I am utterly amazed! Speechless in fact! Thank you so much for taking the time to do the analysis. The formula you say is simple, but is the only one we have to my knowledge, and this makes it so very valuable. I am sure that this will get lots of people thinking and experimenting. I certainly will. As you say it is certainly interesting. I am also glad that a dedicated master such as yourself in understanding the scientific principal of pop pop motors has found it interesting to do, and that the results have seemed to be surprising. In my opinion this is a great step towards a more thorough understanding, and as you say there is much still to learn. I look forward very much to being a part of the understanding of your note 3. How exiting! I look forward to being involved, in some way towards this greater understanding and can not thank you enough for your generosity in time, and for sharing your results with us. This would certainly go a long way to explain why my engine number 3 overheats so readily. I can now go to the drawing board and with all the information you have provided, within my current understanding which, thanks largely to you and many others contributions, is growing at a satisfying rate, and have a better chance of making and enjoying pop pop boats without so much trial and error. Though trial and error is fun and educational, there is nothing more satisfying than planning, designing and building something, and it works as planned and as intended. I suspected there was more to it as I experimented with your 4 dollar motor which worked well, but had better results with 2 pipes, and varied the length as described in your documentation and tried several pipe diameters (8,10 and 15mm). I have found the larger the pipe diameter, the harder and more critical it got to guess the right volume. It is now much easier thanks to you.
Thank you once again.
Very best regards.
--- In email@example.com, "jeanyves_renaud" <boite.de.j-y@...> wrote:
> Hi Tim and others,
> As you were requesting the ratio between boiler volume and pipe inner diameter, I analyzed quickly the data of 18 engines that I though close to optimized. I should have done this a long time ago!!! It is really interesting. The analysis sets as evident that my big engines (above 10mm ID) should or could be more performing.
> The preliminary conclusion of this analysis can be summed up by a simple formula: d being the pipe inner diameter in millimeters, and n the number of pipes, the best boiler volume in cubic centimeters is approx V=n(1+0.06xdxd). For engines with only one pipe it means approx 1.5cc for ID3mm, 2cc for ID4, 2.5cc for ID5, 3cc for ID6, 4cc for ID7 and 5cc for ID8. With 2 pipes every volume is to be multiplied by 2, with 3 pipes by 3
> The boiler volume taken here into account is not only the drum. It is the whole section which is hotter than 100°C.
> What is given by the formula is a sort of good compromise. If the boiler volume of an engine is bigger than suggested, the frequency and thrust are not as high as they could be. If the boiler volume is smaller than suggested the engine is very sensitive to heating power and goes easily to burnout.
> Note 1: This concerns only rigid engines, i.e. without diaphragm.
> Note 2: Pop-pop science is not yet well known. The formula given above is probably too simple, but it is better than nothing.
> Note 3: Many other factors influence the performance: pipe length, slopes, bends, materials, nozzle...