What are Water Fuelled Cars and why are they not more popular: The term water-fuelled car broadly covers all the technologies that use water instead of fossil fuels as their main source of energy to drive the vehicle. Water is not fuel but can produce fuel sources inside an electrolytic cell also called the fuel cell.
When water is separated into hydrogen and oxygen components and oxidized the energy output is 2.5 times more powerful than gasoline, and the byproduct of combustion is only water vapor.
However, water by itself unlike fossil fuels cannot be used in internal combustion engines. Water chemically is made up of 2 molecules of hydrogen and one molecule of oxygen, breaking down water through electrolysis gives us a viable source of hydrogen and oxygen which can be both used to drive the vehicle's internal combustion engine.
The problem is the amount of energy required to separate water into its constituent elements is more than what you get back when the molecules within the cell recombine.
There have been several carmakers who have attempted to make water-powered cars work over the years. Despite many efforts by scientists and inventors, manufacturers lack the infrastructure required for effective deployment of existing technologies and face pushback from automobile industry lobbyists which can make the adoption of clean energy for transport more difficult than other technologies.