Internal combustion engine

An internal combustion engine, comprising:

• a cylinder including an intake opening and an exhaust opening;
• an ignition plug including a distal end located inside the cylinder;
• an intake passage connected to the intake opening of the cylinder;
• an exhaust passage connected to the exhaust opening of the cylinder;
• a water injection valve located inside the intake passage, the water injection valve being configured to inject water into the intake passage;
• an intake valve configured to selectively open and close the intake opening;
• and an exhaust valve configured to selectively open and close the exhaust opening, wherein the intake valve includes:
• a rod-shaped stem;
• and a head that is connected to a distal end of the stem and located inside the cylinder, an outer surface of the head includes an upper surface facing the intake opening, the upper surface includes a curved guiding portion, the guiding portion includes a specific tangent line that, when the intake valve is in a fully open state, intersects with at least one selected from a portion of the ignition plug that is located inside the cylinder, and a portion of the exhaust valve in a closed state that is located inside the cylinder, on an imaginary cross section that is parallel to a central axis of the head and passes through the guiding portion, a line segment that agrees with a tangent line contacting the guiding portion and extends from a contact point of the tangent line with the guiding portion to an intersection point of the tangent line with the central axis is defined as a first line segment, a line segment that agrees with the central axis and extends from the intersection point toward the intake passage is defined as a second line segment, and an angle formed by the first line segment and the second line segment is defined as a specific angle, and the tangent line that minimizes the specific angle is the specific tangent line, and the specific angle is less than 90°.