Precombustion-chamber engine

1. A precombustion-chamber engine, comprising:

a cylinder;

a cylinder head disposed on a top of the cylinder; and

a piston reciprocably disposed within the cylinder, the piston and the cylinder head defining a main combustion chamber therebetween,

the cylinder head including a precombustion-chamber forming part which defines a precombustion chamber communicating with the main combustion chamber through a nozzle,

the precombustion chamber including:

a cylindrical first passage part extending upwardly from the nozzle;

a second passage part extending upwardly from the first passage part and having an upwardly-increasing cross-sectional area; and

a cylindrical space part which extends upwardly from the second passage part and in which a spark plug is disposed,

wherein a center O is eccentric with respect to a straight line L, where the straight line L, is composed of an axis of the first passage part and an extended line of the axis, and the center O is a center of a cross-section, orthogonal to the straight line L, of the second passage part,

wherein, the precombustion-chamber engine further comprises an ignition-fuel-gas supply channel for supplying an ignition fuel gas to the precombustion-chamber,

wherein, provided that

a direction A is a direction in which the center O of the cross-section is eccentric with respect to the straight line L, and

a cross-section S is a cross-section of the precombustion chamber, including the straight line L and the direction A,

the second passage part is symmetric across the cross-section S, and

wherein the ignition-fuel-gas supply channel has a supply port disposed on a space-part forming surface which defines the space part, the supply port being situated away from the cross-section S.

2. The precombustion-chamber engine according to claim 1 ,

wherein a second-passage-part forming surface which defines the second passage part is smoothly connected with a first-passage-part forming surface which defines the first passage part and with a space-part forming surface which defines the space part.

3. The precombustion-chamber engine according to claim 1 ,

wherein, provided that

a first line is a part of a second-passage-part forming surface which defines the second passage part on a direction A side across the straight line L in the cross-section S,

a second line is a part of the second-passage-part forming surface on an opposite side to the direction A across the straight line L in the cross-section S,

θ 1 is an angle between the straight line L and a straight line connecting an upper end and a lower end of the first line, and

θ 2 is an angle between the straight line L and a straight line connecting an upper end and a lower end of the second line,

the second-passage-part forming surface is configured to satisfy θ 1 >θ 2 .

4. The precombustion-chamber engine according to claim 3 ,

wherein the second-passage-part-forming surface is configured to satisfy 30°≤θ 1 ≤45°.

5. The precombustion-chamber engine according to claim 3 ,

wherein the first line extends upwardly with increasing a distance from the straight line L, and

wherein the second line includes: an extended line part extending, from a first-passage wall surface which defines the first passage part, in a direction of extending the first-passage wall surface; and a tapered line part connecting the extended line part with a space wall surface which defines the space part and extending upwardly with increasing a distance from the straight line L.

6. The precombustion-chamber engine according to claim 1 , further comprising an ignition-fuel-gas supply channel for supplying an ignition fuel gas to the precombustion chamber,

wherein the ignition-fuel-gas supply channel has a supply port disposed on a space-part forming surface which defines the space part, the supply port being situated on an opposite side to the direction A across the straight line L, and

wherein the ignition-fuel-gas supply channel is configured to inject the ignition fuel gas through the supply port toward a direction A side.