If those things that actually worked, don't you think cars and trucks would come with it already installed? If a hydrogen kit yielded 50% more mileage (or even 20%), Detroit would be all over it, since it would be the answer to all their problems. The fact is, it requires more energy to separate out the hydrogen than the hydrogen provides (not to mention that at atmospheric pressure and temperature, hydrogen has about 2700 times less energy than gasoline, and 3000 times less energy than #2 diesel). Any cost savings for an increase in fuel mileage (if any at all) is eaten up with alternator replacement costs, because the alternator will be working overtime to recharge those batteries, and the batteries are working overtime to accomplish the electrochemical reactions needed to not only separate out the hydrogen, but to store the electrical current from the alternator. Seriously.
A hydrogen generator can provide modest (5-15%) increase in fuel mileage on older cars and trucks, basically anything with few or no sensors, provided things are tweaked just right. Even an O2 sensor can throw things out of whack. But a modern engine with computer controlled injection and computer controlled air intake, intake air temperature, exhaust gas remix, fuel flow and all that, when you introduce something like hydrogen, everything else within the system compensates. The O2 sensor tells the computer to alter the fuel flow depending on the amount of O2,CO2 in the exhaust system. The basic algorithm might say if there is XX% of O2 in this area, either increase the fuel or decrease the fuel. Everything else changes as well. The ECU messes with the air intake to either retard or increase, when shifting can take place, and on and on. But any gains from the hydrogen are soon lost through the ECU's data which is read that more fuel is needed to overcome the situation of more O2/CO2 in the exhaust system.
That's an oversimplification, since the actual the fuel to air ratio is handled in a different manner but the results are the same. And that's just for one of the sensors. Most vehicles have at least 7, some as many as 20 sensors for the engine to operate efficiently. Every time you change something that effects one sensor, the ECU compensates for it.
There's also the matter of maintenance on the hydrogen generator. And that's why the shelf live of a hydrogen generator for most people is 3-6 months, then they realize the maintenance isn't worth the small gain in fuel economy. Most of those people never get far enough to find out they are replacing alternators at a rate of 3:1, they simply get tired of the maintenance.
But the bottom line is the physics, you don't get something for nothing, and it costs you more energy than you gain from the hydrogen.
