If you try to find the most "secret" engine, then the top five will certainly include the so-called "molecular engine", which, according to the principle of operation, belonged to the internal combustion engine, that is, it was an "internal" combustion, but worked on a fundamentally different fuel than the one to which we are all used to it. Zinc, antimony and water - that's what made him "spin".
This mysterious mechanism was invented by the French inventor Jean Marsol, by the way, a laureate of many scientific prizes and a recognized scientist, in the fifties of the last century.
Marsol did not have the best character - this is recognized by many, and often changed his place of work, without staying anywhere for a long time - he was poached, given the best equipment, and money. I must say that he was married three times and raised four children from all his wives in his house. For a long time, Jean Marsol worked in a government department of unclear specialization, where he headed a laboratory, about which little is known either. Most likely, it was military in nature.
There is very little information about the engine (as well as about Marsol itself - try to find something on the Internet).
In short, the principle of operation is as follows: the pump pumped water into the working cylinder, where electrical resistance covered with a catalytic layer of a mixture of zinc and antimony, heated to hundreds of degrees, decomposes water into hydrogen and oxygen. The vapor formation phase does not occur under such conditions. The volume of the resulting gases is 1000 times greater than that of the water that generated them. The pressure and temperature rise, the times expand further and push the piston.
The quarrelsomeness and independence in decision-making may have played a cruel joke with the inventor: immediately after filing a patent for the "molecular engine", Jean Marsol, his wife, mother-in-law and four children got into a fatal accident. And that same night, a sudden fire broke out in the laboratory, destroying everything to the ground.
Promotional video:
Seven guards died, and three days later, Marsol's first wife died under the train. The inventor's father soon died suddenly. The fate of the laboratory staff is unknown. The remains of manuscripts and documents were seized by military investigators. By the way, the military court also dealt with the fire and car accident.
Video of the Marsol engine operation, of course, no. This is Potapov's engine, "NM" will also tell about it without fail:
In fact, this engine brought energy production to a fundamentally new level, pushing the entire oil and gas industry to the margins. Most likely, it is this fact that should be considered the most important for revealing the secret of the death of the inventor.