For some time, I have done several attempts to build a pop-pop coil made of glass… Following some disappointments, Jean-Yves suggested to me to use a test tube, with a plug and a thinner tube for the outlet pipe. I waited long before starting that job and, some days ago, we were informed of the assembly done by Jorge Hugo Cordero which is totally in accordance with what we intended to do. Thus stitched (!), I decided to start, and I propose you to see how.
Where we start step one !
Nothing, really nothing is complicated for the mounting. The parts being available, in less than 3 minutes it is finished, 5 if we add the time needed to strike a match !
The pipe we used is 160 mm long, with external diameter 16 mm and internal one 13 mm. We can add that it is a classic test tube made of Pyrex (=borosilicated glass) used in laboratories. The plug is made of rubber. The thin pipe (6 ext, 3.5 int) is a pipette made of Pyrex. It has been cut at an appropriate length and the graduations have been removed by scratching.
A dish warming candle suits perfectly. In practice, the tube will not visibly blacken.
Hence, we are looking at a pop-pop engine which works, and which delivers a thrust very likely sufficient to propel a small float… but having an heavy weight due to the useless big amount of water… Therefore, to solve this problem, let's cut the tube!
Where we start step 2
To cut the tube, it must be grooved. As the steel tool that I had got refuses to streak the glass, and as I had no quartz (or diamond!) available, I used a mini grinding disk (corindon) mounted on a Dremel.
Once this is done, the best is to heat locally with a little candle the grooved area, and to put it under cold water… thus the glass cracks and it suffices to exert a slight pull to split both ends. (Hum… as you have to learn before being an expert, forecast some spare parts… To get 2 good parts, I killed 4…)
The tube is now no more than 35 mm long. The rubber bracelet that can be seen on the picture at the bottom of the boiler was there to prevent the enlarging of a crack (detected when the plug has been inserted… ).
Starting is now quicker. During the first test, the oscillating area was located near the plug, and it was difficult to see… A more powerful flame allowed to get a bigger volume of gas, and thus to move the oscillating area after the plug. One could also cut the plug to narrow the shadow area….
The oscillating stroke is longer (x1.5 to 2) than the one we got with the full length tube. It is difficult to ensure that the shortening is responsible of that, because other parameters (flame, distance from test tube bottom to the pipe inlet, immersion depth of the pipe outlet in the tank) are not accurately monitored. However, a cause and effect relationship could exist (we will revert on that later).
Where we conclude for this page
It looks like we are facing a very easy means to study pop-pop engines. In addition, it occurs that it could be the easiest way to build a pop-pop engine, insofar as the components, easy to find (if not, ask for some help), are all available.
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