There are thousands of people using hydrogen in their vehicles right now as a supplement to gasoline and diesel. This type of hybrid conversion in effect turns your vehicle into a hydrogen hybrid. With hydrogen generators, you can build it yourself in a matter of hours. The hydrogen gets mixed with gas or diesel to provide increased horsepower and better diesel mileage.
 
To do a hydrogen conversion, you need an comprehensive guide to safely and correctly install a small container (or 6-pack for increasing a semis diesel mileage) under the hood. This container uses water and electrolyte, plus the electricity from your battery to break down the water into hydrogen and oxygen molecules that your engine can then use as a fuel supplement. This process is also known as electrolysis, producing brown's gas, oxyhydrogen or hho - the contraption being called a hydrogen generator or fuel cell.

Basically, the hydrogen gas then gets sucked up into your intake manifold via the engine's vacuum where it mixes with diesel or gasoline to increase combustion, and your engine's efficiency.  Hydrogen conversion works on injection or carburetor systems, although you may have to trick the computer in fuel injected vehicles to keep the check engine light from coming on. There's no need to make any engine alterations and the system is easily removed, for instance, when you want to resell your vehicle.

Torque is what moves a vehicle. Diesel fuel vehicles are more efficient both thermodynamically and in miles per gallon due to higher compression which yields higher cylinder pressure. When you increase cylinder pressure you get more mpg. If you increase flame speed you will also boost cylinder pressure and you will improve gas mileage even more. The shorter burn time means less heat transfer, which means more mechanical power and better fuel economy.

If you poke around some auto forums, you'll find a great deal of debate surrounding hydrogen hybrid vehicle conversion. Some say it won't work.  But it's hard to argue with actual proof that HHO injection works.  We found a guy on cnet that improved diesel mileage from 17 to 24 mpg in town in his Dodge cummins diesel. 

There are many expensive kits being sold online for hydrogen conversion.  You can spend $1000's of dollars for a hydrogen injection kit, or you can purchase detailed manuals online that explain exactly how to complete a hydrogen conversion, and do it yourself with parts you can pick up at your local home improvement store.