Four-cycle engine

Abstract

To provide a four-cycle engine, in which a pair of intake valve ports is openable/closable by respective intake valves and a pair of exhaust valve ports is openable/closable by respective exhaust valves. The pair of intake valve ports and the pair of exhaust valve ports are provided in a cylinder head such that they are positioned on both sides of a first virtual plane which contains the axial line of the cylinder bore and passes through an approximately central portion of the combustion chamber. A pair of ignition plugs is mounted in the cylinder head. The pair of ignition plugs is mounted such that the degradation of a flame propagation condition is minimized, even if accidental firing of either of the ignition plugs occurs. The pair of ignition plugs is disposed substantially symmetrically with respect to a second virtual plane which passes through the center of a combustion chamber and is perpendicular to the first virtual plane. The ignition plugs are mounted in the cylinder head such that they have axial lines extending substantially in parallel to the first virtual plane. The ignition plugs are tilted with a distance therebetween becoming smaller toward the combustion chamber. End portions of the ignition plugs, which project into the combustion chamber, are disposed in a region surrounded by the intake valve ports and the exhaust valve ports.