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.
whereCDTand AT are the discharge coefficient and area of the venturi throat,respectively. If we assume the velocity at the carburetor inlet can be neglected,the above equation can be rearranged in terms of the pressure drop from upstream conditionsto the venturi throat for the air stream, Δpa = po - pT, as
and accounts for the effects of compressibility. For the normal carburetor operating range, where Δpa / po<=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 CDo and Ao, are the discharge coefficient and area of the orifice, respectively, Δpf, is the pressure difference across the orifice, and the orifice area is assumed much less than the passage area. Usually, the fuel level in the float chamber is held below the fuel discharge nozzle, as shown in Fig, to prevent fuel spillage when the engine is inclined to the horizontal (e.g., in a vehicle on a slope). Thus
where h is typically of order 10mm.
The discharge coefficient CDo 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.