Carburetor venturi size is usually designed by the conventional equation for better performance even though some modern design has changed over the performance of carburetor in the present scenario.

**Venture size**

Carburetor
venturi size is usually designed by the conventional equation for better performance
even though some modern design has changed over the performance of carburetor
in the present scenario.

where**C _{DT}**and A

**and **accounts for the effects of
compressibility. For the normal carburetor operating range,** **where Δp_{a} / p_{o}<=0.1**,** the effects of compressibility which reduce Ф below 1.0 are
small.

**Fuel jet size**

Since the
fuel is a liquid and therefore essentially incompressible, the fuel flow rate
through the main fuel jet is given by

Where CD_{o}
and ** A_{o},**
are the discharge coefficient and area of the orifice, respectively, Δp

where h
is typically of order 10mm.

The
discharge coefficient CD_{o} in fuel flow rate equation represents the
effect of all deviations from the ideal one-dimensional isentropic flow. It is
influenced by many factors of which the most important are the following: (1)
fluid mass flow rate; (2)orifice length / diameter ratio; (3)
orifice/approach-area ratio; (4) orifice surface area; (5) orifice surface
roughness; (6) orifice inlet and exit chamfers; (7) fluid specific gravity; (8)
fluid viscosity; and (9) fluid surface tension. The use of theorifice Reynolds
number, Re, = ρVD,/μ, as a correlating parameter for the discharge co-efficient
accounts for effects of mass flow rate, fluid density and viscosity, and length
scale to a good first approximation. The discharge coefficient of a typical
carburetor main fuel-metering system orifice increases smoothly with increasing
Re.

**Modern Carburetor Design**

The
changes required in the elementary carburetor so that it provides the
equivalence ratio versus air flow distribution are

1. The win
metering system must be compensated to provide essentially constantlean or
stoichiometric mixtures over the 20 to 80 present air flow range.

2. An idle
system must be added to meter the fuel flow at idle and light loads.

3. An
enrichment system must be added so the engine can provide its maximum power as
wide-open throttle is approached.

4. An
accelerator pump which injects additional fuel when the throttle is opened
rapidly is required to maintain constant the equivalence ratio delivered to the
engine cylinder.

5. A
chokemust be added to enrich the mixture during engine starting andwarm-up to
ensure a combustible mixture within each cylinder at the time of ignition.

6. Altitude
compensation is required to adjust the fuel flow to changes in air density.

In
addition, it is necessary to increase the magnitude of the pressure drop
available for controlling the fuel flow. Two common methods used to achieve
they are.

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Mechanical : Advanced IC Engines : Spark Ignition Engines : Venture size |

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