Computers and Control Systems: Description and Operation

Fig. 30 Closed loop system.:

This system consists of an oxygen sensor and a control unit, and is designed to minimize carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) emissions by using a three-way catalyst. The three-way catalyst operates with a precise exhaust gas conversion efficiency under an accurate air/fuel ratio that is controlled within a narrow range around the stoichiometric (ideal air/fuel ratio, 14.7 to 1 air/fuel ratio) ratio.
The oxygen sensor located in the exhaust manifold transmits signals to the control unit to indicate whether the air/fuel ratio at the exhaust manifold is richer or leaner than the stoichiometric air/fuel ratio. The control unit then determines the solenoid operating period according to the sensor signal. During engine warm-up, this system opens until the oxygen sensor reaches normal operating temperature. The system also opens when the engine is operated in wide open throttle, operates at a higher than the preset RPM or when operating at high altitude. The oxygen sensor consists of a closed-end tube of ceramic zirconia. This sensor directs the atmospheric and exhaust gasses to the inner and outer surfaces of the tube, detects the amount of oxygen in the exhaust gas and then transmits an electrical signal to the control unit.
The control unit consists of various transistors and a micro computer. It detects engine speed, coolant temperature, oxygen concentration of the exhaust gas, manifold vacuum and position of the throttle valve (blade).