'Hydrogen on demand' is a system which uses electrolytic cells with a catalyst to split water into hydrogen and oxygen for ingestion by the engine with a view to improving economy.

My Significant Other and I don't doubt that it works with many vehicle manufacturers' engines, but it definitely didn't work on my Audi 80 2.0E auto or on her Golf TDI 90 auto, in fact it DEcreased the economy by 1-4 MPG.

Be cautious if you have a VW Group car. We've had to go to court to get our money back !!