• Why does it need to operate within a certain chemical range?
The gasoline engine catalyst feedgas (exhaust gasses) must remain in a very small window of fuel mixture because the chemical reactions the that reduce NOx and oxidize hydrocarbons are mixture dependent and mutually exclusive. NOx can only be reduced in a fuel rich environment and HC can only oxidized in a lean, oxygen available, environment.
The most important governing equations for HC oxidation are:
H2 + 0.5O2 -> H2O (i)
CO + 0.5O2 -> CO2 (ii)
C3H6 + 4.5O2 -> 3CO2 + 3H2O (iii)
C3H8 + 5O2 -> 3CO2 + 4H2O (iv)
The most important governing equations for NOx reduction are:
H2 + NO -> H2O + 0.5N2 (v)
CO + NO -> CO2 + 0.5N2 (vi)
C3H6 + 9NO -> 3CO2 + 3H2O + 4.5N2 (vii)
C3H8 + 10NO -> 3CO2 + 4H2O + 5N2 (viii)
The Cerium reaction:
Ce2O3 + 0.5O2 -> 2CeO2 (ix)
The only fuel mixture that meets the requirements of both set of reactions is mixture very near stoichiometric. The catalyst will reduce about 80% of the input NOx and HC as long as the mixture is kept within this very small window of fuel mixture, the operating window is AFR’s from 14.55 to 14.69. (Lambda .995 to 1.005). That is a mixture that does stray from stoichiometric by more than ½ of 1 percent.
A graphic representing this concept.
A common misunderstanding is that the mixture is required to cycle from rich to lean for the reactions to complete. This is not true; it needs to stay in the window. In older systems it was required to cycle the mixture to keep it in the window because a narrowband oxygen sensor only reports mixture at stoichiometric. Wide band sensors do not cycle the mixture; they hold steady mixture in an even smaller window which improves pollutant conversion to rates above the 80% that was achievable with the narrowband cycling system.