The Stirling engine, originally known by its originator as the economizer, was invented in 1816 by Robert Stirling. It is an external combustion engine that operates by a sequence of pressure changes of a gas in a sealed enclosure that is converted into mechanical work. Because combustion is external, it is difficult to rapidly change the power output of a Stirling engine since it takes time for temperature changes to propagate throughout the engine. The result is that more of the Stirling engine’s energy is stored in the engine itself, meaning that Stirling engines tend to be larger for the same power output than internal combustion engines. While these facts make the Stirling engine a less attractive option for automobiles and other applications, the engine’s design results in a theoretical thermal efficiency equal to that of the Carnot cycle.
Each of the four stages of the ideal Stirling cycle is a fully reversible process, two of which involve heat transfer, one involving gas expansion, and the last the compression of the gas. More work is done in the expansion stage compared to the compression stage, and the difference is extracted from the system as work. The cycle operates between a maximum temperature at the hot end of the engine and the minimum temperature obtained at the cold end. The gas within is moved between the hot and cold regions by the displacer, resulting in a wave of pressure differences due to the temperature variation of the gas.
The first stage is the isothermal expansion stage (1 à 2). Heat from the external combustion is added to the system causing the working gas (air in this case) to expand and do work. The second stage is the isochoric displacement or cooling stage (2 à 3), in which the displacer piston shuttles the gas from the hot end to the cold end of the engine. Here, the working gas decreases in pressure and temperature. If a regenerator is implemented, the excess heat is absorbed by the regenerator to be used later in “pre-heating” the working gas in the final sequence. In the isothermal compression (3 à 4) stage, the cooled gas is compressed by the power piston in the compression region as heat is rejected to the cold region. During the last stage, known as isochoric displacement or heating (4 à 1), the gas is moved from the cold region to the hot region, thereby increasing the temperature and thus pressure of the working gas. A regenerative engine releases its stored heat from the second stage to help increase the temperature of the gas also. Thus, a fully regenerative Stirling engine can approach the efficiency of the Carnot cycle since all the heat-injection and heat-extraction sequences occur at isothermal places in the cycle. Heat loss stemming from imperfect regeneration is one reason Stirling engines cannot in practice achieve full Carnot efficiency. The P-v diagram for the ideal Stirling cycle is shown below.