Liquid Working Fluids

Closcd-cycle regenerative engines with liquid working fluids were described by John Malone (1931). They conform in every way to the definition for a Stirling engine given in Chapter 1. However, liquid cycle regenerative engines are sufficiently different to be classified separately, perhaps as Malone-cycle engines.

So far as is known. Malone presented only one account of his work. His paper was tantilizingly vague and non-technical but claimed that indicated elliciencies of 27 per cent were obtained. The paper contained photographs of a large two-cylinder engine and a smaller single-cylinder machine. They operated at low speeds of from 24 to 250 revolutions per minute. Very high pressures were used. In the description given, the pressure varied from a low value of 6.8 MN/nr (1.5 Ions per sq in) to a maximum of 27.4 MN/nr (6 tons per sq in).

The mechanical arrangement used by Malone to describe his engine operation was similar to that shown in Fig. 8.18. Two parallel cylinders contained reciprocating elements. One was a displacer including a regenerator and the other was a piston equipped with pressure seals. The arrangement corresponded in every way to the Heinrici/Stirling engine arrangement for a piston-displacer system in separate cylinders. The upper end of the displacer cylinder was heated and the lower end was cooled. The compression cylinder was cooled.

Malone gave the pressure of the working fluid with the elements disposed in the four positions shown in Fig. 8.18. A speculative set of

Hoi space

Displacer (regenerative)

Cold space lr

Fio. 8.IS. General arrangement and operating sequence of Malone engine with liquid n..:.« /-f.-- ----mu»

Stirling Malone Engine

(compression) Volume (arbitrary, units)

FtO. 8.19. Speculative work diagrams for Malone engine with liquid working fluid based on pressure data given by Malone (1931).

(compression) Volume (arbitrary, units)

FtO. 8.19. Speculative work diagrams for Malone engine with liquid working fluid based on pressure data given by Malone (1931).

work diagrams for the system based on arbitrary volume units and the pressures quoted by Malone are shown in Fig. 8.19. The system is clearly capable of producing work in exactly the same way as a conventional Stirling engine.

Liquids are generally thought to be incompressible, that is, to have the same density regardless of pressure or temperature. For most applications this assumption causes no significant error. However, liquids are compressible although, of course, very much less so than gases. It is this change in volume with pressure and temperature that provides the means to operate a liquid-cycle engine.

By way of example consider Fig. 8.20. This shows isotherms for water presented on the pressure-volume plane. Isotherms for 0°C (32 "F) through to b48°C (1200°F) are shown The critical isotherm for 270 °C (700 °F) is shown emphasized and the critical point is marked at the bottom right-hand corner. The critical isotherm is important for it sets the upper limit to the temperature at which a liquid can exist. Above this temperature, whatever the pressure, the fluid is a dense gas with some. but not all. the characteristics of a liquid.

The operating range of pressures for the Malone engine is marked tat the left) in Fig. 8.20. The temperatures of the working fluid in the Malone engine are not known but the furnace gas temperature was given as 1371 °C (2500°F). It is therefore likely that in the hottest parts of the cycle the temperature of the working fluid exceeded the critical temperature of 270 °C (700' F) so the fluid was in fact not liquid. A better description for the cycle might therefore he the Malone dense-phase cycle.

MnlnnA i»»itionf«»rl ihot tiA tiOfl r»r»/««-'i »<•/-! anninnr <«¡1

Specific volume (nv /kgXIO"4) I 2 3

Specific volume (nv /kgXIO"4) I 2 3

Stirling Engines

Specific volume (cu fl/lb)

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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