Hold on science fans... this should be interesting.
"Overunity" is a term that is roughly analogous to perpetual motion, e.g. a mechanism which violates the 2nd law of thermodynamics - and is therefore impossible according to our current understanding of the universe.
That doesn't stop people from working on it, however, with sometimes interesting results.
Most people understand that our carbon-fueled world has a finite, and short, half-life. We're going to have to come up with alternative sources of energy, and alternative ways of living, in the pretty near future. An added benefit is that the Saudis will go broke as long as we don't mortgage our entire civilization to them first (OOPS - too late!).
There have been many recent legendary figures who allegedly invented a "perpetual motion machine"... one of the most famous recent ones was Edwin V. Gray, who produced several supposed overunity devices and was the subject of a great deal of interesting conspiracy theory.
It is a familiar story: ordinary guy invents machine that threatens energy industry and is mysteriously suppressed or done away with (the Ed Gray story has all of that). The story has repeated itself so many times, in so many ways, that it is tempting to believe it. Especially since the equities in the energy status quo are the greatest in the history of humankind. Most of the wealth in humanity comes from control of energy sources, primarily carbon. Consequently that's where most of the power is. The people who control access to carbon energy buy and sell whole governments, including ours.
So while I don't think that any proported violations of the 2nd Law of Thermodynamics have any legitimacy at all, it is possible to build a more efficient engine. In fact, it isn't even hard. But for reasons that look a lot like conspiracy theory even to the most sceptical, consumers are generally not allowed access to the most efficient choices.
For example - a diesel electric hybrid would be significantly more efficient that gasoline hybrids on the market today. But so far you can't buy one. Toyota, Nissan, and Volvo have promised diesel hybrids, but they are yet to appear. (Nissan also promised "supercap" battery technology in their hybrid, 7 years ago. Where is it? Volvo's version is a station wagon that gets 120mpg, scheduled for introduction in 2012, that they DON'T plan to sell in the United States )
So while "overunity" is certainly a myth, according to our current understanding of physics, improvements in efficiency that would almost certainly seem revolutionary or miraculous are almost certainly possible - they are happening "out there", just not for us.
One approach to great improvements in efficiency that I can't quite figure out are hydrogen generators. The idea is that a small electrolytic hydrogen generator can produce hydrogen gas to augment the ordinary fuel-air mixture in your car, greatly increasing gas mileage using only water as the (augment) fuel. These gizmos are also called hydrogen boosters and there are lots of web sites, with plans, formulas, etc.
My big question is whether these small hydrogen generators can realistically add efficiency... John maybe you can help me with the chemistry here. They only use a very small amount of water - on the scale of ounces to pints - during normal operation. It seems improbable to me that you can produce enough hydrogen to really augment the fuel of the vehicle without using a lot more water, and a lot more electricity (which has to come from the car) - which would add weight, use more energy, and decrease the efficiency of the overall system.
I'm very interested to figure out what the real chemistry is behind these gizmos.