cfm-gps

[Patrick G. Moore]

the Fredericks wrote:
> 
> 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.
Oh yes, thank you 2.2E-3 lb = 1 g or more commonly remembered 1 lb = 2.2 kg.
Leave it to me to convert it the hard way and make a mistake.

> 
> 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.
Or more precisely 1 lb = 1 slug * 32.2 f/s2
Therefore lbs/32.2(f/s2) = slugs

> 
> 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
Here is the mistake...
36 /( 32.2 * 3600) slug/s * 14.594E3 g/slug = 4.56 g/s
Yay
But your method is more straightforward ;-)

Please ignore this thrashing about.
---BEGIN-THRASHING----
> 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
---END-THRASHING-----

Let's see, my NA 231ci may have a .8 VE.  It's got ported heads and balanced
so it might be able to run up to 6000rpm.  That's 231*3000*.8 = 554400 in3/m
or 320.8cfm.

So 320.8 / 60s * 2.12E-3 * 14.594E3 = 165.422 gps air.

To run a 12:1 air to fuel ratio I need 165.422/12 = 13.75 gps of fuel.

I have 6 injectors which I think are 32#ers. That's a capacity of
32 / 2.2 * 1000 / 3600 * 6 = 24.24 gps at 100% duty.  So my injectors
should handle that at about a 57% duty cycle.

A BSFC of .5 would then yield about 13.75 * 3600 * 2.2 / 1000 / .5 = 217hp.

Oh, realism is setting in.  I hope to do a little better. 100% on my
injectors could get 383hp, I guess thats why everyone likes boost :-)

Anyone know how to find the stoich of an added nitrous and gas or alcohol?