cfm-gps

[the Fredericks]

Your calculations for air volume to mass air appear correct.   Where you've
apparently messed up is in trying to apply the same volume conversion to
fuel, which has a vastly different density.  Injectors are specified in
lb/hr but this is really a mass flow/hr.   Slugs/hr would be more correct
but no-one would have a clue..  :-)

So, for 255 cfm using your approximation =139 gps, for stoich ratio of
14.7:1, you'd need  approximately 139/14.7 = 9.45 gps of fuel.   Using 36
lb/hr injectors and converting to gps, 36 lb/hr = 36/2.2*1000/3600 = 4.54
gps.  8 injectors gives you  36.36 gps max capacity at 100% duty cycle.  So,
9.45 gps would be approximately 25% duty cycle.

You can take this one step further.  Using a Brake Specific Fuel Consumption
(BSFC) figure of .5 (a good swag for most street motors), those 36 lb/hr
injectors could support a max HP of  36*8/.5, or about 576 HP.  At 12:1 AFR
(it would need to be richer than stoichiometric at WOT), using the above CFM
approximations, it would need to be flowing about 980 CFM to support this.

These numbers match empirical data pretty closely...OK, I'm going to put the
calculator down and step away now...

Kendall Frederick

-----Original Message-----
From: owner-gmecm at esl.eng.ohio-state.edu
[mailto:owner-gmecm at esl.eng.ohio-state.edu]On Behalf Of Patrick G. Moore
Sent: Monday, August 02, 1999 7:53 PM
To: gmecm at efi332.eng.ohio-state.edu
Subject: RE: cfm-gps


Hi All:

Since noone has answered part of this original
question, I got out an old fluids book.

To translate volume flow into mass flow we basically
just need the density of the air.
Rho = 2.51E-3 slugs/f3 at 32F and 1 ATM
    = 2.12E-3 slugs/f3 at 122F and 1 ATM

So a 255 cfm flow at 122F would make
255 /60s * 2.12E-3 = 9.01E-3 slugs/s

1 slug = 14.594 kg -> 131.49 g/s

Of course we have the normal p = Rho*R*T relation
working on the air.  So when under boost, Rho goes up
(unless the MAF is a draw-thru like Dave).

I found R = 1715 f2/(s2 degR) for air where degR is degrees Rankin
degR = degF + 491.69

For those who don't know what a slug is, it is a unit of mass
like a gram.  In the english system, to get pounds, multiply
slugs by 32.2 f/s2. ie.. 1 lb = 32.2 slug f/s2.

Does this look in the ballpark?  Lets see...
A 36 lb/hr injector would be
(36 * 32.2 / 3600) * 14.594 / 1000 = .004699 g/s
Try 8 of these -> .03759 g/s

That's nothing near 14.7:1.

Maybe the MAF measures mass flow directly, I haven't looked at
flow past a hot wire for years.  Then 255 g/m -> 4.25 g/s.
That gives 113:1 ratio.. Huh?

Oh maybe the injectors are rated in lb/s instead. Then 8, at 100%
duty cycle could put out 135.324 g/s.

Then to get a 14.7:1 mix we need 131.49 g/s air / 14.7 = 8.944 g/s fuel.
Therefore the duty cycle of the 8 36lb injectors would be 8.944/135.324
* 100 = 6.6%.  Still doesn't seem right.

If MAF measured directly in g/s then 255 g/s air needs 12.82% duty cycle.
Closer!

Help!  What is the right combo of assumptions. What does a MAF measure.
What are injectors specified at.

Sorry in advance for confusing everyone.

PS: I havn't looked at the excel spreadsheet yet, because I am on unix.

Pat