SPARK-IGNITION TYPE TWO-VALVE ENGINE, ENGINE UNIT, AND VEHICLE

The present teaching provides a spark-ignition type two-valve engine that can improve a thermal efficiency. A spark-ignition type two-valve engine includes a piston part, a crankshaft, only one exhaust port, only one single center tumble port, an offset ignition plug, and a small-diameter long-stroke cylinder that cooperates with the piston part to define a combustion chamber. The combustion chamber is formed in such a manner that: the combustion chamber has a diameter shorter than a reciprocation stroke length of the piston part when viewed in a reciprocation direction; and as the piston part moves toward a bottom dead center by a stroke length longer than the diameter of the combustion chamber when viewed in the reciprocation direction, a tumble flow is imparted to gas taken from the single center tumble port, while as the piston part moves toward a top dead center by a stroke length longer than the diameter, the gas pushed by the piston part flows toward a gas inlet.