an intake duct and an intake valve cooperated with the intake duct, which are provided on an intake side of a cylinder cover;

an exhaust duct and an exhaust side roof, which are provided on an exhaust side of the cylinder cover; wherein

an inclination angle of an axis of an intake valve stem of the intake valve relative to an assembly bottom surface of the cylinder cover is a1, an included angle between a revolution cone surface of an intake valve disc of the intake valve and the axis of the intake valve stem is a2, an included angle between a plane where the exhaust side roof is located and the assembly bottom surface of the cylinder cover is a3, a lower guide surface of the intake duct close to an intake throat of the intake duct is inclined downward relative to the assembly bottom surface, and an inclination angle of the lower guide surface relative to the assembly bottom surface is a4; and

the airway structure satisfies: a3-3°≤a1-a2≤a3+3°; and/or, a4-3°≤a1-a2≤a4+3°.