Calculating Air Density: A Solution for Scientists

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SUMMARY

The discussion focuses on calculating air density and engine idle air flow requirements using specific formulas. The air density formula is defined as Density = P/(R*T), where P is pressure in Pascals, R is the specific gas constant (287 J/KgK), and T is temperature in Kelvin. For engine idle air flow (VAF), the formula VAF = (L/28.317) x (RPM/2) x VE is used, where L is engine capacity in liters, RPM is engine speed, and VE is volumetric efficiency. The value 28.317 serves as a unit conversion factor from liters to cubic feet.

PREREQUISITES
  • Understanding of the Ideal Gas Law and its application in calculating air density.
  • Familiarity with specific gas constants, particularly R = 287 J/KgK.
  • Knowledge of engine mechanics, specifically engine capacity and volumetric efficiency.
  • Basic unit conversion principles, especially between liters and cubic feet.
NEXT STEPS
  • Research the Ideal Gas Law and its implications for air density calculations.
  • Learn about volumetric efficiency and its impact on engine performance.
  • Study unit conversion techniques, particularly for fluid dynamics and gas measurements.
  • Explore advanced air flow calculations for different engine types and configurations.
USEFUL FOR

Engineers, automotive technicians, and scientists involved in thermodynamics, fluid mechanics, and engine performance optimization will benefit from this discussion.

signorvivek
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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.
 
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signorvivek said:
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.

Please show what you did.
 
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:
Density = mP/RT

Rs = R/M

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

It looks pretty close to the molecular mass of air.
 
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...
 
Molecular mass of air is generally 28.97, that's why I said it's close to that of air.
 
  • #10
That's the number of cubic meters in 1000 scf. This is why I cringe when there are no units shown.
 
  • #11
Ahh...

Good point! I'm cringing too!
 
  • #12
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
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.
 

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