Hydrogen as an additive to diesel fuel?

[utimmer43]

Let me start off by saying that I understand conservation of energy and the laws of thermodynamics. I know you can't use water for fuel, and you can't power a vehicle soley on hydrogen produced by an onboard electrolyzer.

But... I have read quite a few claims that using an onboard electrolyzer, and pumping that hydrogen into the air intake will increase the fuel mileage. The claim is that, because hydrogen "burns" very hot and very fast, it will cause the diesel fuel to burn more completely. A more complete burn would mean that less fuel would be required to perform the same amount of work.

Thoughts?

 

[mgb_phys]

Injecting hydrogen will certainly give you power from burning the hydrogen.
I woul dhave thought it would DECREASE the efficency of the diesel since any significant amount of hydrogen will consume some of the oxygen that the diesel could have burnt. Diesels run at pretty much their optimum temperature naturally becuse of the way the ignition operates.

 

[TVP45]

Let me start off by saying that I understand conservation of energy and the laws of thermodynamics. I know you can't use water for fuel, and you can't power a vehicle soley on hydrogen produced by an onboard electrolyzer.

But... I have read quite a few claims that using an onboard electrolyzer, and pumping that hydrogen into the air intake will increase the fuel mileage. The claim is that, because hydrogen "burns" very hot and very fast, it will cause the diesel fuel to burn more completely. A more complete burn would mean that less fuel would be required to perform the same amount of work.

Thoughts?

Getting a fuel to burn completely in general does not depend on temperature but more on mixing and adequate time in the combustion chamber. Just because there are claims doesn't make them true.

 

[brewnog]

You can definitely mix a combustible gas with the intake air for a Diesel engine and have it operate with lower running costs; this is the idea behind dual fuel engines. However, the benefit generally comes from gas (not gasoline) being cheaper per kWb output than Diesel as a fuel.

I don't believe the notion that hydrogen gives a hotter, faster burn causing the Diesel to burn more completely. Diesel has no problem burning completely. Hydrogen on its own is a horrible fuel in an IC engine.

 

[Ulysees]

> Hydrogen on its own is a horrible fuel in an IC engine.

Could you explain how you came to this? Cause I would have thought that mixing hydrogen with the intake air can potentially make the carburator obsolete, as we have an ignitible mixture in the inlet already. Only timing of valves and ignition might need to change a little.

In fact I have a very old car that should soon be withdrawn, and thinking of trying to mix some hydrogen at the air inlet and see what happens. It would be cool if the engine still runs after the gasoline has run out. Even cooler if the car explodes. :)

 

[Ulysees]

I am aware the carburator is obsolete already btw, replaced with injection. So injection can potentially become obsolete if we find a way to produce and store hydrogen, instead of gasoline.

 

[mgb_phys]

> Hydrogen on its own is a horrible fuel in an IC engine.
Could you explain how you came to this?

It has a tendency to pre-ignite when it gets near anything hot - like a piston engine.
It has lower energy/mole than something like LPG simple because there is more hydrogen in the hydrocarbon - so you have difficulty pushing enough gas into the cylinder.
The main reason though is that if you have gone to all the trouble of distributing and storing H2 it is much more efficent to use it in a fuel cell + electric motor.
Using it in an IC engine is like burning batteries on a fire and using the heat to run a steam turbine.

 

[Ulysees]

> if you have gone to all the trouble of distributing and storing H2 it is much more efficent to use it in a fuel cell + electric motor.

Do you have any data to support this statement, any efficiency calculations?

Because I thought electrolysis too is inefficient, fuel-cell inverse electrolysis might be about as bad. And what is the efficiency of an IC engine, after 100 years of development?

 

[Mech_Engineer]

> if you have gone to all the trouble of distributing and storing H2 it is much more efficent to use it in a fuel cell + electric motor.

Do you have any data to support this statement, any efficiency calculations?

Because I thought electrolysis too is inefficient, fuel-cell inverse electrolysis might be about as bad. And what is the efficiency of an IC engine, after 100 years of development?

mgb is correct in that an IC engine isn't a very efficient way to extract energy from gaseous hydrogen. IC engines are inefficient, and hydrogen has a relatively small amount of energy/volume when compared to petroleum based fuels (or even alcohols).

Modern internal combustion engines are still only about 15-20% efficient at extracting power from gasoline or diesel. Some compresion ignition engines (diesels) that have turbochargers or other beneficial aids may have peak efficiencies as high as 40%, but average still works out to about 22%. Gas engines' efficiencies are slightly lower.

New fuel cells have efficiencies approaching and surpassing 60%, while the electrical systems paired to them (motor, batteries, current and throttle control, etc.) bring the total system efficiency to around 40%. In european driving tests, fuel cell cars have exhibited tank-to-wheels efficiencies of around 35%.

Power-plant-to-wheels efficiencies for fuel cell cars have been rated as low at 15%, but the problem is this number cannot be compared to petroleum-based powerplants because their 'energy' already exists and is being mined while any energy put into an electrolysis plant to manufacture hydrogen must first be taken from somewhere else (my vote: nuclear).

 

[brewnog]

> Hydrogen on its own is a horrible fuel in an IC engine.

Could you explain how you came to this?

Yes, hydrogen is very susceptible to knock.

Cause I would have thought that mixing hydrogen with the intake air can potentially make the carburator obsolete, as we have an ignitible mixture in the inlet already. Only timing of valves and ignition might need to change a little.

In fact I have a very old car that should soon be withdrawn, and thinking of trying to mix some hydrogen at the air inlet and see what happens. It would be cool if the engine still runs after the gasoline has run out. Even cooler if the car explodes. :)

I don't know how you arrived at the idea that carburettion/fuel injection could be eliminated.

How would you meter the fuel? How would you mix it with the air? How would you control the engine speed/load and air/fuel ratio? Would you still use gasoline? How would you meter and mix that?

 

[Ulysees]

> Yes, hydrogen is very susceptible to knock.

What is knock caused by with gasoline? I thought it's just a matter of valve timing, and if you adjust them right for the fuel, there's no knock. Is it not so?

> How would you meter the fuel? How would you mix it with the air? How would you control the engine speed/load and air/fuel ratio?

I was thinking that we'd just connect the pedal to the tap of the hydrogen bottle, and that's all. Injection is defined as spraying fuel directly into the combustion chamber, right?

 

[Mech_Engineer]

Here's a good example of an IC engine running on hydrogen:

http://www.greencarcongress.com/2006/07/ford_begins_pro.html
http://www.autoweek.com/apps/pbcs.dll/article?AID=/20070713/FREE/70711005/1006/free

It's a supercharged (18-20psi of boost) 6.8l V-10, that makes 235hp and 310ft-lb of torque; seems like a lot of engine and not a lot of power. That same engine running on gasoline could probably make double those numbers.

 

[Ulysees]

Found a hybrid gasoline/hydrogen one too. The BMW 745h makes 184hp out of a 4.4-liter V8:

http://www.hydrogencarsnow.com/hydrogencars2001.htm

Can someone please tell me, can I work out the energy efficiency of a car just by looking at the consumption and the speed and the power curve and the rpm?

I tried it and got over 100% efficiency so I have made a mistake. Can anyone help me do it right?

 

[Ulysees]

What about this:

efficiency = power at engine output / energy of fuel burned per s

Power I look it up on the power curve, given the rpm reading of the dial.

Fuel burn rate in l/h = fuel consumption in l/100km (from the dial) * speed in km/h from the dial * 100

Is this right?

 

[Mech_Engineer]

Yes, the general method for calculating the engine's efficiency would be to divide the power you're getting out divided by the total possible power you're putting in. So, in the case of a car engine, if you knew how much power it's putting out at a specific rpm, and you also knew how much fuel it's using in time (gal/min, whatever), you could get a gross efficiency by dividing the two numbers.

 

[mgb_phys]

>
What is knock caused by with gasoline? I thought it's just a matter of valve timing, and if you adjust them right for the fuel, there's no knock. Is it not so?

Knock is preignition of the fuel. With a gasoline or dielsel engine it is due to injecting the fuel with the cylinder at the wrong pressure due to bad timing.
Hydrogen will ignite from the temperature inside the cylinder - especially the hot exhast valve. Hydrogen might work better in a rotary/Wankel engine.

 

[utimmer43]

Getting a fuel to burn completely in general does not depend on temperature but more on mixing and adequate time in the combustion chamber.

I would think that all three of those factors would have an influence. Problem is adjusting those factors. Increasing the amount of time in the chamber means you advance the timing. Thats not a good idea in and of itself, but they (the people trying to sell these electrolyzers) actually suggest you retard the timing because of the more rapid combustion. The rapid, hot burn of the hydrogen is supposed to boost the rate of the diesel burn, thus burning it more completely before the end of the power stroke.

Diesel has no problem burning completely.

I beg to differ... Isn't all that black smoke spewing out the tailpipe unburned fuel?

It has a tendency to pre-ignite when it gets near anything hot - like a piston engine.

Yes, hydrogen is very susceptible to knock.

Unless this source http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html" is wrong, Hydrogen has a rather high auto-ignition temperature of 932 oF, almost double that of diesel fuel. Is the air (and surface) temperature inside the chamber higher than this before injection?



BTW, I'm just as skeptical as the rest of you, otherwise I'd be building one right now. I'm just trying to keep an open mind and so far I haven't heard any concrete evidence why it wouldn't work. For me, it's just a gut feeling that it won't, but I need more than that to completely dismiss it.


P.S.

my vote: nuclear.

AMEN!

 

[brewnog]

I beg to differ... Isn't all that black smoke spewing out the tailpipe unburned fuel?

Yes. However, it's not difficult to get this not to happen. (Black smoke is generally a sign of overfuelling for one reason or another).

Unless this source http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html" is wrong, Hydrogen has a rather high auto-ignition temperature of 932 oF, almost double that of diesel fuel. Is the air (and surface) temperature inside the chamber higher than this before injection?

It's not just high temperatures which cause fuel to ignite in an engine, it's the high pressures. This is how an IC engine works; the key is compression of the charge before ignition.

BTW, I'm just as skeptical as the rest of you, otherwise I'd be building one right now. I'm just trying to keep an open mind and so far I haven't heard any concrete evidence why it wouldn't work. For me, it's just a gut feeling that it won't, but I need more than that to completely dismiss it.

As we've seen, it does work. Just not very well, for the reasons that have been stated.

 

[Ivan Seeking]

mgb is correct in that an IC engine isn't a very efficient way to extract energy from gaseous hydrogen. IC engines are inefficient, and hydrogen has a relatively small amount of energy/volume when compared to petroleum based fuels (or even alcohols).

About 1/3.

Modern internal combustion engines are still only about 15-20% efficient at extracting power from gasoline or diesel. Some compresion ignition engines (diesels) that have turbochargers or other beneficial aids may have peak efficiencies as high as 40%, but average still works out to about 22%. Gas engines' efficiencies are slightly lower.

New fuel cells have efficiencies approaching and surpassing 60%, while the electrical systems paired to them (motor, batteries, current and throttle control, etc.) bring the total system efficiency to around 40%. In european driving tests, fuel cell cars have exhibited tank-to-wheels efficiencies of around 35%.

Power-plant-to-wheels efficiencies for fuel cell cars have been rated as low at 15%, but the problem is this number cannot be compared to petroleum-based powerplants because their 'energy' already exists and is being mined while any energy put into an electrolysis plant to manufacture hydrogen must first be taken from somewhere else (my vote: nuclear).

According the the Sci Am article "Questions about a Hydrogen Economy"from May 2004, for a grid electric hydrogen fuel cell powered auto, the fuel chain efficiency is about 22%; the vehicle is about 38% efficient, and the total efficiency is about 8%.

The most efficient option of all is steam reforming - hydrogen from methane using steam -and a H2 fuel cell powered auto. This has a "well to wheels" total efficiency of about 22%.

I believe the fuel chain efficiency for petroleum is est to be about 80%.

 

[Ivan Seeking]

Something else to be considered is the cradle to grave efficiency of the hardware: What is the energy cost of production? What is the life expectancy? What are the recycling energy costs? What is the recycling recovery efficiency? What is the total impact on the environment [mining, smelting, transportation, chemical effluents, all of the workers who commute each day to produce a product, etc]? Also, we have to factor in where the production occurs. For example, would it be produced in China where we have an environmental free-for-all and coal power?

There is often [usually?] a direct relationship between cost and the energy required to produce a technology, but environmental impact may have no serious consequences for cost and can be a hidden variable.

 

[utimmer43]

Isn't all that black smoke spewing out the tailpipe unburned fuel?

Yes. However, it's not difficult to get this not to happen. (Black smoke is generally a sign of overfuelling for one reason or another).

If it is not difficult, then please tell me how. There are smoke suppressors in the additives they put into #2 diesel before it reaches the pumping station. I don't know for sure how they work, but I doubt they help the combustion. After all, the point of all of this is to cause a more complete combustion, thereby increasing the fuel efficiency. A lack of smoke is icing on the cake.

It's not just high temperatures which cause fuel to ignite in an engine, it's the high pressures. This is how an IC engine works; the key is compression of the charge before ignition.

I would not say that is accurate. High pressure does not directly cause combustion in a diesel, nor at all in a gasser. In diesels, increasing the pressure 12 fold or more raises the temperature in the chamber high enough for the fuel to auto-ignite (spontaneously combust). The fuel would never ignite (without some sort of spark or flame) if the temperature were not hotter than the auto-ignition temperature (494 oF), even in a high pressure environment. Likewise, the fuel will auto-ignite if it exceeds 494 oF, even at low pressure.

Hydrogen has an auto-ignition temp. of 932 oF. As long as the surfaces and air inside the chamber are not higher than this, it won't pre-ignite.

The way this is supposed to work is that when the diesel is injected, as soon as the diesel starts to burn, it ignites the hydrogen. In turn, the hot, rapid burn of the hydrogen should cause a more rapid and therefore more complete burn of the diesel fuel before the exhaust valves open. It should be noted that the absence of smoke at the tailpipe is not necessarily an accurate indication of a complete burn. Combustion often continues in the exhaust manifold, after the exhaust valves have opened. Of coarse, any combustion that happens after the valves open is of no use.

As we've seen, it does work. Just not very well, for the reasons that have been stated.

Remember, I'm not trying mearly to make an engine run only on hydrogen. By it "working" I mean that it works as stated above.

 

[brewnog]

I wasn't suggesting that high pressure alone ignites the charge, but that ignition is made far easier by compression, which is why the knock margin of any fuel is reduced by increased pressure. Although the temperature of your trapped (and compressed) charge might be low enough not to cause spontaneous ignition of the hydrogen/air mixture, the exhaust valves/injector tip/piston crown might well be (494°F is only 250°Cish; your exhaust valve head may run at well over 800°C).

You might be able to get round this by playing with nifty AFR control to ensure that the mixture prior to Diesel injection won't exceed the LEL of hydrogen, even when compressed (but I don't know how the LEL varies with pressure, anyone?) You ought to have a look how dual fuel engines are controlled; though of course these are typically subsidised by a methane-based fuel rather than hydrogen, which has a much better knock charecteristic.

Let me know how you get on.

 

[utimmer43]

I had noticed the higher ignition temp of methane and thought to myself "if I could harness cow farts, I'd be set." But if the surface temp. inside the chamber is well over (or even approaching) 800°C, then it would seem that ANY gas that was mixed with the intake air would pre-ignite. I did look into the dual fuel engines as you suggested, and the impression I get is that the methane is injected, like diesel, rather than mixed with the intake air. Perhaps that is why it doesn't pre-ignite? The attractiveness of the hydrogen system is that it is mixed with the intake air, so it's easy to hook up, and is produced onboard for free (after initial cost of the electrolyzer).

Also, it seems that the dual fuel engines are using the alternative fuel as primary (80-90% methane). Not a bad concept if it is cutting costs. But I am not looking for an alternative fuel. What I want to do is add something to the mix that will increase the efficiency of the diesel fuel. In other words, the intention of the hydrogen is not really to directly contribute any useful energy to the drivetrain. It's purpose is to aid the primary fuel (diesel) to burn more efficiently. But if it really is THAT hot inside the chamber, then that entire concept would seem impossible.

 

[brewnog]

Dual fuel engines basically have methane (or methane-based gas) fed into the air inlet, at a concentration less than its LEL, which is why it doesn't autoignite. Yes, they're used for reasons of economy.

 

[Ulysees]

What does LEL stand for?

 

[TVP45]

Lower explosive limit. At concentrations below the LEL, gases will not go BOOM.

 

[brewnog]

Yes, sorry!

 

[utimmer43]

So, a gas that is below LEL can still be lit with a spark or flame?

 

[brewnog]

As I understand it, it won't support combustion on its own, but will still contribute calorifically if another combustion reaction is occurring at the same place (eg the Diesel combustion).

Can a chemist verify that?

 

[Ulysees]

Modern internal combustion engines are still only about 15-20% efficient at extracting power from gasoline or diesel.

Mech_Engineer, check out this calculation:

https://www.physicsforums.com/showthread.php?t=218847

 

[Shahil]

Hey...Interesting topic ... I did some googling and came up with this from the Mythbusters forum.

http://community.discovery.com/eve/forums/a/tpc/f/9551919888/m/2321969559

Every step of all these schemes consumes far more energy than it could possibly release. There is no magic way to get such phenomenal gains in fuel efficiency out of your existing car.

Check out the article though ... I guess that closes the topic here :/

 

[utimmer43]

No mention of diesel engines in that article. The statement that 99% of gasoline does go through combustion doesn't hold true for diesel. Even if it's close to that, not all of the combustion of diesel fuel happens in the chamber. Diesel fuel burns slowly, and some combustion happens after the exhaust valve opens, but by then it's too late to be useful. The hydrogen isn't supposed to add any valuable energy to the driveshaft, rather, it is supposed to cause the diesel to burn more completely before the exhaust valve opens. I think there is something to that, but perhaps not with hydrogen.

As mentioned earlier in this forum, hydrogen tends to preignite. So what if something a little more stable, methane (also mentioned earlier), were used in a similar fashion. Obviously not produced onboard, like the hydrogen (I don't even know where to get methane). Isn't it plausible that adding just a little bit of methane to the intake air might possibly cause the diesel fuel to complete its combustion sooner, when it counts?

 

[schreiz375]

The reason why using hydrogen or HHO gas as they call it makes an engine more efficient is becuase the oxygen for the hydrogen to react with is all right there in the combustion chamber. The burning of the hydrogen is rather explosive, and ignites more of the diesel fuel rapidly, allowing more diesel fuel to burn with the oxygen in the engine. It speeds up the reaction of the diesel. Typically new Tier 3 and 4 engines are losing about 30% of the stored energy to heat and friction. This is why radiators must be used to cool engines. If an engine ran at 100% efficiency, it would be as cold as the air outside. This is why the hydrogen helps to increase the efficiency of diesel or gasoline engines.

 

[Ulysees]

Diesel fuel burns slowly, and some combustion happens after the exhaust valve opens, but by then it's too late to be useful.

One thing that needs to be considered too, is that the burning does not propagate from molecule to molecule, but all molecules ignite at the same time. It's not like gasoline where the burn starts at the spark, in diesel engines the burn starts everywhere where the pressure is high enough.

Therefore combustion outside the chamber must only be an issue at high revs. Is it?

 

[Ulysees]

And I wonder what role hydrogen might play if all molecules ignite at approximately the same pressure. Maybe not much.

 

[pantaz]

One thing that needs to be considered too, is that the burning does not propagate from molecule to molecule, but all molecules ignite at the same time. It's not like gasoline where the burn starts at the spark, in diesel engines the burn starts everywhere where the pressure is high enough.

Diesel combustion is a great deal more complicated than you describe. This presentation deals with the subject in some detail -- http://egweb.mines.edu/faculty/TParker-Primary/Recent%20Presentations/Wyoming.pdf"

The simplest example I can find is on the following web page -- http://public.ca.sandia.gov/crf/research/combustionEngines/AF.php" -- You will find a series of photographs, (as a GIF animation), showing the actual combustion inside a diesel engine. The flame propagation is clearly visible.

Copy of image is attached for convenience...

 

[schreiz375]

I understand that diesel combustion is complicated, even so. The basic concept of why the hydrogen is beneficial remains the same. Even in newer diesel engines, it is beneficial. In newer engines you can have up to 5 injections per cycle. The diesel fuel that is ignited due to the high pressures will ignite the hydrogen. Since the hydrogen is explosive, its ignition instantly ignites the entire combustion chamber, which in theory will help the diesel fuel more completely burn. In a gasoline engine, it is very beneficial becuase gasoline engines are on average not even 50% efficient users of gasoline, the hydrogen helps the gasoline burn more completely, and when you buirn mnore completely, you have lower exhaust temperatures, which also helps reduce emissions. Same with diesel engines. Just a thought.

 

[Ulysees]

But it's not obvious at all why diesel wouldn't burn completely at low revs. Probably completeness of combustion is only an issue at high revs.

 

[russ_watters]

The reason why using hydrogen or HHO gas as they call it makes an engine more efficient is becuase the oxygen for the hydrogen to react with is all right there in the combustion chamber.

No, when people say hydrogen here, they really mean hydrogen. H2. "HHO" is a hoax. It's water by a different name, meant to confuse people about what it really is.

 

[jeffmoss1]

HHO isn't a hoax...it's an H2 and O2 gaseous mixture.

The reason why the presence of hydrogen in small amounts would speed up the combusion process is because it supplies more H and O radicals, sooner, to react with the decomposing hydrocarbons and helps trigger the chain branching reactions sooner.

Combustion is way more complicated than they way you guys are referring to it. There are 1000's of reactions happening all at once and going back and forth until the mixture is finally at a stable equilibrium. Modeling this process is extremely difficult and its part of what my research involves for my graduate degree. Usually there are some critical reactions that one can say dominates the process and you can get a good idea of reaction rates from that and they all involve H2 O2 H O and H2O significantly.

While I haven't yet tried this myself yet (and I am too still very skeptical about it) you can't prohibit it without years of experiments. Trace amounts of hydrogen and oxygen in the intake air can very plausibly make a big difference in allowing for a more complete combustion.

(not making any absolute statements, just providing what I think I know)

 

[TVP45]

HHO isn't a hoax...it's an H2 and O2 gaseous mixture.

But then we'd call it H2, O2 or 57%H2, 43%O2 or something like that. "HHO" is deceptive.

 

[jeffmoss1]

"oxyhydrogen" is what I've read in literature

I agree, HHO is a dumb name.

 

[gambrinus]

existing H2/Diesel engine research

interesting discussion; a couple more link's

(i can't post full URLs on here yet; sorry)

abc.net.au/ra/innovations/stories/s1499183.htm

eprints.utas.edu.au/2781/

 

[kl2345]

I would like to know three things:

Does adding hydrogen into a gasoline engine makes it more efficient?

If you were to use a electrolyzer does the extra load from the altenator decrease the engine efiiciency so that there is no benefit?

And if the electrolyzer load does effect the engine can you then just add another plug in battery to offset the power load.

thank you

 

[mgb_phys]

Does adding hydrogen into a gasoline engine makes it more efficient?

Unlikely, there is an argument for diesel, but only because diesels are pretty efficent at burning aything.

If you were to use a electrolyzer does the extra load from the altenator decrease the engine efiiciency so that there is no benefit?

Yes, we obey the laws of thermodynamics in this house

And if the electrolyzer load does effect the engine can you then just add another plug in battery to offset the power load.

Which would be charged up by?

 

[kl2345]

Which would be charged up by?

I would have a battery that is charged in the house then put it in the car and run the electrolyzer solely from this battery.

 

[mgb_phys]

I would have a battery that is charged in the house then put it in the car and run the electrolyzer solely from this battery.

Is it likely that battery->electrolyzer->hydrogen+gasoline->engine is more efficient than battery->electric motor? And a regular car battery running an electirc motor might run a golf cart but isn't going to do much for a car.

 

[kl2345]

Thank you, i will definitely not buy one.

 

[russ_watters]

Does adding hydrogen into a gasoline engine makes it more efficient?

There is evidence of a very small (under 10%) improvement in combustion and thermodynamic efficiency.

If you were to use a electrolyzer does the extra load from the altenator decrease the engine efiiciency so that there is no benefit?

Yes.

And if the electrolyzer load does effect the engine can you then just add another plug in battery to offset the power load.

Another plug?? I don't know what you mean. Regardless, the first law of thermodynamics applies. That's conservation of energy. You can't get more energy out of a system than you put in. And there are enough sources of inefficiency in the process of electrolysis that you lose a lot of energy.

The hoax in these devices is easy to spot. The people who adverties them directly claim a violation of the first law of thermodynamics that is transparent: they claim that they can get more energy out of a car's alternator without putting more energy into it. But of course, if that were true, you could just couple the alternator to an electric motor and generate electricity that way.

 

[kl2345]

Good point.

i guess i should have explained it better; i understand that i cannot get more energy out of a system than i put in, but it does matter where the energy comes from.

In this situation i want to charge a battery from my power grid and use it to power a electrolyzer to get better gas millage from a gas powered el camino. While i am still putting in the same amount of energy. The cost of charging the battery at home can offset the current cost of gasoline prices. But i only want to do this if it make my engine considerably more efficient.

So in this case the deciding factor is not just science but economics.