Computers and Control Systems: Description and Operation

Closed-loop Feedback System

The closed loop feedback system monitors and controls the air/fuel mixture to provide optimum mixture ratio for economy and emission control by the catalytic converter.

The system consists of an oxygen sensor located in the exhaust manifold, an electronic control unit, and a modified carburetor containing an electro-pneumatic fuel regulator.

Fig.14 - Closed Loop Feedback System Oxygen Sensor:

The oxygen sensor, Fig. 14, measures the amount of oxygen present in the exhaust gasses. The sensor reacts to the oxygen and develops a voltage signal which is sent to the electronic control unit. The control unit determines the air/fuel mixture ratio from the signal and controls the fuel regulator accordingly, in order to maintain the correct mixture.

Fig.15 - Vacuum Solenoid Regulator:

The vacuum solenoid regulator, Fig. 15, consists of two parts. The lower part receives vacuum from the intake manifold through an orifice (ED), which operates the regulator valve through a diaphragm (M), Fig. 15. The upper part of the regulator is a three way electromagnetic solenoid. When non-energized, the core is in the bottom position, closing off passage (SI). the vacuum output to the carburetor is vented to the atmosphere, allowing for a richer mixture. When the solenoid is energized, the core is in the top position allowing the carburetor actuators to be connected to the controlled manifold vacuum passage, and providing leaner mixtures.

When the oxygen sensor registers a rich mixture, it sends a stronger voltage signal to the control unit. A signal from the control unit causes the solenoid core to remain in top (SS) position for a greater percentage of time than it remains in the lower (SI), Fig. 15, position. Vacuum to the carburetor actuators increases, causing leaner air/fuel mixtures. When the oxygen sensor registers a lean mixture, it sends a weaker signal to the control unit. The signal from the control unit then causes the solenoid core to spend a greater percentage of time in the lower (SI) position, decreasing vacuum to the carburetor actuators, causing leaner air/fuel mixtures.