Specific Output

The size and weight of Stirling engines for a given power output is important in many applications, particularly in the automotive, locomotive, space, and under-watcr power fields. It is still important, bul rather less critical in the stationary generator, heal pump and total energy, and surface marine fields.

Savings in size and weight result from elevation of the pressure of the working fluid and from ingenious design. The rhombic-drive single-action. piston-displacer engines of the 1960s were relatively big, heavy machines. However, the conversion lo double-acting Siemens type Stirling engines (see Chapter 13) permitted the construction of machines thai were half the size and weight id' the rhombic-drive units. This brought Stirling engines to sizes and weights comparable with diesel and gasoline motors. Mcijer (1970b) presented the data reproduced in Fig. 9.16 which shows the specific weight as a function of power output for Stirling, diesel, and gasoline engines.

Power output (kW)

Fig. 9.16. Comparison of the specific output nf Stirling, Diesel, and gasoline engines as a (unction of the power output of the engines. Diesel engines lie in the upper shaded band; gasoline engines in the lower shaded band. The Stirling engines represented by lines A, I», C, and D are machines ut dillerent stages ol development (niter Mcijer iy70b>.

Power output (kW)

Fig. 9.16. Comparison of the specific output nf Stirling, Diesel, and gasoline engines as a (unction of the power output of the engines. Diesel engines lie in the upper shaded band; gasoline engines in the lower shaded band. The Stirling engines represented by lines A, I», C, and D are machines ut dillerent stages ol development (niter Mcijer iy70b>.

A comparison oí MAN/MWM Stirling traction motors with diesel engines of comparable power was given with considerable discussion by Zacharias (1974). The pictorial comparison given by Zacharias is reproduced in Chapter 14, Fig. 14.5.

United Stirling engines (see Chapter 15) have been installed in a variety of vehicles, cars, and trucks without modification to the engine compartment (Hallare and Rosenqvist 1977).

The principal disadvantage of the Stirling engine as regards size and weight is the latge. heavy cooling system that must be provided. The cooling system for a Stirling engine has to handle about twice the load of an internal combustion engine of comparable power because the heat lost to the exhaust must be minimized. This unfortunate condition was well illustrated in the comparative heat balance for a Stirling engine and diesel engine presented by Meijer (1970b) and reproduced in Fig. 9.17.


Economic factors are ultimately dominant in (he success or otherwise of new technical developments. At present there is little reliable information about the overall economics of Stirling engines compared with other engines. The use of heat-resistant steels or ceramic components and the relatively sophisticated control systems necessary make it impossible to conceive the production of Stirling engines comparable in cost with conventional internal combustion engines.

Cooling water


Friction lixhnust and mdiniion

Ui.ike horsepower

Friction lixhnust and mdiniion

Cooling water

Ui.ike horsepower

Fin. 9.17. Comparison of the heat batanees for Stirling and Diesel engine» of the same output (after Meijer 19711b).

A reasonable target might be for the Stirling engine to cost as little as twice the cost of an existing diesel motor. Recall here too that the diescl motor itself costs about twice the price of a gasoline engine. The Stirling engine may have economic advantages in terms of higher cllicjency, lower maintenance, and reduced lubricant consumption and so have lower operating costs than internal combustion engines.

Overall economic assessments for Stirling and diesel traction motors have been published by Rosenqvist et al. (1977) and their results emerge in favour of Stirling engines. However, this is not surprising for the study was done bv United Stirling of Sweden. Such comparisons always depend on a number of conjectural assumptions which may or may not be fulfilled so thai various conclusions can be drawn. It is necessary to remember that the diesel engine itself is a moving target. Presently, experimental diesel engines with ceramic components are now running on test beds with thermal efficiencies over 50 per cent. The application of ceramics to Stirling engines will surely improve performance but equal improvements may also result in other types of traction motor.

air engines

All the above discussion has been directed to advanced Stirling engines using hydrogen or helium as the working fluid at very high pressures with emphasis on high elTiciency and specific output. At the present time there is also substantial interest in a quite different class of Stirling engine, the small (less than 1 kVV (1.36hp)) thermal engine required for a variety of power or pumping applications. Reliability is most important and specific power relatively unimportant from both the weight and volume aspect. Thermal efficiencies of 20 per cent or so would be readily accepted, for this would be twice or more the conversion efficiency of the principal competition, the thermoelectric generator.

Willi no critical requirement for high efficiency oi specific output, the use of air as the working lluid is attractive as one approach to the seal problem. A minor leak of working lluid can be tolerated, for the air can be simply replenished by a small compressor driven by Ihe engine. In many applications very simple engine controls might be used, for the engine would operate at constant speed and load using isotope or liquid petroleum gas as the heat source.

The best Stirling cycle air engines were undoubtedly those developed in the first phase of Philips' work on the Stirling engine. These engines arc discussed in some detail in Chapter 12. One well-developed Philips air engine, similar lo that shown in Fig. 12.1. was combined with a power generator into the motor generator set shown in fig. 9.IS. Several hundred of these were made in a pre-production series but no commercial applications could be defined and so work on the engine ceased and further work was concentrated on the larger engines. Many of the small

Sears Roebuck Steam Engine

Cooling; from fnn

Fan-generator unit

F.nginc cylinder

Coolci Compressor

Frame containing compressed air for starting Heater

Eucl tank

Firj. <). 18. Philips Stirling engine/generator set Engine runs at 15110 revolutions per minute using air as the working fluid at a mean cycle pressure of 13.5 bar. Electrical output i:- 2110


engines that had been made were dispersed to educational establishments in Europe. The operating characteristics of this small engine were never published by Philips or any of the recipients.

In the early 1970s the operating characteristics of the engine were carefully measured by Ward (1972) at the University of Rath. This was done as part of a program of development for small Stirling engines for navigation signal beacons, encouraged by ihc Trinity Mouse Lighthouse Service, London, England, and by Atomic Energy of Canada Ltd. Ward's results arc given below in some detail for they have not yet found their way into the open literature and they provide an important point of departure for new development in the area.

'The engine used by Ward was removed front a Philips Type 102C motor generator set and modified to operate with water cooling and liquid petroleum gas (l.PG) as heat source. A cross-sectional view of the engine is shown in Fig. 9.19t, and particulars of the engine are summarized in Table 9.1.

' This remarkably fine drawing was originally prepared by Dr. A. Organ of Kings College, I .ondon. anil redrawn in modified form at the University of Calgary

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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