An introduction to thermodynamic cycles, also discussing the Carnot cycle
Key facts The thermal efficiency of any engine working between the temperatures of T1 and T2 is:
with the equality being satisfied only for the Carnot efficiency. Therefore, the Carnot cycle is the cycle with the highest possible thermal efficiency, while in order to increase the efficiency of all the other engines the (T2-T1) value should be increased.
The thermodynamic cycle consists of a series of operations (e.g. expansion/compression of a volume - see Figure 1) carried out on a gas or steam (which is often called a working substance, WS), after which it returns to its original state. For example, imagine that initially there is an expansion of volume from V1 to V2, corresponding to a work output of W1. Figure 2 shows such a process with a pressure (P) - volume (V) plot, where the work output (W1) equals the area under curve A 9 (shaded blue). Next the gas is compressed back to its original volume, i.e. V2 back to V1, which requires a work input of W2 (the shaded area under the curve in Figure 3). When we combine the two processes on a single plot, we get a closed loop (see Figure 4A). This closed loop corresponds to a thermodynamic cycle. As the net work output is given by: First Law of Thermodynamics ). However, as (the system is reverted back to its initial state), equation (2) becomes: