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.

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🔷 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):

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

Where:

• $Q$ = airflow rate (m³/s)

• $A$ = venturi area (m²)

• $D$ = diameter of venturi (m)

• $V$ = 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

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🔷 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):

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

Where:

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

• $C_d$ = discharge coefficient (~0.6–0.8)

• $A_j$ = jet orifice area

• $\rho_f$ = fuel density

• $\Delta P$ = pressure difference across the jet

🔹 Jet Diameter:

$$d = \sqrt{ \frac{4 \cdot \dot{m}_{\text{fuel}}}{\pi \cdot C_d \cdot \sqrt{2 \rho_f \Delta P}} }$$

• Trial and tuning: Actual jet size is often fine-tuned experimentally for best performance, emissions, and fuel economy.

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📊 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)

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✅ 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.