter increasing with a distance from the small-diameter cylinder chamber 31r and a cylindrical constant diameter cylinder portion 33c connected to the enlarged diameter cylinder portion 33d and having an inner diameter equal to the maximum diameter of the enlarged diameter cylinder portion 33d. Further, the precombustion chamber central axis Cs coincides with the central axis of the small-diameter cylinder chamber 31r.

(21) However, the present invention is not limited to the present embodiments. In some embodiments, the precombustion chamber 3r may have other shape, for instance, a cylindrical shape having a constant inner diameter.

(22) Further, in some embodiments, as shown in FIGS. 1 to 4, the main chamber central axis Cm may coincide with the precombustion chamber central axis Cs. Alternatively, in some embodiments, as shown in FIGS. 5 to 8, the main chamber central axis Cm may not coincide with the precombustion chamber central axis Cs, such as a case where the precombustion chamber central axis Cs is oblique to the main chamber central axis Cm. Further, the central axis of the small-diameter cylinder chamber 31r may not coincide with the central axis of the large-diameter cylinder chamber 33r.

(23) The precombustion chamber gas engine 1 having the above configuration opens the intake valve 17 and closes the exhaust valve 18 when the piston 14 moves downward at the intake stroke, for instance. As the intake valve 17 opens, a lean premixed gas containing a mixture of a fuel gas and air is introduced into the cylinder 13 through the intake port 15 connected to the intake valve 17. As the precombustion-chamber-fuel-gas supply valve 61 opens, a precombustion chamber fuel gas is introduced into the precombustion chamber 3r. Furthermore, at the compression stroke, the precombustion-chamber-fuel-gas supply valve 61 closes when the piston 14 moves upward. Further, the lean premixed gas introduced into the cylinder 13 through the intake port 15 is compressed as the piston 14 moves upward, and a part of the lean premixed gas is introduced into the precombustion chamber 3r through each of the nozzle holes 4 of the precombustion chamber 3r. The lean premixed gas introduced from the main combustion chamber 2r to the precombustion chamber 3r is mixed with the precombustion chamber fuel gas to produce an air-fuel mixture having a concentration suitable for ignition in the precombustion chamber 3r. The air-fuel mixture in the precombustion chamber 3r is ignited by the ignition device 5 at a predetermined timing when the piston 14 arrives at the vicinity of the compression top dead center, and the air-fuel mixture in the precombustion chamber 3r is thus combusted. A combustion flame caused by this combustion is injected into the cylinder 13 through each of the nozzle holes and ignites the lean premixed gas in the cylinder 13, which leads to combustion of the lean premixed gas in the main combustion chamber 2r.

(24) At this time, the present inventors have found through numerical analysis and the like that in the precombustion chamber gas engine 1 according to the present invention, since the ignition portion 51 of the ignition device 5 is spaced from the main chamber central axis Cm at a predetermined distance D, the timing at which the combustion flame produced by ignition of the air-fuel mixture in the precombustion chamber by the ignition portion 51 reaches each of the nozzle holes 4 varies. Further, they found that, if the timing of arrival of the combustion flame produced by ignition by the ignition device varies among the nozzle holes 4 and if, for instance, the nozzle holes 4 have the same shape, variation occurs in injection start timing at which combustion flame (hereinafter, referred to as torch jet as appropriate) is injected to the main combustion chamber 2r from each of the nozzle holes 4 through a corresponding main-chamber-side opening end 41m and in injection strength thereof. If injection start timing and injection strength of torch jet from each of the nozzle holes 4 vary, flame propagation in the main combustion chamber 2r also varies. The variation of flame propagation in the main combustion chamber 2r causes unburned gas to remain in a region where flame propagation delays, which may cause knocking and degrade thermal efficiency and output power of the precombustion chamber gas engine 1.

(25) In view of this, the present inventors have arrived at suppressing the variation of flame propagation in the main combustion chamber 2r by changing the shape of at least one nozzle hole 4 of the plurality of nozzle holes 4 so as to suppress the variation of injection start timing and injection strength of torch jet injected to the main combustion chamber 2r from each of the nozzle holes 4.

(26) The shape of the plurality of nozzle holes 4 in some embodiments of the present invention will now be described.

(27) In the present invention, in a plan view as shown in FIG. 2, the precombustion chamber 3r has a near-ignition region Rn in which the ignition portion 51 is disposed and a far-ignition region Rf opposite to the near-ignition region Rn separated by a borderline Br passing through the precombustion chamber central axis Cs of the precombustion chamber 3r and perpendicular to a straight line (reference line Ba) passing through the precombustion chamber central axis Cs and the ignition portion 51. In other words, in a plan view, the precombustion chamber 3r is divided by the borderline Br into two regions of a near-ignition region Rn relatively close to the ignition portion 51 and a far-ignition region Rf relatively far from the ignition portion 51 having the same size.

(28) First, embodiments relating to connection position between the precombustion-chamber-side opening end 41s of the nozzle hole 4 and the precombustion chamber 3r will be described with reference to FIGS. 1 to 4.

(29) In some embodiments, as shown in FIGS. 1 and 2, a distance rf between the precombustion chamber central axis Cs and a precombustion-chamber-side opening end 41s, connected to the precombustion chamber 3r, of a specific far nozzle hole 43 which is at least one nozzle hole 4 in the far-ignition region Rf is shorter than a distance rn between the precombustion chamber central axis Cs and a precombustion-chamber-side opening end 41s of a specific near nozzle hole 42 which is at least one nozzle hole 4 in the near-ignition region Rn (rf<rn).

(30) Alternatively, in some embodiments, as shown in FIGS. 3 and 4, a distance rn between the precombustion chamber central axis Cs and a precombustion-chamber-side opening end 41s, connected to the precombustion chamber 3r, of a specific near nozzle hole 42 which is at least one nozzle hole 4 in the near-ignition region Rn is shorter than a distance rf between the precombustion chamber central axis Cs and a precombustion-chamber-side opening end 41s of a specific far nozzle hole 43 which is at least one nozzle hole 4 in the far-ignition region Rf (rf>rn).

(31) In the embodiments shown in FIGS. 1 to 9, the precombustion-chamber-side opening end 41s of each of the plurality of nozzle holes 4, or the precombustion-chamber-side opening end 41s of each of the nozzle holes 4 other than the specific near nozzle hole 42 or the specific far nozzle hole 43 is connected to the precombustion chamber 3r at a position above the bottom portion 35 of the precombustion-chamber forming portion 3 (precombustion chamber 3r) (see FIGS. 1, 3, 5, 7, and 9).

(32) Here, the present inventors have found that the combustion flame produced by ignition by the ignition device 5 reaches the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42, a portion (center of bottom portion 35 of precombustion chamber 3r) through which the precombustion chamber central axis Cs passes, and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 in this order. Further, the present inventors have found that, by forming the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 adjacent to the center of the bottom portion of the precombustion chamber 3r, it is possible to advance the timing at which the combustion flame produced by ignition by the ignition device 5 reaches the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43. Conversely, the present inventors have found that, by forming the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 adjacent to the center of the bottom portion of the precombustion chamber 3r, it is possible to delay the timing at which the combustion flame produced by ignition by the ignition device 5 reaches the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42.

(33) Thus, by bringing the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 or the specific far nozzle hole 43 close to the center of the bottom portion of the precombustion chamber 3r, it is possible to reduce the difference between timings at which the combustion flame produced by ignition by the ignition device 5 reaches the respective precombustion-chamber-side opening ends 41s of the specific near nozzle hole 42 and the specific far nozzle hole 43. Thereby, it is possible to adjust and equalize injection start timing and injection strength of combustion flames (torch jets) injected from respective main-chamber-side opening ends 41m of the specific near nozzle hole 42 and the specific far nozzle hole 43, and it is possible to suppress the variation of flame propagation in the main combustion chamber 2r. Further, by suppressing the variation of flame propagation in the main combustion chamber 2r, it is possible to suppress knocking caused by delay of flame arrival, and it is possible to improve the engine efficiency.

(34) More specifically, in some embodiments, as shown in FIG. 2, the specific near nozzle hole 42 may be a nearest nozzle hole 42s which is a nozzle hole 4 closest to the ignition portion 51 in the near-ignition region Rn in a plan view; and the specific far nozzle hole 43 may be a farthest nozzle hole 43s which is a nozzle hole 4 farthest from the ignition portion 51 in the far-ignition region Rf in a plan view. In the embodiments shown in FIGS. 1 to 3, three nozzle holes 4 (4a to 4c) exist in the near-ignition region Rn, and the specific near nozzle hole 42 is one nozzle hole 4b (nearest nozzle hole 42s) extending along the reference line Ba in the near-ignition region Rn. On the other hand, three nozzle holes 4 (4d to 4f) exist also in the far-ignition region Rf, and the specific far nozzle hole 43 is one nozzle hole 4e (farthest nozzle hole 43s) extending along the reference line Ba in the far-ignition region Rf.

(35) Further, in the embodiment shown in FIGS. 1 and 2, as depicted, the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 is shorter than the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 (rf<rn). Conversely, in the embodiment shown in FIGS. 3 and 4, the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 is shorter than the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 (rf>rn). The distance rn may be a distance between the central line Cp of the nozzle hole 4 in the precombustion-chamber-side opening end 41s and the precombustion chamber central axis Cs.

(36) However, the present invention is not limited to the present embodiments. The specific near nozzle hole 42 may be one or more, a plurality of nozzle holes 4, and the specific far nozzle hole 43 may be one or more, a plurality of nozzle holes 4. In a case where the specific far nozzle hole 43 includes a plurality of nozzle holes 4, they may be arranged so that a nozzle hole 4 with a later arrival timing has a precombustion-chamber-side opening end 41s closer to the precombustion chamber central axis Cs, in accordance with the timing of arrival of the combustion flame produced in the precombustion chamber 3r. Moreover, in a case where the specific near nozzle hole 42 includes a plurality of nozzle holes 4, they may be arranged so that a nozzle hole 4 with an earlier arrival timing has a precombustion-chamber-side opening end 41s closer to the precombustion chamber central axis Cs, in accordance with the timing of arrival of the combustion flame produced in the precombustion chamber 3r. Further, the nearest nozzle hole 42s may not be the specific near nozzle hole 42. Similarly, the farthest nozzle hole 43s may not be the specific far nozzle hole 43.

(37) With the above configuration, in a plan view of the precombustion chamber 3r, the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the nozzle hole (farthest nozzle hole 43s) having the main-chamber-side opening end 41m farthest from the ignition portion 51 is shorter than the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the nozzle hole (nearest nozzle hole 42s) having the main-chamber-side opening end 41m closest to the ignition portion 51. Thereby, it is possible to adjust and advance the timing at which the combustion flame produced by ignition by the ignition device 5 reaches the farthest nozzle hole 43s. Thus, it is possible to bring injection start timing of the farthest nozzle hole 43s close to injection start timing of the nearest nozzle hole 42s which the combustion flame reaches earliest. Consequently, it is possible to suppress the variation of inject start timing at which the flame is injected to the main combustion chamber 2r from each of the nozzle holes 4, and thus it is possible to efficiently suppress the variation of flame propagation in the main combustion chamber 2r.

(38) Further, in some embodiments, as shown in FIG. 2, the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 is shorter than an average of distances between the precombustion chamber central axis Cs and the respective precombustion-chamber-side opening ends 41s of the plurality of nozzle holes 4. In the embodiment shown in FIG. 2, the specific far nozzle hole 43 is the farthest nozzle hole 43s, and the respective precombustion-chamber-side opening ends 41s of the other five nozzle holes 4 have the same distance rn from the precombustion chamber central axis Cs. Further, the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 (farthest nozzle hole 43s) is smaller than the distance (distance rn in FIG. 2) between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of each of the other five nozzle holes 4, and is shorter than an average of the distances between the precombustion chamber central axis Cs and the respective precombustion-chamber-side opening ends 41 of the six nozzle holes 4.

(39) With the above configuration, by making the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 shorter than the average, the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 is made shorter than the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42.

(40) In some embodiments, as shown in FIG. 4, the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 is shorter than an average of distances between the precombustion chamber central axis Cs and the respective precombustion-chamber-side opening ends 41s of the plurality of nozzle holes 4. In the embodiment shown in FIG. 4, the specific near nozzle hole 42 is the nearest nozzle hole 42s, and the respective precombustion-chamber-side opening ends 41s of the other five nozzle holes 4 have the same distance rf from the precombustion chamber central axis Cs. Further, the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 (nearest nozzle hole 42s) is smaller than the distance (distance if in FIG. 4) between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of each of the other five nozzle holes 4, and is shorter than an average of the distances between the precombustion chamber central axis Cs and the respective precombustion-chamber-side opening ends 41 of the six nozzle holes 4.

(41) With the above configuration, by making the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 shorter than the average, the distance rn between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 is made shorter than the distance rf between the precombustion chamber central axis Cs and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 (rn<rf).

(42) Further, in the embodiments as shown in FIGS. 1 to 4, in some embodiments, the angle θf of depression of the specific far nozzle hole 43 is equal to the angle θn of depression of the specific near nozzle hole 42. Thus, since the angle θf of depression of the specific far nozzle hole 43 is equal to the angle θn of depression of the specific near nozzle hole 42, it is possible to reduce the difference between timings at which the combustion flame produced by ignition by the ignition device 5 reaches the respective precombustion-chamber-side opening ends 41s of the specific near nozzle hole 42 and the specific far nozzle hole 43, while suppressing the influence on a time required for the torch jet injected from each of the nozzle holes 4 to propagate to an cylinder wall surface 13w.

(43) Second, embodiments relating to shape of the precombustion chamber 3r formed in the precombustion-chamber forming portion 3 (precombustion-chamber cap 19) will be described with reference to FIGS. 5 to 8.

(44) In some embodiments, as shown in FIGS. 5 to 8, a nozzle hole length Lf of a specific far nozzle hole 43 which is at least one nozzle hole 4 in the far-ignition region Rf is shorter than a nozzle hole length Ln of a specific near nozzle hole 42 which is at least one nozzle hole 4 in the near-ignition region Rn. As the nozzle hole length L increases, the combustion flame needs time to pass through the nozzle hole 4. Accordingly, in the precombustion chamber 3r, by setting the nozzle hole length Lf of the specific far nozzle hole 43 which the combustion flame produced by ignition by the ignition device 5 reaches later to be shorter than the nozzle hole length Ln of the specific near nozzle hole 42 which the combustion flame reaches earlier, it is possible to adjust and equalize injection start timing and injection strength of torch jets injected to the main combustion chamber 2r from the respective main-chamber-side opening ends 41m of the specific far nozzle hole 43 and the specific near nozzle hole 42, and it is possible to suppress the variation of flame propagation in the main combustion chamber 2r. Further, by suppressing the variation of flame propagation in the main combustion chamber 2r, it is possible to suppress knocking caused by delay of flame arrival, and it is possible to improve the engine efficiency.

(45) More specifically, in some embodiments, as shown in FIGS. 5 to 8, the precombustion chamber 3r is formed so as to deviate the precombustion chamber central axis Cs from the main chamber central axis Cm so that a distance df between the main chamber central axis Cm and the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 is longer than a distance do between the main chamber central axis Cm and the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42 in the precombustion-chamber forming portion 3. In the embodiments shown in FIGS. 5 to 8, the dotted line shows a case where an outer peripheral surface 36 of the small-diameter-cylinder forming portion 31 is concentric with an inner peripheral surface 37 defining the precombustion chamber 3r (precombustion chamber central axis Cs coincides with the main chamber central axis Cm).

(46) Further, in the embodiment shown in FIGS. 5 and 6, the precombustion chamber 3r (small-diameter cylinder chamber 31r) shown by the solid line is formed so that the center of the inner peripheral surface 37 (precombustion chamber central axis Cs of small-diameter cylinder chamber 31r) is displaced toward the far-ignition region Rf while the diameter of the inner peripheral surface 37 having a circular shape is the same as that shown by the dotted line in a plan view (FIG. 6).

(47) On the other hand, in the embodiment shown in FIGS. 7 and 8, the precombustion chamber 3r (small-diameter cylinder chamber 31r) shown by the solid line is formed so that the diameter of the inner peripheral surface 37 is larger than that shown by the dotted line in a plan view (FIG. 8). More specifically, as shown in FIG. 8, while the position of the farthest nozzle hole 43s is fixed, the center of the inner peripheral surface 37 (precombustion chamber central axis Cs of small-diameter cylinder chamber 31r) is shifted along the central line Cp of the farthest nozzle hole 43s to make the diameter of the inner peripheral surface 37 larger than that shown by the dotted line.

(48) Accordingly, as shown in FIGS. 5 to 8, the wall thickness (distance between outer peripheral surface 36 and inner peripheral surface 37) of the precombustion-chamber forming portion 3 on the side of the far-ignition region Rf is thinner than the wall thickness of the precombustion-chamber forming portion 3 on the side of the near-ignition region Rn by the amount of displacement of the precombustion chamber central axis Cs from the main chamber central axis Cm. Conversely, the wall thickness (distance between outer peripheral surface 36 and inner peripheral surface 37) of the precombustion-chamber forming portion 3 on the side of the near-ignition region Rn is thicker than the wall thickness of the precombustion-chamber forming portion 3 on the side of the far-ignition region Rf by the amount of displacement of the precombustion chamber central axis Cs from the main chamber central axis Cm. That is, the nozzle hole length Lf of the specific far nozzle hole 43 is shorter than the nozzle hole length Ln of the specific near nozzle hole 42.

(49) In the embodiments shown in FIGS. 5 to 8, as shown in FIGS. 6 and 8, among the plurality of nozzle holes 4 disposed in the far-ignition region Rf (three nozzle holes 4d to 4f in FIGS. 6 and 8), a nozzle hole farther from the ignition portion 51 has a shorter nozzle hole length Lf in a plan view. That is, the farthest nozzle hole 43s has the shortest nozzle hole length Lf Conversely, among three nozzle holes 4 disposed in the near-ignition region Rn (three nozzle holes 4a to 4c in FIGS. 6 and 8), a nozzle hole farther from the ignition portion 51 has a longer nozzle hole length Lf in a plan view. That is, the nearest nozzle hole 42s has the shortest nozzle hole length Ln. In the examples of FIGS. 6 and 8, the nozzle hole length of the nearest nozzle hole 42s (nozzle hole 4e) shown by the solid line is equal to the nozzle hole length of the nozzle hole 4e shown by the dotted line.

(50) With the above configuration, for instance, by forming the precombustion chamber 3r to be offset toward the specific far nozzle hole 43 so as to deviate the precombustion chamber central axis Cs from the main chamber central axis Cm in the precombustion-chamber forming portion 3, the wall thickness of the precombustion chamber 3r formed by the precombustion-chamber forming portion 3 becomes thinner in the vicinity of the specific far nozzle hole 43 than in the vicinity of the specific near nozzle hole 42. Thus, the nozzle hole length Lf of the specific far nozzle hole 43 is made shorter than the nozzle hole length Ln of the specific near nozzle hole 42.

(51) Third, embodiments relating to shape of the precombustion-chamber-side opening end 41s of the nozzle hole 4 will be described with reference to FIG. 9.

(52) In some embodiments, as shown in FIG. 9, a specific far nozzle hole 43 which is at least one nozzle hole 4 in the far-ignition region Rn has a precombustion-chamber-side opening end 41s of a shape (bell-mouth shape) having a diameter decreasing with a distance from the connection position with the precombustion chamber 3r. That is, the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 is shaped so as to decrease inlet pressure loss with respect to the combustion flame produced by ignition by the ignition device 5. In the embodiment shown in FIG. 9, the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 has a predetermined curvature Cf, and the diameter thereof decreases in accordance with the curvature Cf from the connection position with the precombustion chamber 3r. For instance, in some embodiments, a precombustion-chamber-side opening end 41s of a specific near nozzle hole 42 which is at least one nozzle hole 4 in the near-ignition region may also have a diameter decreasing in accordance with a curvature Cn from the connection position with the precombustion chamber 3r. In this case, the curvature Cf of the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 is larger than the curvature Cn of the specific near nozzle hole 42. That is, the inlet pressure loss of the combustion flame produced by ignition by the ignition device 5 is smaller at the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 than at the precombustion-chamber-side opening end 41s of the specific near nozzle hole 42. This suppresses deceleration of the combustion flame produced by ignition by the ignition device 5 when the combustion flame flows into the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43.

(53) Thereby, it is possible to increase the flow velocity of the torch jet injected to the main combustion chamber 2r from the specific far nozzle hole 43, and thus it is possible to adjust and equalize the flow velocity of the torch jet from the specific far nozzle hole 43 and the flow velocity of the torch jet injected from the specific near nozzle hole 42 to the main combustion chamber 2r. Consequently, it is possible to suppress the variation of flame propagation in the main combustion chamber 2r. Further, by suppressing the variation of flame propagation in the main combustion chamber 2r, it is possible to suppress knocking caused by delay of flame arrival, and it is possible to improve the engine efficiency.

(54) Further, in some embodiments, the specific far nozzle hole 43 may be the farthest nozzle hole 43s. That is, in this case, the precombustion-chamber-side opening end 41s of the farthest nozzle hole 43s has a bell-mouth shape. Thereby, in the precombustion chamber 3r, it is possible to adjust and equalize injection start timing and injection strength of torch jets injected from the nearest nozzle hole 42s which the combustion flame produced by ignition by the ignition device 5 reaches the earliest and the farthest nozzle hole 43s which the combustion flame reaches the latest, and it is possible to efficiently suppress the variation of flame propagation in the main combustion chamber 2r.

(55) The present invention is not limited to the embodiments described above, but includes modifications to the embodiments described above, and embodiments composed of combinations of those embodiments.

(56) For instance, in some embodiments, the precombustion-chamber-side opening end 41s of the specific far nozzle hole 43 in the embodiments relating to connection position between the precombustion-chamber-side opening end 41s of the nozzle hole 4 and the precombustion chamber 3r as shown in FIGS. 1 to 4 or in the embodiments relating to shape of the precombustion chamber 3r as shown in FIGS. 5 to 8 may have a bell-mouth shape.

(57) Further, although in the embodiments shown in FIGS. 1 to 4 and 9, the angle θf of depression of the specific far nozzle hole 43 is equal to the angle θn of depression of the specific near nozzle hole 42 (θn=θf), the present invention is not limited to the present embodiments. In some embodiments, the angle θf of depression of the specific far nozzle hole 43 may not be equal to the angle θn of depression of the specific near nozzle hole 42. Further, although in the embodiments as shown in FIGS. 5 to 8, the angle θf of depression of the specific far nozzle hole 43 is different from the angle θn of depression of the specific near nozzle hole 42 (θn<θf), the present invention is not limited to the present embodiments. In some embodiments, the angle θf of depression of the specific far nozzle hole 43 may be equal to the angle θn of depression of the specific near nozzle hole 42. Here, as the angle θ of depression of the nozzle hole 4 increases, the flame propagation distance of the combustion flame from the main-chamber-side opening end 41m of the nozzle hole 4 to the cylinder wall surface 13w increases, and it takes longer time for flame propagation to the cylinder wall surface 13w. Therefore, in the precombustion chamber 3r, by setting the angle θn of depression of the specific near nozzle hole 42 which the combustion flame produced by ignition of an air-fuel mixture by the ignition device 5 reaches earlier to be larger than the angle θf of depression of the specific far nozzle hole 43 which the combustion flame reaches later, it is possible to adjust and align arrival timing of the combustion flame to the cylinder wall surface 13w, and it is possible to suppress the variation of flame propagation in the main combustion chamber 2r.

REFERENCE SIGNS LIST

(58) 1 Precombustion chamber gas engine 11 Cylinder liner 12 Cylinder head 13 Cylinder 13w Cylinder wall surface 14 Piston 15 Intake port 16 Exhaust port 17 Intake valve 18 Exhaust valve 19 Precombustion chamber cap 2 Main-chamber forming portion 2r Main combustion chamber 3 Precombustion-chamber forming portion 3r Precombustion chamber 31 Small-diameter-cylinder forming portion 31r Small-diameter cylinder chamber 33 Large-diameter-cylinder forming portion 33c Constant diameter cylinder portion 33d Enlarged diameter cylinder portion 33r Large-diameter cylinder chamber 35 Bottom portion 36 Outer peripheral surface 37 Inner peripheral surface 4 Nozzle hole 41m Main-chamber-side opening end 41s Precombustion-chamber-side opening end 42 Specific near nozzle hole 42s Nearest nozzle hole 43 Specific far nozzle hole 43s Farthest nozzle hole 5 Ignition device 51 Ignition portion 6 Precombustion-chamber-gas supply device 61 Precombustion-chamber-fuel-gas supply valve Rf Far-ignition region Rn Near-ignition region D predetermined distance between ignition portion and main chamber central axis Cm Main chamber central axis Cp Central line of nozzle hole Cs Precombustion chamber central axis Ba Reference line Br Borderline Lf Nozzle hole length of specific far nozzle hole Ln Nozzle hole length of specific near nozzle hole df Distance (between main chamber central axis and precombustion-chamber-side opening end of specific far nozzle hole) dn Distance (between main chamber central axis and precombustion-chamber-side opening end of specific near nozzle hole) rf Distance (between precombustion chamber central axis and precombustion-chamber-side opening end of specific far nozzle hole) rn Distance (between precombustion chamber central axis and precombustion-chamber-side opening end of specific near nozzle hole) Cf Curvature of precombustion-chamber-side opening end of specific far nozzle hole Cn Curvature of precombustion-chamber-side opening end of specific near nozzle hole