Drive propulsion vehicle

[MichiganScott]

Ok, I’m begging you all not to jump on me too hard but I’ve got a few ideas that may seem to be a bit out there but I’d like to share them occasionally to see what everyone else thinks…
The idea I’d like to put out there today is: (and if anyone patents it I’d like some recognition (don’t need a cut but the mention would be nice)recycled air drive vehicle.
The idea would be to incorporate an air piston to supplement an electric underwater propulsion vehicle.
The set up would kinda be like a re-breather except the re-captured air/gasses would be compressed and used to actuate the propeller. Yes in this scenario you’d be tethered to your vehicle, but that’s an argument for another day, and subsequently another “idea” kinda based on the “monkey” diving thing…

 

[JanK]

The problem with this idea is the "...except the re-captured air/gasses would be compressed and used to actuate the propeller..." part. The compressed gas is an energy storage, i.e., to compress gas you need energy. This energy would have to come from the same batteries as for the electric part of the DPV, so you're no better off.

In fact, you're worse off, because you have at least one more energy conversion: from electric to "pneumatic". Each conversion wastes energy in the form of heat so, in the end, you end up with less total available energy for propulsion.

Add to that the compressor you'd need, the extra electric motor to run the compressor, additional plumbing, pressure regulator for the propeller drive,... and the idea becomes quite infeasible.

 

[Packhorse]

You need to redesign it so the gas comes out of your tank as high pressure and drives the DPV engine and then you breath it at ambient.

Still, the little advantage you would get would be out weighed by the cost, complexity and limitations of it all.

 

[DIWdiver]

This is an interesting twist on the “air powered vehicle” concept. It turns out that compared to a battery, compressed air is a much less efficient way to store and carry energy, primarily because of the losses in compressing/decompressing the air, and the size and weight of the tank. But you are talking about using a tank that's already there, and recovering energy that is otherwise going to be wasted. I like the way you think.

Being an engineer, when faced with a proposition like this my first instinct is to do a reality check. Let's try to see whether we're looking at the next Wright Flyer or the next Tacoma Narrows Bridge.

The amount of energy of energy it takes to compress a gas from V1 to V2 (which is the same as the energy available from decompressing the gas from V2 to V1) is:
E = P1*V1*ln(P2/P1), where the pressures are absolute pressure, not gage.

P1 and V1 are known from the tank. P2 is the pressure when we're done with it in our motor, and sending it on to the our regulator. Most regs want 120-150 psig at the second stage. But since we're looking to get lots of energy out of the air before it gets to the reg, lets assume you find a reg that will work with only 90 psig. Converting that to absolute, we get 15 psi additional at the surface, and assuming we will stay shallow (say 10 ft), we get about another 5 psi for depth, making P2=110 psia.

Let's assume we're talking about an AL80. This tank holds 80 ft^3 of gas at around 200 bar. The volume of the tank is thus about 80/200 ft^3, or 0.4 ft^3. To be fair, let's convert our 3000 psig to 3015 psia.

P1=3015lb/in^2 = 144*3015lb/ft^2
V1=0.4 ft^3,
E = 144*3015*0.4*ln(3015/110) = 575,000 ft-lb.

Since one HP = 550 ft-lb/sec, this is equal to 575,000/550 HP-s, or 1045 HP-s, or 1 HP for over 17 minutes. That's nothing to laugh at. But what does it take to capture that energy?

1.You need an air motor that can operate with input pressures from 300 to 3000 psi, with intermittent flow (air only flows when you breathe), and a very consistent and reliable exit pressure of 90 psig.
2.You need a mechanical transmission to link this intermittent output to the propeller, or an electrical generator to recharge the battery. Due to the dramatic difference between the demand for propulsion and the supply of air power, the generator is definitely simpler, and likely more efficient overall.

Air motors are not known for high efficiency, and this one has unusual demands, so it probably won't be more than 50% efficient (likely even less). The generator might be 80% if well designed. So at best you might be able to deliver 40% of the available energy to the motor. That's around 400 HP-s

Now let's compare to a battery.

Turns out a 7 A-H, 12V battery has about 405 HP-s of energy.
(7A-H)*(12V)*(1HP/746watts)*(3600 sec/H) = 405 HP-s.
This battery weighs around 6 lb, and costs about US$30.
Also, you don't have to suck the tank dry to get all the energy from the battery, it doesn't tether you to the DPV, and it's efficiency doesn't fall at increasing depth. If I were at Tacoma Narrows, I think I'd take a different bridge.:depressed:

D

 

[East West]

I believe if you do a historical fact check that a similiar device was on the market

 

[MichiganScott]

Thanks for everyones imput, this is just one of those nagging things I think of on my 65 mile drive to work each day. No fear I'm not through thinking of new and interesting stuff...

 

[JanK]

Heh, I'm in the middle of vacation, reading Douglas Adams' posthumously published book "Salmon of Doubt" when I come across something that would be nice to share. Firstly, for those who don't know who Douglas Adams was, he was a brilliant writer, thinker, conservationist, and the author of the "Hitch-Hikers Guide to the Galaxy" trilogy in five parts. Any of his books are highly recommended.

Anyway, I just found out that he was also an enthusiastic Scuba diver :

Apart from a couple of refresher dives in pools this was my first proper scuba dive in years. I absolutely love it. I'm one of those people who has been tantalised by the flying dream for years, and scuba diving is the closest thing I know to flying. And for someone who is six foot five and less sylph-like than, to pick a name at random, the Princess of Wales, the sensation of weightlessness is an ecstatic one. Also, I usually vomit on the way back which is a good way of working up an appetite.

This quote was taken from a very amusing article Riding the Rays about trying to make a comparison test between riding a Sub Bug and riding a Manta. Check it out! Although I would never ride such a thing as intended (where you can get separated from your air source), the story is an amusing read!

 

[MichiganScott]

Heh, I'm in the middle of vacation, reading Douglas Adams' posthumously published book "Salmon of Doubt" when I come across something that would be nice to share. Firstly, for those who don't know who Douglas Adams was, he was a brilliant writer, thinker, conservationist, and the author of the "Hitch-Hikers Guide to the Galaxy" trilogy in five parts. Any of his books are highly recommended.

Anyway, I just found out that he was also an enthusiastic Scuba diver :



This quote was taken from a very amusing article Riding the Rays about trying to make a comparison test between riding a Sub Bug and riding a Manta. Check it out! Although I would never ride such a thing as intended (where you can get separated from your air source), the story is an amusing read!

Thanks for the article link it was fascinating.

 

[JanK]

No worries, no worries at all

As I said, a brilliant mind. Just two more interesting facts: It is from the HHGTG that the Babelfish translation service got its name. In the books the Babelfish is a small yellow leech-like fish that you stick in your ear and you can instantly understand anything said to you in any form of language. And by removing all barriers of communication between different species and cultures, the Babelfish has caused more and bloodier wars than anything else in the history of creation.


And he more or less predicted Wikipedia 30 years before it actually appeared. The Guide, as described in the book, is a hand-held device that is updated via subether connection to the main office. The entries are written not only by researchers, but also by hitch-hikers for hitch-hikers. The editing post is more or less available to anyone that wants to participate. It is described as having many omissions and that it contains much that is apocryphal or at least wildly inaccurate.