One of the reasons HHO systems work is because the ordinary fuel is not well burned; i.e., most of it is NOT burned and is expelled from the exhaust pipe or is reburned and wasted in the catalytic converter. HHO systems, when properly configured, introduce Hydrogen and Oxygen into the gasoline or diesel fuel mixtures prior to combustion. Because Hydrogen has as much as ten times the laminar burning velocity of the gasoline or diesel it acts as a pre-ignition detonator to ignite the gasoline or diesel vapor in a more complete and timely way thus burning more of it. The result would be a much better fuel usage and mileage.
"All the gaseous fuels can support detonation. However, the burning velocity for hydrogen is much higher than the other fuels: up to 346 cm/sec (depending on the mixture) versus values between 35 and 50 cm/sec for gasoline vapor, methane, and propane. Thus, the process is considerably more likely in a hydrogen mixture. Whether detonation actually happens depends strongly on the geometry and the strength of the initial ignition source."
http://www.osti.gov/energycitations/ser ... 136126.PDF

