Combustion Calculator

Stoichiometric Air-Fuel Ratio (mass basis)

AFR_stoich = (a / 0.21) · 28.97 / M_fuel | with | a = x + y/4

Variables: AFR_stoich = stoichiometric air-fuel ratio (kg air per kg fuel), a = stoichiometric moles of O₂ required per mole of fuel (dimensionless; 2 for CH₄, 5 for C₃H₈, 12.5 for C₈H₁₈, 0.5 for H₂), x = carbon atoms per fuel molecule (dimensionless; 1, 3, 8, 0), y = hydrogen atoms per fuel molecule (dimensionless; 4, 8, 18, 2), 0.21 = mole fraction of O₂ in dry air (dimensionless), 28.97 = molar mass of dry air (kg/kmol), M_fuel = molar mass of the fuel (kg/kmol; 16.04, 44.10, 114.23, 2.016 respectively). Gives 17.20, 15.64, 15.10 and 34.21 kg air per kg fuel for methane, propane, octane and hydrogen. Dry air is modelled as 21% O₂ / 79% N₂ by mole with argon lumped into the N₂ and no humidity. Source: Cengel & Boles, Thermodynamics: An Engineering Approach, 8th ed., Ch. 15 (Chemical Reactions) — Fuels and Combustion, Theoretical and Actual Combustion Processes

Actual Air-Fuel Ratio and Percent Excess Air from Equivalence Ratio

AFR_actual = AFR_stoich / φ = ((a / φ) / 0.21) · 28.97 / M_fuel | excess air (%) = (1/φ − 1) · 100

Variables: φ = equivalence ratio (dimensionless; φ = 1 stoichiometric, φ < 1 lean with genuine excess air, φ > 1 rich with genuinely deficient air), AFR_actual = actual air-fuel ratio (kg air per kg fuel), AFR_stoich = stoichiometric air-fuel ratio (kg air per kg fuel), a/φ = moles of O₂ actually supplied per mole of fuel (dimensionless), a = stoichiometric moles of O₂ per mole of fuel (dimensionless), 0.21 = mole fraction of O₂ in dry air (dimensionless), 28.97 = molar mass of dry air (kg/kmol), M_fuel = fuel molar mass (kg/kmol). Percent excess air is POSITIVE for lean mixtures (φ < 1, e.g. +25% at φ = 0.8) and NEGATIVE for rich mixtures (φ > 1, e.g. −20% at φ = 1.25); the calculator's on-screen excess-air label transposes the lean and rich wording, so take the sign from this equation rather than from the label. Source: Cengel & Boles, Thermodynamics: An Engineering Approach, 8th ed., Ch. 15 (Chemical Reactions) — Theoretical and Actual Combustion Processes; Turns, An Introduction to Combustion, 3rd ed., Ch. 2 (Combustion and Thermochemistry)

Product Balance for Lean and Stoichiometric Mixtures (φ ≤ 1), per mole of fuel

C_xH_y + (a/φ)·(O₂ + 3.76·N₂) → x·CO₂ + (y/2)·H₂O + a·(1/φ − 1)·O₂ + 3.76·(a/φ)·N₂ | [valid for φ ≤ 1]

Variables: x = carbon atoms per fuel molecule (dimensionless; 1, 3, 8, 0 for CH₄, C₃H₈, C₈H₁₈, H₂), y = hydrogen atoms per fuel molecule (dimensionless; 4, 8, 18, 2, so the H₂O yield y/2 is 2, 4, 9, 1), a = x + y/4 = stoichiometric moles of O₂ per mole of fuel (dimensionless), φ = equivalence ratio (dimensionless), 3.76 = 0.79/0.21 = moles of N₂ carried with each mole of O₂ in dry air (dimensionless; the tool applies the exact ratio 3.762). All product quantities are moles per mole of fuel (dimensionless molar ratios), which is the basis the calculator reports. This balance conserves C, H, O and N only for φ ≤ 1. For φ > 1 the calculator continues to report CO₂ = x and H₂O = y/2 while setting O₂ = 0, which is NOT oxygen-balanced and overstates CO₂: a correct rich balance splits carbon between CO₂ and CO and hydrogen between H₂O and H₂, normally closed with the water-gas-shift equilibrium. Source: Cengel & Boles, Thermodynamics: An Engineering Approach, 8th ed., Ch. 15 (Chemical Reactions) — combustion with excess air and incomplete combustion; Turns, An Introduction to Combustion, 3rd ed., Ch. 2 (Combustion and Thermochemistry)

Adiabatic Flame Temperature — Tool's Empirical Scaling (not an enthalpy balance)

T_ad = T_ad,stoich · (0.7 + 0.3·φ) for φ ≤ 1 | T_ad = T_ad,stoich · max(0.5, 1 − 0.25·(φ − 1)) for φ > 1

Variables: T_ad = estimated adiabatic flame temperature (K; also displayed in °C as T_ad − 273.15), T_ad,stoich = tabulated stoichiometric adiabatic flame temperature in air (K; 2230 for CH₄, 2267 for C₃H₈, 2274 for C₈H₁₈, 2480 for H₂), φ = equivalence ratio (dimensionless), 0.7, 0.3, 0.25 and the 0.5 floor = dimensionless fitting constants hard-coded in the tool. Treat this tile as an order-of-magnitude indicator only: there is no enthalpy balance, no species Cp(T), no dissociation and no inlet-temperature input, the fuel's lower heating value is not used, the lean-side slope is far too flat (φ = 0.5 methane gives about 1896 K against roughly 1600 K from an equilibrium solution), and the peak is placed at φ = 1.0 rather than the real φ ≈ 1.05–1.10. Source: No published correlation — an in-tool heuristic. The rigorous definition is the constant-pressure adiabatic enthalpy balance H_reactants(T_in) = H_products(T_ad) with temperature-dependent species Cp and dissociation: Cengel & Boles, Thermodynamics: An Engineering Approach, 8th ed., Ch. 15 (Adiabatic Flame Temperature); Turns, An Introduction to Combustion, 3rd ed., Ch. 2