Explain how the jet size & venturi size are determined for a carburetor used in SI engines

 

🔧 Determination of Jet Size and Venturi Size in a Carburetor (SI Engines)

A carburetor in a spark-ignition (SI) engine is responsible for preparing an air-fuel mixture by drawing fuel through jets and mixing it with air flowing through a venturi. The sizing of the jet and venturi is critical to ensure proper mixture strength, throttle response, and engine performance.


🔷 1. Venturi Size Determination

🔹 Purpose:

  • The venturi creates a low-pressure area to draw fuel through the jet.

  • Its size directly affects airflow capacity, engine breathing, and fuel-air mixing.

🔹 Factors Considered:

  • Engine displacement (cc)

  • Maximum RPM (engine speed)

  • Volumetric efficiency

  • Air density (altitude, temperature)

🔹 Formula (Basic Flow Consideration):

Q=AV=π4D2VQ = A \cdot V = \frac{π}{4} D^2 \cdot V

Where:

  • QQ = airflow rate (m³/s)

  • AA = venturi area (m²)

  • DD = diameter of venturi (m)

  • VV = air velocity at throat (m/s), usually around 80–100 m/s

🔹 Thumb Rule:

  • Venturi diameter ≈ 70–85% of intake valve diameter

  • Too small → restriction of airflow at high RPM

  • Too large → weak vacuum, poor fuel draw at low RPM


🔷 2. Jet Size Determination

🔹 Purpose:

  • The main jet meters fuel flow into the venturi throat.

  • Must ensure the correct air-fuel ratio (typically around 14.7:1 stoichiometric).

🔹 Factors Considered:

  • Venturi vacuum (ΔP) created at the jet

  • Fuel density and viscosity

  • Desired A/F ratio

  • Airflow rate (from venturi size)

🔹 Formula (Orifice Flow Equation):

m˙fuel=CdAj2ρfΔP\dot{m}_{\text{fuel}} = C_d \cdot A_j \cdot \sqrt{2 \rho_f \Delta P}

Where:

  • m˙fuel\dot{m}_{\text{fuel}} = fuel mass flow rate

  • CdC_d = discharge coefficient (~0.6–0.8)

  • AjA_j = jet orifice area

  • ρf\rho_f = fuel density

  • ΔP\Delta P = pressure difference across the jet

🔹 Jet Diameter:

d=4m˙fuelπCd2ρfΔPd = \sqrt{ \frac{4 \cdot \dot{m}_{\text{fuel}}}{\pi \cdot C_d \cdot \sqrt{2 \rho_f \Delta P}} }
  • Trial and tuning: Actual jet size is often fine-tuned experimentally for best performance, emissions, and fuel economy.


📊 Summary Table

Parameter Venturi Size Jet Size
Depends on Engine airflow, RPM, valve size Fuel flow required for target A/F ratio
Governing factor Air velocity and pressure drop Pressure difference and fuel density
Key impact Air intake capacity, fuel draw strength Fuel delivery rate
Typical range 20–35 mm (small engines) ~0.8–1.5 mm jet diameters (main jet)

Conclusion:

  • Venturi size is chosen to optimize airflow and vacuum signal for fuel draw.

  • Jet size is selected to deliver the right amount of fuel for the required air-fuel ratio.

  • Both are interdependent and are often calibrated experimentally for best engine performance and drivability.

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