Help with calculating air mass through throttle body/orifice

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Discussion Overview

The discussion revolves around calculating air mass flow rates through a throttle body (TB) in an engine, particularly under wide open throttle (WOT) conditions and in boosted applications. Participants explore theoretical models, practical calculations, and the variables involved in airflow dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to model engine flow rates and mentions using the Saint Venant equation for mass flow rate calculations based on ambient air conditions and pressure.
  • Another participant inquires whether the calculations are theoretical or based on real data and suggests the importance of tuning or data collection software for accurate results.
  • A third participant offers a simplified approach, suggesting that at WOT, mass flow can be estimated using cylinder displacement and volumetric efficiency, while noting that this method may overlook losses and variations in air density.
  • The original poster expresses a desire to create a versatile equation that accommodates multiple engine configurations and variables such as displacement, throttle body size, air temperature, and density, particularly for boosted applications.
  • The original poster provides specific engine details, including bore and stroke measurements, and seeks to understand airflow through the TB across different pressure conditions, from maximum vacuum to 6 psi boost.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the best approach to calculate airflow through the throttle body, with multiple competing views on the methods and variables involved remaining unresolved.

Contextual Notes

Limitations include the need for assumptions regarding volumetric efficiency, the impact of losses in the system, and the dependence on specific engine characteristics that may vary widely.

my8950
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I am in the mindset right now of trying to model my engine and calculate various flow rates. What I'd like to do is make a calculator to show various rates of flow through the TB. I know it will be hard to determine the amount of opening anything other then full open/closed.
I'm at WOT, and I'm also boosted...
I have some info that I was reading that mentions Saint Venant equation to calculate the mass flow rate, using Ambient Air and Temp Upstream / PMan.
Can anyone give some help on how I can do this? I'd like to be able to get velocity, mass flow, and whatever else I can take from it as well.

Thanks!
 
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Are you interested in a theoretical calculation or real data? I can dig through Heywood and find you some info, but probably not for a while since I'm working this weekend. How about some more info on the engine and what you're wanting to do?

I'm hoping this is a fuel injected application (just for myself). If so, do you not have access to tuning or data collection software? If you don't then you're going to have to approximate certain things for the calculations.
 
I guess I am giong to give an over simplfed answer to this, because at face value it seems a very simple calculation. (depending of level of accuracy needed)

At WOT you just calculate cylinder displacement * volumetric efficency to find the mass used in a given cycle.

Assuming a single throttle body you can take it that this is the mass flow through it per cycle. You start to get problems if you want to include losses and variations of air density in the plenum.
The above is all very back of the envelope stuff which will suffice for a rough calculation, such as, "is my throttle body big enough?"

If you acually want to model the flow, or find more details such as losses, then you are going to need more info because the above calcualtion is woefully simplified.What are you tring to achieve?
 
Thanks for the info. I've got a Heywood here that I should look through again too.
Right now I am trying to get a equation worked out that I can use for multiple engines. So having the displacement would be a variable, as well as TB size, air temp, density, etc would be variables. Also I'd like to have it work for a boosted application as I'm running a turbocharger on my current setup. I am using my engine for the basis of all my work, but I'd like to have that as a variable so I can apply this all to other applications.

I've got a 4.030" bore, 3" stroke, not sure of the Ve on the engine at any specific loads, so that is a tough one to hone in on.

What I'd like to know is how much airflow is moving through the TB across the spectrum, from maximum vacuum to my current boost of 6psi.

Hopefully this gives some more insight...Any other info would be greatly appreciated...
Thanks!
 

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