Air-to-Fuel Ratio Control of Gas Engines Using Response Characteristics of Three-Way Catalysts under Dynamic Operation 912362

This paper presents a new method of controlling the air-to-fuel ratio (A/F) in gas engines with three-way catalysts. In the method under discussion, the center of the λ-window is detected directly through the dynamic response of an O₂ sensor installed at the rear of the catalysts. As the result, the A/F remains stable at the center of the λ-window for long periods of time, even in cases where static response characteristics of the O₂ sensor change due to the deterioration of the O₂ sensor, or in instances where the λ-window shifts or decreases due to the aging of the catalysts.

The responses of the O₂ sensor outputs have delay times in both the step changes of lean-to-rich and rich-to-lean A/F. Varying delay times were observed, and it was determined that the differences depended on the mean value of the A/F of the step change. Moreover, the delay times equalized when the mean A/F was at the center of the λ-window. These phenomena are considered to be caused by O₂ storage, CO adsorption, HC retention, and the water shift reaction of the catalysts. When the A/F is modulated sinusoidally, these phenomena appear as symmetrical response waveforms in the O₂ sensor outputs. The waveforms are asymmetrical when the mean A/F is not centered in the λ-window. Therefore, the A/F can be controlled at the center of the λ-window by feedback operation that keeps the response waveforms symmetrical.

Experiments with new control method demonstrated that less than 80, 150, and 150 ppm emissions of NO_x, CO, and CH₄, respectively, were achieved even for older catalysts. This method is easily applied to conventional gas engine controllers.