Venturi Calculation for a Carburettor

[Sachamaddison]

Hi there, I am in my first year at uni studying aeronautical engineering and I am doing a group project where we have to design and build a carburettor.

We have to calculate the venturi size for the narrower section


The capacity of the engine is 166cc and it is 4 stroke
We are going to run the engine at 3600 RPM
The entrance and exit size of the venturi is going to be 22mm (size of the manifold)

Thats pretty much all we have to work on, so far I have tried to calculate the volume flow rate
and from that work out the mass flow rate of the air and then the mass flow rate of the fuel.

I could not find out how to calculate volume flow rate of a 4 stroke engine so I did this.

I divided 3600 by 60 to get RPS = 60 then divided that by 4 as only 1 every 4 strokes actually draws air in, to get 15 then multiplied that by the capacity 15 x 0.000166 to get 0.00249 metres cubed per second.
from this I multiplied it by the density of air to get the mass flow rate of air then divided it by 14.7 (stoichiometic ratio) to get the fuel mass flow rate.

I'm not sure if any of this is right and from here I am stuck. I do not know how to figure out the air speed for bernoulli's equation or how to figure out the pressure difference.

Any help would be much appreciated, I'm working in a big group and we are all struggling.

Thanks, Sacha.

 

[Achy47]

Hello Sacha,

Thank you for reaching out for help with your project. It sounds like you and your group are tackling a challenging task, and I commend you for taking on a project like this in your first year of university.

Firstly, let's talk about the venturi size for the narrower section. The size of the venturi is determined by the desired air speed and pressure difference. In order to calculate these values, you will need to use Bernoulli's equation, which relates the velocity, pressure, and height of a fluid.

To calculate the air speed, you will need to know the volume flow rate, which you have already calculated correctly. However, instead of multiplying it by the density of air, you will need to divide it by the cross-sectional area of the venturi. This will give you the velocity of the air passing through the venturi.

Next, to calculate the pressure difference, you will need to know the velocity of the air and the density of air. You can then use Bernoulli's equation to calculate the pressure difference between the wider and narrower sections of the venturi.

I understand that you are struggling with determining the air speed for Bernoulli's equation. You can use the equation v = Q/A, where v is the velocity, Q is the volume flow rate, and A is the cross-sectional area. This equation will give you the velocity in meters per second.

Once you have calculated the pressure difference, you can use this value to determine the size of the narrower section of the venturi. You can use the equation ΔP = 0.5ρv^2, where ΔP is the pressure difference, ρ is the density of air, and v is the velocity. Rearranging this equation to solve for the cross-sectional area A, you get A = Q/v, where Q is the volume flow rate and v is the velocity.

I hope this helps you and your group make progress with your project. Remember to always double-check your calculations and assumptions, and don't hesitate to reach out for help if you need it. Good luck with your project!