Engines using Hydrogen as primary fuel
Creating enough HHO to act as the primary fuel for a vehicle would require extreme amounts of electrical current – much much more than the charging system could provide.
Even if this were possible, energy is lost upon every conversion step, since no energy conversion can be 100% efficient.
These losses occur during the combustion process that makes mechanical energy, then the alternator that converts mechanical energy into electrical energy and finally, the HHO system that converts electricity into chemical energy.
This theoretical system would violate the first law of thermodynamics because not only is it a perpetual motion machine, but would need to create energy sufficient enough to power the vehicle as it travels.