Modulation of F/A Window

Oscillations in fuel flow rate around the set point widen considerably the F/A window, thereby adversely affecting the conversion efficiency. To maintain high conversion efficiency for all the three pollutants, the adverse effect of these oscillations in F/A ratio is countered by use of an oxygen storage/release system in the catalyst wash coat. Components like cerium oxide (CeO₂ ) and zirconium oxide (ZrO₂ ) are added to the washcoat. CeO₂ acts as oxygen storage system and widens the air-fuel ratio window
when high conversion rates of all the three pollutants are possible. CeO₂ undergoes the following chemical changes as the mixture transits from rich to lean and back due to variations in fuel-flow rate.

Rich Operation:

(5.4)

Lean operation:

(5.5)

During fuel rich operation, CeO₂releases oxygen for oxidation of CO and HC and in the process gets itself reduced to Ce₂O₃When the engine operation becomes lean as a result of control of fuel flow by engine fuel management system, Ce₂O₃ gets oxidized back to CeO₂by reacting with excess O₂ or NO. These reduction-oxidation (Redox) reactions in cerium oxide continue and effectively even in a wider F/A (nearly 0.06 F/A ratio) widow high conversion efficiency of all the three pollutants is obtained.