K15

"with ring for ixot ope heat bui less isotope capsules. "** alt c'.iitii without comb- system ' 200 Shp. 3000 rpm. 1600

larger military submarines incorporating aluminum oxide heut storage.

(d) It designed rhombic engines for installation in buses.

(e) It included conceptual designs of a swash-plate automobile engine for Oldsmobilc.

(0 At the Allison Division, it included conceptual design studies of a solar-heated power-plant, a chemically fuelled space power-plant, an isotope-heated space power-plant, a rhombic-drive torpedo engine, an ASW power-plant to operate from hydrogen peroxide, a residential air-conditioner engine, and a back-pack power-plant to operate from indigenous fuels.

17. The program published eight engineering papers on Stirling engines, including their heat sources and application studies.

IS. It also produced internally 33(1 research reports and technical memoranda at the Research Laboratories.

Table 13.2 is a summary given by Percival (1974) of test or design data for many of the General Motors Stirling engines.

closure

Work on Stirling engines ai General Motors started in 1958 and formally ceased early in 1.970. It stands as the most sustained and concentrated effort ever made outside the Philips Research Laboratories and the achievements realized are impressive. The big question, of course, is why did they stop? Why did they stop, suddenly, in February 1970, with the Model 4L23 bus engine on dynamometer test and projected for vehicle installation a month or two later?

It is a question that has never been addressed in the literature but is important, for it has profoundly affected the development of Stirling engines in North America and the world. It is clear that the people working on the program were taken by surprise when the decision was made to terminate it. They were by no means at the end of the road. In fact, the program was rapidly accelerating with the culmination of many independent development studies in the design of optimized engines.

Hearsay has it that, at the time, the top management of General Motors was preoccupied by incipient problems concerning the brakes on G.M. school buses. Thus, when the time for renewal of the Philips licence came, it was easier to stop the work than to evaluate the situation properly. So it was that one day. out of the blue, the Stirling engine team was told to 'stop work on engines; tomorrow start on emission controls'. If hearsay is correct, then, in light of developments in the seventies, the decision must qualify as one of the most crass and stupid that industrial management has ever made.

Percival (1974) attributes the stimulation of interest in Stirling engines at General Motors to Mr. Arthur Underwood, not only for the initial interest in 1948 but also for successful negotiation with Philips of a licence agreement in 1958. In the light of the abrupt termination of work in 1970, it is not without interest that Mr. Underwood retired in 1969 as Manager of the General Motors Research Laboratories; he remains a public champion of Stirling engines (Underwood 1976).

The fate of the Stirling engine hardware developed in the course of the General Motors program is unknown. At least one of the GPU3 engines came into the possession of Wayne Stale University, Detroit, Michigan, and was used for the combustion and emission studies reported by Davis eí al. (1971 and 1972). The other GPU3s developed for the U.S. Army were acquired in 1976 by the NASA Lewis Research Centre as a part of the Stirling engine program for highway vehicles. Cairclli and Thieme (1977) have reported on tesLs carried out with these engines.

In May 1978 a collection of about 100 internal reports and memoranda prepared by General Motors during the tenure of their licence period was transmitted to NASA Lewis Research Center as a G.M. Special Publication (HefTner 1978). The reports are expected to be reproduced and circulated by the Stirling Engine Project OtFice of NASA Lewis or by the National Technical Information Service.

Superficial examination of a copy of the reports indicate that they contain much useful and interesting information but that they have been heavily expurgated of much material that would have added to their value. Further, it is well to recall that although a hundred reports have been made available this is in fact less than one-third of the 330 items referred to above. Many of the reports included appear to contain material referred to by Percival (1974). and if PercivaPs report provided the basis of selection for the material now released by General Motors, it therefore becomes the 'executive summary' of the new Special Publication. I; is to be hoped that these reports will be widely distributed and further, that the remaining two-thirds of the G.M. technical reports and internal memoranda will eventually be released to the public domain.

14 MAN/MWM STIRLING ENGINES

introduction

In l()67, two West German diesel engine manufacturers. Maschinenfab-rik Augsburg-Niirnberg (MAN) and Motorcnwerke Mannheim (MWM) formed the 'Entwicklungsgruppe Stirlingmotor MAN-MWM', and a licence and cooperation agreement was negotiated with Philips. The new company at Augsburg. West Germany, undertook scientific and development work directed to the development of heavy vehicle engines and underwater power systems.

Single - acting engines

Initial work included the acquisition of a 7.3 kW (10 hp) Philips 1-9S engine. Some test results obtained with this engine were included in the important review paper by Neelen et al. (1971) containing reviews of the Dutch, German, and Swedish developments on Stirling engines for vehicular use. The German contribution to the paper included the results for torque and efficiency characteristics, a ttoise spectrum distribution and an assessment of the significance of the dead space on the efficiency and power output of the 7.3 kW (10 hp) engine.

Following this early experience MAN/MWM designed a single-cylinder rhombic-drive engine, designated the Type 1-400, which developed 22 kW (30 hp) at 1500 rpm. The engine was equipped with all the auxiliaries for independent engine operation and was intended principally as a test unit for the development of a four-cylinder 90 kW (120 hp) engine designated the Type 4-400.

Aim et al. (1973) gave further information about the Type 1-400 and Type 4-400 engines. This paper was presented jointly with the Swedish licensees. United Stirling, but did not include a contribution from Philips. Ii was principally a recitation of the favourable environmental characteristics t»f Stirling engines (low noise and exhaust emissions). For vehicular use. however, it is noteworthy because it contains recognition by both MAN/MWM and United Stirling of their inability to reduce the production costs of rhombic-drive engines to an acceptable level. Both were therefore forced to seek cheaper alternatives in the form of multiple-cylinder double-acting engines of the Siemens type.

Double-acting engines

A cross section of the MAN/MWM four cylinder double acting engine is shown in Fig. 14.1. Two important features incorporated in the design are the simplified heater head and the air preheater assembly. Fig. 14.2 is a section of the heater-head configuration and Fig. 14.3 is a further detail

10hp Stirling Engine Heating Head

FlO. 14.1. Cross-section of MAN/MWM four-cylinder double acting Stirling cnjtinc (Aim ct ai 1973).

FlO. 14.1. Cross-section of MAN/MWM four-cylinder double acting Stirling cnjtinc (Aim ct ai 1973).

of the heater tubes. The heater consists of straight tubes of heat-resistant steels, one set of which is finned, joined in pairs at the upper ends by brazing a cast U-shaped tube element. The lower ends of the tubes are brazed to the regenerator casing or the expansion space cylinder. The tubes are arranged to form a plane heater screen.

The accordion-type preheater developed by MAN/MWM is shown in Fig. 14.4. This can be made at relatively low cost by simply folding a sheet metal strip to produce the plate-type recuperative heat exchanger shown. It will be recalled that low oxides of nitrogen in the engine exhaust have been achieved by recirculation of a significant proportion of the combustion products. One way to achieve this might be to use a 'leaky* recuperative exchanger. The accordion-type preheater is clearly

Heater

Cylinder

Heater

Cylinder

Etc;. '4.2. MAN/MWM heater-head configuration (Aim ef ul. 1973).

Etc.. 14.3. Detail of heater tubes (Aim ft ul. 1973).

Air inlet

Exhaust gas

Air inlet

Exhaust gas

Burner air

Fig. Í4.4. Accordion-type air prcheater (Aim el al. 1973), well suited for Ihis application. If rigorous separation of the Iluid stream is not necessary, the unit could be made at very low cost indeed. It provides an economically attractive alternative to the more complicated brazed plate-type recuperative exhaust heat exchangers featured elsewhere.

The new heater head and air preheater could be used on multiple or single cylinder rhombic-drive engines but it is clearly evident from their publications that MAN/MWM plan their future developments to be principally double-acting Siemens engines.

Zacharias (1974) drew a comparison of diesel and Stirling engines in terms of size and volume. Some of his data is summarized in Fig. 14.5. In the crank-driven, double-acting Stirling engine it was necessary to use a crosshead. Furthermore, it was customary to use a long piston/displaccr so the displacer seal operated always in the cold space of the engine. Good sealing of very high pressure hydrogen or helium was necessary where the piston rod leaves the cylinder. All this conspired to increase the height of the engine measured from the oil pan to the crown of the piston to be about 25 times the crank throw. In contrast, conventional diesel practice called for a height of about 10 times the crank radius. Parenthetically Zacharias noted the corresponding height for rhombic-drive and swash-plate engines was about 32 and 25 times the crank radius, respectively.

Some savings in height for the Stirling engine could be achieved by the

COMPARISON OF STIRLING AN Li DIESEL ENGINES

Bus Philips Stirling Engine

Jfj.-H'p Width of crank gear W'D

1^5 - 0.002 mVlcW Volume combust, syst. I'D - 0,001 mVkW

Jfj.-H'p Width of crank gear W'D

1^5 - 0.002 mVlcW Volume combust, syst. I'D - 0,001 mVkW

l-io. M.5. Size proportions of Stirling mid Diesel engines (Zachnrias 19~.|).

use of ;» small oil sump. The lubrication requirements were far less demanding than for the diesel engine and a reduced quantity of oil was required.

The cylinder centre-line distance of Stirling engines was larger (1.6x cylinder diameter) than that of diesel engines (1.3 X diameter) because the very high pressures used required thick walls. Also, the cylinders were divisible into hot and cold regions with heat-resistant steels used for the hot parts and a cheaper metal for the lower temperatures.

The width of both Stirling and diesel engines was found to be about the same with the inclusion of all the auxiliaries.

The work done per unit piston displacement or the brake mean effective pressure (bmep) was quoted by Zacharias to be about 0.8 MN/tti* (8 bar) for non-turbocharged diesel engines and 1.8MN/m* (18 bar) for Stirling engines having a mean cycle pressure of 11 MN/nr (110 bar). This significant difference (in the ratio of 18/8) was almost exactly the inverse ratio of the difference in height in terms of crank radius (from above HJHd = 25R/10R). Thus for engines of the same height the power output was about the same despite the smaller crank radius (and hence piston displacement) of the Stirling engine.

Zacharias estimated the combustion chamber and inlet-air preheater of a Stirling engine to be about 2 cm3/W. (90in Vhp) For the non-turbocharged diesel vehicle engine he estimated the cylinder heads, gas manifolds, mufllers and related parts to be about half this volume, i.e. 1 cm3/W (45 in*/hp). Zacharias also pointed out that the Stirling engine does allow some flexibility in arrangement so that, in fact, the increased size of the equipment related to combustion could be accommodated such that the space requirement of the engine would be no greater than that required for a diesel engine.

In vehicular use a further handicap for the Stirling engine compared with a diesel was that the cooling system had to handle about twice the load. Furthermore, the efficiency of the Stirling engine was directly related to the cooling water temperature and increased as the temperature was reduced. This was in contrast to the diesel engine where the efficiency increased with increase in the cooling temperature. In the Stirling engine therefore the quantity of heat to be transferred was double and the temperature limitations were more stringent.

Zacharias was less precise in regard to a weight comparison but made the point that the specific weights of both diesel and Stirling engines were comparable. With regard to Cost he was able to do no more than reiterate the attention MAN/MWM have paid to cost reduction by careful design for series production of a range of engines using Common components, connecting rods, crossheads, piston rods, piston seals, heater head assemblies, regenerator/cooler units, air preheaters, etc. Aluminum was used for the cold parts of the engine, heat-resistant steels foi the hot parts and. for the heater tubes, cast super alloys joined by vacuum brazing.

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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