cfm-gps

[Ward]

Insid the software the injectors and MAF are rated in grams/sec.

Ward

On Mon, 02 Aug 1999 19:52:40 -0400, Patrick G. Moore wrote:

> 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





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