HHO generators don't work because the amount of electricity needed to get the hydrogen to separate from the oxygen puts a generous load on your alternator. The power has to come from somewhere. When you load your alternator like that it draws more power from the engine. In order for an HHO generator to work in a car you need to be able to separate something like 5cc/min to get the energy that you are drawing off the motor back.

http://wiki.answers.com/Q/How_does_a_HHO_generator_work_on_vehicles

Read that for a more detailed explanation. Trust me, if HHO worked every single car manufacturer would have been on this years ago. If HHO worked efficiently, we would have fusion reactors.