A float-type carburettor meters fuel from a bowl into an air passage. This illustration shows the main jet, discharge nozzle, venturi and open throttle as one connected circuit. The calculator estimates the bore of one equivalent fuel orifice at a specified airflow, air–fuel ratio and effective pressure difference.
Carburetter Interactive Calculator
Enter the air mass flow and desired mass air–fuel ratio, then the effective pressure across one equivalent fuel jet, its discharge coefficient and fuel density. The calculated bore changes in the enlarged inset. Flow tracers show direction; this is a steady operating-point calculation.
Equation Used
- Steady liquid flow through one equivalent circular orifice.
- Air–fuel ratio is a mass ratio, with both flows on the same time basis.
- Density, discharge coefficient and effective jet pressure difference are specified independently.
- The pressure input is not automatically equal to the air-side venturi depression; head and other passage losses must be accounted for separately.
- The body, nozzle, float and open throttle are schematic; only the enlarged jet bore follows the calculated diameter.
- The animation does not model atomization, evaporation, float transients, idle circuits or throttle response.
Retain all five inputs and four equations. Draw a connected updraft main-metering circuit. Pressure is now explicitly the effective jet pressure difference; the previous jet-number output is only a diameter conversion. Correct the worked example and unsupported tuning claims.
A connected main-metering circuit
The FAA float-type carburettor illustrations show a vented float chamber feeding a discharge nozzle through a metering jet. The nozzle discharges into the venturi. A float and needle valve regulate the bowl supply; the downstream throttle regulates the air passage.
The new cutaway follows that arrangement with upward airflow. The fuel passage stays connected from the submerged bowl pickup, through the main jet, to the nozzle. The float remains at a steady level and the throttle is shown open. Moving marks explain flow direction, not calculated fluid velocity.
The enlarged jet inset is linked to the calculated bore. Other body dimensions are illustrative because no body geometry is entered. Additional carburettor circuits are outside this simplified main-metering model.
What this calculator compares
Use the inputs to compare a single equivalent fuel-orifice requirement at different air mass flows, target mass ratios and effective jet pressure drops. Every input participates in the mass-flow and orifice equations.
The resulting bore is not a recommendation for a particular engine, branded jet or multi-barrel carburettor. The calculator does not distribute fuel between multiple jets or determine an engine operating map.
Fuel demand and equivalent orifice area
First compute fuel mass flow as air mass flow divided by the mass air–fuel ratio. Convert g/s to kg/s before using the liquid-orifice equation: ṁ = Cd A √(2ρΔp). Rearranging gives A = ṁ/[Cd√(2ρΔp)] and circular bore d = √(4A/π).
Here ρ is liquid density in kg/m³ and Δp is effective jet pressure drop in Pa, converted from the kPa input. Volume flow is ṁ/ρ, converted from m³/s to cc/min by multiplying by 60000000. The fourth result multiplies the bore in mm by 100; it is explicitly a unit conversion.
At unchanged fuel demand, increasing effective pressure reduces required bore with the inverse fourth root of pressure. At unchanged pressure and coefficient, doubling fuel demand increases bore by √2. The calculator does not derive pressure from the air flow or venturi geometry.
Default operating-point example
With 30 g/s air and a mass ratio of 14.7:1, fuel demand is 2.0408 g/s. At 740 kg/m³ that is 165.47 cc/min. With Cd = 0.70 and an effective jet pressure drop of 3 kPa, the equivalent circular bore is 1.3273 mm.
Multiplying that diameter by 100 gives 132.73 hundredths of a millimetre. It is not a universal jet number. For unit comparison, 1 inch equals 25.4 mm; a 3.50 mm bore is approximately 0.138 inch, not 0.069 inch.
Limits of a single-orifice estimate
The fuel demand and equivalent bore are algebraic results at one entered operating point. A real carburettor also has pressure losses, fuel lift, changing air density, air bleeds and additional circuits. These are not determined by the five inputs.
Discharge coefficient is an entered flow parameter, not a constant assigned here to every jet. Likewise, a selected mass ratio does not establish a recommended mixture for an engine. The animation clarifies the fuel route without claiming to simulate those missing effects.
Questions about the main-jet calculation
Is the fourth result a manufacturer jet number?
No. It is the bore expressed in units of 0.01 mm. Manufacturer designations may follow different conventions and require their own dimensional or flow data.
Does venturi depression equal the pressure input?
Not automatically. The input is the effective pressure across the fuel jet. Fuel lift and pressure losses elsewhere in the circuit are not separately modelled.
Why does the float not bob continuously?
This is a steady operating-point illustration. The float is shown maintaining a constant level; no bowl-filling transient is calculated.
Do the moving dots represent actual velocity?
No. They identify the connected flow paths. The calculated quantities are fuel mass flow, volume flow and equivalent bore.
Linear actuator checks for Carburetter Mechanism
An actuator-driven version of carburetter mechanism should be reviewed for stroke, linkage angle, and peak force. The mechanism may feel light through most of the travel and still bind or spike in force at one position.
Where springs or counterbalances are involved, calculate the actuator force over the full travel. The highest load may occur at a different position than the most obvious one.
If the application needs synchronized motion, feedback should be treated as a requirement rather than an accessory. Two open-loop actuators wired together can drift apart when friction, load, or mounting geometry differs between sides.
- Map actuator stroke to the actual output travel before choosing a model.
- Check for binding, changing leverage, and peak load near the ends of travel.
- Decide whether timed motion is acceptable or position feedback is required.
For a FIRGELLI design review, calculate the load case with the FIRGELLI actuator force calculator and plan the switching or controller arrangement with the linear actuator wiring diagram generator. If an actuator or cylinder is already installed, the linear actuator replacement finder helps keep the comparison tied to real dimensions.
References & Further Reading
- FAA Aviation Maintenance Technician Handbook—Powerplant, chapter 2, figures 2-10 and 2-12 (pages 2-9 and 2-10): float-type carburettor components and main-metering fuel route. The new cutaway simplifies this arrangement to one steady main circuit.
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