As you can see in the graph above, HC and CO levels are relatively low near the theoretically ideal 14.7 to 1 air/fuel ratio. This reinforces the need to maintain strict air/fuel mixture control. However, NOx production is very high just slightly leaner than this ideal mixture range. This inverse relationship between hC/CO production and NOx production poses a problem when controlling total emission output. Because of this relationship, you can understand the complexity in reducing all three emissions at the same time.
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