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I have heard a few claims about water-fueled cars. Being a student of Engineering I totally understand that water-fueled cars (using electrolysis to generate Hydrogen) are nothing more than a hoax, unless you are giving electricity from an outer source.

However, I have heard quite knowledgeable people that water hybrid cars are quite a reality and you can save up to 60% on your fuel costs using some kind of kits. I have difficulty agreeing with this claim as well. I think inclusion of another step (hydrolysis) in the energy cycle of car power system will only make it less efficient. Reason being you can achieve at most as much energy from making a bond (burning Hydrogen) as much you spent on breaking it (Electrolysis).

Can someone please help me understand if I am missing some point? Or I am correct?

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The idea of splitting water using electrolysis and pumping it into an engine's air intake has been around for a long time. The idea is that it's supposed to increase the efficiency of burning gasoline. However, even assuming that this is true (there are many reasons why it may not be—modern gas engines are in the range of 95–98% efficient at combustion), electrolysis is very inefficient, so I'm quite dubious of any potential gains.

Hunting around on the internet, it's obvious that a lot of people seem to think this works (and that people who say it doesn't are oil company shills or some such), but I wasn't able to find any compelling evidence to support it. How this kind of system interacts with a modern computer controlled engine may perhaps even make efficiency worse. There doesn't seem to be much academic research into it, though a Google Scholar search brings up a lot of kooky patents. This kind of thing is pretty difficult to test well—tests need to be performed under very well controlled conditions to get accurate results and is thus always subject to confirmation bias and the like.

I did find a single report done by Environment Canada on one of these products for diesel engines and they found that adding the hydrogen generator device didn't make any statistically significant change to combustion efficiency, exhaust emission rates, or fuel consumption. They calculated that the device was about 60% efficient at making hydrogen, at best, and used about 8 mL of diesel fuel every 100 km, so it doesn't really make efficiency worse to any appreciable degree, but doesn't help either. (Source: ERMD Report # 2004-032 If you search, it's on scribd, but I couldn't find any permanent sources)

In short, a water-fuelled car on its own or as a hybrid with gasoline or diesel don't seem to be possible. Water is a very stable compound and it takes a great deal of energy to break it into hydrogen and oxygen gas. At most, you can get the energy you put in back, and realistically, a lot will be lost along the way. This makes a solely water fuelled car a perpetual motion machine of the first kind as it violates the law of conservation of energy—one cannot burn hydrogen and oxygen to produce energy to make the same amount of hydrogen and oxygen consumed and still have energy left over to move a vehicle. A hybrid vehicle faces a similar problem. Because the part of the system that involves water consumes energy rather than contributes it to motion, the gas or diesel part has to produce more energy to make up for the difference. Given that modern gas and diesel engines burn almost all the fuel put into them, there doesn't seem to be any mechanism by which adding a small quantity of hydrogen and oxygen could produce significantly more energy.

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  • $\begingroup$ Thanks for your answer. Modern IC engines are 25-30% efficient. And yes, my research on the topic concluded that it is not possible to have a hybrid car with water. Definition of hybrid vehicle tells us why. Water is not being used as a source of power. So we can not call above mechanism as hybrid. Furthermore, Inclusion of electrolysis and burning of Hydrogen loop is only going to make efficiency worse, so it is not feasible either. $\endgroup$
    – Junaid ur Rehman
    Commented Dec 26, 2014 at 1:41
  • $\begingroup$ In short, there is no possibility to make an engine which can use water as a fuel (neither as only source nor as hybrid). Please read this and include your findings in your answer and I will accept it as correct answer. So that it can help someone in the future. :) $\endgroup$
    – Junaid ur Rehman
    Commented Dec 26, 2014 at 3:48
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    $\begingroup$ By 95–98% efficient, I'm not talking about thermal efficiency, I mean that 95–98% of fuel added to the cylinder is actually combusted. $\endgroup$
    – Michael DM Dryden
    Commented Dec 26, 2014 at 4:09
  • $\begingroup$ A small amount of hydrogen injection greatly increases the octane rating of the fuel. The idea may well beat carrying around a hydrogen tank. $\endgroup$
    – Joshua
    Commented May 7, 2018 at 19:34
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    $\begingroup$ @joshua I have nothing on which to dispute that claim, but even if true: higher octane is not necessarily any better for the car. I mean perhaps adding hydrogen will let you use a lesser grade of gasoline on a car with a high compression engine that needs (or at least benefits from) premium gasoline, but a car not designed to take advantage of higher octane fuel may not really benefit at all. $\endgroup$
    – Kevin Cathcart
    Commented May 8, 2018 at 16:53
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For any engine that does not have a sophisticated ECU then the addition of hydrogen into the air intake does produce significant savings on fuel and reductions in pollution. The modern ECU will offset this effect but older engines will perform well. The greater risk is the hydrogen that remains in the engine, but this is present at 0.001% parts per air and since I believe (though please correct me if I am wrong) hydrogen is explosive at a min of 18% even this should not be an issue

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