The Stirling cycle is a thermodynamic cycle that describes the operation of a class of equipment (generating or operating machines). The cycle describes the original Stirling engine that was invented and patented in 1816 by the Reverend Robert Stirling, helped substantially by his brother engineer.
The Stirling engine is an external combustion engine. This characteristic differentiates it from other types of engines such as the Otto engine or the diesel engine which are internal combustion engines. Both engines operate according to the Otto cycle and the diesel cycle respectively.
The Stirling cycle is reversible. This cycle can be used by generators to obtain mechanical energy from the application of heat and a cold source (a heat pump). You can also use this cycle to obtain thermal (heat) or cold energy by applying mechanical energy.
The Stirling cycle is a closed cycle, that is, the fluid that causes the cycle to be permanently contained in the apparatus that performs the cycle and does not exchange with the outside. A specific characteristic of the original cycle is that it is regenerative. A regenerative res cycle when you use a particular internal device called a regenerator. A regenerator is a heat exchanger-accumulator that increases efficiency.
The cycle is similar to many other cycles, where there are basically four phases:
As often happens in the comparison between ideal cycles and real cycles, the real cycle is not so perfectly separated into distinct and distinct phases. In the Stirling cycle, the superpositions of the different phases are particularly striking.
Ideal Stirling Cycle
The Stirling ideal cycle consists of four thermodynamic phases that act on the cycle fluid (see the diagram on the right):
- From point 1 to point 2: isothermal expansion. The expansion compartment is heated from the outside and the contained gas has an isothermal expansion.
- From point 2 to point 3: transfer of hot gas to constant volume or isochoric transformation; the gas passes through the regenerator yielding to this a part of the heat, which will remain available for a later phase.
- From point 3 to point 4: isothermal compression, the fluid in the compression space is cooled, the compression is imagined isothermal.
- From point 4 to point 1: heat transfer at constant volume; the fluid flows back through the regenerator, recovering the heat from the same regenerator.
Although the theoretical criterion is conceptually simple, the real thermodynamic analysis has involved physicists for a long time. The creation of an analysis model of the real cycle has not turned out to be a trivial task, since the ideal cycle has only a distant resemblance to the real thing.
The analytical problem of the regenerator (the central heat exchanger in the Stirling cycle) was judged as one of the most complex levels that can be found in Engineering.
Movement of mechanical devices in Stirling engines
Most of the texts dealing with the Stirling cycle follow the very simplified model of Stirling's ideal cycle. This way of proceeding is misleading given that if we calculate the areas of the ideal cycle (theoretically) appear very high energy yields in work. However, this would require mechanisms that are impossible to perform physically.
Actually, it is necessary to imagine a practical mechanism that achieves something that resembles the ideal cycle, using the real and habitual mechanical parts, such as the pistons, and the crank mechanisms linked to them.
The use of kinematics related to rotation produces, as is understandable, movements of sinusoidal-type parts. The set of sinusoidal movements, often with "crossed" pistons, transform the cycle, represented by straight lines or pure curves. in a kind of flattened "bean", in which the internal area (and, therefore, work) is drastically reduced.
Some kinematics, such as the so-called "Ross yoke" (Ross's rod), (a compromise link between the crossed head and a simple lever transmission), produce an almost sinusoidal movement. Other cinematisms produce different movements, the possible kinematisms govern the possible solutions, but most of the possible movements are not always compatible with all the contrast conditions of an ideal system.
Inverse Stirling Cycle
On the one hand, it is difficult to establish an efficient heat with pulsation, and effectively extract the energy from the pulsator system. On the other hand, it is also even more difficult to practice the reverse cycle. The reverse cycle involves obtaining heat or cold by the administration of mechanical energy.
With mechanical energy a pressure is generated to a confined fluid. The pressure to the fluid implies a compression and the generation of heat. On the other hand, the mechanical energy supplied can generate a depression to the fluid, an expansion thereof. This expansion absorbs heat energy, that is, a cooling. This is what is achieved in the Stirling refrigeration machine, obtained with conventional mechanical devices, with (cranks and pistons), or with the inverse use of the thermo-acoustic engine, where the mechanical pulsation is provided by resonant systems (linear motors), plates piezoelectrics) that operate at much higher frequencies.
PV diagram of a real cycle
PV diagram of a real Stirling cycle; four angular positions of the crank of the machine running the cycle are indicated
The real cycle can be represented in a pressure-volume diagram (PV) with a closed curve with a shape; this curve represents, with different values of pressure and temperature, most of the real Stirling cycles.