Interesting topic - as an aside, Ian and I had a look at a similar system in a petrol Paj at Hogmoor a while back. As you've said, this is a project for an enthusiast!
I guess I could dig my anorak and a few old lab coats and do the sums as to whether the theory supports this. The hypothesis seems sound enough. My gut feel is that there is nowhere near enough hydrogen going into the system to make a difference to the energy balance - same goes from the oxygen generated.
Crude calcs: Your 4.2l is sucking in 4.2l of air/fuel mixture per rev. At 2000rpm, the engine is sucking in 8,400l/min of air/fuel mixture. So the hydrogen concentration is pretty low in that volume. The question is whether 1:840 parts is meaningful. My gut feel is that from an energy perspective the hydrogen contribution is at
best a zero net gain (you're simply reversing the hydrolysis reaction for all intents and purposes!) - chances are that you're inputting more energy to generate the hydrogen because no process is 100% efficient and you're certainly not capturing 100% of the energy released by the hydrogen combustion.
So the real issue is what benefits does hydrogen offer to the combustion process? Diesels are limited in performance by the flame speed during combustion - I would speculate that hydrogen would assist with this. Increased flame speed means higher rpm and higher power delivery, so we could assume improved performance. But more power means more fuel consumed all else being equal (and IMHO the hydrogen isn't contributing anything in terms of the energy balance) so I would think this increase in performance comes with a higher fuel bill. So a diesel might seem a bit more lively with hydrogen but you'd be feeding in more diesel too - maybe less additional diesel for the increased performance than would be needed for the same increase in performance without using hydrogen...
Some guys in South Africa reported better fuel economy adding 500ml ether per tank of diesel but the empirical data suggested that it was only/most significant when filling up and using the tank of fuel immediately (i.e. on a long trip). The speculation was that the ether evaporated out if left in the tank for an extended period. The theory behind this was that the ether helped with a cleaner, more efficient burn. So a similar theory but no energy consumption in generating the ether.
IMHO a net 10% improvement in fuel consumption with the hydrogen is a pipe dream. The fact that there might be claims of 30-40% makes me seriously question the veracity of the seller. A potential 30-40% increase would have every motor company pouring tens of billions into the technology!
I'll be very interested in your results once you've had a chance to get a few measurements. Hopefuly us sceptics will be proven wrong!
Hats off to you for at least giving it a bash :handgestures-salute: , rather than being an armchair "expert" like some of us
Cheers,