The various “Run your car on water” schemes rely on using electrolysis to generate a hydrogen/oxygen gas mixture that is used to augment the vehicle’s normal fuel. But the basic chemistry I remember holds that the electrical energy needed to split the water will always be at least the same as you get by burning the resultant gases.
Also, as electrolysis is not 100% efficient, it seems to me that the vehicle would in fact use slightly more fuel in charging the battery to make up for the electricity used in the system.
Can someone with more knowledge of thermodynamics please comment on this.
Back in WWII they discovered turbocompounding: the exhaust would drive a turbine, and through gear reduction, the turbine’s recovered power would be sent back to the crankshaft, boosting engine output by a good 20-25 %.
If you were to inject water into the hot exauhst manifold you could in effect make the engine a dual-cycle IC/Steam engine.
The enthalpy of formation of water is -285.83 kJ/mol
Meaning - the reaction of hydrogen and oxygen to form water releases energy to the environment. This is easy to observe. Ignite some hydrogen gas in the presence of oxygen and it BURNS - producing H2O and a good deal of energy.
The reverse of this - going from water to the constituent gases of hydrogen and oxygen - will obviously take AT A MINIMUM the same energy that is released in the combustion of hydrogen in the presence of oxygen. So you end up net even energetically in the best case. Because of activation energies of the reaction and the thermodynamically delicious concepts of entropy and gibbs free energy, you’ll always need to put more energy into the H2O -> gas reaction then you’ll get out of the gas -> H2O reaction.
If you were in a situation where electricity was completely free (in a monetary sense) and the energy you got from the combustion of hydrogen was salable, then yes - I suppose you could make money electrolyzing water, combusting the gas, and selling the resultant energy.
But in the big picture, there’s no such thing as getting energy for nothing. You’ll never come out ahead energetically in converting water to hydrogen and oxygen and then combusting the hydrogen+oxygen back to water.
We’ve done this several times in the recent past. Baldly stating that it violates the laws of thermodynamics is too simple an answer.
It’s perfectly true that the universe will not permit you to get more energy out of burning hydrogen and oxygen than you spent breaking apart water to obtain it, so that part is true.
But being discussed is an engine that burns hydrocarbons - it’s possible that injecting hydrogen and oxygen might cause it to use the hydrocarbon fuel more efficiently, to the extent that more energy is extracted from it than is spent on cracking the water. No violation of physics is strictly necessary there.
Now it may be the case that there is no capacity for such improvement of efficiency, or that this process doesn’t offer the right kind of chemistry to achieve it, or it may be that the process/device is just plain fraud, or something else like that - so in practice, it may well be the case that this doesn’t work.
But the point is that you can’t treat the water > hydrogen and oxygen > water part of the process as a closed system - because it is not the whole picture.
For the record, I don’t expect it works (so make no mistake about the nature of this post, please)
This being general questions, I think we need to stop letting these questions get away with the term burning water or water for fuel in a car. Water doesn’t burn. It’s an end product of combustion. Water is only a fuel when it’s heavy water and then you use it for fission. The process of electrolysis is the way the first heavy water for bombs was collected. Heavy water is harder to convert to hydrogen and oxygen, so it concentrates in the vessel you use for electrolysis.
A not unheard-of nitpick to this is burning water in an atmosphere of fluorine, not that this is ever really relevant to claims about water-powered engines.
You could utilize only ‘wasted’ energy, e.g. use a dynamo for braking rather than waste the energy as heat in brake disks - don’t hybrids do this? Unless you brake very frequently it would be hard to imagine generating enough H to overcome the extra energy spent in just carrying the extra equipment about.
In a broad sense, Mangetout is exactly right: use of hydrogen can change the combustion in the cylinder, and that could be enough of a change to increase efficiency to the point where the additional power produced offsets losses in the electrolysis system. Possibly an interesting research topic.
However, the “run your car on water!” ads I see always claim an implausibly high mpg gain, cite completely bogus physics, or (usually) both. If you see any of the following statements:[ul][li]Claims of over 10% improvement in efficiency (even worse is 30%, or 50%)[/li][li]Claims that water is not the lowest energy state for H and O, and additional energy can be extracted from same.[/li][li]Statements that large percentages of the fuel (like 30% to 50%) is never burned in the cylinder (and addition of hydrogen decreases this large percentage)[/ul][/li]…then it’s a sure bet the technology is bogus.
Thanks. Just to be doubly clear, I agree that the currently-marketed solutions are probably fraudulent. I just wanted to chime in on the thermodynamics issue, which seems to be used as an inappropriate refutation with some frequency on this board.
I think economic arguments are probably at least as persuasive in this case: Auto manufacturers (at there are quite a few competing ones these days) do indeed have an economic incentive to improve fuel efficiency; of course they also have an economic incentive to make powerful and comfortable cars, and so they try and balance those various things, but if they could improve fuel efficiency with no drawbacks to size/comfort/performance/cost, they’d do it in a second.
Since auto manufacturers haven’t adopted this technology, I must assume either it significantly affects performance or cost, or it doesn’t work.
It would be really unecononomical to combust hydrogen as a fuel since hydrogen has a good HHV but poor LHV. What this means is that a lot of energy released by burning hydrogen is stored in the form of water vapor that is formed - that cannot do significant work. Combustion engines will have to let the combustion products out at higher temperature (hydrogen burns at a higher temperature and will form much higher NOx) to avoid dew point corrosion issues. This may result in reduced overall efficiency.
If hydrogen is available and relatively pure (purity is critical here since catalyst poison is an issue) fuel cells are a far better option.
We had a guy in the shop today who is building a hydrogen generator for large diesel engines. (He had some Cummins manuals)
Anyway this guy has gone so far as to design circuit board for the controller for the hydrogen generator.
Just for laughs I asked him what he hoped to gain by burning the hydrogen in the engine.
His reply was “Twice the gas mileage”
:rolleyes:
Later as he was preparing to leave, I thought I would have some fun with him.
Me: So just is the mechanism that improves the gas mileage?
Him: Well if you burn something that has more energy than gasoline you will get better gas mileage.
Me: You mean hydrogen
Him: Yes
Me: But where are you getting the hydrogen to burn?
Him: From water
Me: And how are you tearing the water apart into hydrogen and oxygen?
Him: From the car
Me: How?
Him: With electricity
Me: Where does the car get the electricity?
Him Huh?
Me: Where does the car get the electricity?
Him: From the alternator
Me: And how does the alternator create electricity?
Him: Huh?
Me: It uses mechanical energy and converts it to electrical energy. Since that conversion is 100% efficient, you have a net loss.
Him: No, the alternator is working all the time, there is no extra load
Me: Sorry but that is not how an alternator works, it turns all the time, but it is a demand item and only produces as much current as the car needs. If you impose an additional 40 amp draw the alternator will have to work harder, and your fuel mileage will go down as a result.
At this point I got a phone call, and he drove off before it could get real fun.
Maybe he will come back tomorrow.
