Two-cycle engine of the spark ignition type

Abstract

A two-cycle engine of the spark ignition type is provided wherein a mixture in a combustion chamber can be compressed and self-ignited at least in a low load operation region to effect an activated heat atmosphere combustion. In a transition operation region between the activated heat atmosphere combustion region and an ordinary spark ignition combustion operation region, the mixture can be ignited by a spark ignition at a timing earlier than the spark ignition timing in the ordinary spark ignition combustion operation region to stabilize the combustion condition and eliminate abnormal combustion noise and the harmful effects of heat. A throttle valve opening of a throttle valve manually operated is detected by a throttle valve opening sensor formed from a potentiometer or the like while an engine speed is detected by an engine speed sensor, and they are inputted to a CPU. The CPU operates in accordance with a control graph of FIG. 5 which defines an exhaust air passage opening ratio in response to the engine speed Ne and the throttle valve opening, and transmits to an exhaust control servomotor a drive signal at which the exhaust air passage opening ratio based on the graph is provided. An ignition plug 22 generates, in an ordinary combustion operation region, a spark at 10.degree. (crank angle) prior to the top dead center. When it is discriminated by the CPU that the engine is operating in a transition operation region A between the ordinary combustion operation region and an AR combustion operation region, a spark is generated at 20.degree. prior to the top dead center in response to a control signal of the CPU.