Equations..

[Bruce Bowling]

~ 
~ Jim Staff, try this instead:
~ 
~ PV=mRT, where
~ 
~ P=pressure in pascals(not kPa)
~ V=volume in m^3(cubic meters, not liters)
~ m=mass of air in the volume, kg
~ R=287 kJ/(K * kg) for air
~ T=absolute temperature in K
~ 
~ This one does away with moles. I'll make a bold assumption that you
~ know what a derivative is(if not, we'll explain). Take the derivative 
~ of each side of that equation with respect to time and you get:
~ 
~ P(dV/dt)=(dm/dt)RT, where
~ 
~ t=time, seconds
~ dV/dt=volume flow rate in m^3/s. A more common unit is CFM, but you
~        can perform the conversion
~ dm/dt=mass flow rate in kg/s. A more common unit is lbs/hr, but...
~       
~ Rearrange to solve for dm/dt, which is what you are really after:
~ 
~ dm/dt=P(dV/dt)/(R*T)
~ 
~ Now, R is constant, you can measure P with a MAP, T with an air charge
~ temperature(ACT) sensor.
~ 
~ Throw in Volumetric Efficiency(called eta, but I can't type in greek,
~ so I'll call it N which looks similar) as a correction factor for
~ dV/dt:
~ 
~ dm/dt=P(N*dV/dt)/(R*T)
~ 
~ Now dV/dt becomes a constant(it's your engine or cylinder 
~ displacement), and you need to predict N(that's you asked in the first
~ place, huh?). That's tough to predict without any measuring equipment 
~ at all. You could assume a relatively safe, constant number like 70%, 
~ but without measuring SOMETHING, you'll never know how close that is 
~ or how it changes with rpm at wide open throttle(WOT).
~ 
~ What are you building, kart? Powerplant? Tell us and I'm sure someone 
~ can help you figure out a way to qualitatively determine your WOT 
~ volumetric efficiency. I'm thinking you could try to monitor exhaust 
~ gas temperature(EGT) vs rpm, which can be done without a dyno.
~ 
~ Hey, glad to see a high school student trying stuff like this. Life
~ doesn't suck after all!
~ 
This explanation is EXCELLENT!!!!  This should go on the DIY_EFY WWW page.
Good work, Mr. R)   (whoever you are.....).

- Bruce



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               Bruce A. Bowling
  Staff Scientist - Instrumentation and Controls
 The Continuous Electron Beam Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
                bowling at cebaf.gov  
        http://devserve.cebaf.gov/~bowling
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