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In a conversation with a friend recently, she was telling me about a new car that "some Japanese company" had developed, apparently a working prototype that's about to be put into production. It has a rather astounding fuel source: water.

Apparently it operates by using electrolysis to separate the water into hydrogen and oxygen, and then burning the hydrogen as fuel to power the car. Some of the surplus energy runs an alternator, which produces electricity like in a conventional car, some of which is used for electrolysis to produce more hydrogen fuel...

Now, I hear that and my mind goes on yellow alert. That's not technically a description of a perpetual motion machine, but the concept of running both halves of a reversible reaction and turning a profit on the energy involved has "stop in the name of the Second Law!" written all over it.

And yet, according to my friend at least, a working prototype exists and they're about to put it into production.

Is that possible? Is there some missing detail in the description? And is some car company actually producing it?

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    Do you have a citation for this claim, aside from a casual conversation? The purpose of this site is to verify or refute notable claims.
    – Goodbye Stack Exchange
    Commented Apr 25, 2012 at 4:54
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    This is a common claim. en.wikipedia.org/wiki/Water-fuelled_car Also this question discusses similar concepts skeptics.stackexchange.com/questions/8067/…
    – Andrey
    Commented Apr 25, 2012 at 13:23
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    A very common claim. The famous book series 'How Things Work' features this such apparent prototype.
    – Anti Earth
    Commented Apr 26, 2012 at 9:35
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    Just to note, this is breaking the first law of thermodynamics, as well as the second. Going from Water -> Water + electricity is creating energy from nowhere, and so violates the first law. The second law is about entropy, eg not being able to suck heat out the cool air to power your hot engine.
    – Nick
    Commented Aug 21, 2012 at 16:20
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    @neilfein: I'm fairly new to this site, but as far as I remember all the claims I've seen have been from casual conversation. Perhaps the intended use of the site and the actual utility of the site are not well aligned? (The analogy comes to mind of paving sidewalks where people actually walk...)
    – iconoclast
    Commented Sep 14, 2012 at 18:16

2 Answers 2

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This system implies that the burning of hydrogen & oxygen into water generates more energy than is required to electrolyse water, as there needs to be enough energy left over after the electrolysis, to move the car. Unless the car is not a "closed" system, (e.g. they're using power from the grid to run the electrolysis, and then run the car on the resultant hydrogen) this is impossible by the most stringent law of physics: the second law of thermodynamics

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  • You are assuming that the engine must be efficient. That is not really a requirement of the engine. The power could come from a solar collector. While not strictly closed it would not require connection to a grid to energize the catalyst
    – Chad
    Commented May 6, 2012 at 14:58
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    @Chad True, but whether it uses solar energy or the power grid, the end result is the same: the car does not run on water, it runs on power obtained externally.
    – hdhondt
    Commented May 13, 2012 at 23:40
  • It could run off of the waste energy of the electrolysis. Which would technically be internal fuel source of the water. That it would be incredibly inefficiency to collect electricity to perform electrolysis then capture the waste energy does not mean it is impossible.
    – Chad
    Commented May 14, 2012 at 20:24
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    @chad - How does your idea not run afoul of the laws of thermodynamics?
    – user3344
    Commented Aug 21, 2012 at 19:37
  • @woodchips - Because there is no gain of free energy. In that scenario you spend more energy splitting the water into H and O molecules than you get in conversion to thermal energy which is then converted to kinetic energy. Its incredibly inefficient but if the goal is to run a car using water as a fuel source you could. I expect you would have a MPG equivalent rating of a bout 1/1000 but thats just a guess it could be worse.
    – Chad
    Commented Aug 21, 2012 at 20:29
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Technically yes.

Though in the opposite of the way you are describing.

The Hydrogen Fuel Cell harnesses waste energy from the chemical reaction between hydrogen and oxygen that produces water.

The electrons flow out of the cell to be used as electrical energy. The hydrogen ions move through the electrolyte membrane to the cathode electrode where they combine with oxygen and the electrons to produce water

Water is a stable compound. So it is not going to split into its respective compound with out a catalyst. Because of this water will never act as the sole "Fuel." For Water Electrolysis to occur like you have suggested would also require an electrical input. This would more correctly be the termed the fuel that would run an Electrolysis Motor. There is currently no evidence of a motor that can do this more efficiently than a standard electric motor.

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    A hydrogen fuel cell car runs on hydrogen, not water.
    – endolith
    Commented May 7, 2012 at 18:00
  • A hydrogen fuel cell car can indeed run on water in the way described. The difference is that the electrolyzer isn't in the car. And it requires far more electricity than you'd need to simply power the car with electricity. But yeah, split water, recombine water, whatever.
    – Ernie
    Commented Mar 21, 2016 at 20:34
  • @Ernie, such a car would runs on electricity (the primary source of energy). The fact that water is involved in the process does not make the car run on water. Similarly, a steam engine does not run on water either, it runs on heat (provided by whatever mean, coal, solar, geothermal, nuclear ...). The water is merely a component part of the engine, not the fuel source.
    – Hoki
    Commented May 27, 2022 at 14:08

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