Fundamentals
5-9
Oxygen (O
2
)
The level of O
2
in exhaust gas is an indicator of air-fuel ratio
leanness. The O
2
is present in the air the engine inducts and mixes
with the HC for combustion. Since the atmosphere is about 21%
O
2
, the percentage of O
2
in the exhaust gas after combustion
indicates air-fuel ratio leanness.
In
Figure 5-7
, the O
2
concentration is at a steady low level when
the fuel mixture is richer than about 15:1, because all available O
2
is consumed in the combustion process. As the mixture gets
leaner, O
2
steadily increases, because less is used in combustion.
Higher concentrations of O
2
in the exhaust are therefore directly
proportional to leaner air-fuel ratios.
Figure 5-7
: Oxygen Versus Air-Fuel Ratio
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