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Provided is a combustion chamber applied in a gas engine including: a main combustion chamber including, at a central portion of its ceiling, an auxiliary combustion chamber; and a piston. A peripheral portion of a top surface of the piston is a flat surface. A raised portion, which has a curved sur

Provided is a combustion chamber applied in a gas engine including: a main combustion chamber including, at a central portion of its ceiling, an auxiliary combustion chamber; and a piston. A peripheral portion of a top surface of the piston is a flat surface. A raised portion, which has a curved surface and which is a solid of revolution, is formed at a central portion of the top surface, the raised portion being joined to the flat surface of the peripheral portion via a curved surface. When the piston is positioned at a top dead center, nozzle holes of the auxiliary combustion chamber jet out flame jets to a portion at which the raised portion of the top surface of the piston and the flat surface of the peripheral portion of the top surface of the piston are joined together.

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1. A gas engine combustion chamber applied in a gas engine including: a main combustion chamber including, at a central portion of its ceiling, an auxiliary combustion chamber having nozzle holes exposed within the main combustion chamber; and a piston inserted in the main combustion chamber in a sl

1. A gas engine combustion chamber applied in a gas engine including: a main combustion chamber including, at a central portion of its ceiling, an auxiliary combustion chamber having nozzle holes exposed within the main combustion chamber; and a piston inserted in the main combustion chamber in a slidable manner, wherein a peripheral portion of a top surface of the piston is a flat surface,a raised portion is formed at a central portion of the top surface of the piston, the raised portion being raised upward from the peripheral portion,the nozzle holes of the auxiliary combustion chamber are formed such that, when the piston is positioned at a top dead center, central axes of the respective nozzle holes are directed to a portion, of the piston, at which the raised portion and the flat surface are joined together, andwhen the piston is positioned at the top dead center, the nozzle holes of the auxiliary combustion chamber jet out a nozzle gas to the portion, of the piston, at which the raised portion and the flat surface are joined together. 2. The gas engine combustion chamber according to claim 1, wherein a distance to an edge of the piston from the portion, of the piston, at which the raised portion and the peripheral portion are joined together is a distance that allows a flame occurring at an upper surface of a cylinder head due to the nozzle gas ejected from the nozzle holes of the auxiliary combustion chamber to propagate to an inner wall surface of the cylinder head before an inner wall surface of a cylinder liner is exposed within the main combustion chamber due to the piston starting lowering. 3. A piston for use in a gas engine, wherein a peripheral portion of a top surface of the piston is a flat surface,a raised portion which is a solid of revolution, whose axis of revolution is a central axis of the piston, is formed at a central portion of the top surface, the raised portion being continuously raised upward from a portion at which the raised portion and the flat surface are joined together to a center of the central portion, the raised portion including an upward protruding top portion with a curved surface, andthe raised portion and the flat surface are smoothly continuous with each other. 4. The piston for use in a gas engine, according to claim 3, wherein the raised portion has a side formed as a conical surface inclined by an angle of 5° or more, and the angle is smaller than an angle of depression of nozzle holes of an auxiliary combustion chamber. 5. The piston for use in a gas engine according to claim 3, wherein if the piston is sectioned by a plane containing the central axis of the piston, a profile line of the top surface of the piston is a smooth line represented by a graph, in which: an inclination of the profile line with respect to the flat surface of the peripheral portion is zero at the peripheral portion; the inclination starts increasing continuously from zero near a starting point of the raised portion and thereafter decreases; and the inclination becomes zero again at a peak of the raised portion. 6. The piston for use in a gas engine, according to claim 3, wherein a distance to a side edge of the piston from a position where the flat surface and the raised portion are joined together is a distance that allows, in a combustion stroke of the engine, a flame occurring at an upper surface of a cylinder head due to a nozzle gas ejected from nozzle holes of an auxiliary combustion chamber to propagate to an inner wall surface of the cylinder head before an inner wall surface of a cylinder liner is exposed within a main combustion chamber due to the piston starting lowering. 7. The piston for use in a gas engine according to claim 4, wherein if the piston is sectioned by a plane containing the central axis of the piston, a profile line of the top surface of the piston is a smooth line represented by a graph, in which: an inclination of the profile line with respect to the flat surface of the peripheral portion is zero at the peripheral portion; the inclination starts increasing continuously from zero near a starting point of the raised portion and thereafter decreases; and the inclination becomes zero again at a peak of the raised portion. 8. The piston for use in a gas engine, according to claim 4, wherein a distance to a side edge of the piston from a position where the flat surface and the raised portion are joined together is a distance that allows, in a combustion stroke of the engine, a flame occurring at an upper surface of a cylinder head due to a nozzle gas ejected from nozzle holes of an auxiliary combustion chamber to propagate to an inner wall surface of the cylinder head before an inner wall surface of a cylinder liner is exposed within a main combustion chamber due to the piston starting lowering. 9. The piston for use in a gas engine, according to claim 5, wherein a distance to a side edge of the piston from a position where the flat surface and the raised portion are joined together is a distance that allows, in a combustion stroke of the engine, a flame occurring at an upper surface of a cylinder head due to a nozzle gas ejected from nozzle holes of an auxiliary combustion chamber to propagate to an inner wall surface of the cylinder head before an inner wall surface of a cylinder liner is exposed within a main combustion chamber due to the piston starting lowering. 10. The gas engine combustion chamber according to claim 1, wherein the raised portion is a conoid body, and a portion around a peak of the raised portion is formed in a spherical shape. 11. The piston for use in a gas engine, according to claim 3, wherein the raised portion is a conoid body, and a portion around a peak of the raised portion is formed in a spherical shape.