Computers and Control Systems: Description and Operation

Closed loop emission control system is a system that precisely controls the air-fuel ratio near stoichiometry (ideal mixture), allowing the use of a three-way catalyst to deoxidize oxides of nitrogen and to oxidize hydrocarbons and carbon monoxide.

The essential components are a heated oxygen sensor (HO2S) an engine control module (ECM), fuel injectors and a three-way catalytic converter (TWC).

The ECM receives the voltage signal from the HO2S and generates a controlling signal to the fuel injectors to control Air Fuel ratio near stoichiometry.
During closed loop operation, the ECM monitors the voltage output of the oxygen sensor. As the sensor voltage output increases and passes through the ECM threshold set point, the proportional gain immediately changes the fuel injector operating time. This operating time is further changed at a constant rate (integral gain) until sensor output voltage decreases and passes through the ECM threshold set point.

Integral and proportional gains of the ECM are based on engine operating conditions.