Spark ignition engine and method for operating spark ignition engine

A spark ignition engine, comprising:

• an engine body that has a cylinder extending in a first direction;
• a piston that reciprocates in the first direction in the cylinder;
• an intake port that communicates with an inside of the cylinder, and causes intake air at least containing air to flow into the cylinder when being opened at least in an intake stroke;
• an exhaust port that communicates with the inside of the cylinder in a manner to oppose the intake port in a second direction orthogonal to the first direction, and discharges exhaust gas from the inside of the cylinder when being opened at least in an exhaust stroke;
• and
• a first ignition plug and a second ignition plug, each of which is attached to the engine body and forcibly ignites an air-fuel mixture in the cylinder during a compression stroke or an expansion stroke, wherein
• the first ignition plug and the second ignition plug are spaced apart from each other in a third direction that is orthogonal to the first direction and the second direction,
• in the cylinder around compression top dead center, with ascent of the piston during the compression stroke, two vortex flows of the air-fuel mixture, each of which swirls around an axis extending in the second direction, are aligned in the third direction with a center of the cylinder being interposed therebetween and swirl in opposite directions from each other, and
• the first ignition plug and the second ignition plug forcibly ignite the air-fuel mixture at a timing around the compression top dead center.