PURPOSE: To prevent worsening of operating efficiency during low load running, by a method wherein, when an absolute pressure in a suction pipe located down a line from a throttle valve is below a reference value, an on-off valve is closed despite of the intensity of an output from an O2 sensor, and the reference value is set to a value responding to the intensity of an atmospheric pressure.
CONSTITUTION: An air cleaner 2 is communication with a suction manifold 4 located down a line from a throttle valve 6 through an intake secondary air feed passage 8, and an electromagnetic on-off valve 9 is located in the passage 8. Based on an output from an O2 sensor 14, situated to a suction manifold 15 and generating an output proportioning O2 concentration when an air-fuel ratio is at least within a theoretical air-fuel ratio, duty control is made on an opening time of the valve 9 by means of a control circuit 20. A suction pipe is situated down a line from the throttle valve 6, and when an absolute pressure in the suction pipe, detected by a pressure sensor 10, is below a reference value, the valve 9 is closed despite of the intensity of an output from the O2 sensor 14. The reference value is set to a value responding to an output from an atmospheric pressure sensor 19.
ASAKURA MASAHIKO
MUROYA MINORU
MATSUURA KOJI
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