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Delta O2 Table. The wet versus dry reporting of NOx is best resolved by how the
NOxCAN(g) is zeroed and spanned. If you want wet numbers, calibrate using wet (i.e.
water-compensated) CLA data. If you want dry numbers, calibrate using dry CLA data.
Equilibrium versus Non-Equilibrium O2
Gases exiting the combustion chamber of an Otto or Diesel cycle engine are not at chemical
equilibrium. As the exhaust valve opens, the gas temperature and pressure drop so quickly
that many chemical reactions still occurring are slowed down or “frozen” at non-equilibrium
values (for the specific temperature and pressure they are at). This degree of chemical non-
equilibrium is affected by exhaust valve timing and ignition timing. The Lambda and NOx
sensors are hot and highly catalytic and will bring the exhaust they sample closer to chemical
equilibrium. The result of this is that the %O2 measured by the Lambda and NOx sensor
will be close to equilibrium levels (typically 0.5% lower than actual engine-out). For
example, at Lambda = 1 conditions, the %O2 reported by the Lambda and NOx sensor will
be 0 (the chemical equilibrium %O2 value) as opposed to the actual frozen equilibrium %O2
value of approximately 0.5%. Keep this in mind when comparing gas-bench measured %O2
with Lambda and NOx sensor-measured O2. This difference can be corrected for by a Delta
O2 Table.
Lambda Sensor-Measured O2 versus NOx Sensor-Measured O2
Both the Lambda sensor and the NOx sensor will report %O2. However, the NOx sensor
will not measure %O2 (and hence Lambda, AFR, FAR, and PHI) as accurately as the
Lambda sensor. This is because in the Lambda sensor, the electrodes measuring %O2 are
pure platinum. Pure platinum electrodes give best %O2 measuring performance. In the NOx
sensor, the %O2 measuring electrodes have some gold in them which keeps the electrodes
from destroying the NOx in the exhaust. NOx is measured in a second set of electrodes
downstream of the %O2 measuring electrodes. If the first set of electrodes were pure
platinum, the NOx would not reach the second set.
Pressure
The main source of error influencing O2 (Lambda, AFR, FAR, PHI) measurements is
exhaust pressure. This error is more significant the further from Lambda = 1 the exhaust
stoichiometry gets. Pressure compensation (P-comp) data is stored in the memory chip of
every Lambda and NOx sensor. All that is required is the addition of a calibrated pressure
sensor to the module and the entering of the correct pressure sensor constants to activate the
pressure compensation. It is highly recommended that pressure compensation be used with
any lean burn, HCCI, or diesel engine.