makeUsayHmm, there is energy in hydrogen, but none in water. Water is the “ashes” that you’re left with when you burn hydrogen, just as carbondioxide is left over when you burn carbon (e.g. coal). A car that runs on water is just as impractical as a car that runs on CO2.
To make hydrogen from water (for example, through electrolysis) requires energy input. Where will this energy come from? Because no process is 100% efficient, the amount of energy invested to extract hydrogen as an energy carrier from water will always exceed the usable energy gained from burning hydrogen in a fuel cell or internal combustion engine.
To give you some concrete figures, it takes about 5 kWh of electricity to extract one cubic metre (1000 litres) of hydrogen. If you then run a car engine on this to drive a generator to make electricty, you’ll get back about 0.5 kWh at best. You’re stuck with a net loss of 4.5 kWh. A car with a “hydrogen generator” that sucks power from your alternator actually uses more fuel, not less.
In practise it makes no difference because these devices are so inadequate in size, their negative impact is not even noticeable, except in your wallet when you purchase this junk. The last part is what it’s all about.
“Water for fuel” or “the car that runs on water” is a hoax that enriches snake oil salesmen and their shills who get kickbacks from advertising scam sites via affiliate schemes.