This Timeline of heat engine technology describes how heat engines have been known since antiquity but have been made into increasingly useful devices since the 17th century as a better understanding of the processes involved was gained. They continue to be developed today. In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred to the sink from the source, and in this process some of the heat is converted into work.