The efficiency of a fuel is expressed in terms of _____

Concept

A fuel releases heat when it burns completely. Calorific value is the heat energy released by complete combustion of a unit mass of fuel, usually expressed in kilojoules per kilogram (kJ/kg). It therefore provides a common basis for comparing the energy yield of fuels.

Application

The blank asks for the property used to express fuel efficiency. Density measures mass per unit volume, and volume measures occupied space; neither states the heat obtained from a unit mass. Calorific value directly measures that heat yield.

Contrast

• Density: mass per unit volume; it describes compactness rather than combustion energy yield.

• Calorific value: heat released by complete combustion per unit mass; it measures fuel energy yield.

• Volume: space occupied by the fuel; it does not normalize the released heat by mass.

• More than one of the above: only calorific value uses heat released per unit mass, so at least two listed quantities do not share the required measure.

• None of the above: calorific value is present in the list, so the list is not empty of the required measure.

Cross-check

A larger calorific value means more heat is obtained from the same mass of fuel, which is exactly the comparison the question asks for. Therefore, fuel efficiency is expressed in terms of calorific value.