# Simplified Carburettor (Venturi Principle) Problem

1. Apr 9, 2013

### EulerLeonhard

1. The problem statement, all variables and given/known data

A carburettor in a petrol engine works on a Venturi principle as sketched below.

The pressure difference across the contraction draws fuel up from the reservoir and ejects it through a circular cross-section pipe into the main airstream, where it mixes with air. A particular carburettor has a contraction area ratio on the air side of 0.75. The fuel side has a coefficient of discharge of 0.9 and is required to discharge 1.25 cc/s of fuel of relative density 0.7 when the airflow at the carburettor inlet is at a velocity of 50 m/s.

Calculate the diameter of the fuel pipe at discharge.
2. Relevant equations
Cd= Vactual/Vtheoretical
P + 0.5 x ρ x u2 + ρ x g x h = constant
min=mout

3. The attempt at a solution

1) A2/A1=0.75
2) Apply the Bernoulli Principle between points 1 & 2 ( where both fuel holes are located ) for the airstream. P1-P2 = 0.5 x ρair x [ V2-u2] , we also know that
uA1=VA2, therefore V=66.67 m/s . Now we can calculate the pressure difference P1- P2 = 1196.1 Pa.

I am unsure as to how to proceed from here.

Your guidance would be greatly appreciated !

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