A long time ago i worked out a 'scam scenario' for the Papp engine, and a reason for its explosion, with its tragic outcome. I post it here for no particular reason beyond checking to see if there are any holes in it.
I0.5 kg of hydrogen at 100Bar has a volume of 0.06CM3. With a custom-built tank I think that is small enough to be concealed within the sump of a Volvo engine that, since it was idling and doing no work would not need a serious oil-supply. Since 0.5 kgH contains 16.5 kW/H of energy, it would run such a motor at low speed for a few hours and only produce clean but steamy exhaust.
Switching off the control system power allowed hydrogen to fill the free space in the sump resulting in a hydrogen-assisted pressure explosion, and a fatality.