Quick questions.

Will increaing the volume of air flowing though a fixed area, also increase the pressure, even if the air temp/density remained the same?

Volume - it's my understanding that volume is a measurement of capacity, which is stated in fixed units or fixed cubed units (liters, cms cubed, inched cubed, ect,..) and that 1 cm cubed is 1 cm cubed regardless of the material content or its properties. Of course materials with different masses,weights, and densities, will need to be contianed in vessels with a different volumes, but the actual area measured for each unit of space (unit of volume) does not change - again 1cm cubed is still equal to 1cm cubed, regardless of density, mass, or weight. Correct?

 

more volume at the same desnity = more mass flowing.

volume flow = cross sectional area * average velocity so if you increase volume flow at a fixed desnity, presure differenital is up



you are correct.

1 gallon of milk, 1 gallon of ice cream, one gallon of air- all same volume. different denisty, different mass.

now if you heat them up they will expand or contract to different volumes but the mass will be the same.

 

My $.02

 

Thanks for the replys guys.

I asked because I was having a FI discussion with a buddy, and wanted to check my theories.

Here's the criteria of my discussion

1st - given the same engine at approx the same engine rpm
2nd - 2 different size compressors
3rd - taking psi and temp measurements in the intake

It's my opinion that a small compressor providing 10psi at 150deg, is flowing the same volume of air (cfm as you mentioned) through the intake, as a larger compressor flowing 10psi at 100deg. Although the larger compressor would be flowing a larger mass of air, since it would be at a lower temp/density. Same volume, differnt mass...

And if both compressors were putting 10psi at 100deg into the intake, then both the air volume and the mass would be the same with either compressor. Same volume and same mass (I know the smaller compressor would need to be spun higher and the air charge would need to be cooled to accomplish this)

Am I still correct, or way off?

I'm not trying to agrue which compressor size is better or will provide more end horsepower, or even which will operative better through the motor's or compressor's rpm range. Just want to know what happening with the air flow, within the given criteria.

Thanks

 

http://web.camaross.com/forums/show...947#post2581947

That's a good thread about temps and mass flow in NA and FI motors.

Bret

 

Maybe. The same mass (number of molecules) in a closed container exert more pressure as temp is raised. So to achieve the same pressure with a higher temp gas, fewer molecules would be needed, so mass of gas would be less. PV=nRT expresses this. It's called the equation of state for an ideal gas or something like that.

I wouldn't assume the same volume of air for the two conditions you mentioned. Maybe your concern should be the mass of air moved, not the volume which is temp and pressure related. I think I'd get into the compressor maps of each one to determine which would be the more efficient for what I needed.

Good luck.

 

Yep, I know for hp, mass is what's important, as are impeller efficiency and impeller drive efficiency, but I'm mostly arguing semantics and conditions in the intake, at a snapshot point in the motors rpm (or limited rpm range).

I couldn't get your link to work, but so far all the formulas I've seen have supported my theories, especially the second instance where I claim that intake CFM and mass would be the same with either compressor, and long as PSI and temp/density were the same.

So far I've got:

"more volume at the same density = more mass flowing.

volume flow = cross sectional area * average velocity so if you increase volume flow at a fixed density, pressure differential is up"

from Boost It!

and

"if the volumes are the same past the turbo, the pressure should only be the same if the mass flow is the same... PV=m(dot) rT where m(dot)=mass flow."

from Teeeman

 

We all pretty much agree that if temp and pressure are the SAME, you have the same mass flow. It's this part where it gets sticky:

"It's my opinion that a small compressor providing 10psi at 150deg, is flowing the same volume of air (cfm as you mentioned) through the intake, as a larger compressor flowing 10psi at 100deg. Although the larger compressor would be flowing a larger mass of air, since it would be at a lower temp/density. Same volume, differnt mass...

FWIW: flow is volume per time like cubic feet per minute. "Volume flow" is kinda shaky to define. "Mass flow" is number of molecules per unit of time, like pounds per minute. Semantics again, but we need to be on the same page.

 

So volume is a fixed "area" measurement. Which also changes due to temperature increases

"With ASTM D1250/API Table 6A as an example, a 1{degree} F. error in temperature produces a 0.04% error i net volume for that single measurement, regardless of the size of the tank.^As a rule of thumb, every .5{degrees} F. temperature error cause a 0.1% volume error"

BUT the content in a measured volume CAN change, with more or less density

Say you have a fixed 1 cu/ft volume.

Fill it with air at standard sea level density,

then fill it with air compressed to 10 psi.

Still has the same volume (area), but isn't there more air in the same space? Measuring flow by volume alone is not going to tell you the "amount" of air that you have.

 

Not exactly - A single unit of volume never changes, regardless of the density or material in that volume. If a mass (we'll say liquid) is heated it will take up exisiting volume (if available) by displacing air (or whatever other matter was in that existing volume), but the total existing volume was never changed, just the volume of the particular liquid. In an enclosed tank with one liquid (full tank), there is no additional volume to displace, so the volume of the liquid could not change with temperature.

That's a little out of context and doesnt apply here. Arent you talking about the measurement of displacement volume of a single liquid inside a tank (partially filled and vented tank)? That's not the total volume of the tank if it was full of the same liquid. The total volume of the tank doesnt change, unless tank temp changes enough to alter the physical dimensions of the tank or the weight of the fluid changes the physical dimension of the tank. If the tank remains at the same dimensions, there is a set volume of any fluid that can be put into the tank, regardless of temp or density.



Yes, It's got the same volume of air regarless of the pressure or vaccum, but you have a more mass (more air if you want to call it that). That's was I've been saying all along.

I never said you could get the mass by using only the volume. I said with volume, density, and resistance, you should be able to get a realitive mass.