The D-90 Engine

The excellent performance of the D-60 engine convinced me to proceed with the next step, which was to redesign the engine for limited production. l decided that any production engine would have a cast block, rather than one carved from bar stock, to decrease the time necessary to machine it. The expense of the pattern for such a cast­ing suggested that due care should be exercised to select a swept volume for this new engine that would satisfy all my long term aims. After considerable indecision, I settled on 90 cubic centimeters swept volume, which offered the possibility of becoming a 500 watt air-charged engine, weighing under 5 kg, with a charge pressure of only 5 bar absolute.

The overall design of the D-90 is quite similar to that of the D-60, and its construc­tion presented no major challenges. The engine ran on the first try, as expected, and it was extremely quiet as a result of the pains I’d taken to reduce the clearances in the needle bearings by grinding custom oversized shafts for each bearing.

The power and speed on the initial runs on the brake, however, were extremely disappointing. The engine was expected to produce a peak power of about 100 watts for each atmosphere of pressurization, at a speed of about 3300 rpm. In fact, the engine produced only 48 watts at 2880 rpm at one bar, and 77 watts at 2724 rpm at 1.7 bar pressure.

Subsequent inspection revealed that the dome of the hot piston was heat discol­ored only to a pale yellow, not the dark brown that would be expected, indicating that the working gas was probably not getting above 275° C (550°F). There obviously was a problem with the heater.

The problem involved the method of brazing the fins to the inside wall of the heater can. The fins are corrugated out of nickel 0.015 inch (0.38 mm) thick, and are fur­nace brazed to the type 310 stainless steel heater can. In the heater ofthe D-60 engine, similar (but 33% thinner) fins were held against the wall of the heater can for brazing by an inner plug made of stainless steel. This plug was coated with stop-off material to

The D-90 Engine

The D-90 engine (above and on the following three pages)

The D-90 Engine

The D-90 Engine

Г—«

The D-90 Engine

Prevent it from being brazed to the fins, but there remained the risk that some of the stop-off would be scraped off during assembly, leaving the plug permanently attached to the fins.

With the thicker, stronger fins of the D-90, it seemed possible that the fin material could be fit tightly inside the heater can, fixturing itself during brazing, and dispensing with the plug. Unfortunately, this procedure did not work. Apparently gravity and the softening of the nickel fins at brazing temperature combined to allow the fins to ride up slightly on the internal radius at the closed end of the heater can. Consequently, the filler metal attached the fins to the can only at either end; their midsections stood away from the can wall, forming long, thin triangular gaps. Initial visual inspection did not reveal the flaw, since the visible end of the fins were nicely filleted with filler metal. Only after the discouraging initial test run of the engine was a proper visual inspection made, with a magnifying glass and a small flashlight to illuminate the fins from the closed end of the heater can. Then, the obvious triangles of light leaking from fin to fin revealed the cause of the poor performance.

A new heater assembly was promptly made, and the performance improved to a satisfactory level. Power tests were not conducted above 2.7 atm for the lack of an adequate test cell.

Originally, I had intended to construct two D-90s concurrently; one for me and one for Briggs & Stratton, for some work they hoped to pursue with John Hoke on catalytic combustion. Many duplicate parts had accumulated on my bench, but the work was go­ing so slowly that l decided to proceed with just one engine. No sooner was it running right than l had to deliver it to its new owners under my prior agreement. Fortunately, it would eventually return home.

In the meantime, there were other ideas I wanted to pursue, such as the rocker hot piston and the magnetic shaft drive.

For some time William Beale and I had realized that a yoke drive mechanism where the rocking lever length was made close to the length of the opposite yoke arm would produce a motion with that yoke arm that was nearly linear (US Patent 4,738,105). By attaching the hot piston, with its extended insulation dome to this yoke arm, one could eliminate the upper wrist pin and bearing (which hardly oscillates 2°, and is tough duty for a needle bearing), lighten the hot piston assembly considerably, and provide su­perior guidance for the piston, all at once. The challenges would be to provide proper lateral guidance for the piston tail rod, and to provide some means of initial adjustment to center the slight oscillations of the insulation dome. My solution was to put the entire hot cylinder (which could now be quite short) in the cylinder head, then properly locate the head and tix its position with dowel pins. This method had the additional advan­tage of permitting an unusually large upper rib to strengthen the one piece pressurized crankcase casting.

Magnetic drives, on the other hand, were not new. They had been used in centrifu­gal pumps for some time, and with the availability of super magnets they seemed to be

The D-90 Engine

The D-90 EngineThe E-70 engine (above) used the rocking expan­sion piston and the magnetic shaft drive ideas.

Its expansion cylinder did not extend beyond the cooler (upper left), which allowed very close cylinder spacing. The magnetic drive was later adapted to the D-90 engine.

A good way to get shaft power out of a sealed high speed stirling engine.

The D-90 engine block did not lend itself to incorporating either of these ideas, so I designed a 70cc engine, using the 2.250 inch (57.2 mm) bore of the D-90, with the 0.285 inch (7.2 mm) crankthrow and 2.250 inch (57.2 mm) cylinder spacing of the D-60. This engine was named the E-70, and it was intended to become the production engine that had proved so elusive.

The E-70 was relatively easy to make since it was simply a blend of the two pre­ceding engines. lt runs quietly, and apparently well, but some still unsolved problem prevents it from performing as it should. At atmospheric, for example, it produces a mere 49 watts, whereas it should be producing about 70 watts. Its free speed is 4690 rpm, which is 400 rpm short of the D-90 and over 600 rpm short of the D-60. All kinds of minor modifications to the regenerator, heater, pistons, seals, etc., have been tested without improvement in performance. This engine seals well, has no excessive dead vol­ume, appears to be reaching high internal temperatures, and has no apparent friction problems, and yet something is still quite wrong with it.

I managed to reacquire the D-90 about the same time I learned that my brazer had, at one stroke, ruined two modified heaters for the E-70 by mistakenly brazing the short fins into the long can, and the long fins into the short can. These events convinced me to put the enigmatic E-70 on the shelf for awhile, and refocus my attention on the reli­able and powerful D-90, and several variants thereof.

The V arrangement for the alpha engine has long seemed to me (and many others) an ideal form for small stirlings. lt is simple, robust, and easy to balance. The disadvan­tages are the side loadings on the pistons, which cause friction and wear problems in oil-less machines, and the relatively long distance between the cylinder heads that must be connected by the heat exchangers, which leads to excessive dead volume. Long connecting rods would reduce the side loading, but increase the distance between the cylinder heads.

Two different approaches to an oil-less V alpha had been simmering in my mind, and the return of the D-90 spurred me to try both in metal, based on the burner, heat exchangers, and hot piston of the D-90.

The first approach was a conventional V alpha with an unconventional stepped piston (US Patent 5,103,643). The idea was to combine the low dead volume of annular heat exchangers and short connecting ducts, with the low side loading and excellent balance provided by long connecting rods. This engine was named the V-90.

The other approach was to make a double 90cc engine, using two D-90 power heads mounted 90° apart from each other above a common crankshaft. In this arrange­ment conventional pistons with connecting rods of any desired length could be used, since each power head was a complete and separate engine, and no heat exchangers needed to cross the valley of the V. This engine was named the Double V-90. For simplic­ity, the prototype was made with only one power head (the only one I had), and the

The D-90 Engine

Connecting rods for the other power head merely drove dummy bob weights to allow proper balancing.

Both engines ran without noticeable vibration. Their power and speed, however, were very disappointing.

The V-90 produced 78 watts at 3120 rpm, at 1.3 atm., whereas the D-90 produced 123 watts at the same speed and pressure. Free speed for the V-90 was 4000 rpm, over 1000 rpm below that of the D-90. On each succeeding tear-down there were ever more teflon flakes on the stepped compression piston’s small wear band. Eventually, particles of hard coat anodizing from the small extension cylinder were being transferred to this wear band. Despite numerous theories and modifications, these problems were never solved.

The Double V-90 proved even weaker. Its free speed never exceeded 3000 rpm, indicating it had no power to spare at a speed where the D-90 would produce over 90 watts of excess power, atmospheric. This lackluster performance was very puzzling at the time. Only recently have I discovered that serious fretting had occured beside the crank-pin bearings of both connecting rods, which most likely would account for all the missing power.

These experiences with the V arrangement were helpful in re-convincing me that the yoke drive was truly worthy of my full attention. The D-90 was reassembled, and further testing proceeded with it.

Considerable thought was now given to speed control means. The first idea tried was a variable poppet valve interrupter gear. This mechanism allows one to variably open the work space to the buffer, starting at the low pressure portion of the cycle. In this way, one can delay the onset of the compression stroke, and thereby change the ef­fective swept volume, power, and speed of the engine. The mechanism required a cam or eccentric driven at crankshaft speed, and a variable fulcrum rocker to actuate the poppet valve, so it was not simple. lt was effective, but the nature ofthe alpha engine’s piston phasing is such that a great deal of the bottom of the cycle must be opened up to the buffer before any noticeable speed control occurs. But as one continues to open up more of the cycle, the opposite effect begins to be seen, and minute differences in control input make for great differences in speed.

I then tried a needle leak valve. This was very simple, but unstable in operation. lf the valve was adjusted to provide, say, 2000 rpm free speed, then any slight load on the engine would slow it down and allow more leakage per cycle, which would further slow the engine allowing yet more leakage, etc., and engine speed would rapidly decay to a stall. To be satisfactory, such a device would require some sort of feedback means, and thereby loose its sole virtue of simplicity. An attempt to couple this valve with a fixed dead volume was unsuccessful.

Another control means tested involved variably cutting off the high pressure por­tion of the cycle with a spring-loaded check valve. This approach was simple and effec­tive, but inefficient. An attempt to couple this valve with a fixed dead volume was also unsuccessful.

Various dead volumes without valves were made and tested. These proved very satisfactory and highly stable. As the dead volume was increased the speed and power dropped, but torque remained strong. The engine was quite resistant to stalling even at very low idle speeds. A three valve manifold was made to variably connect three dead volumes in proper sequence to provide eight steps of control.

A simpler approach involved a single large dead volume connected to the en­gine by a spring-loaded, hand-actuated poppet valve. This system allowed a surprising degree of control by merely varying the pressure on the valve, and by fully opening the valve a reliable idle was immediately available. Closing the valve would rapidly restore full power.

Several other promising speed control devices were tried, and I am confident that developing a simple, useful control will not be difficult. This work was great fun, because it is easy to come up with ideas to try, and a few hours of machine work is usually all that stands between the idea and its trial.

Another loose end was dynamic balance. With the preferred inverted yoke drive, the standard balance shaft (US Patent 4,138,897) necessary for complete primary bal­ance is located directly below the crankshaft, adding substantially to overall engine height. This balance scheme also requires piston masses to be equal, but the hot pis­ton in an alpha would normally outweigh the cold piston by a factor of 2 to 3. Adding mass to a lightweight piston naturally goes against the grain of any mechanician. It intuitively seemed that some sort of offset of the balance shaft toward the heavier hot piston could solve both of these problems, but a solution eluded me. Gary Wood finally found a means to achieve complete primary balance with pistons of different masses (US Patent 5,146,749). I promptly made a test rig for this idea, and it seemed to work, but I wanted to test it on a real engine at higher speeds before making a new crankcase incorporating it into the D-90. For this purpose, an upgraded version of the B-20 was made, incorporating the new balance scheme, an inverted yoke, and Rulon J cup seals. The dynamic balance proved to be excellent, and so a new crankcase for the D-90 was made that incorporates the balance shaft and also the magnetic shaft drive system originally made for the E-70 engine. This shaft drive system can readily be replaced with an alternator, if desired. Starting is accomplished by an o-ring sealed key incorporated into the engine, which can engage the crankshaft and turn it over compression.

Test runs showed the balance shaft doing a fine job, but the magnetic drive was absorbing about 50 watts of power. This loss was the result of hysteresis within the stainless steel pressure barrier used between the magnets. After a new pressure barrier was made from Delrin plastic, the magnetic drive losses diminished to insignificance.

In its current form, the D-90 is being periodically field tested in both an outboard rig and a mountain bike, which brings the story of my stirling work more or less up to the present time. Future plans include additional field and life testing, as well as inves-

The D-90 Engine

The poppet valved dead volume speed controller (below), and mounted on the D-90 engine (above).

The D-90 Engine

The D-90 Engine

The D-90 Engine

Tigation of a liquid fuel recuperative burner, air cooling, and increased pressurization. Then, perhaps, l can finally pursue my elusive aim of getting the engine into some kind of limited production, for sale to the many people searching for a small stirling for use in their various projects.

The D-90 EngineENGINE:

GENERALSPECS:

Type

Crank mechinism Crankthrow (dia)

подпись: generalspecs:
type
crank mechinism crankthrow (dia)

Beta Beta Alpha

Bell-crank Rhombic Yoke

-1.094 in 1.000 in 0.596 in

-27.8 mm 25.4 mm 15.1mm

Alpha

Alpha

Alpha

Inv. yoke

Inv. yoke

Inv. yoke

0.570 in

0.570 in

0.670 in

14.5 mm

14.5 mm

17.0 mm

Bore:

Exp or pwr cyl

Comp or displ cyl

Stroke:

Exp or pwr piston

Comp or displ pstn

подпись: bore:
exp or pwr cyl
comp or displ cyl
stroke:
exp or pwr piston
comp or displ pstn

-1.094 in

1.125 in

0.843 in

0.806 in

0.806 in

0.947 in

-27.8 mm

28.6 mm

21.4 mm

20.5 mm

20.5 mm

24.1 mm

-1.044 in

Same

Same

Same

Same

Same

-26.5 mm

62.3 cm3

65.4 cm3

35.4 cm3

60 cm3

60 cm3

89.6 cm3

62.3 cm3

75 cm3

25.1 cm3

42.5 cm3

42.5 cm3

63 cm3

Air

Air

Air

Air

Air

Air

12 to 15

2.7

1

2.3

3

2.7

450-688

81

44

104

200

230

25

26.7

45.8

41.7

55

55

650-800

-650

-650

-700

-650

-650

-17.3 Kg

3.4 Kg

2.2 Kg

подпись: -1.094 in 1.125 in 0.843 in 0.806 in 0.806 in 0.947 in
-27.8 mm 28.6 mm 21.4 mm 20.5 mm 20.5 mm 24.1 mm
-1.044 in same same same same same
-26.5 mm 
62.3 cm3 65.4 cm3 35.4 cm3 60 cm3 60 cm3 89.6 cm3
62.3 cm3 75 cm3 25.1 cm3 42.5 cm3 42.5 cm3 63 cm3
air air air air air air
12 to 15 2.7 1 2.3 3 2.7
450-688 81 44 104 200 230
25 26.7 45.8 41.7 55 55
650-800 -650 -650 -700 -650 -650
-17.3 kg 3.4 kg 2.2 kg

Net swept vol. Max. hot vol. Working gas Mean buffer press, (atm) Max pwr (watts) Speed at max. pwr (hz) Max temp (°C) Weight

HEATER:

(Inside)

Type

Heating

Material

Number of slots Slot width

Slot depth

Slot length

Length/width Fin aspect ratio

Cross sect. area

подпись: net swept vol. max. hot vol. working gas mean buffer press, (atm) max pwr (watts) speed at max. pwr (hz) max temp (°c) weight
heater:
(inside)
type
heating
material
number of slots slot width
slot depth
slot length
length/width fin aspect ratio
cross sect. area

-2.156 in 2.125 in 1.520 in

-54.8 mm 54.0 mm 38.6 mm

Same 2.280 in same

57.9 mm

Annular

Annular

Annular

Fins

Plain

Plain

Flame

Flame

Flame

Al. bronze

304 ss

304 ss

180

1

1

.0118 in

.020 in

.016 in

0.3 mm

0.51 mm

0.41 mm

.0984 in

N/a

N/a

2.5 mm

1.48 in

1.5 in

1.25 in

37.6 mm

38.1 mm

31.8 mm

125

75

78

2.9-1

N/a

N/a

CD

CD

CJ

Co

.09 in2

1.35 cm2

.955 cm2

.58 cm2

2.025 in 2.025 in 2.280 in

51.4 mm 51.4 mm 57.9 mm

Same same same

Annular

Annular

Annular

Plain

Fins

Fins

Flame

Flame

Flame

310 ss

Nickel

Nickel

1

224

197

.022 in

.0245 in

.030 in

0.56 mm

0.62 mm

0.76 mm

N/a

.090 in

.125 in

2.29 mm

3.18 mm

1.875 in

1.5 in

1.75 in

47.6 mm

38.1 mm

44.5 mm

85

61

58

N/a

9-1

8.3-1

.169 in2

.469 in2

.739 in2

1.09 cm2

3.03 cm2

4.77 cm2

Surface area 58.6 in2 11.1 in2

378 cm2 71.6 cm2

Dead Volume.309 in3 .222 in3

5.06 cm3 3.64 cm3

(Pressure wall)

Material ss ss

Heat trnsfr area 12.6 in2 11.1 in2

81.3 cm2 71.6 cm2

(Outside)

Type fins plain

Surface area 58 in2 11.1 in2

374 cm2 71.6 cm2

Material al. bronze ss

REGENERATOR

Type annular annular

Material sswire ssfoil

Mtrl thickness.0015 in

.038 mm

Height 1.25 in.625 in

31.8 mm 15.9 mm

Cross sect. area 1.42 in2 1.21 in2

9.16 cm2 7.81cm2

Total volume -1.6 in2 .91 in2

-10.3 cm2 5.87 cm2

Fill factor -25% -11%

Dead volume 1.25 in3 .76 in3

20.5 cm3 12.5 cm3

COOLER:

(Inside)

Type annular annular

Fins fins

Cooling air water

Material copper al alloy

Number of slots 180 120

Slot width.0118 in.020 in

0.3 mm 0.51 mm

Slot depth.0984 in.063 in

2.5 mm 1.6 mm

Slot length 1.48 in 1.5 in

37.6 mm 38.1 mm

Length/width 125 75

Fin aspect ratio 2.9-1

Cross sect. area.209 in2 .150 in2

1.35 cm2 ,968 cm2

Surface area 58.6 in2 29.7 in2

378 cm2 192 cm2

Dead Volume.309 in3 .225 in3

5.6 cm3 3.69 cm3

(Pressure wall)

Material al alloy

Heat trnsfr area 12.6 in2

81.3 cm2

7.1 in2

14.5 in2

68.7 in2

98.7 in2

45.8 cm2

CO

CO

СЛ

О

3

443 cm2

637 cm2

.113 in3

.316 in3

.704 in3

1.293 in3

1.85 cm3

5.18 cm3

11.5 cm3

21.2 cm3

Ss

Ss

Ss

Ss

7.1 in2

14.5 in2

11.7 in2

15.9 in2

45.8 cm2

93.5 cm2

75.5 cm2

102.6 cm:

Plain

Plain

Plain

Plain

7.1 in2

14.5 in2

11.7 in2

15.9 in2

45.8 cm2

93.5 cm2

75.5 cm2

102.6 cm:

SS

Ss

Ss

Ss

Annular

Annular

Annular

Annular

Ss foil

Ss foil

Ss foil

Ss foil

.0015 in

.001 in

.001 in

.001 in

.038 mm

.025 mm

.025 mm

.025 mm

.625 in

1 in

1 in

1 in

15.9 mm

25.4 mm

25.4 mm

25.4 mm

.662 in2

CO

1.43 in2

2.26 in2

4.27 cm2

9.23 cm2

9.23 cm2

14.6 cm2

.414 in2

1.43 in2

1.43 in2

2.26 in2

2.67 cm2

9.23 cm2

9.23 cm2

14.6 cm2

-11%

-11%

-11%

-11%

.368 in3

1.27 in3

1.27 in3

2.01 in3

6.0 cm3

20.8 cm3

20.8 cm3

32.9 cm3

Flat plate

Flat plate

Annular

Annular

Fins

Fins

Fins

Fins

Water

Water

Water

Water

Al alloy

Al alloy

Al alloy

Al alloy

25

33

160

192

.028 in

.028 in

.016 in

.020 in

0.71 mm

0.71 mm

0.41 mm

0.51 mm

.120 in

.185 in

.080 in

.100 in

3.0 mm

4.7 mm

2.0 mm

2.54 mm

1.75 in

1.96 in

1.094 in

1.938 in

44.5 mm

49.8 mm

27.8 mm

49.2 mm

62.5

70

68.4

97

2.7-1

4.3-1

.084 in2

.171 in2

.205 in2

.384 in2

.542 cm2

1.1 cm2

1.32 cm2

2.48 cm2

11.7 in2

25.7 in2

30,8 in2

81.9 in2

75.5 cm2

166 cm2

199 cm2

528 cm2

.147 in3

.335 in3

.224 in3

.744 in3

2.41 cm3

5.49 cm3

3.67 cm3

12.2 cm3

Al alloy

Al alloy

Al alloy

Al alloy

C-60 D-60 D-90

Crankcase

Speed

Power

Speed

Power

Speed

Power

Inlet pres.

Hz

Watts

Hz

Watts

Hz

Watts

1.0 atm.

38.7

54.2

1.3 atm.

46.4

81.2

54

102.6

51.4

128.5

1.7 atm.

42.9

94.4

52.9

133.3

52.8

155.8

2.0 atm.

41.7

104.3

54.9

159.2

54.3

184.6

2.3 atm.

42.2

105.5

53.8

173.2

53.4

208.3

2.7 atm.

54.4

190.4

54.8

230

3.0 atm.

55

200.8

TABLE 3

Dead Volume Experiments D-90 Engine (engine’s own dead volume=~148cc)

Added Volume Free Speed (hz) @ Approx Heater Temperature

~550°C ~600°C ~650°C

TOC o "1-5" h z 60cc 58 66 69

290cc 30 32 36

503cc 18.1 18.5 19.4

1025cc ~5 — -10

This much-modified engine was the first of the "Rocker — V" type alpha machines. As the name implies, the engine is balanced like a 90° V-twin, and employs a rocking lever to connect the compression piston to the single crankpin. v

A recently completed model of the Denney im­proved Ericsson (left) made by the author and his son Bryce. What began as a father-son project was blatently taken over by Dad when it started looking good. The engine has a one inch bore, and it runs nicely from the heat of a short candle concealed within the firebox. No special provision for cooling has proven necessary.

The D-90 EngineA stirling engine (or compressed air) driven model of a Swiss railcar (below), made by the author and described in the March 2002 ModelTec magazine.

The D-90 Engine

The D-90 Engine

The author (center) with Harald Berg and their interpreter Hilde, aboard Berg’s wood-fired stirling-powered boat near Askim, Norway, one summer day in 1984.(photo courtesy of Sigmund Kydland).

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