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Old 02-06-2013, 08:25   #31
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Re: SCUBA tank to drive a batt charger motor

A typical alternator might use 1hp or so on a smaller engine. I doubt that you could get the air out of a tank fast enough to start a 1hp alternator.

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Old 02-06-2013, 08:53   #32
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Re: SCUBA tank to drive a batt charger motor

Wrap a bit of line around your prop and shaft like pull rope then hop in your dinghy with the rope and take off, in the correct direction
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Old 02-06-2013, 12:27   #33
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pirate Re: SCUBA tank to drive a batt charger motor

I need an impact wrench pretty often. And air for tires. My tank travels in the truck with a flexi hose and is perfect. Air goes a long ways. I know of a guy who spray paints with one. Seems a heavy piece of equipment to have just a single use for diving.

I bought a crank handle for my YSM12 but I'd think the torque wrench would start it easily. Just need a machined interface between the wrench and crank. No sweat.
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Old 02-06-2013, 14:31   #34
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Re: SCUBA tank to drive a batt charger motor

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Originally Posted by Blue Crab View Post
I need an impact wrench pretty often.....

... I bought a crank handle for my YSM12 but I'd think the torque wrench would start it easily. Just need a machined interface between the wrench and crank. No sweat.
Bingo. One of those "why didn't I think of that" moments. A $20 3/8" drive air drive wrench from Harbor Freight. Fabricate/buy the crank-fitting for my flywheel. Engage decompression lever on engine, crank, release and go! (Maybe no need for decompression). No need for complex/dubious charging setup for tank
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Old 02-06-2013, 17:18   #35
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Re: SCUBA tank to drive a batt charger motor

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Bingo. One of those "why didn't I think of that" moments. A $20 3/8" drive air drive wrench from Harbor Freight. Fabricate/buy the crank-fitting for my flywheel. Engage decompression lever on engine, crank, release and go! (Maybe no need for decompression). No need for complex/dubious charging setup for tank
If you use an impact wrench on your crank bolt 2 things will happen....

1. The rotation of the engine while doing this would be in the neighborhood of 1 rev/min.... Plenty of time to have one or two sips from your cocktail before TDC passes twice... It's called an impact wrench for a reason...

2. You will tighten the crank bolt so much that it will never come off again...
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Old 02-06-2013, 17:37   #36
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I would have thought a small air driven stately motor would be possible. The generator idea is nuts, simply not enough run time.

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Old 02-06-2013, 18:41   #37
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Re: SCUBA tank to drive a batt charger motor

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If you use an impact wrench on your crank bolt 2 things will happen....

1. The rotation of the engine while doing this would be in the neighborhood of 1 rev/min.... Plenty of time to have one or two sips from your cocktail before TDC passes twice... It's called an impact wrench for a reason...

2. You will tighten the crank bolt so much that it will never come off again...
OK but besides that ... ?
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Old 02-06-2013, 19:04   #38
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Re: SCUBA tank to drive a batt charger motor

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A more efficient use would be an air starter...
BINGO!

Running a generator off of the tank isn't going to last long enough to make a difference. But I'm willing to bet that you could put together some sort of compressed air powered starter that could start your auxiliary engine when the batteries are dead--and then, obviously, use the engine to re-charge the batteries.

This seems to me like the most feasible way to approach this.
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Old 02-06-2013, 19:23   #39
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Re: SCUBA tank to drive a batt charger motor

The OP's basic proposition: can a charged SCUBA tank start an engine, has merit.

The advantage, apart from the fact he already has it, is that it will stay charged almost indefinitely.

There is a way to directly exploit the considerable energy potential of a charged high-pressure air tank which would be affordable if you had the knowledge and the time to buy the necessary hardware at sacrifice prices on eBay: the high pressure air from your tank could be applied to one side of a piston in a 'free piston accumulator' (other types are available but this is the easiest to make). On the other side of the piston would be hydraulic fluid, of which a small portion could be drawn off rapidly at virtually the same pressure as your air, and used to run a hydraulic starter motor.


Using the stored air to drive either a pneumatic starter motor, or an air motor to generate electrical power has a problem: the air is at MUCH too high a pressure to be usable. There is no affordable way I'm aware of to exchange pressure for volume, which is what you need to do here (the air analogy to an transformer for AC)electricity, which drops the voltage while increasing the current demand).

Furthermore, pneumatic motors are notoriously inefficient, especially the sort fitted to inexpensive hand tools.

Thermodynamics, the gas laws (scientific rather than legislative), and the frictional losses using intensification devices in reverse are all against you.

Generating electrical power is the least sensible option, it seems to me. Apart from the time factor and the inevitable conversion losses, it means that you are still entirely dependent on a battery holding charge, and an electrical starter motor delivering it to cranking.

Someone scammed Tata (Indian corporation which makes motor vehicles among other things) with a pseudo-solution to the problems of direct conversion of high pressure air to mechanical prime mover. They wasted billions of rupees and years of 'research' to no avail trying to make it happen.

It was immediately obvious from the 'inventor's' claims that it had elements of a perpetual motion scam, purporting to be able to extract energy gratis from the environment, with a bit of hand-waving.

Sure, heat pumps appear to do this, but the energy they 'extract' (actually, move from point A to point B) is low grade (low temperature, high volume) energy, much lower grade than the electrical energy they consume in order to do the moving. Low grade energy has very limited uses in automotive applications, once you've heated the interior sufficient for comfort...

Returning to viable solutions:

However you do it, it will make a mockery of 'saving money by using a SCUBA tank I already have', but it could be fun. To be useful in the long term, you would want to fit your engine room with a dive compressor, to close the loop and recharge the bottle for next time. And/or a bit of diving on the side, like when you foul an anchor or wrap a pot line around a prop ...

Some emergency generator sets use exactly this technique to start large diesel engines very reliably after months or years of inactivity, except that they use compressed nitrogen rather than air (better, especially for material longevity, eg seals, but not crucial AFAIK)
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Old 02-06-2013, 19:40   #40
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Re: SCUBA tank to drive a batt charger motor

Just for the fun of it, since we are on the subject of compressed air starting a diesel, here is an M.A.N. diesel, circa 1917, being started up:

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Old 02-06-2013, 20:16   #41
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Re: SCUBA tank to drive a batt charger motor

Seems like it would easier and a much snaller proposition to use a shot gun shell ( Flight of the Phoenix) to start your engine than to build a compressed air/ hydraulic starter system. Most hyd starter systems have a hand pump and an accumulator. An air over hyd system with just 1 bottle wold be pretty much a 1 shot deal. What if it did not start the first time?
You need a way to recharge the cylinder.
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Old 02-06-2013, 20:31   #42
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Re: SCUBA tank to drive a batt charger motor

The Coffman Starter, which used a blank shot shell like cartridge, was pretty common and pretty reliable as long as you had a ready supply of cartridges.
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Old 02-06-2013, 20:59   #43
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Re: SCUBA tank to drive a batt charger motor

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Running a generator off of the tank isn't going to last long enough to make a difference.
According to the energy storage figures, even at 10% efficiency there is plenty
of air to charge enough to start. At 1% efficiency there would be enough to start it a couple or three times. So it is feasible, just not very practical.
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Old 02-06-2013, 21:59   #44
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Re: SCUBA tank to drive a batt charger motor

I have an aircraft inertial starter of WWII vintage that I plan to fit to the 32 H.P. Universal diesel in my boat. I got it from a friend who used one on his 75-foot schooner. He says it worked fine. It contains a flywheel that is spun up with a hand crank. It is about 8" in diameter by 8" high. Made by Eclipse Aviation, a division of Bendix Aviation. The spring starter sounds interesting too, but since I have this, I'm going with it.
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Old 02-06-2013, 21:59   #45
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Re: SCUBA tank to drive a batt charger motor

If a piston could handle a single explosion of sufficient energy to start a diesel, which is a big "If", *
then conceivably a single shot of high pressure air into one of the cylinders might also do the trick.

Better yet would be to decompress the other cylinders, start the engine turning slowly by cranking it over by hand, with a timed pulse of HP air each time the driven cylinder was at TDC. Then recompress another cylinder when up to cranking speed. Then another .....

However I think it would be pretty unlikely to work without a lot of development effort, if at all.

* to my admittedly limited knowledge, direct cartridge starting was only rarely done for diesels, in the early phases of their development. Even in cases where it was an option, use of it was discouraged because it was detrimental to the bearings. And AFAIK it only worked when used in conjunction with some external heat source to bring the combustion chamber closer to ignition temperature.

Diesels rely on cranking sufficiently fast and for sufficient duration to compress air up to ignition temperature: it's not like starting an aircooled aero engine on high octane petrol, with a spark to provide ignition, and low thermal mass in any case -

Coffman starters were capable of starting diesel engines, even quite large ones, but that was not a direct explosion into the combustion chamber: these starters were a separate starter motor, which had a separate cylinder and piston, whereby the piston ran down a multistart threaded spindle, causing that spindle to spin and turn the engine over multiple times.
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