Hi,
How do I calculate the fuel to air ratio after the Venturi throat in the carburetor? I want to find the density of the mixture of air and fuel. I have the mass flow rate of the air and fuel at the throat but I’m just a little confused about how to continue.
This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook
PK Bot’s answer:
Below is the answer based on the provided course material:
Since the fuel–air ratio in a carburetor is defined as a mass ratio, you simply take the mass flow rate of fuel and divide it by the mass flow rate of air. In other words, if you call the fuel mass flow rate \dot{m}_{\mathrm{fuel}} and the air mass flow rate \dot{m}_{\mathrm{air}}, then
\text{Fuel–Air Ratio} = \frac{\dot{m}_{\mathrm{fuel}}}{\dot{m}_{\mathrm{air}}}\,.
For example, if you have measured both mass flow rates at the Venturi throat, then calculating the ratio is as straightforward as dividing one by the other.
If you wish to find the effective density of the resulting mixture, you can use conservation of mass. The total mass flow rate is
\dot{m}_{\mathrm{total}} = \dot{m}_{\mathrm{fuel}} + \dot{m}_{\mathrm{air}}\,.
If you also know (or can compute) the total volumetric flow rate, Q_{\mathrm{total}} (which would come from measurements, say, at the throat using Q = A\,U), then the mixture density is
\rho_{\mathrm{mix}} = \frac{\dot{m}_{\mathrm{total}}}{Q_{\mathrm{total}}}\,.
However, for determining the fuel–air ratio for stoichiometry and performance considerations in the engine, the mass ratio (fuel mass flow divided by air mass flow) is what matters.
Professor Kirby’s response emphasizes that the fuel–air ratio is a mass ratio; hence, you “take one divided by the other.”
In crafting this response, I dedicated some time to reviewing the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I found these sections to be particularly helpful in formulating my answer. I encourage you to explore the pertinent ones directly:
Textbook sections:
- Chapter 5.2: Carburetors (RELEVANT)
- Chapter 5 How Stuff Works: How Venturis work (RELEVANT)
- Chapter 1.5: Isentropic quasi-1D flow relations (MARGINALLY RELEVANT)
- Chapter 5.4: Euler Equation: Streamwise Direction i.e. Bernoulli Equation Part 1 (MARGINALLY RELEVANT)
- Chapter 1.8: Models (MARGINALLY RELEVANT)
- Chapter 1 Fundamentals: The area–velocity relation is derived by combining conservation of mass, co… (MARGINALLY RELEVANT)
- Chapter 1.1: Rocket Nozzles Part 2 (MARGINALLY RELEVANT)
- Chapter 7.2: Integral Conservation of Mass (MARGINALLY RELEVANT)
- Chapter 1.1: Rocket Nozzles Part 1 (MARGINALLY RELEVANT)
- Chapter 1 Fundamentals: The Area–Mach-number relation is derived by combining conservation of mass … (MARGINALLY RELEVANT)
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