Calculating Air Intake Pressure: A Beginner's Guide

In summary, the other day the author was using their OBD-2 scanner to find out various information about their car, including the intake's peek flow rate and the velocity of the air passing through the intake. They then calculated the pressure difference between the intake and the atmosphere, which they were unable to solve.
  • #1
Max_VQ
2
0
(First post!)

The other day I was using my OBD-2 scanner and found out that my car's intake has a peek flow of 25 lbs/min. I converted this to ~317 cubic feet (at 5.5 deg C)

I then calculated the velocity in the 3.5" diameter intake to be ~36 feet/sec.
The intake has a 2" venturi orifice (sound muffler) and calculated the flow at this point to be ~64 feet/sec.

Here is where I am stuck. How do I calculate the pressure difference?

Thank you in advance!
 
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  • #2
To accurately calculate the expected delta P you need to have the venturi's discharge coefficient. The coefficient is a function of the geometry and is calculated during tests. Venturis usually have pretty high Cv values when compared to an orifice (usually close to 1) but they do vary with Reynolds number.

The standard calculation for flow through a venturi device is

[tex]Q = C_v A_t \sqrt{\frac{2 \Delta P}{\rho(1-\beta^4)}}[/tex]

Where:
[tex]Q[/tex] = Volumetric flow rate
[tex]C_v[/tex] = Discharge coefficient
[tex]A_t[/tex] = Throat area
[tex]\Delta P[/tex] = Pressure difference
[tex]\rho[/tex] = Flowing density
[tex]\beta[/tex] = Diameter ratio
 
Last edited:
  • #3
I think the standard Cv is taken as 0.975 but varies with the Reynolds number.
 
  • #4
Just to add my 2 cents...since this is compressible flow, you'll probably need to include a gas expansion factor in Fred's equation. Although, if the delta P is less than 10% (if memeory serves me correctly), Crane TP410 say's that you can use incompressible flow as an approximation. So you'll probably be ok with the first equation.
 
  • #5
FredGarvin said:
The standard calculation for flow through a venturi device is

[tex]Q = C_v A_t \sqrt{\frac{2 \Delta P}{\rho(1-\beta^4)}}[/tex]

Where:
[tex]Q[/tex] = Volumetric flow rate
[tex]C_v[/tex] = Discharge coefficient
[tex]A_t[/tex] = Throat area
[tex]\Delta P[/tex] = Pressure difference
[tex]\rho[/tex] = Flowing density
[tex]\beta[/tex] = Diameter ratio

Wow! Thanks for the quick replies.

Are my units correct:
[tex]Q[/tex] = Volumetric flow rate in CFM
[tex]C_v[/tex] = Discharge coefficient
[tex]A_t[/tex] = Throat area in inches squared
[tex]\Delta P[/tex] = Pressure difference in inches of water??
[tex]\rho[/tex] = Flowing density in pounds/min
[tex]\beta[/tex] = Diameter ratio 3.5:2
 

1. What is air intake pressure and why is it important?

Air intake pressure refers to the amount of air that is entering an engine. It is important because it directly affects the combustion process, which ultimately determines the engine's power and performance.

2. How do I calculate air intake pressure?

Air intake pressure can be calculated by multiplying the density of air (which is dependent on temperature and altitude) by the velocity of the air entering the engine. This formula can be found in most physics textbooks or online resources.

3. What factors can affect air intake pressure?

The main factors that can affect air intake pressure include temperature, altitude, and engine speed. As temperature and altitude increase, the density of air decreases, resulting in a lower air intake pressure. Engine speed also plays a role in air intake pressure, as higher speeds can increase the amount of air entering the engine.

4. How can I improve air intake pressure?

There are a few ways to improve air intake pressure, including using a cold air intake system, increasing the size of the air intake, and adding a turbocharger or supercharger. These methods can help increase the density and velocity of air entering the engine, resulting in a higher air intake pressure and improved engine performance.

5. What are some common issues with air intake pressure?

Some common issues with air intake pressure include a dirty air filter, which can restrict airflow and decrease pressure, and leaks in the air intake system, which can result in lower pressure and potential damage to the engine. It is important to regularly maintain and inspect the air intake system to ensure optimal air intake pressure for your engine.

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