# Thread: Quick question concerning energy needed to run car engine with hydrogen

1. Can someone explain, in simple terms, how I can determine the math to find out how much hydrogen I'd need to keep a car running the same as with using gasoline? Is their some type of formula that I can use to find out if the car is so many pounds, has a 200 HP engine and needs to be able to travel at different speeds as it moves along? The reason is that gasoline and hydrogen have different types of power ratings as to how far each fuel can actually go at certain speeds and under certain conditions. I'm just trying to understand the main differences as to how much hydrogen is needed to replace the gas in a car to get the same distances as a gas powered motor would get. Thank you in advance for any help on this subject.

2.

3. Originally Posted by cosmictraveler
Can someone explain, in simple terms, how I can determine the math to find out how much hydrogen I'd need to keep a car running the same as with using gasoline?
That's not really a math problem. To first order, neglect the fuel weight difference (you can check the validity of that assumption later). Just look up the energy densities of the two fuels. From there it's trivial to compute how much hydrogen would be needed to give you the same range as gasoline.

4. If you just look at the energy density per kilogram, hydrogen looks pretty good - about 3 times the energy of gasoline per kilogram. But then if you look at volume you see that it is about 1/4 the energy density of gasoline (for liquid hydrogen) to about 1/8 the density of gasolie for hydrogen compressed to 690 bar (~10,000 psi). As you can imagine, the tank to contain 10,000 psi, or the refrigeration system to maintain it as a liquid will add a lot of weight and that's going to kill your fuel mileage.

Energy density - Wikipedia, the free encyclopedia

5. I'm trying to determine how much I'd need to make the hydrogen as I'm driving instead of having it containerized. So if say gas gets about 30 MPG how much hydrogen would I need to make to equal that number?

6. If you don't have to store it, then figure you will need 1/3 the weight of the gasoline. So, if 1 liter of gasoline weighs 0.75 kg, then you would need 0.25 kg of hydrogen to replace the energy in a liter of gasoline.

7. But how much will I need to make to keep up with the fuel requirements of the gas engine ? I thought there would be some way to find that out with a formula of some kind. If not, how do I determine how much I need to produce as I am driving to keep the car running as it were running ongasoline?

8. I've moved this to the engineering forum, where I think it fits better.

I have to wonder how you would be generating the hydrogen. If it's from solar power, this will be a big loser. You will have inefficiencies in generating the hydrogen, and you will probably only recover about 30% or so in the internal combustion engine. Much better to use the solar power to drive an electrical motor directly, especially since you are not planning on storing any hydrogen.

To answer the question, though, let's say you are getting 30 mpg with gasoline. Then if your car goes 30 mph, you are using 1 gallon of gasoline every hour. Since hydrogen has 3 times the energy per unit weight, that means you need to generate hydrogen equal in weight to 1/3 gallon of gasoline per hour. As I calculated above, a liter of gasoline weighs 0.75 kg. 1/3 gallon is equal to 1.26 liters, so we have 1.26 liters gasoline*0.75 kg/liter gasoline = .945 kg. You need to generate a bit less than 1 kilogram of hydrogen per hour to travel 30 mph.

9. I'm using electricity from the alternator to make the hydrogen.

10. That being the case, then what you are trying to invent is a perpetual motion machine. That is forbidden by the first law of thermodynamics.

Laws of thermodynamics - Wikipedia, the free encyclopedia

11. Originally Posted by Harold14370
That being the case, then what you are trying to invent is a perpetual motion machine. That is forbidden by the first law of thermodynamics.

Laws of thermodynamics - Wikipedia, the free encyclopedia
Run the engine (alternator) on gasoline until it can fill the hydrogen storage tank, using water as a source, dipped out of ditches along the way, then revert to use of the hydrogen thus produced? Ha! jocular

12. Lets look at it strictly from an energy content comparison. One pound of gasoline (about one pint) has 114,000 BTU of energy. One pound of hydrogen has about 52,000 BTU of energy but occupies 200 cubic feet at one atmosphere of pressure. You can see that the gasoline takes up much less space per BTU. If you start compressing the hydrogen to take up less space that opens up a whole new set of problems.

13. Originally Posted by thinkatron
Lets look at it strictly from an energy content comparison. One pound of gasoline (about one pint) has 114,000 BTU of energy. One pound of hydrogen has about 52,000 BTU of energy but occupies 200 cubic feet at one atmosphere of pressure. You can see that the gasoline takes up much less space per BTU. If you start compressing the hydrogen to take up less space that opens up a whole new set of problems.
But I was trying to manufacture hydrogen as you are traveling along in your vehicle. That way all you need do is make enough to keep the vehicle moving at whatever speed you need to travel at. As I was told by Harold this is "perpetual motion" and cannot be achieved.

14. Well, there are plenty of ways to manufacture hydrogen for fuel, but to include a powerplant in the car which does so would be impractical and dirty.

15. According to the author of the book The Hydrogen Age, "It takes about 50Kw of electricity and just over a gallon of water to produce the energy equivalent in hydrogen of one gallon of gasoline. If you pay four cents/kilowatt, your gallon of equivalent energy will cost two dollars. There are many ways to produce clean electricity renewably, and some of them can be had for four cents/Kw or even less."

16. Originally Posted by MarcusCarey
According to the author of the book The Hydrogen Age, "It takes about 50Kw of electricity and just over a gallon of water to produce the energy equivalent in hydrogen of one gallon of gasoline. If you pay four cents/kilowatt, your gallon of equivalent energy will cost two dollars. There are many ways to produce clean electricity renewably, and some of them can be had for four cents/Kw or even less."
Does the book really say kilowatts instead of kilowatt-hours?

17. I was quoting from a comment the author made in a Forbes forum discussing Could-Hydrogen-Breakthrough-Revive-the-fuel-cell-car.

18. Does this mean the gasoline in the tank cannot generate enough energy to drive the car and produce enough hydrogen to drive the car an equal distance at the same time? The alternator does generate electricity. It just cannot produce enough to convert water to hydrogen.

19. Originally Posted by MarcusCarey
Does this mean the gasoline in the tank cannot generate enough energy to drive the car and produce enough hydrogen to drive the car an equal distance at the same time? The alternator does generate electricity. It just cannot produce enough to convert water to hydrogen.
Forget the electricity generated by the alternator. It cannot be used for propelling the car. By generating more electricity from the alternator, you will put more drag on the engine. This will not be made up by using the electricity to drive the car. That violates the laws of thermodynamics.

Not having read the book, I don't know if the author has made a credible case for a hydrogen car. Just based on what is quoted above, I'm not convinced.

He says "It takes about 50Kw of electricity and just over a gallon of water to produce the energy equivalent in hydrogen of one gallon of gasoline. If you pay four cents/kilowatt, your gallon of equivalent energy will cost two dollars. There are many ways to produce clean electricity renewably, and some of them can be had for four cents/Kw or even less."

But there will be additional costs associated with making hydrogen besides the cost of the electricity. You have to invest in the equipment, operate it and maintain it. I don't know what that costs, but it isn't zero. Then there are some very formidable problems in transporting hydrogen, because of its low volumetric density. I mentioned some of the issues in an earlier post.

If you go with liquid hydrogen, then you have a problem with keeping it cold. If not refrigerated, it will eventually boil off creating a fire hazard. You couldn't keep it in a garage. If you have a refrigeration system, that adds weight to the car and continually uses energy.

If you go with compressed hydrogen, then you have massive storage tanks that contain very little fuel. The weight of the tanks will have a huge adverse effect on fuel mileage and the range would be severely limited.

20. Originally Posted by MarcusCarey
According to the author of the book The Hydrogen Age, "It takes about 50Kw of electricity and just over a gallon of water to produce the energy equivalent in hydrogen of one gallon of gasoline. If you pay four cents/kilowatt, your gallon of equivalent energy will cost two dollars. There are many ways to produce clean electricity renewably, and some of them can be had for four cents/Kw or even less."
But this is typical of these half-baked gee-whiz notions. If you can get the calorific equivalent of 1 USG gasoline for \$2, so what? It's a bit like saying hydroelectric power is "free" because the water in the rivers doesn't cost anything. Or that nuclear energy holds out the promise of power "too cheap to meter" (this was actually said, back in the 60s - causes a hollow laugh today though, eh?). The devil, as always, is in the engineering detail.

Surely the problem of compact, low weight and safe storage of hydrogen has been recognised as the big stumbling block for decades, hasn't it? I recall reading of experiments with powdered metal hydrides and things to overcome it, back in the 1980s. No go, apparently. One of the more recent ideas seems to be this: Hydrogen fuel closer to reality because of storage advances

But apparently this problem still is far from solution today. Does this book devote any space to proposing solutions to it? If not, it is hard to see how it can be making a worthwhile contribution.

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