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Does anyone actually calculate carburetors air to fuel ratio when designing one in practice? You would have to calculate the flowrate of air, the pressure drop, the rate of flow due to pressure difference etc. Do engineers just adjust them empirically until it's right?

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  • $\begingroup$ You can know the fuel flow at e.g. the "stoichiometric" point from the cylinder displacement, RPM - that determines air flow to the engine - and then the fuel chemistry . Although this isn't telling you anything about how to design the carb itself $\endgroup$ – Pete W Mar 3 at 21:24
  • $\begingroup$ Look at the number of features in the design of a fairly standard carb like an SU or Weber (e.g. sucarb.co.uk/technical-h-type-carburetter-diagram or carbparts.eu/exploded_view/WEBER_32-36_DGAV-DGEV). Then ask yourself how anybody would arrive at that design "empirically." $\endgroup$ – alephzero Mar 4 at 2:45
  • $\begingroup$ I don't design carburetors but I'd be amazed if a thing that is built by the tenthousands, and is critical for the operation of a car, is developed without some CFD studies /and or experiments of the actual airflows. $\endgroup$ – mart Mar 5 at 11:20
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Yes.

The range of airflow required is established by the swept volume of the engine and the max rpm. That will then define the pressure drop and venturi needed - which is often controlled by a moving piston to give a larger orifice for higher airflow.

Based on the air/fuel ratio the orifice for supplying the fuel can be designed and some (most) carbs use a tapered needle in the orifice to provide more fuel at higher airflow - the needle being moved by the same piston. Reece-Fish carbs had a different operation.

Some carbs had two venturies to cope with higher airflow, but various strategies have been used to cope with feeding fuel to larger engines as issues of a piston far from the carb such as an always weak mixture sometimes happened.

So, 2 or more carbs, or carbs with one venturi per cylinder - usually in pairs (Weber comes to mind) but then tuning the carbs so all is balanced becomes important. Matching airflow between carbs and also match fuel delivery is needed. A v8 where 4 cylinders are rich and 4 weak is not a happy thing.

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As a disclaimer before answering - I am NOT an actual engine or fuel system designer, but I tinker with them extensively, including adjusting A/F ratios for different fuels w/ different stoichiometric ratios and heat capacity ratios.

Short answer: It is most likely that carburetor designers do an initial sizing based exactly how Solar Mike described, by an effective swept volume of cylinder(s) + maximum designed RPM, giving them a maximum flow rate. With max air flow rate, they can use the air:fuel ratio that they want to determine maximum fuel flow rate. This would get them in the ball park for carburetor size and jet sizing. They would then likely follow up with empirical testing to fine tune based on combustion quality & characteristics.

This is where things get too complicated for simple calculations. A short list of major design variables:

  • Carburetor size (pressure drop & velocity for given flow rate)
  • Air flow mechanism (piston, butterfly etc)
  • fuel flow mechanism (single jet, multi jet, needle style & taper, injectors or port injection etc)
  • dynamic A/F ratio requirements across an RPM band

and many many more all depending on the various configurations of an internal combustion engine.

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