Doc Science and the Airmobile

“Doc,” I said, “I read an interesting article about a new car they are building in India and I wanted to talk to you about it.”

He looked at his bottle of cold Blatz, and the chip I placed in front of him good for another, then replied, “Go ahead Wes. Don’t see as how I can stay polite and stop you.”

“It’s powered by compressed air. No need to buy gasoline. I saw a picture of it so it’s already built. It looks like a little van. They say it will be able to be used like a taxi around town, drive all day and you can charge it overnight. It even has a built in air compressor you can plug into your house circuit. How is it that they can build something like that in India and we are still trying to make an electric car battery that works?”

“It’s the Oil Companies Wes,” Aldo Brimley, owner of the Gas n’ Go, sitting on the other side of the bar said, “You should see what it cost me to stay in business. They buy up all the good batteries to keep them off the market. I could switch to selling air. Back when I did full service I had a lot of air tool so I know that air motors work. Powered them of my own compressor I did. People couldn’t always wait to charge their cars tanks at home and compressors are noisy. I could do just fine. What about it Doc?”

“It all comes down to foot-pounds,” the Doc started.

“You say that about everything” I said, thinking about his speed of light drive.

“Only because it’s true. Gasoline, batteries, compressed air; all of these are energy and energy is nothing but stored foot-pounds. So what you want to know is are there enough foot-pounds in a tank of air to run a car that does what you want it to do. Tell me what kind of performance you need and I can tell you how much compressed air it will take.”

“Well there are cars around that get 50 miles to a gallon of gas and all I really need to do is get to Marquette and back so the equivalent of two and a half or three gallons and I am good to go.”

“Three gallons ain’t much. I got portable tanks that hold that much easy.” Aldo said.

“Wes, tell Aldo how many foot-pounds in a gallon of gasoline.” the Doc commanded me.

“Gee Doc; I thought that was your department. Let me think.”

“Don’t hurt yourself there Wesley boy!” Aldo laughed at his own joke.

“Ok, I remember that a lb of high explosive has about 2 million foot pounds and gasoline is around 8 lbs per gallon so 8 times 2 carry the 1 multiply by 2 and a half. I rolled my eyes. About 40 million foot pounds will do it.”

“Not so fast Aldo,” even if your algebra is right those cars that get 50 are more powerful than we really need to run around town and go to the big city.” Marquette, biggest town in Michigan’s Upper Peninsula has ten times the 2800 of Munising where we were talking at our Moose Lodge. “So say you had half the horse power engine and used half the fuel. By your numbers Wes, 20 million foot pounds might be enough. Right Doc?”

“By Wes’s number you have a point Aldo, but Wes got his number wrong. He remembered from when we talked before about 2 million foot pounds of energy in a pound of explosive but there is a lot more than that in a lb of gasoline. You see the thing is that explosives explode because they burn so fast. And they burn fast because they have oxygen combined in the materials they are made from. And that adds a lot to their weight. Gasoline gets the oxygen it needs to burn from the air so you need to figure the weight of the gas plus the weight of the air before you know how much energy you are going to get.”

Doc traded his chip for another Blatz and then continued, “I am going to make this all simple by letting you know that a gallon of gasoline will get you about 40 KWH of electric power equivalent. And to give a little better number than Wes and I used last time .75 KWH equals 1 Horse Power Hour. For now we can forget all that and just say that if we can do the drive for 60 HPH we need 120 million ft lbs.”

“Consider it forgot!—But how can you figure how many ft lbs in a tank of air?”

“Easier than you think. It depends on the size of the tank and how much the air compressed. You know PSI pound per square inch. In metric it would be in kilopascals and nobody knows what they are. So how big is your tank and how much pressure?”

“The article said it was about a cubic meter, but it didn’t say how much pressure.”

“I’ll tell you what then. You try and figure it out and we can all come back tomorrow and see what you can come up with.”

To be Continued.

* * *
Doc Science and the Airmobile – Part 2

The next day back at the Moose I said, “I give up Doc. I don’t see any way simple to get foot pounds out of a compressed air tank. You gotta tell me.”

“You will hate yourself when I show you how easy it is.” The Doc was grinning from ear to ear, never a good sign. “Instead of a cubic meter we will say 35 cubic feet. Just imagine a tank 5 by 7 and 1 ft high. It will hold the same amount of air as any other shaped 35 cu ft tank. Follow me so far.”

“Sure, but I still don’t get where you’re going.”

“The tank is a foot high and the top surface area is 5000 square inches. If I wanted to support 5000 lbs without the tank squashing down any how much air pressure, how many PSI would that take?”

“Only one would be my first guess. One more PSI than an empty tank.”

“Ring a ding a ding! That’s it. We can forget about atmospheric pressure of close to 15 PSI because it acts on everything. It’s only the air we pump into the tank that we need to consider. Now then, if I put enough air into the tank so a pressure gauge reads 2 PSI that tank will support 10,000 lbs. If I let the air out and that 10,000 lbs drops one foot how many ft lbs of energy will I get?”

“I see now, 10,000 ft lbs. So if I need to get 120 million ft lbs out of that tank to power my car I need to put in 120 million divided by 5000 sq inches worth of pressure. Lets see, that’s 12 divided by 5 subtract 10 and write down a 2. . . 24,000 PSI.”

“There ya’ go! Think you can do that, or they can do it in Indiana?”

“India the country Doc, not India the state, and I don’t know. How about you tell me?”

“Sure Wes, cost you a Blatz.”

“I knew that was coming and was already reaching for my wallet.”

The Doc waited till it was in front of him (suspicious nature) before continuing. “The short answer, to twist a phrase, is — No They Can’t! Not to live and talk about it anyway. Gases compress well, but only up to a point. The most compressible gas, hydrogen—or maybe helium—can’t remember which but it starts with an ‘H’ will compress up to about 1000 atmospheres. Say 15,000 PSI. And a tank holding that much pressure is more likely to move on tracks than wheels.”

“I see your point. Welding tanks are darned heavy and they hold what? — 2000 PSI or a little more right?”

“That’s it Wes. Even if they make the tank out of some high strength miracle material and get a light one to hold 4000 PSI they can only store 20 million ft lbs worth of energy inside. One sixth of the minimum we are looking for. That’s a lot though. Remember about a lb of TNT and it releasing 2 million ft lbs when it explodes.”

“Yeah Doc, and imagine the accident where the tank burst and 10 lbs of TNT go off all at once!”

“How much are they supposed to cost?”

“The article said less than ten thousand dollars.”

“Put me down for one. I never go to Marquette anyway. Just the thing for the Moose and back.”