A heat engine has three components namely the heat source, the heat sink and the object where the work is done. Heat engines are used to convert heat to work. A 100% percent efficiency means all heat energy is converted into work energy. This means there is no heat lost to the heat sink. So, in this situation, the temperature should still stay the same. However, this cannot be done. A heat lost is needed in order for the heat engine to keep running continuously. Thus, in reality, a 100% efficiency is impossible.
