it seem obvious to me that adding extra fuel to the engine will result in extended milage.
Maybe it is obvious, if the additional hardware involved weighs nothing. Additional weight added to a vehicle means more gas needs to be burnt to move it the same distance.
The question now is, does the improvement to fuel efficiency offered by such a system exceed the additional fuel cost of moving whatever it weighs? Part of this will be how much water you intend to carry, as water is not light.
Some relevant factors: Stirling engines are ~15-30% efficient. I can't find hard numbers for the efficiency of electrolysis as it's seemingly measured as a function of hydrogen produced over time or something, but every source I have found says "low efficiency". This is the reason why nearly all industrial Hydrogen is produced from natural gas reformation rather than electrolysis.
It's a good idea to recapture waste heat from internal combustion engines for various uses in other parts of the drive train to improve overall efficiency, and indeed many automakers do that. But it would seem none of those methods involve using a stirling engine to power electrolysis of water.
This means either the math doesn't work out (it contributes less in the way of fuel savings than the fuel cost of moving the additional weight) or no auto engineer employed by any car manufacturer in the world has thought of this idea.
obviously the weight of the system needs to be taken into consideration and would need to be minimised. a simple stirling can be made with little more than 2 tin cans and some wire wool.. really doesn't weigh much at all... perhaps an electrolyser could be formed using graphene coated plastic instead of the usual metal plates. it would also not weigh much. by having a small water tank, that could also be kept at a reasonable level.
if you just want to dismiss the idea because no one has done it yet, fair enough... that's up to you i guess...
Can it be minimized enough that the energy output contributes meaningful improvements to MPG, though?
That doesn't sound terribly efficient.
"Minimized", "simple" and "reasonable" sound good, but are not math demonstrating that the total weight of such a system does not consume more fuel to move than the fuel it saves.
There is nothing to dismiss yet. I encourage you to build such a device, test it and write an article about your results.
btw. here's a quick vid showing how to build the tincan stirling...if you're interested... it was designed by NASA so is probably quite good. don't know the numbers though...
Neat. So in your opinion, what is the most likely reason that auto makers who have built waste heat recapturing systems into their cars have used every method except stirling engine powered electrolysis?
i really wasn't aware that any cars were in production that did so. a quick google has showed me that they seem to mostly use a type of steam engine called a rankine engine. this is a new one on me . will have to look into it. another option is thermoelectric devices, presumably some kind of peltier module, something i have contemplated in the past.
as these are being used to directly charge the batteries in the case of hybrids, or add power directly to the driveshaft they are not things that would be practical for someone attempting a DIY improvement to an old car.
from wiki: Researchers at Loughborough University and the University of Sussex, both in the UK, also have concluded that using waste heat from light-duty vehicle engines in a steam power cycle could deliver fuel economy advantages of between 6.3% and 31.7%"
although this is done using a rankine engine it shows that there is validity to the concept of using waste heat to improve efficiency, even with the added weight of the system.
they probably use these approaches to avoid the inefficiency of electrolysis. they are making brand new machines and want to use the optimal solution.
for a DIY mechanic looking to improve the efficiency of his old car, electrolised hydrogen may be the only option. even if the limitations of the electrolyser reduce the impact to only 2 or 3% instead of the 30% offered by a shiny new rankine steam system, it's still better than nothing in my opinion.
There is absolutely merit to recapturing waste heat, that was never in dispute. What's being disputed is whether the method you've described can save more gas than it uses.
Only if it actually delivers any benefit. If it did, probably auto makers would be selling these kits themselves.
It may deliver no benefit at all, or a net loss.
there's obviously no point continuing this conversation. your mind is made up. anything HHO is bunk... not worth considering... ok. you should get on with the flatearthers really, the same unswervable confidence that you are entirely correct in your assumptions and refusal to listen to any contrary arguments... oh well.. peace.
perhaps i will one day, but i'm more of a dreamer than a do-er... just putting it out there as an option for anyone that's into DIY experiments... good luck in your flag war, all the best...