Heat is transferred from the source , through the `` working body '' of the engine , to the sink , and in this process some of the heat is converted into work by exploiting the properties of a working substance -LRB- usually a gas or liquid -RRB- .

Heat is transferred from the source , through the `` working body '' of the engine , to the `` sink '' , and in this process some of the heat changes into work by using the qualities of the gas or liquid inside the engine .
