What is a Hot Air Motor / Stirling Motor?

The hot air motor is a periodic thermal energy engine, which transforms thermal energy into mechanical energy. There are three different types of Stirling motors: alpha, beta and gamma. In the picture below, a beta type Stirling motor is displayed. All consist of a cylinder, in which a working piston and a displacement piston with a momentum phase shift of 90°.

In the upper part of the cylinder, gas is heated, while gas in the lower part is cooled.

The displacement piston has the task of shifting the contained air from the upper into the lower portion of the cylinder, viz. from the lower into the upper portion. This occurs via a hole in the displacing piston, which is filled with copper wool. During "air discharge" the air flows through the whole with the copper wool, which transduces the thermal energy from the air to the copper and vice versa. Because of this characteristic of thermal discharge reversal, the copper wool can be called a regenerator.

In an ideal Stirling process, there are 4 consecutive processes that occur:

Click on the Picture to start

Click on the picture to start an animated sequence in a separate window.


1 .

The air expands isothermically (also at constant temperature).

2 .

It becomes isochoric after the expansion (meaning at a constant volume) and cooled by the regenerator.

3 .

After this, it becomes isothermically compressed.

4 .

The air again becomes isochoric, while heated in the regenerator back to the original temperature.

These four consecutive steps describe the ideal Stirling process.
Unfortunately, an ideal Stirling process cannot be realized, since a continual run of the machine cannot be constructed in which isochoric state changes can remain intact.

In the animation of the shown motor with continual crank shaft movement, one can come close to the ideal process through the phase shift during the movement of the working pistons and dischargers. The four steps overlap in this process: Thus the expansion and simultaneous gas exchange of hot for cold takes place, while most of the air is in the cold portion of the motor during compression.

The real Stirling process is represented by the ovular curve in the P-V diagram.

The motor only functions when sufficient temperature differences are present between the hot and cold sides. This difference is displayed in Kelvin (K) (Explanation Kelvin). In 1990, Dr. Senft at the University of Wisconsin and of Professor Kolin set the record for the practical lowest operation level of a Stirling motors at only 0.5° Kelvin, which remains the world record.

The produced mechanical energy is equivalent to the difference between added thermal energy and the energy emitted from the cooling element. Also noteworthy, is that a portion of the energy is a natural loss due to friction.

Further information about the “Function of Stirling Motors” can be found here.