Heat engines and refrigerators

Heat engine is a device which is used to convert of heat into work or mechanical energy
Refrigerator is a device which is the reverse of heat engine.
Heat engine represents the following figure
In the above figure, source indicates the hot reservoir and sink indicates the cold reservoir. A body contains the engine is called working substance. Engine extracts heat from the source and some process done at it, after certain amount of heat is released to the sink. Again source produces heat to engine, engine use it and it do some work, and then certain amount of heat released to the sink. Like this, heat engine undergoes cyclic process.
Whatever heat extracted by the engine is used to work done by the engine transferred to the environment via some arrangement to drive vehicles.
The efficiency of heat engine is defined as the amount of heat turns to work done by the working substance per given heat to the engine or the ratio of work done by the engine to the amount of heat absorbed by the engine, From the above figure
Here Q1>Q2, T1>T2
In case of the refrigerator indicates the following figure.
In a refrigerator the working substance extracts an amount of heat Q2 from the sink at lower temperature T2. An amount of external work done W on the working substance and then the amount of heat Q1 given to the source at a higher temperature T1.