Phased Air/Fuel Ratio Perturbation - Effect of Air/Fuel Ratio Perturbation Frequency, Amplitude, and Phase Shift on Underbody Catalyst Efficiency, With and Without Oxygen Storage, 08-9149

Principal Investigator
Cynthia C. Webb

Inclusive Dates: 07/01/99 - Current

Background - The mechanism of phased perturbation was initiated under an earlier quick-look IR&D project entitled, "Phased Air/Fuel Perturbation - A Fuel Control Technique for NO_x Reduction." These program results revealed that phased perturbation produced a significant impact on mixed air/fuel ratio (A/F) control and catalyst hydrocarbons (THC), carbon monoxide (CO) and oxides of nitrogen (NO_x) conversion efficiencies. However, in studying the data collected during the quick-look study, it was noted that the standard techniques for measuring A/F (using a UEGO sensor) and raw gaseous emissions (using standard gas analyzers) created some uncertainty in the data. It appeared that the continuous data were extremely accurate and precise at low A/F perturbation frequencies (<1 Hz), but as frequency (and also amplitude) increased, the data accuracy decreased.

Approach - The objective of this project was to study the mechanism of phased perturbation using a more stable method of measuring catalyst conversion efficiency and exhaust A/F. The mixed A/F and exhaust emissions were assessed using a dry raw bag sample analysis and the Urban A/F calculation method (a dual point bag sampling technique was developed under this study). The experimental levels of frequency and amplitude used in this work were closely modeled after the studies found in literature, with the A/F perturbation amplitudes and frequencies covering the ranges typically evaluated. This procedure was followed to allow comparison, verifying that the typical trends with amplitude and frequency were maintained for the 0° phase shifted control, while also providing data to examine the effect of phase shifted control.

Accomplishments - Examination of the catalyst conversion efficiencies (NO_x conversion efficiency graphs are shown in the illustrations below) shows that, for a 0° phase shift (top illustration), efficiency is maximized as perturbation amplitude is reduced and frequency is increased. These results and trends agree with those results found in the literature. In comparing these results for the varying phase levels, it is seen that as A/F phase shift is increased, NO_x conversion efficiency becomes insensitive to changes in frequency and amplitude for all operating conditions tested. In addition, the peak efficiency for 180° phase shift (96 percent) was slightly higher than the peak efficiency for the 0° phase shift condition (94 percent).

The test procedure development has been completed, and the procedure appears to produce stable measurements. Currently the research team is studying the interaction between A/F phasing and catalyst oxygen storage capability, and its subsequent effect on catalyst conversion efficiency. This work is scheduled for completion in January 2000.

This illustration shows measured NO_x conversion efficiency for a 0° phase shift

This illustration shows measured NO_x conversion efficiency for a 180° phase shift 

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