[Diy_efi] WB O2 system

[Garfield Willis]

On 13 Jun 2002 16:49:27 -0000, "Tom Sharpe" <twsharpe at webmail1.mtco.com>
wrote:

>Thanks for the straight scoop, Gar. For us DIYers, how far off are our =
home=20
>groan (pun intended) meters when running on 10% alcohol (gasahol) here =
in=20
>the midwest???  Any rough estimates or guesses as to correction factors =
or=20
>rules of thumb that you can share??  How about Amoco 93 octane premium?=20

=46rom the gasohol or from the design of your 'dweeb-o-meter'? I think
you'll find your fueling variations are the least of your problems.
Sorry about that.

In general, if you want to answer questions like this for any particular
mix, like I said in the previous post, you need the sensor cal'd for
individual component sensitivities (hydrogen, oxygen, and unburnt
hydrocarbons; I'm looking at a sensor cal sheet for an ECM product, and
it also lists them individually as we do, so you can punch them into
their better meters). PLUS, you need to have the so-called C:O:H ratios
for your fuel to punch into the instrument. Examples listed below. If
you look at any curve for these sensors from the mfg., take NTK's or
Bosch's published stuff, it will have a cryptic little note somewhere
saying something like "(assumes H:C ratio 1.85, O:C ratio 0.0)" or just
"H:C=3D2.0", etc.

This is a clue that (1) the curve applies to petrol ONLY, because the
fuel is assumed to have no oxygenates, that's why the O:C ratio says
0.0, and (2) with an H:C ratio of 1.85, the AFR at stoich will be about
14.6.  All this 'stuff' is referred to as 'stochiometry', which is to
chemistry as arithmetic is to mathematics. IOW, the basics.

Our EGORpro meter, as do the higher end Bosch, Horiba, and ECM devices
that support arbitrary fuel composition, come with all the
training/tutorials/calculators necessary to figure out AND *understand*
these issues when running fuel mixes. All the AFR sensing mfgs mentioned
above also let you input these figures into the instrument, so the
corrections are made without repeated hand calcs. But we take our
clients thru the hand calcs anyway, so they understand what's going on.
It's mostly just straightforward stoichiometry, but if that's foreign to
you, it takes some tutoring. Once you have that under your belt, you can
handle ANY type fuel measurements.

Now of course, if one of the dweebites that professes some scientific
training, like say GregH for example, would just step up to the plate
and do the homework, you guys wouldn't have to be asking another vendor
for such customer support. RulesOthumb won't get it in this case. (Even
recently had someone ask me if I'd be willing to 'fix' the dweeb's
design; da noiv a some people :).

Now for some examples:

petrol		(1.70<H:C<2.10, O:C=3D0.0)
			(1.75 and 1.85 are common pump gas values)
methanol	(H:C=3D4.0, O:C=3D1.0)
ethanol		(H:C=3D3.0, O:C=3D0.5)
propane		(H:C=3D2.67, O:C=3D0.0)
nat. gas		(H:C=3D4.0, O:C=3D0.0)

I posted some time back (gawd, I think it was on this list, dunno) a
list of stoichiometric AFRs for various common fuel mixes, but this
doesn't tell you the whole story, since the shape of the curve away from
stoich is also major affected by the fuel composition. Here they are,
for what they're worth, as but part of the story:

>Fuel						stoich AFR
>
>LPG   							15.5
>avg. Petrol 					14.7
>10% ethanol (aka gasohol)		14.2
>straight butyl alc.				11.2
>straight ethanol  				  9.0
>straight methanol			  	  6.5
>straight nitromethane			  1.7
>
>As they say, "Gas is for cleaning parts. Alcohol is for drinking.
>Nitro is for racing!." :)

HTH,

Gar Willis
Principal Engineer
EGOR Techno
3491 Edison Way
Menlo Park, CA 94025
650-216-9874
garwillis at msn.com (e-mail & PayPal transfers)
www.egortech.com (best viewed with IE)



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