Following one of the possible ways to explore about pop-pop engines, Jean-Yves used pipes of bigger and bigger diameter…
The first escape from the classic field for this kind of building (diameter 2, 3, 4 or 6mm) was an engine using a brass pipe of inner diameter 12mm (length approx 700mm, copper evaporator of ID 28mm). It worked perfectly at the first attempt and encouraged to follow the exploration towards bigger diameters.

A curtain rail made of steel became the next engine… Inner diameter 23.5mm, length 1.70m… A “big” blowtorch supplied with butane… And it worked… Frequency approx 2Hz. Thrust approx 130mN… A video of this engine while running is available here ("Right-Clic/Save the target as").

Titillated by this diameter increasing competition, Loïc, using some pieces of motor car exhaust pipes and a big diameter steel pipe built what could be (please note the carefulness of the wording) the biggest pop-pop engine in the world (Hum, hum…).
A steel pipe of inner diameter 40mm, 3m long, an evaporator of 90x150… and the first run was a little bit difficult… But finally it worked… Not very fast and, compared with what we could have expected based on the result of previous engines, without a big thrust…
Let’s stop words! Look preferably at a video Look preferably at a video ("Right-Clic/Save the target as").

  

In short, it can be seen that a pop-pop engine of inner diameter 40mm works… Look on the following graph what is its thrust compared with other similar engines. It looks like we have reached a plateau.

Don’t forget that if the diameter and the pipe wall surface per length unit are multiplied by 2 when the diameter evolves from 20 to 40, the volume of the engine is multiplied by 8. Therefore, considering a progressive increase of the diameter, the volume evolves quicker than the pipe surface (the ratio volume/surface grows). But the surface is an important parameter because it ensures the cooling of the system… Cooling that would become insufficient with the hot water/steam amount increase… And, as a matter of fact, on this 40mm engine we got a permanent working by forcing the pipe cooling (with water) ; thing that was not needed on previous engines…

It remains to run some complementary tests to know if it is possible to increase again the size… But, whatever, the thrust that we go was so weak that these “engines” will remain of the entertaining physics, as we have no doubt for a long time…

Our engine with a pipe of 40mm was modified. We removed the large boilling chamber and we increased the curvature (on small engine this one seems useful). The results in terms of thrust were not better than before... The engine is just less beautiful... But the "science" is sometime like this !

In February, 2008, Jean-Yves received some photos of an impressive engine. It was made by two engineering students from Netherlands, Eljoh Sneep and Bjorn Goldenbeld. They gave us their agreement for the publication on this website of some images. Apparently this engine, made with a 100L drum and a 60mm pipe, works... with a frequency of 0,25 Hz... i.e. one pulse every 4 seconds.
These twos guys are currently working on an interesting project... We will talk about it when it will become a reality.