[Diy_efi] What is stoichometric....really?

[Mike]

At 02:53 AM 18/10/2002 +1000, you wrote:
>At Lambda 1.1 NOX is at it's maximum as the temperatures are
>also at a maximum. But then so is the efficiency.

mmmm, I udnerstand NOx rises somewhat the leaner you go and then
drops off as the ignition falters due to too lean mixtures,

>The real problem lies in the legislation adopted for testing vehicles
>and their emission compliance.

<cough> Partly true but a bit of propoganda here :(

>The std drive cycle test such as the NY27 or the Australian ADR27
>set down a max. speed of 92 km/hr.

>Up to that speed all run closed loop so as to minimize emissions.

Yes, with the understanding the CAT is reducing NOx back to N2 and O2.

>Over that speed the ECU knows that it's not in a test so it's free
>to implement lean burn and save the fuel.

I really think thats conjecture, to suggest on a public forum that
manufacturers 'cheat' or close to it is tad provocative, rather the
interpretation is the engine can tolerate some leaning out at higher
rpm on part load to maximise econony ~`"o

>The Catalytic converter can not be damaged by these exhaust temps.
>Lean cruise exhaust temps are in the 350 to 400 deg C range.
>The std 3 way cat can handle temperatures of 1100 degrees C.

Please understand its *not* the exhaust temperatures that damage
a CAT - its the amount of NOx which the CAT then reduced to N2 and O2,
this generates heat within the CAT which raises the temperature of the
ceramic monolith, this does not necessarily  melt the cat but does scour
the monolith of the precioius metals making the CAT virtually
ineffective to handle further NOx and subsequently HCs and CO,

>Most of the high temp damage occurs during oxidization, this is
>simply combustion that takes place within the cat's substrate and
>thus releases a lot of heat.

Yes thats also partly true, in that 'most' catastrophic damage
occurs in terms of melting from oxidation not from reduction,

>These instances of damage occur when a condition of rich offset
>occurs in the  fueling regime. A rich offset is a condition whereby
>the ECU cycles around Lambda 0.9 something instead of
>Lamda 1.0.  Melt down of the ceramic will often follow.

Yes this is true *and* the worst case is when someone runs from
very rich to lean or rich to very lean - this places the greatest
thermal loads on the CAT and thats where it has the highest chance
of melting the ceramic monolith and can obstruct the flow causing
even greater thermal damage and outright failure necessitating
replacement...

>All it takes is a dirty connection with a 0.05 volt drop to achieve
>this offset. If combined with a moderate load such as a small
>caravan and sustained 85 to 90 km/hr cruise it won't be long
>before there is a catastrophic failure.

Possibly, more likely is the non tow sceanrio where its overly
rich then lean - either by people experimenting with AFR or any
connection failure of the O2 sensor,

>Very small Japanese cars with 600 cc engines are also susceptible
>to this type of failure as they are at half throttle when they cruise.

Well, not sure about that, they use real small CATs, made cheaply
in China and real cheap ECUs also made in china that are rough at
best - there is a lot of mechanically unsound engineering in the
cheapest smallest cars - vibration is also a source of damage from
a 600cc 3 or 4 cylinder - wild mixture variation is the least cause.

>As there is no excess fuel during lean burn there is no reduction of
>NOx with out resorting to a storage type cat. These cats store the
>NOX and release it again during suburban driving or period of reduced
>speed where the reduction can again be achieved with very little loss
>of economy.

Huh ! Whats this storage stuff ? There aint no storage, the NOx is
reduced to N2 and O2 on its way through - thats it - simple as that !
To suggest NOx is stored then released flies in the face of EPA
requirements re online continuous emission control, there aint no
money for auto companies to store NOx - cheaper to handle it as
it is presented to the CAT - provided mixtures are reasonable
so the CAT doesnt get thermal damage from oxidising HCs and CO and
from reducing NOx to N2 and O2 - oh and by reducing SOx back to
elemental sulphur and O2 from the bad fuels we have in Australia ;-)

rgds

Mike




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