Calculate combustion air volume from fuel mass, air-to-fuel ratio, temperature, pressure, gas constant, and compressibility factor.
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Combustion Air Formula
The calculator uses the air mass required by the fuel and then converts that air mass to volume using the real gas form of the ideal gas law.
Rearranged forms used when a different field is left blank:
- m_air = mass of combustion air
- m_fuel = fuel mass or fuel mass flow
- AFR = air-to-fuel ratio on a mass basis, such as kg air per kg fuel
- V_air = combustion air volume or volumetric flow
- Z = compressibility factor
- R = specific gas constant of air
- T = absolute combustion air temperature
- P = absolute combustion air pressure
If you leave combustion air volume blank, the calculator finds the air volume needed for the entered fuel mass and air-to-fuel ratio. If you leave fuel mass blank, it solves how much fuel corresponds to the entered air volume. The same equation is rearranged to solve for air-to-fuel ratio, temperature, pressure, gas constant, or compressibility factor when exactly one of those fields is left blank.
Temperature is converted internally to kelvin, pressure to pascals, fuel mass to kilograms, gas constant to J/(kg·K), and air volume to cubic meters before the calculation is made.
Typical Air-to-Fuel Ratios and Input Reference Values
Use mass-based air-to-fuel ratios. Actual combustion systems may use excess air, so the entered AFR may be higher than the stoichiometric value.
| Fuel | Approximate Stoichiometric AFR by Mass | Notes |
|---|---|---|
| Methane | 17.2 kg air/kg fuel | Main component of natural gas |
| Propane | 15.7 kg air/kg fuel | Common LPG fuel |
| Gasoline | 14.7 kg air/kg fuel | Typical engine reference value |
| Diesel | 14.5 kg air/kg fuel | Approximate value, varies by composition |
| Hydrogen | 34.3 kg air/kg fuel | High AFR because hydrogen has low molecular weight |
| Quantity | Common Value | Use in the Calculator |
|---|---|---|
| Specific gas constant of air | 287.05 J/(kg·K) | Use for dry air in SI units |
| Atmospheric pressure | 101,325 Pa, 1 atm, or 14.696 psi | Enter absolute pressure, not gauge pressure |
| Room temperature | 20°C, 68°F, or 293.15 K | Temperature is converted to kelvin internally |
| Compressibility factor near ambient conditions | About 1.0 | Use 1.0 for ideal-gas air at low pressure |
Example Calculations
Example 1: Calculate combustion air volume
You burn 2 kg of methane and use an air-to-fuel ratio of 17.2 kg air/kg fuel. Air is at 20°C and 1 atm. Use R = 287.05 J/(kg·K) and Z = 1.
The required combustion air volume is about 28.54 m³.
Example 2: Calculate fuel mass from air volume
You have 10 m³ of combustion air at 300 K and 101,325 Pa. The air-to-fuel ratio is 14.7, R = 287.05 J/(kg·K), and Z = 1.
The corresponding fuel mass is about 0.800 kg.
FAQ
Should pressure be absolute or gauge pressure?
Use absolute pressure. If you have gauge pressure, add atmospheric pressure before entering it. For example, 0 psig at sea level is about 14.7 psia, 1 atm, or 101,325 Pa.
What value should you use for compressibility factor?
For air near atmospheric pressure and ordinary temperatures, use Z = 1. At high pressure or unusual temperature conditions, use a compressibility factor appropriate for the air state. A higher Z increases the calculated air volume, and a lower Z decreases it.
Is the air-to-fuel ratio stoichiometric or with excess air?
Enter the actual mass-based air-to-fuel ratio you want to model. If you are calculating theoretical combustion air, use the stoichiometric AFR. If the burner, furnace, or engine uses excess air, enter a higher AFR that includes that excess air.
