How does a Stirling engine work? Design and operation of an alpha-type hot-air engine

In this video, we look at the design and operation of an alpha-type Stirling engine. We show the advantages and disadvantages of such hot-air engines and focus in particular on the important function of the regenerator.

The Stirling engine essentially consists of two cylinders. One cylinder is constantly heated and cooled. In a Stirling engine of the so-called alpha type, the two cylinders are at right angles to each other, with the cylinder axes intersecting the axis of the crankshaft. The two cylinders are connected by a regenerator (a gas-permeable heat accumulator). The working gas is pushed back and forth between the cylinders via this pipe connection.

In the 45° position of the crankshaft, the gas volume is at its smallest and the gas is therefore maximally compressed. The total gas volume is made up of the volume of the heated cylinder, the cooled cylinder and the volume of the regenerator.

When the crankshaft moves half a revolution further, the total volume of gas is at its highest. The gas is then compressed again during the next half turn of the crankshaft. So during one revolution of the crankshaft you have the typical phases of expansion and compression.

However, for the engine to produce useful work, more work must be released during expansion than is required for compression. To achieve this, the pressure on the pistons during expansion must be higher than the pressure during compression. The pressure is primarily determined by the temperature. The higher the temperature of a gas in a closed system, the higher the pressure. Therefore, during the expansion of the gas, there must be a greater amount of heated gas to exert a higher pressure on the pistons. During compression, on the other hand, there must be a greater amount of cooled gas in the cylinders so that the pressure on the pistons is lower.

This is ultimately achieved by the cylinders being arranged 90° apart. During expansion, the volume of the heated cylinder increases much more rapidly than the volume of the cooled cylinder. As a result, there is more gas mass in the heated cylinder than in the cooled one. Therefore, the pressure during this expansion process is relatively high due to the relatively large heated gas portion. The expansion work provided by the gas is correspondingly large.

During the next half turn of the crankshaft, the expansion turns into compression. In this phase, the volume of the heated cylinder decreases more rapidly than the volume of the cooled cylinder. As a result, there is more gas mass in the cooled cylinder than in the heated one. Therefore, the pressure during this compression process is relatively low due to the relatively large cooled gas portion. The compression work done on the gas is correspondingly small.

The offset arrangement of the permanently heated and cooled cylinders ensures that the expansion work is greater than the compression work. As a result, useful work can be effectively extracted from the process. This useful work comes from the heat supplied by the heated cylinder, which transfers heat to the expanding gas, thereby raising its temperature.

However, even at this point, the supplied heat cannot be fully converted into useful work. Part of the supplied heat energy is removed from the gas during compression by the cooled cylinder to bring it to a lower pressure level. The difference between the heat supplied in the heated cylinder and the heat removed in the cooled cylinder corresponds to the useful work. In the volume-pressure diagram, the useful work is represented by the enclosed area within the cycle.

00:00 Desing and structure of a Stirling engine
01:09 How a Stirling engine works
02:33 Expansion
03:05 Compression
03:32 Heat input (heat supply)
04:22 Advantages and disadvantages of a Stirling engine
06:12 Regenerator (heat accumulator)
07:27 Outlook: Idealized Stirling cycle