While Ford experimented with the idea of using alcohol in the Model T, there was never a production Model T sold that could have been operated on alcohol. Henry Ford is quoted as saying: “We have found that 160-proof alcohol works very well in the ordinary gas engine on our cars and tractors”. And “Using alcohol in an ordinary Ford car, we are able to get 15 per cent more power than with the present gasoline”. This statement is often cited out of context as proof that Ford made a production Model T capable of operating on Ethanol, when in fact it was only an unsuccessful experiment that was conducted many years after the design of the engine.
References :

Car engines run at about 20% efficiency, so if you use a car engine to make electricity, use that electricity to crack water into Hydrogen and Oxygen, and feed that as fuel into the car, you will use 80% more energy than you generate by burning the hydrogen.

In diesel engines, adding a small amount of hydrogen to the intake manifold (as is also done with natural gas injection) will cause a small pre-ignition in the combustion chamber, this raises the temperature and means the diesel is injected into a flame-front. In some circumstances that would result in more complete burning, possibly more power and more thermal efficiency.

Where this story really comes from is water injection. During WWII, Allied bombers used both superchargers and turbochargers in series, which made quite high effective compression ratios–which created major problems with pre-ignition. To raise the effective octane of the fuel, we added water injection. Adding water to the combustion chamber cooled it, but also generated steam (which raises the pressure and hence the power) and also prevented pre-ignition of the fuel, which can cause major damage. The net result was planes with more power, an ability to run at higher altitude, and more range on the same amount of fuel. Some creative people have adapted this idea to cars, possibly some even succeed in making a huge boost in mpg. As long as you don’t count the long-term effect of extra water in the crankcase. But the people who advertise that you can run your car with your engine on nothing but water–those folks are scammers.
References :

Water is H20 , so H2 comes from water , not the other way round .

Joe invented this cell many years ago , of course the oil company have him made silence so the product never get to surface on the market .
Check this out !

http://www.joecell.com.au/index.html
References :

Now, you can build a hydrogen powered car that has water as a waste product, but there are various reasons to prefer one buring a hydrocarbon instead (Easier fuel handling and better volumetric energy density mainly).

Anyone who has ever worked in the automotive industry in any sort of design capacity can tell you stories of the total kooks who inhabit this space, do a search on ‘water arc’ to see some more truly horribly bad "science". One company I worked for had a "museum" of the weird things people had sent in for us to "evaluate"…..
My favourite was a magnet you were supposed to clip over the HT lead from the coil that would "improve fuel efficiency", it quite obviously could do no such thing..

The internal combustion engine has had a LOT of development over the years, and a modern small bore diesel is a very tough act to beat in terms of energy conversion efficiency.

The Indians (Tata) are trying semi seriously for a compressed air powered car, but the overall efficiency will suck due to the thermodynamic non reversibility inherent in storing high pressure gas. I think the win of no pollution at point of use may mitigate this for some applications however.

Regards, Dan.
References :
I worked on a alternative fuel vehicle project for a major car company for a while (Back during the "Hydrogen economy" sillyness).