5-10
Fundamentals
Interpreting Air-Fuel Ratios and
Emissions
The relationship between the air-fuel ratio and exhaust gases
monitored by the analyzer are:
•
HC is lowest when the air-fuel ratio is ideal because most of
the fuel is consumed in combustion. Richer or leaner
mixtures, or ignition problems cause HC to increase
because of incomplete combustion.
•
CO is lowest when the air-fuel ratio is nearly ideal because
there is less O
2
and C left over. This is due to more complete
combustion occurring at stoichiometric ratios. Richer than
ideal mixtures cause CO levels to increase; leaner mixtures
have little effect.
•
CO
2
levels are highest when air-fuel ratios are close to ideal,
and decrease when the mixture becomes richer or leaner.
•
O
2
levels are near zero when the air-fuel ratio is near
stoichiometric, since most of the O
2
consumed in
combustion. It remains low with richer mixtures, and
increases when the mixture leans out.
•
NO
x
is lowest when the air-fuel ratio is either very rich or
very lean and highest when the air-fuel ratio is slightly lean
and when the engine is under load.
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