These documents provide extremely useful information about the viability and credibility regarding the on-board electrolysis of water and results from feeding the resulting gases into the engine.

The International Journal of Hydrogen Energy (a respected, peer-reviewed journal) published a paper by Dulger & Ozcelik in which they found that the production of hydrogen on-the-fly was able to increase fuel economy by 35-40% and reduce emissions by 40-50% in four carburetted vehicles. Log in to see the document.

Obviously this is only possible if the engine is not making full use of the energy of the fuel it is supplied with, e.g. due to a slow/incomplete burn (hydrogen combusts at an extremely high speed, making any fuel it is mixed with also burn faster and more thoroughly).

The document below is produced by the US Department of Transport (2007) and makes specific reference to on-board electrolysis (Hydrogen Injection) and provides affirmative results from limited testing.