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Running a petrol generator underwater with compressed air tanks

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Andy
Sat Mar 28 2015, 02:41AM Print
Andy Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
Hi
I'm trying to find out if its fessiable to run a petrol generator underwater in a submarine with the air coming from a compressed tank.
I'm thinking about 3 * 80L tanks at about 150psi.

I don't know how much air a petrol genny will use, but will be looking for 20kw of output, and assuming 3kwh per litre of fuel.

What do you lot think would be the run times?, would batterys be better if charged on the surface for dives?
Any thing im missing.

Cheers
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Sulaiman
Sat Mar 28 2015, 04:41AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
mathematically;
air volume intake per revolution is 1/2 engine capacity for a four stroke at full throttle,
e.g. if 20 kW = full power output of a 1L engine at 3600 rpm = 60 Hz, .... 30 litres of air per second
exhaust gas would be hot = higher volume, all to be vented out underwater.

3x 80L x (10 - 1 atm). = 240L x 9 = 2160 litres of air available = 1 min. 12 sec. (less in practice)

logically;
if compressed air could easily be a good/safe store of energy it would have been done already
application sounds somewhat governmental (free roaming) so use silver batteries, tried, tested, approved,available

I am not an expert in underwater power, just random thoughts.
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Hazmatt_(The Underdog)
Sat Mar 28 2015, 05:09AM
Hazmatt_(The Underdog) Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
How are you going to vent low pressure exhaust gasses when the surrounding water pressure is high in comparison?

I'd think you would need to run a compressor off the same shaft to compress the exhaust gases to a point that they would vent past a check valve without back-pressure from the water.

As far as I know, the submarines only ran combustion engines during a surface run.
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Bjørn
Sat Mar 28 2015, 07:08AM
Bjørn Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Some submarines can run the engines submerged using a snorkel.

Instead of compressed air you could use more exotic fuels that contains the oxidiser at higher density. It will be very expensive and dangerous.
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Dr. Slack
Sat Mar 28 2015, 08:08AM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
So many issues to consider

a) Your exhaust pressure will increase 1 bar per 10m of submerged depth, so 1 bar at the surface, 2 bar at 10m etc. An engine is also a pump (the old mom'n'pop garage would often re-purpose an old 4 stroke motorcycle engine as a compressor that was good to 10 bar), so there's no need to increase the pressure of the exhaust gas. However, engine efficiency suffers when it's working into a back pressure. It would not make sense to try to pump up the exhaust gases into the water, as the pump would take energy to run, just take the hit on engine efficiency.

b) You will need a lot of air, much more than you think. 150psi, 10 bar, doesn't even come close to what you could do. Ideally, you would want to re-purpose COTS (Commercially Off The Shelf) equipment, at least for initial experiments with prototypes. Sport divers use air tanks pressurised to 200 to 300bar. That's not an uncertainty range, older equipment is good to 200, newer more expensive stuff can operate at 300bar. As divers upgrade, 200bar stuff is readily available second hand. You can buy refills over the counter. There may be certification required that you can handle this stuff, but for the difference between 10bar and hundreds of bar, it may be worth it. Probably 10 pool hours, to get a basic diver qualification.

c) You will need so much air, can you use a snorkel? Breathing surface ambient pressure and exhausting at depth will give greater hit in engine efficiency, but it may be worth it to not have to carry all that air.

d) It might be worth doing the stociometric sums for the burning of hydrocarbons, and coming up with a weight ratio of oxidiser to fuel, then notice that air is 5x heavier for the same oxiding power as O2. Putting the air requirement as a mass per mass of fuel might concentrate your mind on the design problems.

e) Air is 80% nitrogen. That's a lot of ash to carry with you and heat up, spoiling efficiency. Pure oxygen might be an ideal oxidiser, but that is seriously scary stuff to deal with and you might well not be able to get it commercially. OTOH, Nitrox (see wikipedia) is used by sport divers to extend bottom times, less nitrogen means less susceptibility to the bends. A typical mix would be 36% O2,stronger mixes are uncommon.

f) While we are riffing on this problem, higher pressure air into the inlet of an engine is a well known way to boost volumetric efficiency (turbos and blowers). This would allow you to use a smaller engine for the same power. First stage regulators used by sport divers reduce the pressure to 7bar above local ambient, still way more than enough to feed the engine a boosted inlet pressure. Note that running a commercial ambient-pressure engine at a boosted pressure would over-heat it in minutes. A boosted engine has to be designed for higher heat removal, and higher torque than a normally aspirated one. If you use a normal pressure engine, you could be really smart and drop the 7 bar excess pressure through a turbo to boost the exhaust gas pressure into the local ambient pressure, clawing back your engine efficiency, but that's extra complexity to carry and go wrong.

g) The corollary of (f) is that if you are using a COTS generator set, you must regulate its input air fairly accurately to 1 bar, any significant over pressure and you risk toasting the engine.

h) The cooling on expansion of compressed air will set a limit on how fast you can pull air out of the cylinders and into the engine. For a engine-sized flow rate, rather than a person-sized one, you might need additional heat delivered to the cylinder and first stage regulator. Hmmm, I think the flow rate is the killer there, you may not be able to use a COTS first stage regulator. Now several in parallel, from several tanks ...

i) Did you mention surface charged batteries? I think that's the way to go. Subs can have huge buoyant carrying power at little additional cost, a totally different proposition to aircraft, so whereas you might choose lithiums for a plane, you could use NimH or even SLA for a sub.
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Andy
Sat Mar 28 2015, 09:42AM
Andy Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
I don't mind charging battery's on the surface and then submerge , but lead acid prices for a 120Ah battery is about 300-400 each, with a 1 hour run time(hoping) it will blow the budget.
A snorkel could be a good way to go, i'll proable be looking at a max of 20meters depth, before major designs problems with nature bounacy level and surface ability (hard to get right...and i'll be on board :) )

I plan to do a dive course, they have one locally, mainly if i have to exit the sub due to failure I would like a way to get to the surface.

Cheers all for the input.
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Sulaiman
Sat Mar 28 2015, 12:17PM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
From memory ... basic scuba courses only go to 15m depth,
for insurance purposes you may need more than the basic scuba certification,
if Iwere you I'd check this early in the design and planning stages
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nzoomed
Sat Mar 28 2015, 08:39PM
nzoomed Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
I dont know if im missing something here, but how did they do it with the early diesel submarines?
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BigBad
Sat Mar 28 2015, 08:55PM
BigBad Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Diesel would be more likely to work, because the engine itself runs at much higher pressure.

With petrol, the engine doesn't run at very high pressure, and may stall at high backpressures.
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nzoomed
Sat Mar 28 2015, 10:49PM
nzoomed Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
BigBad wrote ...

Diesel would be more likely to work, because the engine itself runs at much higher pressure.

With petrol, the engine doesn't run at very high pressure, and may stall at high backpressures.

Yes quite true, but im unsure how their air supply was set up, they must have had some large compressed air tanks or something to have supplied the engines.
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