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Explanation and Analysis of Commercially Available Stirling Engine Demo Unit by StirlingEngine.co.ukTranslated by AltaVista,
June 14, 2004. Posted here for historic archive purposes.
It is undoubtedly very amusing to
build oneself its Stirling engine with weak variation in temperature, but it
is necessary well to acknowledge that that takes a little time and that that
does not function inevitably of the first blow as it would be liked...;o)
If you want to quickly try out an engine of this type, you will currently find on Internet a finished model ready to go, with decent performance and the reasonable price of £95.00, port included/understood - (and there are interesting bonds on the site!) One year ago, I had succeeded in
with me getting one of them on ebay.com
for £79,50, but the last left a few days ago with £127,00
(for which it is necessary to add £12.50 of carriage
costs for Europe) - if you are a player, try to obtain it in the
conditions of last year: there were of them then one or two on sale each
month, but they are done today increasingly rare - it is what perhaps explains
the rise of the prices... (seek "Stirling
engine") Of size about identical to the model simplified 1.1, its design much more mechanical ensures him of better performances. All the parts are carried out with precision, out of brass or anodic aluminium - (there exists in blue, red or green). The principal axis is assembled on ball bearings, and the polyamide piston is appairι with the drive roll to guarantee a reduced type font. As one sees it on the image of right-hand side, the sealing of the principal cylinder is ensured by two O rings. I did not go further in disassembling, but the originator of this engine ensures that the central pillar, the drive roll and the slide of the deplacor finely are threaded and screwed in the higher plate, and that the sealing is, lΰ-aussi, ensured by O rings. ___
Subjected to the same tests as the preceding models, this engine proves at the same time very sensitive and very rapid. It as well agrees to turn posed on the dish of the hand as to exceed 430 rpm on a boiling cup of water.
However, the first tests carried out with this Stirling enabled me to highlight some small weaknesses (it cannot have only qualities...;o): - initially, under penalty of having to dismount the engine completely, one should not absolutely lubricate the stem of the deplacor. The least oil molecule at this place, I hardly exaggerate, makes instantaneously fall of half the number of revolutions - and that whatever the fluidity of oil employed (you can believe me, I dismounted it and cleaned 3 times...;o) (1) - an advantage, maintenance is at least reduced... - then, this engine was designed to function with weak differences in temperatures, as those which are obtained by posing it on the palm of the hand, and the good direction would like that one does not deviate too much from these operating conditions. In spite of that, at the time of certain tests, I did not hesitate to apply electric resistances directly to the higher plate. These resistances heat much, and can even become extreme if guard there is not taken, and I could note that under these quite particular conditions, it could happen that the polyamide piston tightens and that the engine stops - in fact, it is not serious, he is enough to let cool the engine so that he sets out again: in normal use, that does not occur. If one wants to have fun with the play of the comparisons, one can estimate that its performances locate it between models 1.1 and 3.6. What is most astonishing, it is that in spite of its reduced size, it functions in a stable way as from a difference in temperature of only 5,5°C and approximately a minute after having posed it on the dish of the hand - to note the modesty of the originators who, in the note, speak about a threshold of operation of 12-15°C and about a possible time of 5-10 minutes before it does not start to turn... These two small restrictions put on share, one can consider that it acts of a model sympathetic nerve, rather sensitive, and especially particularly succeeded from the aesthetic point of view!
___
Θvaluation of the output, layout of the diagram (p,V) Which is the fraction of quantity of heat transformed into mechanical energy? The formula of the ideal output of the thermal engines can be written in the form: 1 - Tf/Ti, with Ti = initial temperature, and Tf = final temperature of gas used. The difficulty immediately is seen that represents the temperature measurement of a gas varying continuously and quickly inside the cylinder of an engine. In a first approximation, one will thus assimilate Ti and Tf at the temperatures of the lower and higher plates, being quite conscious that, in this way, the ideal output will undoubtedly be over-estimated from 30% to 50% - to see comment image 5 The measurement of the quantity of really produced energy, obtained by plotting the diagram of work (p,V), will be probably more precise. This diagram which represents volumes in X-coordinate and the pressures in ordinate is in general a closed curve, and provided work
is equal to the surface of this curve (when the layout is traversed in the direction of the needles of a watch).
To take these readings, the lower plate is heated by two electric resistances adding up a power of 4,8 W. The difference in temperature between the two plates is established with 21°C (54°C - 33°C) and the number of revolutions with 198,6 rpm. The pressure is measured by a piezoresistive sensor MPX2010 from Motorola, and the position of the operating piston by means of a laser diode and of a phototransistor (thus without mechanical taking away of energy) Below: 1 - the whole of the device which will undoubtedly make smile some professors of metrology...;o) (2) 2 - variations of internal pressure and travel of the operating piston, cold, while making turn the engine to the hand - when the piston is in bottom, pressure is maximum and the two curves are perfectly in phase. 3 - turning engine with 240 rpm fed in thermal energy by electric resistances - one notices a dephasing of approximately 70 degrees between the low position of the piston and the maximum pressure. 4 - the engine, idle with the hand, do not turn any more but to approximately 100 rpm. The air has more time to be heated, the pressure increases by 30 %, and tends to being in phase with the movement of the deplacor. Dephasing is of almost 80 degrees with the operating piston - the reference marks "have" and "B" show a suspension in the movement of the piston - in "A", the internal pressure exceeds the weight of the unit piston-rod and the play passes from the lower part of the axis, with the higher part - in "B" it is the reverse - in "C", probably of small vibrations which one finds amplified on the diagram (p,V) 5 - layout of the diagram of work (p,V), under the conditions described above. The raised power is approximately 1 MW, whereas the formula of the ideal output gives 150 MW (by supposing that 50 % of the energy of resistances are dissipated towards outside in the form of radiation, by convection, etc, and that only 2,4 W take part in the operation of the engine) 6 - plotted diagram whereas the engine is in extreme cases of operation (gradient of 5,5°C, number of revolutions of approximately 20 rpm). Turning with vacuum, measured work (2,46E-4 J by turn), corresponds mainly to internal frictions - to compare with the 6,5E-5 J by turn measured under similar conditions on version 3.6, and which let suppose that the losses by friction (guidance deplacor, piston, rods) are 4 times higher on Blue Stirling...;o)
___ (1) if it sometimes happens to you to have to dismount and reassemble your engine, tighten gently, and without forcing, the screws which maintain the two plates: initially the polyamide spacers are rather fine and it would be a shame to damage them, then the internal pressure being only of a few grams, the sealing will be just as easily ensured in this way! (2) if you want to acquire a more professional unit, equipped appreciably with the same possibilities, American Stirling Company markets Physics Lab Engine KY-2000 at the cost of $4950.00. It is true that for CE price you have Stirling, the sensors, and programs it installed on a PC. On the other hand, according to the downloadable pdf handbook on the site (420k), it would seem that the layout of the diagram (p,V) is not automatic and that the procedure passes by the manual input of the data in Excel...
See also
Page posted by Sterling
D. Allan, June 14, 2004 |
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