Determining the amount of "power" in a liter of HHO is not clear cut. It is clear that a liter of hydrogen gas contains 9.54 BTUs, but I have read that HHO contains 6 times more BTU's than hydrogen alone. Personally, I think this is probably true, but only if your looking at the explosive capacity of a container of HHO vs a container of pure hydrogen. Nothing burns without oxygen, so if you already have the needed oxygen inside the container, it will burn/explode faster and hotter.
However, if your looking at the mixture inside the combustion chamber I think it's a totally different story because the HHO has already been mixed with so much air that the minute amount of oxygen coming from the HHO gas is of no consequence. So at this time, I'm not going to count it.
Here are the values I came with. 1 liter of hydrogen gas contains 9.54 BTUs and 1 gallon of #2 diesel contains 140,000 BTU.
So, it you are driving down the road in your truck at 60 MPH getting 7 MPG, you are burning about 8.6 gallons per hour, or 1,204,000 BTUs/hour from the diesel,
IF the engine was operating at 100% efficiency. Since the only about 50% (?) of the diesel is actually burned, even with HHO assist, that reduces the BTU/hr to 602,000.
If you are injecting HHO into the engine at the rate of 6 LPM, that would be 4 liters/minute of hydrogen or 240 liters per hour, or 2,290 BTU/hour at 100% efficiency, which is correct for the hydrogen . If you add the BTUs from the diesel that was combusted to the hydrogen that was combusted, you get a total of 604,290 BTUs/hr. So the percentage of that total that was produced directly from the combustion of hydrogen would be only .37%!!