GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 80)

COMBUSTION THEORY
The content of an engine’s exhaust stream
depends on the efficiency of the combustion
process. Ideally, pure fuel (hydrogen and
carbon) and oxygen would enter the
combustion chamber in precisely the right
amounts (called stoichiometry), and the
mixture would be ignited at just the right
moment so that all of the oxygen would
combine with all of the fuel. In this perfect
example, the hydrogen (H) in the fuel would
combine with oxygen (O
2 ) to form water
(H 2 O) and the carbon (C) in the fuel would
combine with oxygen to form carbon dioxide
(CO
2 ). Those two compounds and heat
would be the only products of combustion,
and there would be no undesirable
emissions. Combustion would be complete.
In reality, an engine is fed air, which is mostly
nitrogen, and fuel of varying composition and
quality. The air/fuel ratio may not be ideal,
and the spark may not occur at precisely the
right moment. These conditions, and many
others, will cause less than complete
combustion. Subsequently, the exhaust
stream will contain hydrocarbons (HC), and
carbon monoxide (CO), in addition to Carbon
Dioxide (CO
2) and traces of Oxygen (O 2),
Oxides of Nitrogen (NOx), Sulfur Dioxides
(SO2), and soot. The goal is to operate the
engine as efficiently as possible, to minimize
undesirable emissions.
The Engine Diagnostic Center will measure
HC, CO, CO
2 , O 2, and NOx levels in the
exhaust stream. The relative amounts of
these compounds provide information about
the efficiency of the combustion process and
clues about the causes of abnormal levels.
Three harmful compounds we want to
minimize are carbon monoxide (CO),
hydrocarbons (HC) and oxides of nitrogen
(NOx). To achieve this, the engine
management system controls the content of
the exhaust that enters the catalytic
converter by adjusting the air fuel mixture
and spark timing based on exhaust manifold
O2 levels, intake air flow and pressure,
engine temperature, RPM, and several of
other inputs.
There are a lot of things that can go wrong,
and understanding the results of different
component failures will go a long way toward
helping you use your Engine Diagnostic
Center to its full potential.
AFR (Air Fuel Ratio), LAMBDA
and STOICHIOMETRY
Stoichiometry or the stoichiometric point 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 represents 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.
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