4.7 - Fuel Efficiency

Fuel efficiency refers to how effectively a fuel is used to produce energy. It is an important concept in combustion as it directly influences the amount of useful energy we can obtain from a fuel compared to the energy lost during the combustion process. In the context of fuels, efficiency can be determined by various factors, including the energy content of the fuel, the completeness of combustion, and the technology used to burn the fuel.

When a fuel burns, it releases energy in the form of heat. The amount of energy released is called the calorific value of the fuel. The calorific value is typically measured in kilojoules per gram (kJ/g). The more energy a fuel releases per unit mass, the higher its calorific value and, generally, the more efficient it is as a fuel source.

Calorific Value

The calorific value of a fuel is defined as the amount of heat energy released when a unit mass or volume of fuel is completely burned. The unit of calorific value is expressed as joules per kilogram (J/kg) or kilojoules per gram (kJ/g). The formula to calculate the calorific value is:

C.V. = \dfrac{E}{m}

Where:

• C.V. = Calorific Value (in kJ/g)
• E = Energy released during combustion (in kJ)
• m = Mass of the fuel burned (in grams)

Complete and Incomplete Combustion

The efficiency of a fuel also depends on how completely it burns. When a fuel burns completely, it reacts with oxygen to produce carbon dioxide and water, releasing maximum energy. This is known as complete combustion. However, if the combustion is incomplete due to insufficient oxygen, carbon monoxide and soot (carbon particles) may form, reducing the efficiency of the fuel. Incomplete combustion is undesirable as it leads to wastage of fuel and release of harmful gases like carbon monoxide (CO).

Factors Affecting Fuel Efficiency

Several factors affect the fuel efficiency of a combustion process:

• Oxygen Supply: The availability of oxygen determines how efficiently the fuel burns. A higher oxygen supply ensures complete combustion and better efficiency.
• Temperature: Higher temperatures tend to increase the rate of combustion, leading to more efficient fuel use.
• Fuel Type: Different fuels have different calorific values. For example, LPG has a higher calorific value than wood, making it more efficient for burning in household stoves.
• Burning Technology: The design and technology of combustion devices (like stoves, engines, and boilers) also influence the efficiency. Modern engines and stoves are designed to optimize fuel use and reduce energy loss.

Efficiency of Common Fuels

The efficiency of fuels varies depending on their composition and calorific value. Some common fuels and their calorific values include:

• LPG (Liquefied Petroleum Gas): 55,000 kJ/kg
• Coal: 24,000 kJ/kg
• Wood: 15,000 kJ/kg
• Kerosene: 45,000 kJ/kg
• Natural Gas: 50,000 kJ/kg

By comparing these values, we see that fuels like LPG and natural gas have higher calorific values compared to coal and wood, which makes them more efficient for energy production.

Improving Fuel Efficiency

To improve fuel efficiency, steps such as using high-calorific fuels, ensuring complete combustion, and using advanced combustion technologies should be considered. Additionally, using efficient appliances that minimize energy loss, such as high-efficiency engines or stoves, helps in reducing the overall fuel consumption while maximizing the energy output.