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Combustion and Flame - Fuel Efficiency and Calorific Value

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Fuel efficiency is measured in terms of its calorific value, which determines how much energy a fuel can provide upon combustion.

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The amount of heat energy produced on complete combustion of 1 kg1\text{ kg} of a fuel is called its calorific value.

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The unit of calorific value is kilojoule per kilogram, represented as kJ/kgkJ/kg.

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An ideal fuel should have a high calorific value, be cheap, readily available, easy to transport, and should not produce any poisonous gases or ash after burning.

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Combustion of fuels like coal and diesel releases sulphur dioxide (SO2SO_2) and nitrogen oxides (NOxNO_x), which dissolve in rain water to form acid rain.

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Incomplete combustion of carbon-based fuels (like wood, coal, petroleum) releases carbon monoxide (COCO), a very poisonous gas that can be fatal if inhaled in closed rooms.

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Increased concentration of carbon dioxide (CO2CO_2) in the air is believed to cause global warming, leading to a rise in the temperature of the Earth's atmosphere.

📐Formulae

Calorific Value=Total Heat Produced (in kJ)Total Mass of Fuel burnt (in kg)\text{Calorific Value} = \frac{\text{Total Heat Produced (in kJ)}}{\text{Total Mass of Fuel burnt (in kg)}}

Total Heat Energy (kJ)=Mass (kg)×Calorific Value (kJ/kg)\text{Total Heat Energy (kJ)} = \text{Mass (kg)} \times \text{Calorific Value (kJ/kg)}

💡Examples

Problem 1:

In an experiment, 4.5 kg4.5\text{ kg} of a fuel was completely burnt. The heat produced was measured to be 1,80,000 kJ1,80,000\text{ kJ}. Calculate the calorific value of the fuel.

Solution:

Given: Mass of fuel =4.5 kg= 4.5\text{ kg}, Total heat produced =1,80,000 kJ= 1,80,000\text{ kJ}. Using the formula: Calorific Value=1,80,000 kJ4.5 kg=40,000 kJ/kg\text{Calorific Value} = \frac{1,80,000\text{ kJ}}{4.5\text{ kg}} = 40,000\text{ kJ/kg}

Explanation:

The calorific value is obtained by dividing the total energy released by the mass of the fuel consumed.

Problem 2:

Compare the efficiency of 2 kg2\text{ kg} of LPG (Calorific Value =55,000 kJ/kg= 55,000\text{ kJ/kg}) and 2 kg2\text{ kg} of Coal (Calorific Value =25,000 to 33,000 kJ/kg= 25,000\text{ to } 33,000\text{ kJ/kg}). Which produces more heat?

Solution:

Heat from LPG: 2 kg×55,000 kJ/kg=1,10,000 kJ2\text{ kg} \times 55,000\text{ kJ/kg} = 1,10,000\text{ kJ}. Heat from Coal (max): 2 kg×33,000 kJ/kg=66,000 kJ2\text{ kg} \times 33,000\text{ kJ/kg} = 66,000\text{ kJ}.

Explanation:

LPG is more efficient as it produces significantly more heat energy (1,10,000 kJ1,10,000\text{ kJ}) compared to Coal (66,000 kJ66,000\text{ kJ}) for the same mass.

Fuel Efficiency and Calorific Value - Revision Notes & Key Formulas | CBSE Class 8 Science