Which use combustion to release thermal energy? furnaces and heat engines : this is the answer heat engines and heat pumps heat pumps and bimetallic coils bimetallic coils and furnaces

Heat of combustion. The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat.

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.