Let me share my own definition of hydrogen hybrid cars. With the term hybrid, it means a mixture of two different elements. In the case of cars, it is the mixture of or using both hydrogen (actually what they call hydroxy, HHO, brown's gas) and fossil fuel. This is attained by installing a hydrogen generator injection system connected to the present engine combustion mechanism.
Watch the following video:
These cars were first done by DIYers using whatever car they owned. So it can be from Honda, Toyota, Ford, or any other brand of car.
Now, a wiser car builder, Ronn Motors, adopted the system and produced the Scorpion HX™ eco-exotic supercar. "It's twin-turbo 450-horsepower Scorpion™ supercar features the latest in green fuel technology through its proprietary H2Go™ Hydrogen On-Demand system. H2GO™ creates hydrogen as a vehicle is driven on the road and does not use any on-board high pressure hydrogen storage tank. Ronn Motors and the DIYers common goal is to increase mileage and minimize CO2 emission.
The one that's made by Honda, the FCX Clarity, which is now on test drive by some private entities, is solely fueled by hydrogen with the use of fuel cell technology. This type of cars, along with:
BMW hydrogen 7 series
Necar 4 of Daimler-Chysler
E-450 shuttle bus and Focus Fuel Cell (FCVs) from Ford
Those belongs to Fuel Cell Cars. And are not necessarily powered by hydrogen. They can be fueled by bio-diesel, ethanol, methane or other forms of gas but not petroleum. Fuel cell cars also don't use internal combustion engine, it is actually an electric car, since fuel cell converts those gasses into electricity. So it has its own mechanism.
Fuel Cell Cars function like a gasoline powered car. It has storage tanks (for the hydrogen or other fuel), needs refueling through a fueling stations, whether hydrogen or other gas. But these cars are yet very expensive to produce. Mainly because of the fuel cell and the storage tank.
Here's one more. The one I called, Hydrogen Car. These are Daniel Dingel, Stanley Meyers, among others' invention, which they claim as their water-car. Daniel Dingel made use of an existing car while Stanley Meyers made his own prototype. It garnered lots of controversies and remains somewhat like a fantasy. But funny though, coz hydrogen is in water and water is 75% of earth. H2O.
For the meantime, do-it-yourself hydrogen hybrid car would do, or wait up for Ronn Motors positive outlook to happen, as they surely will be producing eco-friendly cars that's accessible to the masses. For me, it's a good start.