Abstract: A coolant passage structure for an internal combustion engine includes a coolant communication member. The coolant communication member includes a centrifugal water pump, a housing portion, a scroll portion, a second coolant passage portion, a first coolant passage portion, a direction of a center line of the second coolant passage portion, and a rib. The first coolant passage portion includes a downstream region connected to an upstream region of the second coolant passage portion. A direction of a center line of the first coolant passage portion is parallel to a direction of a rotation shaft of the centrifugal water pump. The direction of the center line of the second coolant passage portion is orthogonal to the direction of the center line of the first coolant passage portion. The rib is disposed on an internal circumferential surface of the first coolant passage portion.

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: A coolant passage structure for an internal combustion engine includes a coolant communication member. The coolant communication member includes a centrifugal water pump, a housing portion, a scroll portion, a second coolant passage portion, a first coolant passage portion, a direction of a center line of the second coolant passage portion, and a rib. The first coolant passage portion includes a downstream region connected to an upstream region of the second coolant passage portion. A direction of a center line of the first coolant passage portion is parallel to a direction of a rotation shaft of the centrifugal water pump. The direction of the center line of the second coolant passage portion is orthogonal to the direction of the center line of the first coolant passage portion. The rib is disposed on an internal circumferential surface of the first coolant passage portion.