Isothermal heat cycle, pressure-volume

Fig. 13.1. Pressure-volume diagram of an isothermal heat cycle, from [1]...

The Carnot cycle starts at state 1 with temperature db, volume V and internal energy U = U db) and it expands reversibly and isothermally to state 2, volume V2 with the same temperature db. Internal energy U2 = Ui = U db) does not change during this isothermal expansion. Balance (1.5) (here and in the following we use J = 1), in the form (1.12) for this reversible expansion (pressure follows from the ideal gas state equation (A.3)) gives at this db the heat Qb (the component of heat distribution at db of Carnot cycle)... 

Carnot cycle The idealized reversible cycle of four operations occurring in a perfect heat engine. These are the successive adiabatic compression, isothermal expansion, adiabatic expansion, and isothermal compression of the working substance. The cycle returns to its initial pressure, volume, and temperature, and transfers energy to or from mechanical work. The efficiency of the Carnot cycle is the maximum attainable in a heat engine. It was published in 1824 by the French physicist Nicolas L. S. Carnot (1796-1832). See Carnot s principle. 

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

In Fig. 5 the isobaric and isothermal pVT curves of P4SC are shown. Because of the very small changes in specific volume the first phase transition can hardly be seen in the isobaric experiment. The two transitions are resolved only in the heating runs. The phase transition temperature, Td, is shifted to higher values and a decrease in AVsp is observed for both phase transitions. The two transitions are separated by only about 7°C for all pressures investigated. In the isothermal experiment (Fig. 5, right) pressure-induced crystallization can be seen for temperatures above 120°C, concurrently with a decrease in Ksp. Crystallization experiments performed with different pressure cycles show no distinct changes in the phase behavior in particular, the pure J. phase could not be observed. 

---