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Category Engineering Calculators

Fuel Selection

Fuel TypeSelect fuel to auto-fill stoichiometric AFR

Stoichiometric AFRAuto-filled from fuel type (editable for custom)

Calculation Mode

Input Mode

Lambda (λ)Measured lambda value (0.5 – 5.0)

Actual AFRMeasured air-fuel ratio by mass

Air Mass FlowMass airflow rate in kg/h

Lambda (λ)Operating lambda for this calculation

Actual AFR—:1 by mass

Lambda (λ)—

Equivalence Ratio (Φ)—

Excess Air—%

Fuel Mass Flow—kg/h

Mixture—

Lambda Gauge

0.5 Rich1.01.5+ Lean

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About

Incorrect air-fuel ratio causes catalytic converter failure, increased NO_x emissions, detonation knock, and up to 25% fuel waste. The stoichiometric AFR for gasoline is approximately 14.7:1 by mass, meaning 14.7kg of air per 1kg of fuel. This calculator derives the actual air-fuel ratio from lambda (λ) or vice versa, computes the equivalence ratio Φ, and quantifies excess air percentage for 15 common fuels. All stoichiometric values are sourced from SAE J1829 and ISO 16183 combustion standards. The tool approximates ideal combustion assuming complete oxidation, dry air composition of 21% O₂ by volume, and no dissociation at high temperatures.

Pro tip: real-world engines rarely operate at λ = 1.0 continuously. Most gasoline ECUs target λ = 0.85 - 0.90 under wide-open throttle for cooling, and λ = 1.00 - 1.05 at cruise for efficiency. Diesel engines run lean by design (λ = 1.3 - 2.0). Note: this model does not account for humidity, altitude corrections, or fuel blend variations such as E10/E85 intermediate mixtures.

Formulas

The air-fuel ratio represents the mass of air divided by the mass of fuel entering the combustion chamber. Lambda expresses this ratio relative to the chemically ideal (stoichiometric) mixture.

λ = AFR_actualAFR_stoich

The equivalence ratio is the reciprocal of lambda.

Φ = 1λ = AFR_stoichAFR_actual

Excess air percentage quantifies how much additional air beyond stoichiometric is supplied.

EA = (λ − 1) × 100%

Fuel mass flow rate from known air mass flow.

m_fuel = m_airAFR_actual

Where λ = lambda (relative AFR), Φ = equivalence ratio, AFR_actual = measured air-fuel ratio kg/kg, AFR_stoich = stoichiometric AFR for selected fuel kg/kg, EA = excess air %, m_air = air mass flow rate kg/h, m_fuel = fuel mass flow rate kg/h.

Reference Data

Fuel	Chemical Formula	Stoich AFR	Density kg/L	LHV MJ/kg	Octane (RON)	Carbon %	Hydrogen %
Gasoline (Petrol)	C₈H₁₈	14.7	0.745	43.4	95	84.2	15.8
Diesel (No. 2)	C₁₂H₂₃	14.5	0.835	42.5	-	86.1	13.9
Ethanol (E100)	C₂H₅OH	9.0	0.789	26.8	108	52.2	13.1
Methanol (M100)	CH₃OH	6.4	0.792	19.7	108	37.5	12.5
E85 (85% Ethanol)	Blend	9.8	0.783	29.2	105	57.0	13.5
LPG (Propane)	C₃H₈	15.7	0.493	46.4	112	81.8	18.2
LPG (Butane)	C₄H₁₀	15.4	0.573	45.7	92	82.8	17.2
CNG (Methane)	CH₄	17.2	0.656	50.0	120	75.0	25.0
Hydrogen (H₂)	H₂	34.3	0.071	120.0	130	0.0	100.0
Biodiesel (B100)	C₁₉H₃₆O₂	13.8	0.880	37.3	-	77.0	12.0
Kerosene (Jet A)	C₁₂H₂₆	14.5	0.810	43.0	-	84.7	15.3
Acetylene	C₂H₂	13.2	1.097	48.2	-	92.3	7.7
Nitromethane	CH₃NO₂	1.7	1.137	11.3	-	19.7	4.9
Ethane	C₂H₆	16.1	0.546	47.8	115	80.0	20.0
Toluene	C₇H₈	13.5	0.867	40.6	120	91.3	8.7
Wood Gas (Syngas)	CO + H₂ mix	1.0	Varies	5.0	-	Varies	Varies

Frequently Asked Questions

Modern engines run slightly lean at idle (λ = 1.01-1.05) to reduce hydrocarbon emissions and improve catalytic converter light-off efficiency. This is normal closed-loop behavior controlled by the ECU using the narrowband or wideband oxygen sensor feedback. If lambda exceeds 1.10 at idle, suspect a vacuum leak, faulty injector, or fuel pressure issue.

At higher altitude, air density decreases (approximately 12% per 1000 m elevation). A naturally aspirated engine ingests less air mass per cycle, effectively richening the mixture if fuel delivery remains constant. Turbocharged engines compensate via boost control. This calculator uses sea-level stoichiometric values. For altitude correction, multiply the actual AFR by the density ratio (ρ_altitude / ρ_sea_level).

Lambda (λ) is the ratio of actual AFR to stoichiometric AFR. Values above 1.0 indicate lean, below 1.0 indicate rich. Equivalence ratio (Φ) is simply 1/λ, so Φ > 1.0 means rich and Φ < 1.0 means lean. Aerospace and combustion research typically use Φ, while automotive industry uses λ. Both convey identical information in inverse form.

Yes. Select E85 from the fuel presets, which uses a stoichiometric AFR of 9.8:1. For custom ethanol blends (e.g., E20, E30), use the Custom fuel option and interpolate: AFR_stoich = (ethanol_fraction × 9.0) + ((1 − ethanol_fraction) × 14.7). Flex-fuel ECUs perform this interpolation in real time using an ethanol content sensor.

Hydrogen (H₂) has an extremely low molecular weight of 2 g/mol. Burning 1 kg of H₂ requires 8 kg of oxygen (stoichiometric), and since dry air is only 23.2% oxygen by mass, approximately 34.3 kg of air is needed. Despite this high AFR, hydrogen's gravimetric energy density of 120 MJ/kg means less fuel mass is required per unit of power output.

Below λ = 0.75, combustion becomes unstable. Unburned hydrocarbons saturate the catalytic converter, causing overheating and potential substrate melting. Spark plugs foul with carbon deposits. Fuel washes cylinder walls, diluting oil and accelerating bore wear. Exhaust temperatures paradoxically drop because incomplete combustion absorbs less heat. The misfire threshold for gasoline typically occurs near λ = 0.65-0.70.