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Appendix C: Interpreting Data from LambdaCAN and
NOxCAN(g) Modules
Comparing to Spindt and Brettschneider Calculations
LambdaCAN and NOxCAN(g) calculate 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 and NOx sensors operate 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 and NOxCAN(g) assume 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 and NOxCAN(g) 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 and
NOxCAN(g) 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