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Appendix D: Interpreting Data from LambdaCAN*
Modules
Comparing to Spindt and Brettschneider Calculations
LambdaCAN* calculates Lambda, AFR, FAR, and PHI numbers comparable to those
determined by mass flowrates of air and fuel into the engine. Lambda calculations based on
gas-bench analyzer data (i.e. measurements of CO, CO2, etc) do not always give these same
values. This fact is well documented in the literature.
However, some organizations prefer to call their specific (and often unique) Spindt or
Brettschneider calculations of Lambda the “true” values. To satisfy such needs, ECM has
provided the Delta Lambda Table and Delta O2 Table features.
Before and After Catalyst Measurements
The Lambda sensor operates on a diffusion mechanism. Molecules leaving the combustion
chamber (O2, CO, CO2, H2O, H2, HC, NOx, N2, etc) diffuse into the sensor where
oxidation, oxygen liberating, and oxygen pumping occurs. O2, Lambda, AFR, FAR, and
PHI are determined from the oxygen pumping rate and assumptions about the combustion
products entering the sensor. One such assumption is that the exhaust exists in a certain
chemical composition. Exhaust gases entering and exiting a catalytic converter have
different chemical compositions. LambdaCAN* assumes that the gases are engine-out (not
catalyst-out). Therefore measurements made at the exit of a catalyst will require a Delta O2
Table and a Delta Lambda Table correction to be accurate. This is sometimes called “H2
Correction”.
Wet versus Dry Measurements
Percentages of components in the exhaust of an engine are expressed as percentages
(molecule count or volume) or ppm. The numerator used to calculate these percentages and
ppm contains the molecule-of-interest count and the denominator contains the total number
of molecules in the sample containing the “count”. One of the molecules produced by the
process of combustion, and in significant quantities (ex. 15%), is H20.
Percentages and ppm calculated by LambdaCAN* consider the water molecules in the
denominator and are called “wet” percentages (or ppm). This only makes sense since the
sensors are directly in the exhaust where the H2O is present. In contrast to this, classical gas-
bench analyzers almost always remove the water before the gas sample reaches the analyzers.
This is because the analyzers cannot tolerate condensed water. Therefore, gas-bench
analyzers will report O2 percentages and NOx ppms as “dry” percentages (or ppm) (i.e.
without H2O molecules counted in the denominator).
Therefore, when comparing “wet” (or “true”) O2 and NOx data from LambdaCAN* to “dry”
O2 and NOx data from gas-bench analyzers, realize that the dry percentages and ppm will be
5% to 10% higher than the wet (or true) readings. The wet to dry conversion for %O2 is a
function of %O2 (or Lambda) and can be performed by using a Delta O2 Table.
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