Fundamentals
5-11
Stoichiometric Fuel Mixture
The stoichiometric air-fuel ratio (14.66:1) is near the point where
the emission levels drastically change. The stoichiometric air-fuel
ratio, where the HC and CO levels are lowest, is as close to perfect
combustion as can be attained.
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The term lambda is often used instead of air-fuel ratio.
Lambda is a numerical value of the measured air-fuel
ratio relative to the ideal air-fuel ratio. Lambda equals one
(1) when the air-fuel ratio is stoichiometric. Lambda is
less than one when the air-fuel ratio is rich, and is greater
than one when lean.
Because combustion temperatures and the air-fuel ratio
requirements can change in engines under dynamic load, the only
way to ensure that the air-fuel ratio remains close to stoichiometric
under most operating conditions is to use a feedback system to
monitor oxygen content of exhaust gas. This feedback system
reports back to a computer that calculates fuel delivery and
commands the fuel injector or carburetor to deliver the necessary
amount of fuel required to maintain the correct air-fuel ratio.
Figure 5-8
: Major Combustion By-products Versus Air-Fuel Ratio
Figure 5-9
: Carbureted Fuel Feedback System
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