Combustion is a chemical reaction in which a fuel reacts with oxygen and releases thermal energy, often as a visible flame or a rapid pressure rise. Engineers use combustion in engines, gas turbines, furnaces, boilers, and burners, so controlling it affects power, efficiency, emissions, and safety. The air-fuel ratio compares how much air enters a system to how much fuel enters, usually by mass.
A correct mixture helps the fuel burn predictably and deliver useful energy.
Key Facts
- Air-fuel ratio by mass: AFR = mass of air / mass of fuel
- Equivalence ratio: phi = (fuel-air ratio actual) / (fuel-air ratio stoichiometric)
- Lambda: lambda = AFR_actual / AFR_stoichiometric = 1 / phi
- Stoichiometric methane combustion: CH4 + 2O2 -> CO2 + 2H2O
- Approximate stoichiometric AFR for gasoline: AFR_stoich = 14.7 kg air / 1 kg fuel
- Lean mixtures have lambda > 1, rich mixtures have lambda < 1, and stoichiometric mixtures have lambda = 1
Vocabulary
- Combustion
- Combustion is a chemical reaction between a fuel and an oxidizer that releases heat and usually produces light, hot gases, or pressure.
- Air-fuel ratio
- Air-fuel ratio is the mass of air supplied divided by the mass of fuel supplied to a combustion system.
- Stoichiometric mixture
- A stoichiometric mixture contains exactly enough oxygen to completely react with the fuel with no excess fuel or oxygen left over.
- Lean mixture
- A lean mixture contains more air than the stoichiometric amount, which usually leaves excess oxygen in the exhaust.
- Rich mixture
- A rich mixture contains less air than the stoichiometric amount, which can leave unburned fuel, carbon monoxide, or soot in the exhaust.
Common Mistakes to Avoid
- Using volume ratio when the problem asks for mass ratio is wrong because AFR is usually defined by mass in engineering calculations.
- Calling any high-air mixture stoichiometric is wrong because stoichiometric means exactly enough oxygen for complete reaction, not simply a lot of air.
- Assuming rich mixtures always make more power is wrong because too much fuel can reduce flame speed, waste fuel, and increase incomplete combustion.
- Ignoring nitrogen in air is wrong for exhaust and temperature analysis because most intake air is nitrogen, which carries heat and affects emissions even though it does not normally burn.
Practice Questions
- 1 A gasoline engine uses 0.050 kg of fuel and 0.735 kg of air during a short test. Calculate the actual AFR and decide whether the mixture is stoichiometric if AFR_stoich = 14.7.
- 2 A burner is supplied with 2.4 kg of methane and the actual AFR is 20.0. How many kilograms of air enter the burner, and is the mixture lean or rich if the stoichiometric AFR for methane is about 17.2?
- 3 Explain why a rich flame can produce carbon monoxide and soot even though combustion is still releasing heat.