A popular idea for improving automobile fuel economy with Do-It-Yourselfers is the HHO system, which people usually describe as a means of running your car on water. The system relies on the vehicle's charging system to supply the electricity to electrolyze water (H2O) into Oxyhydrogen, which consists of 2 parts Hydrogen gas (H2) and one part Oxygen Gas (O2). Oxyhydrogen is also known as Brown's Gas. Electrolyzing water requires a huge amount of energy because water is at a very low energy state and is very stable as a result.

Proponents of HHO Systems often acknowledge that the Water Fuel Cell requires more energy to operate than what is gained by combusting the gas. However, they claim that the addition of HHO to the air inducted by an internal combustion engine enhances combustion, thereby increasing the engine's fuel economy by an amount greater than what it cost to electrolyze the water. The problem is that this constitutes a perpetuation motion machine, which violates the first law of thermodynamics: the law of conservation of energy.

Although there are plenty of testimonials that HHO systems work, working HHO systems have never been verified in an accredited laboratory. In contrast, they has been discredited many times:

I am not aware of a single HHO system that been documented to work as advertised by an automotive test facility. Lots of testimonials but no hard proof. One of the people interviewed in the Dateline NBC Story (Eric Krieg) even has a real reward (an open $10,000 prize offer for proof of a free energy machine) for anyone who an prove their system works and he's still offering a $1000 commission to anyone who talks an inventor into submitting a winning design to him.

On March 8, 2014, Eric Krieg confirmed to me that there have been no takers since he started offering the reward in 1996.