Originally Posted by
UselessPickles
Since water is incompressible (in practicality, at least), but air IS compressible, the garden hose is a bit oversimplified.
With water, there IS a direct relationship between volumetric flow rate (e.g., CFM) and mass flow rate (e.g., lbs/ft^3).
With air, the relationship between volumetric flow rate and mass flow rate depends on density, which depends on temperature, pressure and volume.
To make things more confusing, in the world of engine dynamics, they speak of the volumetric efficiency of the engine. It is often explained as the percentage of "volume" of the cylinder that gets filled with air. As I discussed before, the volume of the displacement of the cylinder never changes. So volumetric efficiency is a bit of a misnomer (but not entirely incorrect, either, as I'll explain later). It's really a ratio of the AMOUNT (mass) of air that gets into the cylinder compared to the AMOUNT of air that *would* fill the cylinder *if* it were allowed to reach full equilibrium with the intake manifold.
At 50% volumetric efficiency (number chosen arbitrarily to make my later example easier to comprehend), it doesn't mean that the cylinder is only filled 50% of the of the way with air. That's impossible, because whatever AMOUNT of air enters the cylinder will expand to fill the entire volume.
But if you think of it from the perspective of the intake manifold, the term "volumetric efficiency" actually makes sense: 50% efficiency means that 50% of the cylinder's volume worth of air in the manifold has filled the cylinder. For the Pentastar, a cylinder is 3.6L / 6 = 0.6L. Let's say the manifold pressure is 10 psi (easy number to work with). At 50% volumetric efficiency, that means 50% of 0.6L = 0.3L worth of the manifold's 10 psi air enters the cylinder. That AMOUNT of air will fill the entire 0.6L in the cylinder (when the piston is at the bottom of the stroke). Volume are inversely proportional. That means when that 0.3L of the 10 psi air expands by double to fill the 0.6L cylinder, the pressure will be cut in half to 5 psi* (see disclaimer below). The air in the cylinder now has 50% the pressure of the manifold, AND 50% the amount (mass) of air that would have filled the cylinder if it were allowed to completely "fill" with air from the manifold.
So because of the relationships between volume, pressure, density and mass of air, it can really be thought of as either volumetric efficiency (from the perspective of the manifold) OR mass filling efficiency. It all works out mathematically the same, and will produce the same result in the calculation to determine the amount of air in the cylinder.
*Disclaimer: I simplified a bit for the explanation. When that 0.3L of 10 psi air expands into the 0.6L cylinder, the pressure may not become exactly half, because I believe the temperature of the expanding gas will decrease as well. This gets into some complications that I don't understand. So some of my previous statements in earlier posts may be incorrect about the relationship of how much pressure gets into the cylinder, but the concepts would be correct if you worked in terms of mass rather than pressure.
Connect With Us