Calculation help

[Red Admiral]

Can someone check this for me;

Trying to calculate the mass flow rate through a piston engine

displacement (in m^3) * rpm / 60 = m^3 / s = Volume flow rate

Volume flow rate * density = mass flow rate

The problem comes in calculating the density (1.2kg/m^3 at s.l.), how do I account for the increased boost pressure from the supercharger?

Cheers

[Wuzak]

You need the Ideal Gas Law.....

http://en.wikipedia.org/wiki/Ideal_gas_law

pV = nRT

p = pressure, V = volume, T = temperature (in Kelvin - above absolute zero). R is a constant and n depends on the material.

In the Wiki article it defines n = m/M, where m is the mass of the substance, and M is the Molecular weight.

If you assume 1m^3 of free air has been compressed, then m = 1.2kg. M can be calculated by adding the molecular masses for gaseous nitrogen (N2) and oxygen (O2) which make up almost all of the air - figure 21% oxygen, 79% Nitrogen.

I assume that you are working on a 4 stroke cycle engine - in these the cylinder only ingests air once for every crank revolution. Thus the volume taken in by the engine in one revolution is half its capacity. Also need to account for volumetric efficiency, if you can. For a supercharged engine the VE will be over 100% - that is it will take in more air than the volume of the cylinder. In unsupercharged engines the number is generally below 100%.