Short Answer:

Combustion is a chemical process in which a fuel reacts with oxygen to produce heat and light. It is commonly known as burning. During combustion, chemical energy stored in the fuel is converted into thermal energy. This process is used in engines, power plants, and various heating systems to produce energy for mechanical work or heat applications.

Combustion may be complete or incomplete depending on the amount of oxygen available. In complete combustion, carbon dioxide and water are the main products, while incomplete combustion produces carbon monoxide, soot, and other harmful gases due to a lack of oxygen.

Detailed Explanation :

Combustion

Combustion is one of the most important chemical processes in mechanical engineering and energy systems. It refers to the rapid chemical reaction between a fuel and an oxidizer, typically oxygen from the air, resulting in the release of heat and light energy. This process is exothermic, meaning it gives out energy in the form of heat. Combustion plays a key role in internal combustion engines, gas turbines, boilers, furnaces, and many other energy conversion systems.

1. Basic Concept of Combustion
   The basic principle of combustion is the combination of fuel with oxygen to produce heat. The general chemical reaction can be written as:
   Fuel + Oxygen → Products + Heat + Light
   For example, the combustion of methane (a common fuel) can be represented as:
   CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
   Here, methane reacts with oxygen to form carbon dioxide and water vapor while releasing heat.
2. Conditions Required for Combustion
   For combustion to take place, three essential elements are required:

• Fuel: Any substance that can burn, such as coal, wood, petrol, diesel, or gas.
• Oxygen (Air): Oxygen supports the burning process. Without sufficient oxygen, combustion becomes incomplete.
• Ignition Temperature: The minimum temperature required to start burning the fuel.

These three elements are often represented by the fire triangle—fuel, oxygen, and heat. Removing any one of them stops the combustion process.

3. Types of Combustion
   Combustion can be classified into different types depending on how it occurs:

• Complete Combustion:
  Occurs when the fuel burns completely in the presence of sufficient oxygen. The products are carbon dioxide and water vapor. It is efficient and clean.
  Example: Burning of natural gas in sufficient air.
• Incomplete Combustion:
  Occurs when the supply of oxygen is insufficient. The products include carbon monoxide, soot, or unburned hydrocarbons. This type of combustion is less efficient and produces harmful gases.
  Example: Burning of wood or coal with limited air.
• Rapid Combustion:
  Happens very quickly and produces heat and light almost instantly, such as in a gas stove or engine cylinder.
• Spontaneous Combustion:
  Occurs without an external ignition source when a material self-heats to its ignition temperature. Example: coal dust or oily rags can self-ignite.
• Explosion:
  It is an extremely rapid form of combustion that produces a large amount of gas and pressure in a very short time.

4. Stages of Combustion
   Combustion occurs in several stages:

• Preheating Stage: The fuel is heated up to its ignition temperature.
• Ignition Stage: The fuel starts to react with oxygen and the flame appears.
• Flame Propagation: The combustion continues as the reaction spreads to the unburned parts of the fuel.

5. Importance of Combustion in Engineering
   In mechanical engineering, combustion is the foundation of most power-producing systems.

• In internal combustion engines, fuel burns inside the engine cylinder to produce power.
• In steam power plants, combustion of coal or oil generates steam to run turbines.
• In industrial furnaces and boilers, combustion provides the heat necessary for various processes.

Understanding combustion helps engineers design efficient systems that use less fuel and produce fewer pollutants.

6. Combustion Efficiency and Pollution
   Efficient combustion ensures maximum energy output and minimal emission of harmful gases. Incomplete combustion produces carbon monoxide (CO) and unburned hydrocarbons, which are harmful to humans and the environment.
   To achieve clean and efficient combustion, engineers control the air-fuel ratio, temperature, and mixing of air and fuel. Advanced systems like catalytic converters and exhaust gas recirculation are used in vehicles to reduce pollution from combustion.
7. Energy Released During Combustion
   The amount of heat released during combustion is called the calorific valueof the fuel. It depends on the type of fuel and its composition. For example, hydrogen has a high calorific value, while wood has a lower one. Fuels with higher calorific value are preferred in engines and power plants for better performance.

Conclusion:

Combustion is a vital process that converts chemical energy in fuels into heat and power. It is widely used in engines, boilers, and many industrial systems. Efficient combustion provides high energy output and reduces air pollution. By understanding and controlling combustion, engineers can design systems that are both energy-efficient and environmentally friendly.