[Diy_efi] WBO2 and AFR

[Klaus Allmendinger]

Hi,

According to Haywood, partial pressure of oxygen in exhaust at 750 degC:

AFR  partial pressure O2 in Pascal
19     7000
17.6  4000
16.2  1500
14.7+ 50
14.7-  5e-11
13.2   2e-13
11.8   3e-14
10.3   1e-14

Partial pressure of O2 has a large jump by many orders of magnitude at 14.7
AFR (Lambda 1.0 in this case). 
A WB sensor adds/removes oxygen from it's measurement chamber using it's
pump cell. The controller keeps the WB sensors Nernst cell at stoich (jump
point) by adding O2 (thereby catalytically oxydizing the reducing
combustion products) when rich of stoich and removing O2 when lean of
stoich. The pump current is more or less proportional to the oxygen flow
and therefore proportional to K * (1 - 1/lambda). The factor K varies
depending on wether the exhaust gas is lean or rich of stoich and is more
or less constant, but varies slightly with Lambda, the gas composition,
pressure and temperature. At higher pressures more O2 molecules (lean case)
or reducing molecules (rich case) are present at the pump surface (lean
case), requiring a larger positive or negative pump current (this makes the
readings richer in a rich gas and leaner in a lean gas). But the diffusion
rate through the diffusion gap reduces roughly with pressure (~1/P)
depending on the construction and size of the gap. This offsets the
pressure sensitivity somewhat depending on sensor construction.
When saying that an O2 sensor is sensitive to partial O2 pressure, is that
correct because it DOES hold the Nernst cell at the stoich point where the
it has a jump. This does NOT mean it is linearly and directly related.


Regards,
Klaus