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

[Garfield Willis]

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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