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

[Mike]

At 03:44 AM 22/10/2002 +1000, Phil Lamovie <phil at injec.com> wrote:
>Mike wrote...
>
>> 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
>
>It's what I do for a living. Conjecture is what you read in magazines.

No. You are guessing on a lot of what you have written, some of which
is correct - most of which is just way off the mark... read on...

>It's not "some leaning out" it's exactly Lambda 1.1 for the very
>reasons

Some engines and control systems can tolerate higher AFR - they do not
just go for a Lambda of 1.1 - of course there is diminishing returns
and off course driveability is an issue and there are heaps of
compromises... Arguing a general case for a specific issue is not
helpful here. A lot of people I know go for an AFR of from 12 to 13
so they can get maximum power and 'some' exhaust valve cooling at
higher boost levels - some also go for water injection to reduce
chance of pinging etc etc These same people occasionally go for
as lean as their engine can tolerate for long country trips, not
helpful for the CAT reliability but saves a heap of fuel costs.

>he then wrote...
>
>> 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,
>
>If the Engine is running constantly rich there is simply no O2 left
>to oxidize the exhaust stream. As such there is no conversion and
>no temperature rise. The cat just gets sooty. No damage will occur.

Well that may be but - note, I said that when you go from rich *to* 
too lean then damage can occur, and this is quite a common property
of most ECUs. Rich for dynamic changes in throttle then leaned (somewhat)
to cruise and the cycle repeats - this means soot/carbon on the cat,
then in a lean moment this gets burned off - if it happens to be too
lean then there is chance of damaging the cat *also* from reduction
of NOx coupled with higher exhaust temperatures if you do go too lean...

>> 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
>!
>
>The storage was introduced thus ...

Well its wrong. Standard CATS dont have effective 'storage' as such.

>phil wrote...
>
>>As there is no excess fuel during lean burn there is no reduction of
>>NOx with out resorting to a storage type cat.
>
>and now adds...
>
>Time to check your info, there is no way to reduce NOX without adding
>hydrocarbon. The cat ONLY works in closed loop. Exactly as it was
>designed. It's a catalyst and can only enhance an already possible
>chemical reaction. It cannot magically reduce NOX.

Wrong!		It is designed to do exactly that !

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>

This is where you are planting sheer conjecture and this is false :(

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
but it does help to keep the cat hot enough so the reduction can
proceed at maximum efficiency (which I might add is not 100%,
ie. Not all NOx is reduced even at perfect tune).

The production of NH3 for example is done with control of equilibrium
conditions and catalysts - the equation of N2 + H2 to give NH3 is
not favoured at STP.

Similarly the reduction of NOx is not favoured at STP but requires
higher temperatures and a precious metal catalyst, now one can
improve the heat operation of the cat with some HCs which get burned
by the cat to maintain temperature - helpful but definitely not
essential as one aim of a properly tuned engine is to reduce HCs
where possible.

Also SOx is reduced somewhat by a properly functioning CAT *but* at
the expense of less NOx reduction as the two reactions are somewhat
competitive and the catalyst must 'relax' after handling some SOx
to properly handle NOx - this is one main reason low sulphur fuels
are favoured...

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 ?

Rgds

Mike


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