# Calculation of Air Density

1. Nov 22, 2007

### signorvivek

Hai,

I have the pressure value (which is less than atm) and temperature value. How to calculate Density of the air?
I calculated but the result is not satisfactory.

2. Nov 22, 2007

### TVP45

3. Nov 22, 2007

### signorvivek

P = Pressure at the region : in Pa
R = Specific gas constant R : 287 J/KgK
T = Air Temp : in K

Density of air = P/(R*T) in Kg/m3

Last edited: Nov 22, 2007
4. Nov 23, 2007

### stewartcs

5. Nov 23, 2007

### BlackWyvern

Density = mP/RT

Rs = R/M

:)

6. Nov 23, 2007

### signorvivek

Can anyone tell me the method to calculate the engine idle air flow requirement?

Engine rpm = 1200 rpm = RPM
Rev/cyle = 2
Engine capacity = 395 cc = 0.395 Lit = L
Volumetric Efficiency = 90% = VE

Then VAF (Volume of Air Flow) is,

VAF = (L/28.317) x (RPM/2) x VE.

I want o know what is the meaning for 28.317
Can anyone tell be the derivation for it?

7. Nov 23, 2007

### stewartcs

It looks pretty close to the molecular mass of air.

8. Nov 26, 2007

### signorvivek

Hai

Molecular mass of air is 29. But this value is 28.317 which is 0.6 lesser than molecular mass of air.
I am confused on this 28.317...

9. Nov 26, 2007

### stewartcs

Molecular mass of air is generally 28.97, that's why I said it's close to that of air.

10. Nov 26, 2007

### TVP45

That's the number of cubic meters in 1000 scf. This is why I cringe when there are no units shown.

11. Nov 26, 2007

### stewartcs

Ahh...

Good point! I'm cringing too!

12. Nov 26, 2007

### signorvivek

Yes it is the unit conversion data only.
But what is the purpose of the 28.317 there.
See i am giving the Engine capacity in Litre only,

And my end unit of Volume of air is Lit/min only.

And also VAF = (Engine Displacement/1728) * (RPM/2) * VE

I am still confused on the 28.317?

13. Nov 27, 2007

### TVP45

That's just a unit conversion. It changes litre/minute to cubic foot/minute. I don't know why you do that, but....

When you do problems, try to always use units. That will reduce your effort by at least half.