Lambda Measurement after CAT

[Martin]

How will the reading on a PLM change if measured after the CAT?

Has anybody tested this? Motec?

[Holmz]

This thread showed a last post by Martin on May-4 (Today), but no reply was able to be seen?

[Martin]

I can see your reply,,,its the only one though....


Im not sure what you are meaning?

[RossB]

I am not aware of anybody here testing this but... Lambda does not change during the combustion process but the oxygen content of the exhaust does and this can have an effect on the lambda readings when using a lambda sensor(PLM or otherwise). This is because the Lambda sensor is using oxygen to measure the difference between Lambda 1 and the gas being sampled. For example we would see a small difference between the PLM reading from a sensor mounted close to the engine (where mixture is still burning in the exhaust) compared to one mounted say a meter further down the pipe where the oxygen content is lower. When a catalyst is functioning there is a differeence between pre and post cat of about 0.5% oxygen so I would expect to see a difference even though the actual Lambda value hasn't changed.
The PLM (and Mx00) lambda tables can be adjusted to give different lambda values as a function of pump current so that if the sensor location is not ideal you can still have the correct values. I would use a 5 gas analyser to do this.

[gruntguru]

Although the paper below refers only to narrow band sensors the priciples are the same. The full text of the paper further reinforces that lambda sensor output correlates more closely to AFR (Lambda) than to oexhaust xygen content. This is in spite of the fact that the sensor output is driven by movement of oxygen ions through the sensor electrolyte.

"Operating characteristics of zirconia galvanic cells (lambda sensors) in automotive closed-loop emission control systems
Bozek, John W | Evans, Richard | Tyree, Clifford D | Zerafa, Kenneth L
SAE Special Publications , no. 910, pp. 1-17. 1992

Simple tests were performed to investigate the operating characteristics of zirconia galvanic cells (lambda sensors) in automotive closed loop 'three-way' emission control systems. Commercially available cells were exposed to typical gaseous components of exhaust gas mixtures. The voltages generated by the cells were at their maximum values when hydrogen, and, in some instance, carbon monoxide, was available for reaction with atmospheric oxygen that migrated through the cells' ceramic thimbles in ionic form. This dependence of galvanic activity on the availability of these particular reducing agents indicated that the cells were voltaic devices which operated as oxidation/reduction reaction cells, rather than simple oxygen concentration cells. Such operation explains why a cell that is used as a lambda sensor in a closed-loop control system exhibits a sixfold or greater decrease in voltage output when the exhaust gas composition changes from a slightly rich condition (lambda identical with 0.995) to a slightly lean condition (lambda identical with 1.005). It also explains why the voltage of a cell that is located downstream of a properly operating catalyst normally remains at a low level as the air/fuel ratio oscillates around the stoichiometric value but increases to a high level when ignition misfire occurs at a rate that exceeds a certain value."

[Martin]

Great answer, thanks!

[RossB]

The narrow band sensor provides an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V represents a lean 0.8 V represents a rich mixture, one which is high in unburned fuel and low in remaining oxygen. At approximately 0.45 V the mixture is slightly lean but near to stoichiometric. The voltage produced by the sensor is not linear with respect to oxygen concentration, because of the way the sensor reacts with other components in the exhaust gas, which is ok for most automotive (gasoline) applications as the sensor is most sensitive near the stoichiometric point and less sensitive when either very lean or very rich, but useless when tuning a race engine or where very lean mixtures are used (e.g. Diesels).

The wideband sensor operation is completely different. It is based on a zirconia cell and incorporates an electrochemical gas pump. An electronic circuit loop controls the gas pump current to pump enough oxygen in to (or out of) the measurement cell to keep the output of the electrochemical cell constant. The the pump current directly indicates the oxygen content of the exhaust gas. We then apply a table (Lambda Characteristic Table) to this to translate this in to a Lambda value. Because the pump current is fairly linear as a function of oxygen content we are able to get acurate Lambda readings over a very wide range.

The limitation with any lambda sensor is that, because they rely on O2 measurement to detrmine Lambda (Air Fuel Ratio), they are reliant on “complete” combustion. For example if an engine is running at Lambda 1 the Lambda sensor will measure this ok but if the engine then missfires the Lambda sensor will give a lean reading. The Air-Fuel ratio has not changed but the fuel hasn’t ignited so there is an excess of oxygen in the exhaust. There is also excess un-burnt fuel in the exhaust but the Lambda Sensor can not detect this so it just reads lean. Similarly, as stated in the previous post, a lambda sensor voltage on a narrow band sensor will be lower on a sensor mounted downstream of a functioning catalyst. This is because the catalyst operation uses oxygen the lambda sees this as a richer mixture even though (of course) the air-fuel ratio has not changed.

[vmrracing]

Hi, I am new in this.. just only one question. using the PLM after the cat, it should give you a leaner value than with out the cat....
The CAT burns the excess..

thanks!

[Holmz]

Hi, I am new in this.. just only one question. using the PLM after the cat, it should give you a leaner value than with out the cat....
The CAT burns the excess..

thanks!

I would think probably richer, as the unburnt fuel will be consumed - so will be using O2 or producing more CO. But either way the O2 gets used up if more fuel gets burned in the CAT.