Short Answer:

Stoichiometric combustion means the perfect burning of fuel using the exact amount of oxygen needed so that all fuel and oxygen are completely used with no leftover air or fuel. This condition gives maximum heat output with minimum harmful emissions like carbon monoxide or unburnt hydrocarbons.

In mechanical engineering, stoichiometric combustion is an important concept for designing engines, boilers, and burners. It ensures clean and efficient combustion by maintaining the correct air-fuel ratio, especially for fuels like petrol, diesel, or natural gas in controlled environments.

Detailed Explanation:

Stoichiometric combustion

The term stoichiometric combustion comes from chemistry and thermodynamics. It refers to a chemical reaction where a fuel burns with the exact amount of oxygen required for complete combustion. In this process, no oxygen is left unused, and no fuel remains unburned. The only products formed are usually carbon dioxide (CO₂) and water vapor (H₂O).

This ideal condition is used in theory and practice to design efficient combustion systems in internal combustion engines, power plants, industrial furnaces, and even household gas appliances.

How stoichiometric combustion works

To understand this, we must know the basic components involved in combustion:

1. Fuel – A substance like petrol, diesel, coal, natural gas (mostly hydrocarbons).
2. Oxygen (from air) – Air contains about 21% oxygen by volume.
3. Heat or spark – To start the combustion process.

The goal of stoichiometric combustion is to balance the chemical equation so that all carbon (C) becomes CO₂, and all hydrogen (H) becomes H₂O, with no formation of carbon monoxide (CO) or unburnt fuel.

Example: Combustion of Methane (CH₄)

Methane is a simple hydrocarbon. Its balanced combustion equation is:

CH₄ + 2O₂ → CO₂ + 2H₂O

Here:

• 1 molecule of methane reacts with 2 molecules of oxygen.
• It produces carbon dioxide and water vapor only.
• No CO, soot, or unburnt fuel is formed.

This is the stoichiometric combustion of methane.

Air-Fuel Ratio (AFR)

In real systems, we often express stoichiometric combustion in terms of Air-Fuel Ratio:

• AFR is the ratio of the mass of air to the mass of fuel.
• Each fuel has its own stoichiometric AFR.

Examples:

• Petrol (gasoline): Stoichiometric AFR ≈ 14.7:1
  (14.7 kg of air for 1 kg of petrol)
• Diesel: Stoichiometric AFR ≈ 14.5:1
• Natural gas: Stoichiometric AFR ≈ 17.2:1

Maintaining this ratio ensures complete and clean combustion.

Importance of Stoichiometric Combustion

1. Maximum Efficiency:
   • All fuel is converted into useful heat.
   • No energy is wasted in unburnt fuel.
2. Minimum Pollution:
   • CO, NOx, and hydrocarbon emissions are reduced.
   • Produces less smoke and particulate matter.
3. Controlled Combustion:
   • Used in engine tuning and fuel injection systems.
   • Improves power output and reduces fuel consumption.
4. Safer Operation:
   • Prevents the formation of explosive gas mixtures.
   • Ideal for closed combustion chambers and controlled environments.

Real-World Conditions

In real applications, perfect stoichiometric conditions are hard to maintain. So engineers use:

• Lean combustion: More air than stoichiometric (excess air) → cleaner but cooler burn.
• Rich combustion: Less air than stoichiometric → more power but more emissions.

Sensors and controllers in modern engines help adjust fuel injection to stay close to stoichiometric conditions.

Conclusion:

Stoichiometric combustion is the ideal fuel-burning condition where the exact amount of oxygen is used to burn all the fuel completely, producing only carbon dioxide and water vapor. This concept is crucial in designing efficient and clean combustion systems in mechanical engineering. By maintaining the correct air-fuel ratio, we can achieve maximum energy output, minimal pollution, and safe operation of engines and boilers.