Help with calculating air mass through throttle body/orifice

[my8950]

I am in the mindset right now of trying to model my engine and calculate various flow rates. What I'd like to do is make a calculator to show various rates of flow through the TB. I know it will be hard to determine the amount of opening anything other then full open/closed.
I'm at WOT, and I'm also boosted...
I have some info that I was reading that mentions Saint Venant equation to calculate the mass flow rate, using Ambient Air and Temp Upstream / PMan.
Can anyone give some help on how I can do this? I'd like to be able to get velocity, mass flow, and whatever else I can take from it as well.

Thanks!

 

[fireslave]

I dont know how you would get a precise answer, but you could get an approximation if you had a decent idea what the volumetric efficiency of the engine was.

Most cars are somewhere around 80-90% efficient, which means that if a cylinder displaces 100cc, for example, there will be 80-90cc of air that enters that cylinder on every intake stroke. Of course, VE is different at different RPMs as well.

for boosted engines, just make sure to take into account the actual air pressure, ex 10 psi boost + 14.7psi atmospheric = 24.7 psi absolute pressure.


After that, there should be a formula or table somewhere that relates the different amounts of oxygen content per cubic foot of air at different temperatures and pressures. I have been looking for such a formula myself, actually, and havent found it yet.

Sorry if I just repeated what you already knew. I need that formula too!

 

[black2003cobra]

Not sure if this is what you're looking for or will help you, but at WOT, it's pretty simple to show that the mass-flow rate (mdot) for a 4-stroke is approximately,

mdot = 0.5*?*VEe*Vd*(1 + Pb/Pa)*N

? = air density
VEe = volumetric efficiency referred to intake manifold p & T
Vd = engine displacement
Pb = boost pressure
Pa = atmospheric pressure
N = engine speed

Leave out ? to get volumetric flow rate. Watch your units.

 

[black2003cobra]

P.S. Should have added that ? is air density at the inlet, and the approximation assumes ideal intercooling.

 

[patprimmer]

Don't forget, for a four stroke it takes two revolutions to obtain the quoted displacement.

Regards
Pat
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[black2003cobra]

Yes, hence the factor of 0.5 out front. So Vd as it appears is the quoted displacement.

 

[patprimmer]

Sorry

I was looking for /2 and missed the X 0.5

Regards
Pat
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[black2003cobra]

No worries, Pat. Easy to miss when looking for something else.

 

[fireslave]

found a website that has a few web-applications that are useful for calculating air density

input is pressure in mb (1000mb = sea level... i didnt even relize b was metric... makes sense though) temperature in C and dewpoint, as moist air is somewhat less compressable

http://wahiduddin.net/calc/density_altitude.htm

 

[patprimmer]

I'm not sure what you mean by somewhat less compressible. Steam engines seem to work OK

Regards
Pat
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[fireslave]

what i mean is when you compress a cubic foot of air, but only .99 cubic feet of that is actually air, the reduction of volume would be closer to what you would expect from .99 cubic feet of air, not 1 cubic foot of air. that .01 percent which is water will only compress slightly, where the air will compress more.

that is all. doesnt have a huge effect because air at earthly temperatures can only hold so much water before it just falls out of suspension i.e. dew.

you could probably ignore it for this purpose, i was just mentioning that the calculator includes it should you want to be ultra precise.

 

[patprimmer]

You are wrong.

Regards
Pat
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