| 主权项 |
1. An exhaust device for a multicylinder engine, comprising a plurality of cylinders each including an intake port, an exhaust port, an intake valve capable of opening and closing the intake port, and an exhaust valve capable of opening and closing the exhaust port,
wherein the exhaust device comprises a plurality of independent exhaust passages each having an upstream end connected to an exhaust port of one cylinder or a plurality of cylinders that are non-sequential in exhaust order, and a mixing pipe connected to downstream ends of the respective independent exhaust passages and into which exhaust that has passed through the respective independent exhaust passages flows, the downstream ends of the respective independent exhaust passages are connected in a bundled form to the mixing pipe, a pre-silencer and a main silencer including a space section in which a flow path sectional area is increased are disposed in an exhaust passage downstream from the mixing pipe in such an order that the main silencer is located on a downstream side of the pre-silencer, a distance from a seating position of the exhaust valve in the exhaust port of each cylinder to an upstream end of the pre-silencer is set to a distance such that, in a first operation region including at least an intermediate speed range in which an engine rotation speed is greater than or equal to a first reference rotation speed set in advance and less than or equal to a second reference rotation speed higher than the first reference rotation speed, a negative pressure wave is generated due to an exhaust pressure wave generated by opening the exhaust valve reaching and being reflected by the pre-silencer and the negative pressure wave reaches the exhaust port of the cylinder during an overlap period in which an open period of the exhaust valve and an open period of the intake valve of the cylinder of which the exhaust valve has been opened overlap for a predetermined period, a distance from the seating position of the exhaust valve in the exhaust port of each cylinder to an upstream end of the main silencer is set to a distance such that, in a second operation region including at least a low speed range in which the engine rotation speed is less than the first reference rotation speed, a negative pressure wave is generated due to an exhaust pressure wave generated by opening the exhaust valve reaching and being reflected by the main silencer and the negative pressure wave reaches the exhaust port of the cylinder during an overlap period in which an open period of the exhaust valve and an open period of the intake valve of the cylinder of which the exhaust valve has been opened overlap for a predetermined period, and a valve driving device for driving the intake valve and the exhaust valve of each cylinder is provided so that an open period of the exhaust valve and an open period of the intake valve of each cylinder overlap for a predetermined period and, for cylinders that are sequential in exhaust order, the exhaust valve of a subsequent cylinder opens during the overlap period of a preceding cylinder, in a predetermined operation region including at least low and intermediate speed ranges in which the engine rotation speed is less than or equal to the second reference rotation speed. |
