I am beginning to understand that the Papp engine was a cavitation based device.
In the 1960's Papp used water for his fuel. Papp must have produced water crystals in the compression part of the cylinder cycle where the volume of the cylinder was decreasing. During this increasing pressure environment inside the cylinder, cavitation bubbles must have formed thereby producing ultra dense water crystals.
For example, some larger diesel engines suffer from cavitation due to high compression and undersized cylinder walls. Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall. The result is pitting of the cylinder wall, which will eventually let cooling fluid leak into the cylinder and combustion gases to leak into the coolant.
Cavitation seems to be a problem inherent to the design of all sorts of internal combustion engines.
From about the 1980s, new designs of smaller gasoline engines also displayed cavitation phenomena. One answer to the need for smaller and lighter engines was a smaller coolant volume and a correspondingly higher coolant flow velocity. This gave rise to rapid changes in flow velocity and therefore rapid changes of static pressure in areas of high heat transfer. Where resulting vapor bubbles collapsed against a surface, they had the effect of first disrupting protective oxide layers (of cast aluminium materials) and then repeatedly damaging the newly formed surface, preventing the action of some types of corrosion inhibitor (such as silicate based inhibitors). A final problem was the effect that increased material temperature had on the relative electrochemical reactivity of the base metal and its alloying constituents. The result was deep pits that could form and penetrate the engine head in a matter of hours when the engine was running at high load and high speed. These effects could largely be avoided by the use of organic corrosion inhibitors or (preferably) by designing the engine head in such a way as to avoid certain cavitation inducing conditions.
To stop the cavitation based erosion of the cylinder walls and the subsequent loss of compression over time, Papp went to noble gases which produce ultra dense noble gas crystals during the compression stage of the cylinder cycle but the formation of ultra dense noble gas crystals did not damage the cylinder walls. Papp used noble gas additives in his fuel formula in the design of his water fueled engine. In his development work on that fuel type, Papp must have discovered the noble gases can form ultra dense crystals just like water does.
When Papp fired a spark, the ultra dense noble gas crystals exploded as happens in the Holmlid experiment when the ultra dense hydrogen crystals produce atomic particle fragments that move outward at 3/4 the speed of light. Currently, Holmlid does not capture that huge amount of energy inherent to his expanding plasma.