1

Combustor blower and air filter

FiG. 15.19. United Stirling PI50, V8 Stirling engine installation in 13-ton medium-duty truck (alter Hatlarc and Roscnqvist 1977).

installation had a weight advantage of 40 kg (88.2 lb) compared with the dicscl installation. The higher initial cost of the Stirling engine was offset by the lower operating cost so that a pay-back period of 0.5 to 5 years will be achieved depending on utilization in the range 16 000 lo 1(10 000 km/year (9942 to 62 137 miles/year). The economic analysis depends on predicted price levels for fuels and lubricants in the 1980s and on performance penalties imposed on the diesel by anticipated regulatory emission requirements. In both these areas prognostication is so speculative that the credibility of such comparisons must be low. It would be better if specific information tor the Stirling engine in terms of original and operating costs could be published for other, disinterested, parlies to undertake cost comparisons.

The main thrust of United Stirling effort is clearly directed to vehicle application but other uses are foreseen. Marine applications, both surface and underwater, have always been of interest. Some aspects of the marine and stationary applications were discussed by Carlqvist et ai. (1977). In such applications it is possible to use lower water cooling temperatures (10—20 °C (50-68 C'F)) compared with automotive use (60-70 °C (140-158 °F)), but the gain in performance is largely offset however by the preference for helium as the working fluid rather than hydrogen. It is to be expected that marine versions of the P40, P75, and PI50 engines will be available.

Stationary applications will no doubt include electric power generation. Hallare and Rosenqvist (1977) touched briefly on the attractiveness of the Stirling engine for heat pumps. They outlined a district heating scheme for spacc heating (and cooling) and hot water production for large industrial or commercial residential buildings and estates of single or multiple family dwellings.

Lia and Lagerqvist (1973) have discussed experimental work on Stirling engines heated by heat pipes rather than direct combustion heating. Such a programme is indicative of interest in high-temperature, high-performance engines and of engines used in association with thermal energy storage systems for emission-free vehicles or underwater power systems.

Recently it was announced (NASA, 1977) that United Stirling was to be a partner with Mechanical Technology Inc. and American Motors in one of the two teams to be funded by the Department of Energy in the United States for development of an automotive Stirling engine.

Recent developments

Recent developments at United Stirling include the engine configuration shown in Fig. 15.20. This is a double-acting engine having four cylinders connected as shown in Fig. 15.7. It is therefore similar to the four-cylinder Vee engines described earlier. The big difference between the new engine and the previous Vee engines is that the cylinders are parallel and two crankshafts are used with two pistons coupled to each crankshaft. Twin gears are used to couple the crankshafts.

The relative merits of parallel versus Vee cylinders were discussed by Kitzner (1977a) as part of the study carried out by Ford on behalf of ihe U.S. Department of Energy, for an 60 to 74 kW (80 to 100 hp) automotive Stirling engine. The parallel cylinder design leads to a more complicated drive mechanism but allows a very compact heater and air preheater arrangement. Machining costs for the parallel cylinder arrangement arc likely to be much reduced.

It is not known which of the two engine forms is projected for production series engines.

Prcliciilcr 1-1 enter

Comhustei

Piston

C ross head-Drive 'Shaft-

Fuel-injector

Cylinder

Connecting roil Crank shaft

Oil puntp

Turbulrttor

Cooler

Piston rod

Piston roil seal

Fig. 15.20. Four-cylinder double-acting Stirling engine with parallel cylinder axes. {Courtesy United Stirling)

Conclusion

United Stirling now has a decade of experience on Stirling engine design, development and operation and they appear an aggressive, practical, commerically oriented team. This accumulated experience coupled with the substantial funding for development available from U.S. sources provides the greatest possibility that commercially viable Stirling engines will become available in the 1980s.

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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