15 United Stirling Engines

introduction

Tm Swedish company. United Stirling A.B., was formed in 1968 by Forenade Fabriksverken (FF"V) and Kockums Mekaniska Verkstads A. B. FFV is a defence-related industrial group owned by the Swedish government. Kockums is a large publicly-owned Swedish company having its main business in shipbuilding and the lumber industry.

The declared aim of United Stirling is to commercialize the Stirling engine. Following a decade of development they appear to be well on the way to achieving this aim with both the technology and the will to do it. United Stirling will not manufacture engines on a production basis, but rather, will serve as design and development consultants for established engine manufacturers. An important development announced in 1977 by the U.S. Department of Energy was the formation ol' a second major Stirling engine development group for automotive engines including United Stirling, of Malmo, Sweden, Mechanical Technology Inc.. of Latham, New York, and American Motors Inc., of Detroit, Michigan.

single-acting vehicle engines

Since inception of United Stirling a decade ago, the vehicular application of Stirling engines has dominated their papers in the open literature. Principal emphasis has been given to the engines' advantageous characteristics from the aspect of atmospheric pollution, noise, and more recently, omnivorous fuel capacity and high thermal efficiency. Marine applications, in particular underwater power systems, have also occupied substantial effort and attention, but little in this field has been reported.

Soon after it was founded, the company became a Philips licence holder. In the important paper by Neelen et al. (1971), outlining work by United Stirling, MAN/MWM, and Philips, it was mentioned that work on the four-cylinder in-line rhombic-drive engine Type 4-235 was started at Philips in mid-1968 at the request of United Stirling. This engine has been described by deWilde de I.igne (1971). Swept volume was 235 cm3 per cylinder, the nominal speed and power were 3000 rpm and 145 kW (200 hp) at a mean pressure of 22.3 MN/nr (220 atm) helium with heater tube and cooler temperatures of 700 °C and 60 °C (1292 °F and 140 °F). respectively. I he early engines of this type were limited to a mean pressure of 11.1 MN/nv ( 110 atm) because the heater heads were made from conventional stainless steels rather than the high nickel alloys. Because of this low mean pressure the engines delivered only half power.

At United Stirling, engines in this form were installed for evaluation in a pleasure boat and a medium-sized bus (Aim at al. 1973). In the boat.

Bus Philips Stirling Engine

Fig. 15.1. Stirling engine Type 4-235 installation in pleasure boat. (Courtesy United

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Fig. 15.1. Stirling engine Type 4-235 installation in pleasure boat. (Courtesy United

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FlG. 15.2. Stirling engine Type 4-235 installation in bus. Note the cooling system installation in luggage compartment. (Courtesy United Stirling).

shown in Fig. 15.1. the engine was rated at 75 kW (101) hp), and at a speed of 2200 rpin drove the boat at 5.66 m/s (11 knots).

The bus installation is shown in Fig. 15.2. The engine was installed at the rear, in the same place as the conventional diesel engine, and was matched to an automatic split-torque type gear box comprising a hyd-rokinetic torque converter and two mechanical gears. Cooling was obtained by a hydrostatically-driven radiator fan. The cooling system could not be accommodated in the engine compartment but was installed in the underfioor baggage compartments. The grilles for this large cooling system can be seen in Fig. 15.2. Even with this it was necessary to increase the cooling water temperature and hence reduce power of the engine in the bus from 75 kW (100 hp) to 65 kW (SO hp) (Hallare and Rosenqvist 1977). No performance details of the bus or boat have been published but, no doubt, both provided invaluable installation and operational experience.

United Stirling also had available for laboratory use a number of the Philips single-cylinder rhombic-drive Type 1-98 engines. One of these was installed in a pleasure boat for evaluation. Another was engineered into the 2 kW (2.72 hp) generator set shown in Fig. 15.3 under a contract

Philips Stirling Engine Cooler

Fig. 15.3. 2kW generator set incorporating u Philips Type 1-98, single-cylinder rhombic-drive Stirling engine. This unit has been in service with tin- Swedish Navy lor 10 years.

(Courtesy I MJted Stj-fev-V

Fig. 15.3. 2kW generator set incorporating u Philips Type 1-98, single-cylinder rhombic-drive Stirling engine. This unit has been in service with tin- Swedish Navy lor 10 years.

(Courtesy I MJted Stj-fev-V

with the Royal Swedish Navy. Much later (Hallare and Roscnqvist 1977). this unit was said to be still in use on a Navy patrol boat. This machine must therefore qualify for the record 'time in service' for modern Stirling engines. A report on the maintenance and operating experience with the engine would be illuminating.

Neelen et at. (197 1) identified the first engine to be produced by United Stirling as the Type 4-615 engine shown in Fig. 15.4. 1'his engine was similar to the Philips Type 4-235 engine in that they were both four-cylinder displacer-type rhombic-drive engines of 145 kW (200 hp) rating, but the United Stirling engine was much larger.

In the Type 4-615 engine the efficiency was accorded a high priority and. to achieve this, a large slow-running engine was designed. Whereas the speed of Philips Type 4-235 engine was 3000 rpm, the speed of the United Stirling Type 4-615 engine was only 1550 rpm with internal gears to increase the shaft output speed to 2400 rpm. This slow speed and conservative values for the heater temperature and mean cycle pressure made necessary the large swept volume of 615 cm3 (37.5 in5) compared to 235 cm' (14.34 in ') of the smaller high speed engine. Despite these obvious handicaps Neelen et at. (1971) were sufficiently optimistic to

Bus Philips Stirling Engine
Fio. 15.4. United Stirling engine Type 4-615.

project prototype Operation in 1971 and for development to series production of the engine in 1976. Installation in a city bus was projected for 1973. As in so many other things, however, economic realities prevailed and these projections were never realized.

The optimism of the 1971 paper was tempered two years later with the publication of the milestone paper by Aim ei ai. (1973). There it was wryly noted:

The evaluation of the 4-615 engine with regard to performance, emission and costs has led to the following conclusions:

(a) although performance is satisfactory and emission characteristics are excellent the production cost of the engine makes it suitable for special applications only;

(b) major simplifications regarding basic engine design as well as components and control systems are necessary to make the engine generally competitive.'

A conversation in the early '70s is recalled at which a target figure for the cost of a production series Stirling engine was said to be twice the cost of a bus diesel engine of corresponding power. Typically, in a bus, the cost of the engine is about ten per cent of the gross vehicle cost. It was

Stirling Engine
ElO. 15.5. United Stirling double-acting engine Type V4X. (Courtesy United Stirling}.

said that for a ten per cent increase in the bus price (allowing the engine cost to double) operators would be glad 10 have the quiet, low-pollution characteristics of the Stirling engine. The target ligure of twice the cost of a diesel engine could not be achieved. It is understood the most realistic estimates for production costs for multi-cylinder displacer-type rhombic-drive engines were calculated to be nearer three times the cost of diesel of the same power.

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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