Abstract: An internal-combustion engine includes a cylinder, a piston, a spark plug, and a fuel injection device. The piston includes a top surface, a guiding protrusion, and a cavity. The cavity is provided at a position closer to the fuel injection device than the guiding protrusion when viewed from above the top surface of the piston. The guiding protrusion includes a guiding surface to guide the fuel mist toward the spark plug when the fuel mist is injected from the fuel injection device at a first predetermined timing in a compression stroke. The cavity includes a guiding wall portion to guide the fuel mist toward the spark plug when the fuel mist is injected from the fuel injection device at a timing in the compression stroke between the first predetermined timing and a second predetermined timing.

Abstract: In order to increase in concentration of HC in an exhaust gas due to an axial gap between a ring groove and a piston ring, when an internal combustion engine is at a lower temperature, particularly, during the starting or the low-speed operation of the internal combustion engine in which the moving speed of a piston is lower, a restricting ring having a radially outward tension is inserted in a wedge shape into between axially opposed surfaces of a ring groove and a piston ring so that in a lower-speed operational range of a piston, the axial movement of the piston ring within the ring groove is restricted by the wedging action of the restricting ring, and in a higher-speed operational range of the piston, the wedging action of the restricting ring is removed by a floating force of the piston ring generated with the descending movement of the piston.

Abstract: In order to increase in concentration of HC in an exhaust gas due to an axial gap between a ring groove and a piston ring, when an internal combustion engine is at a lower temperature, particularly, during the starting or the low-speed operation of the internal combustion engine in which the moving speed of a piston is lower, a restricting ring having a radially outward tension is inserted in a wedge shape into between axially opposed surfaces of a ring groove and a piston ring so that in a lower-speed operational range of a piston, the axial movement of the piston ring within the ring groove is restricted by the wedging action of the restricting ring, and in a higher-speed operational range of the piston, the wedging action of the restricting ring is removed by a floating force of the piston ring generated with the descending movement of the piston.