In practical energy calculations, the calorific value of a fuel is usually expressed in which unit?

Introduction / Context:

This question is about the unit used for calorific value of fuels. Calorific value measures how much energy is released when a unit mass of fuel is completely burned. It is widely used in evaluating coal, petrol, diesel, LPG, and other fuels for engines, stoves, and power plants. Using the correct unit is important in thermodynamics and energy engineering.

Given Data / Assumptions:

• Calorific value refers to energy released per unit mass of fuel on complete combustion.
• We are working in the SI framework and also familiar with some practical units.
• Options list kN/kg, kJ/kg, kW/s, and kcal/s.
• We assume mass based calorific value, not per unit volume.

Concept / Approach:

In SI units, energy is measured in joules and mass in kilograms. Therefore, calorific value is naturally expressed as kilojoules per kilogram (kJ/kg) when dealing with large amounts of energy. This tells us how many kilojoules of energy are obtained from one kilogram of fuel. Occasionally, older systems use kilocalories per kilogram, but in strict SI, kJ/kg is preferred. Units involving newtons, watts or per second are not appropriate for calorific value, as they relate to force, power or rate rather than energy per mass.

Step-by-Step Solution:

Verification / Alternative check:

Standard tables of fuel properties list calorific values such as 42000 kJ/kg for petrol or around 33000 kJ/kg for coal. These values confirm that kJ/kg is widely used. Engineering handbooks may also show values in kcal/kg, but when converted to SI they become kJ/kg. None of these sources use kN/kg or kW/s as units for calorific value, since newton and watt represent different physical quantities.

Why Other Options Are Wrong:

• kN/kg: Newton per kilogram is related to acceleration (g) and is not an energy per mass unit.
• kW/s: Kilowatt per second would be a rate of change of power, not energy per unit mass.
• kcal/s: Kilocalorie per second would represent a power output, similar to watt, not energy released per kilogram of fuel.

Common Pitfalls:

A common mistake is to confuse power and energy. Power units involve time in the denominator, like watt (joule per second), while energy units do not. Calorific value is an energy per mass concept, so time should not appear in the unit. Another confusion occurs between kilojoules and kilocalories, as both are used in nutrition and fuel tables. Remember that one kilocalorie is approximately 4.2 kilojoules, and in pure SI usage, kJ/kg is preferred for calorific value.

Final Answer:

The calorific value of a fuel is usually expressed in kilojoules per kilogram, that is kJ/kg.