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

[Mike]

Thanks Gar,

I stand corrected - or rather an incremental correction ;-)

I like your end quote - maybe we'll see better zeolytes and
catalyst combos so my current inland home doesnt turn into
beach front - or cliff front property for my grandkids,

Tah

rgds

Mike



At 02:16 PM 23/10/2002 -0700, you wrote:
>On Wed, 23 Oct 2002 14:47:07, Mike <erazmus at iinet.net.au> wrote:
>
>> 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!
>>...
>>Standard CATS dont have effective 'storage' as such.
>
>Here's a quote from Heywood: (pg. 654)
>"NO is removed by reduction using the CO, hydrocarbons, and H2 in the
>exhaust. ***No catalyst is available for the decomposition of NO to O2
>and N2, which is sufficiently active for use in engine exhausts***
>[emphasis mine, Gar]." He then goes on to explain how the current/modern
>single-bed catalyst, aka three-way catalyst (aka your 'Std. Cat', Mike),
>makes use of both 'redox' combinations, but even in the 'std. cat',
>what's used to reduce the NOx *isn't* a purely direct catalytic
>reduction equation of 2NO -> N2 + O2, but rather the CO, HC, H2 are used
>to combine for the reduction.
>
>Secondly, it's true, 'std' three-way cats don't have storage, but Phil
>is right in that (all?) modern lean-burners ARE equipped with an
>*additional* special NOx cat that does indeed store NOx and relys on the
>ECU to switch to rich-burn periodically for a moment, to reduce the NOx
>before the storage cat saturates. It used excess CO/HC/H2 to accomplish
>this reduction, not purely catalytic direct NO->N2 + O2.
>
>>At 03:44 AM 22/10/2002 +1000, Phil Lamovie <phil at injec.com> wrote:
>>> there is no way to reduce NOX without adding hydrocarbon.
>
>>It seems to me you either havent read about CATs from an authoritative
>>source (not all stuff on the next is correct - there are heaps of
>>people that also spread conjecture ;) or you didnt do first year
>>university chemistry or possibly both... <sigh>
>>...
>>It does seem like magic to some, that a precious metal catalyst with
>>the addition of heat can reduce NOx to N2 and O2 - but, this is what
>>they are designed to do and its not necessary that HC's be present ...
>
>Well, I cited an authoritative source, and I also logged into Stanford's
>SAE index and doing a search on "NOx Storage Catalysts" yielded dozens
>of hits. Every description I've ever read in SAE papers of how these NOx
>cats work says you have to go rich-burn for a moment to reduce the NOx
>and clear them out, which is what I took Phil to be referring to when he
>said you had to have hydrocarbons to reduce NOx.
>
>>So - I really suggest you dont spread conjecture.
>>
>>I'm now curious which source of info you are relying on which states
>>that:-
>>
>>a.	CATS only work in closed loop
>>and
>>b.	They cannot 'magically' reduce NOx
>>
>>because both statements are wrong.
>>
>>1.	CATS do operate in open loop - but of course not as
>>	effectively, they do catalyse HCs and they do reduce NOx,SOx
>>	at many ranges of AFR - some of which raise propensity to
>>	damage such as running too lean or too rich (ie Lots of HCs
>>	and some NOx can cause hot spots in the CAT and can cause
>>	damage).
>>
>>2.	CATs do (not so magically) reduce NOx and its not essential
>>	HCs be present.
>>
>>So which authoritative source of info says the contrary ?
>
>Once again, here's a citation from Heywood:
>
>"Fig. 11-57 (pg. 656) shows the coversion efficiency for NO, CO, and HC,
>as a function of the AFR. There is a narrow range of AFR near stoich in
>which high converison efficiencies for all three pollutants are
>acheived. The width of this window is narrow, about 0.1AFR for a
>catalyst with high mileage use..." [However, he goes on to say that...]
>"Experimental data show that there is a considerable widening of the AFR
>window where all three pollutants are effectively removed, with cyclic
>variation of the fuel flow [he's talking about the bang-bang hunting
>across stoich that comes from normal C-L feedback operation]. The
>maximum conversion in the middle of the window is reduced, however, from
>it's value when there are no fluctuations. The effect of the
>fluctuations depends on the frequency; frequencies of about 0.5 to 1.0
>Hertz are most effective and the usable window (at lower conversion
>efficiencies) can be broadened to about 1AFR." [ie, about 0.5AFR on
>either side of stoich].
>
>The MOST revealing thing comes from actually looking at how the
>efficiency of the Cat dramatically falls of as you move just slightly
>away from Stoich. That's in Fig. 11-57 Heywood cites above. That's the
>very reason tailpipe probes with WB AFR measurement work quite well,
>despite contrary 'intuition'. For most of the areas we're interested in
>tuning for (except driveability regions right around stoich), the Cat is
>effectively asleep.
>
>As to an authoritative and convincing quote on the need for rich
>combustion byproducts to provide the NOx storage cat with what's needed
>to perform the NOx reduction and clear them out, I need only refer you
>to the countless articles that discuss the current lean-burner's need to
>have it's ECU go periodically into a rich-burn regime for NOx cat
>purge/regeneration. AFAIK, this is the way ALL lean-burners achieve
>their NOx emissions requirements.
>
>This IS all pretty new stuff; could you be suffering from a case of
>emissions future-shock, Mike?
>
>Gar
>
>
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