Abstract: An engine intake system includes a primary port for introducing intake gas throughout an entire engine operating condition. A secondary port is provided with a gate valve which is opened in a high engine load condition for introducing the intake gas. A ratio of a tumble flow to a swirl flow is gradually increased as the gate valve is operated from an entirely closed condition to a fully open condition. According to this invention, ignition and combustion characteristics can be improved.

Abstract: A combustion chamber for an engine is provided between a cylinder bore formed in a cylinder block, a generally conically-shaped wall formed in an under wall of a cylinder head, and a piston in the cylinder bore. The combustion chamber has a structure into which an intake port opens so as to introduce an intake air flow closer to a marginal portion than a central portion of the combustion chamber, thereby generating a swirl flow of intake air in the combustion chamber. The combustion chamber also includes a curved air flow guide wall formed in the generally conically-shaped wall which extends from near the intake port so as to approach the cylinder bore surface gradually, thereby generating what is termed a squish flow of intake air.