Exhaust control system in two-cycle internal combustion engine

Abstract

It is intended to provide an exhaust control system in a two-cycle internal combustion engine capable of suppressing the occurrence of irregular combustion when an opening timing of an exhaust port is set to a maximum delay timing, thereby improving the combustion stability and affording a high engine output when the opening timing of the exhaust port is set to a maximum advance timing. In the exhaust control system, an exhaust control valve has valve elements movable between their maximum projected positions at which a maximum delay timing of the exhaust ports is set and their maximum retracted positions at which a maximum advance timing of the exhaust ports is set. The valve elements are provided with control surfaces, that face a cylinder bore at the maximum projected positions of the valve elements to close upper port portions of the exhaust ports. Lower edges of the control surfaces are formed in a shape that substantially coincides with the shape of upper edges of the exhaust ports at the retracted positions of the valve elements. A pair of communication passages are formed in the valve elements, respectively, having first end portions, at the maximum projected positions of the valve elements, open to lower control surface portions positioned lower than top portions of the lower edges, and second, opposite end portions which are directly open to the exhaust passage.