GxT V016-01, Ferret 16 GasLink II 5-Gas Analyzer - Manual (Page 18)

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Gas Theory
AFR (Air Fuel Ratio), Lambda and Stoichiometry
Stoichiometry is where air and fuel are in the proportions that provide the
greatest combustion efficiency. The AFR (air to fuel ratio) is a number that
describes the proportions of air and fuel. For gasoline, the AFR that repre-
sents the stoichiometric point is 14.7 parts of air to 1 part of fuel. Lambda
is a normalized scale where 1.0 is the stoichiometric point for the fuel being
used. So, for gasoline, you can multiply lambda by 14.7 to get the air fuel
ratio. Why have both? It turns out that other fuels have a different air/fuel
ratio for the stoichiometric point. For example, the stoichiometric point of
propane occurs at approximately 11.8 Air/Fuel Ratio. The Lambda is still
1.0 at stoichiometry for propane as with any other fuel.
3 Way Catalytic Converter
The three-way catalyst uses precious metals to cause the reactions needed
to lower emissions of HC, CO and Oxides of Nitrogen. HC and CO emis-
sions are lowered by a chemical reaction called ‘oxidation’ and Carbon
dioxide (CO2). Emissions of Oxides of Nitrogen (NOx) are lowered by a
chemical reaction called ‘reduction’.
The ‘oxidation’ of CO and HC works best when the mixture is lean and
the ‘reduction’ of NOx into N2 and CO2 works best when the mixture is
rich. The Engine Control Module controls the fuel delivery, and after the
engine is warmed up, a properly operating system will alternate between
rich and lean.
If the rate at which the system changes between rich and lean is too slow,
the catalyst runs out of stored oxygen, leaving HC and CO not oxidized.
If the rate is too fast, not enough time is allowed for oxygen to be stored,
leaving HC and CO unoxidized in the following rich cycle. In either case, HC
and CO emissions will be higher than expected. If the system is running
mostly rich, NOx will be lowered at the expense of higher HC and CO. If it
is running lean, HC and CO will be lowered at the cost of higher NOx.
Hydrocarbons (HC)
Petroleum based products are made up of hydrocarbons. Any fuel that is
not burned in the combustion chamber will be pumped into the exhaust,
where it may be converted to water and carbon dioxide in the catalytic
converter. Any remaining fuel will show up as HC at the tailpipe. Hydro-
carbons are measured in parts per million (ppm). Too many hydrocarbons
in exhaust gas as are caused by: Incorrectly timed spark, Insufficient
spark, Misfire due to rich or lean mixture. HC is an excellent indicator of
problems in the ignition system.