There are two bodies at two different temearatures | 16 2. THERMODYNAMIC ENGINE CYCLES Figure Ideal cycle process There are two bodies at two different temperatures Thermal energy q is exchanged by conduction from the hot body at temperature to the cold body at temperature 72 The change of entropy within the system after the transfer of thermal energy can be described by f - 2 1 Even when 71 72 there is a positive change of entropy NS due to the irreversible process of energy flow from the hot to the cold body. Thermal energy can only be transferred from hot to cold media and not in the opposite direction. The entropy of the cold body 2 is larger than the entropy of the hot body 1 as the entropy is inversely proportional to the temperature. A reversible state change is characterized by AS 0 and an irreversible state change is characterized by AS 0 which is also called the second thermodynamic law. It means that the entropy within the system always increases for real state changes. Ideal Cycle Process Assuming reversible state changes the following ideal cycle process can be described The gas absorbs thermal energy 71 in an isothermal and reversible state change at high temperature . The gas then loses the thermal energy 72 in an isothermal and reversible state change at low temperature 2- Kinetic energy wij2 is delivered by such an ideal engine which is equivalent to the difference of the thermal energies wij2 71 72- The entropy change shall be zero AS - 0 02 01 and therefore f 2 02 . INTRODUCTION TO THERMODYNAMICS 17 Figure pV-diagram and 5-diagram of Carnot Cycle. The thermal efficiency of this ideal cycle process is equivalent to the ratio of mechanical work v iv to the absorbed thermal energy gp. 1 2 91 - 92 . 2 1J ------------ ------1---- 1 91 91 1 1 It can be seen that the thermal efficiency p only depends on the ratio of the absolute temperatures to 2- is always smaller than 1. Please note that in non-ideal cycle processes the entropy change is always non-zero i. e. NS 0. .
