Well LG, I don't know about breaking the laws of thermodynamics, all I do know is that Stan had something and there are many around the globe reverse engineering his invention. I am a moderator on a few other forums and have personally talked with several I know who are on the brink of cracking that egg. I have seen their equipment, their applications, their experiments, read Stan's Patents enough to know, they are very close.
It depends how you make and use it, I have tried because I was interested and the following is based on that.
IF you have say an extra battery and use that to produce gas, you can feed the gas into a engine (was a 4 stroke generator for me) and you save some petrol (well obviously). But the saving is only because its cheaper to use electric to make gas than the price of petrol.
If you use the generator OR another alternator (I tried both) connected to the machine your feeding gas into, then you save nothing. Also I noticed that HHO wasnt great for the engine, if you split water and feed HHO in an engine and burn it you get............ Water, water isnt great for an engine, What I did find was a bit more power from feeding just the oxygen in, but I couldnt produce enough to keep up with revs. When I used more current and got some more Oxygen I used more petrol.
So yes you cant argue against it if you use an outside source to generate the gas, after all your adding the outside power to the engine so using less petrol. But that is way different from claiming a closed system can somehow give a net gain or something for nothing.
Often things look like they work because people dont understand the underlying mechanism, a great example was a you tube video with a demo of a special electric engine that took 12V and made 35V!! It was a super coil thing but basically a motor that floated on magnets. What the guy was excited about was the fact that he was feeding 12V in and could measure 35V on the output mosfet's.
What he didnt understand was he was measuring the inductive spike as he didnt have flyback diodes! But had he measured current instead of voltage he would have realized he wasnt gaing anything in terms of saving. But again I do think if you had a ultra efficient solar panel roof on a car then you could power a HHO cell from that and save a small amount of fuel, but your not gaining anything as such. All that happens is your turning an external power source (sunlight) into energy (electric) and using that energy to make a gas, as you feed that gas in of course you use less petrol!
But the fact remains if you use the cars alternator or even a extra one added to the engine to make the gas you use more petrol, I noticed the more gas I produced (higher current needed) the harder the engine worked(more revs needed to compensate from the alternator), as I said mine was a generator but the same works on a car. The best saving I have made by far was for our tiny tractor, I now use converted waste veg oil and mix with 20% red diesel. I save money in red diesel but it costs to make the bio fuel and it takes time, but there is a overall gain cost wise.
Simply looking at chemistry will show why it dosnt work with HHO, you use energy when you burn the gasses together and they make water. I cant remember the electron Volts and all the details but clearly there is a net loss. Shame it dosnt work as it would mean a secure energy future.