| 摘要 |
PURPOSE:To prevent the deterioration of a ternary catalyst by closing and opening a solenoid valve for supplying secondary air to the upper stream of the ternary catalyst at the frequency of 1-2Hz, periodically varying an air-fuel ratio within the range of + or -0.2-+ or -1.0, and closing the solenoid valve when an engine load exceeds a prescribed value over a prescribed period of time. CONSTITUTION:A secondary air supplying hole 60 is provided on an exhaust manifold 3, and secondary air is supplied to the upper stream of a ternary catalyst 5 via a control valve 50 and a check valve 59. Negative pressure is introduced into the negative pressure chamber 53 of the control valve 50 via a solenoid valve 51, and this solenoid valve 51 is closed and opened by means of a solenoid driving circuit 70 at the frequency of 1-2Hz, allowing an air-fuel ratio to be periodically varied via the control valve 50 taking a theoretical air-fuel ratio as a center. A negative pressure switch 72 for introducing the negative pressure via a negative pressure delay valve 80 is connected to the solenoid driving circuit 70, and when an engine load exceeds a prescribed value over a started period of time, the negative pressure in the negative pressure chamber 76 of the negative pressure switch 72 is reduced, and the negative pressure switch 72 is turned off, allowing the solenoid valve 51 to be closed via an AND gate 73. |
