Understanding duct physics Part one
I created a little drawing to help the viewer understand what goes on inside an inlet runner when physical changes are made to it.
Bike modifiers often change the air filter from stock to pod type filters or Velocity stacks, without understanding how that effects the carb settings. This is an attempt toward understanding of why the carbs have to change in order to adapt to such a change.
The drawing was made to relate the basic concepts. It is not the final word on duct interaction in a physical environment. But rather, it is foundational in conceptional understanding of duct physics. It deals with duct pressures rather than the air flow volume or speed that results from pressure differentials.
We begin with a duct 30 inches in total length. One end of the duct is open to the atmosphere. Although it is really under pressure from the miles of atmosphere stacked on top of it, for our purposes, we will use it here as a point of reference, as most automotive pressure gauges show, and label it zero PSI. In reality, this point would be the air filter chamber entrance opening. Or, if the filter membrane is exposed, as with pod type filters, the outside of the filter membrane.
The other end of the duct ends at the intake valve, where negative pressure is applied via the falling piston on the intake stroke.
For this illustration, I've chosen -30 PSI as what the piston has presented to the outlet side of the duct. It should be obvious that there must exist a gradient inside the duct representing a change in pressure between -30 a zero PSI. I've shown that as the red line in this idealized drawing. Note that at 10 inches from the negative pressure source, the pressure has changed (raised to -20 PSI) due to the effects of the air moving towards the partial vacuum source. (For this illustration, we will ignore the fluid viscosity and resistance to flow from the duct's features.) For illustrative purposes, we'll say the gradient is a linear relationship of change over length.
The next drawing shows the duct shortened to 20 inches (representing the change from the stock air induction to pod filters or velocity stacks). Still measuring 10 inches from the negative pressure source, we see that the pressure has changed from -20 PSI to -15 PSI, as it is now closer the pressure equalization source (inlet). The carburetor position didn't physically move in the duct relative to the negative pressure source. The duct was just cut off so it's relative position in the duct DID change.
Why is this significant?
On the lower drawing, I have added a carb fuel bowl and a fuel jet tube. At the bottom of the jet tube is zero PSI as is what the atmosphere presents to the fuel level and jet tube inlet. The negative pressure at the carb air duct pulls the fuel through the jet tube at a rate proportional the differential pressure applied to the jet tube ends. For a given tube size (or orifice size inside the tube), the volume of fuel delivered reduces when the duct throat pressure reduces its drawing power.
Note that the volume of air transferred in the duct can be any value, as duct resistance to flow is not a factor. If the piston and valve timing are the same in both duct length examples, the air volume remains constant in both examples of duct length. However, a change in duct pressure DOES change the volume of fuel mixed with the air, and thus changes the air fuel mixture delivered to the engine. To restore the mixture ratio that was correct for the 30 inch duct in the 20 inch duct, the fuel jet tube orifice must increase in size to allow more fuel to pass with a lower pressure differential applied to it.
There's more to it of course. Inlet duct entrance design, filter membrane characteristic, duct obstructions, venturi effect, turbulence, etc.
If there's interest, perhaps I will continue with inlet duct physics. But, that's all for now.
Cheers,
