Discussion
Consider the following pairs of mixture condition and its emission effect:
Rich of stoichiometric - High carbon monoxide and hydrocarbons
Slightly lean - Peak nitrogen oxides
Deeply lean - Lower nitrogen oxides but poor catalyst reduction
Exactly stoichiometric - Peak nitrogen oxides
How many of the pairs given above are correctly matched?
Answer: A
Correct answer: Only three
The first three are right. Rich running gives incomplete combustion and therefore high carbon monoxide and hydrocarbons; slightly lean gives peak nitrogen oxides, having both flame temperature and oxygen; deeply lean lowers them but takes the exhaust outside the window in which a three-way catalyst can reduce.
The fourth is wrong and contradicts the second. The nitrogen oxide peak lies slightly lean of stoichiometric, not at it - which is what makes precise control around stoichiometric a compromise rather than an optimum for any single pollutant.
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