Re: Running a petrol generator underwater with compressed air tanks
Sulaiman, Sat Mar 28 2015, 04:41AM

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.

Re: Running a petrol generator underwater with compressed air tanks
Hazmatt_(The Underdog), Sat Mar 28 2015, 05:09AM

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.

Re: Running a petrol generator underwater with compressed air tanks
Bjørn, Sat Mar 28 2015, 07:08AM

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.

Re: Running a petrol generator underwater with compressed air tanks
Dr. Slack, Sat Mar 28 2015, 08:08AM

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.

Re: Running a petrol generator underwater with compressed air tanks
Andy, Sat Mar 28 2015, 09:42AM

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.

Re: Running a petrol generator underwater with compressed air tanks
Sulaiman, Sat Mar 28 2015, 12:17PM

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

Re: Running a petrol generator underwater with compressed air tanks
nzoomed, Sat Mar 28 2015, 08:39PM

I dont know if im missing something here, but how did they do it with the early diesel submarines?

Re: Running a petrol generator underwater with compressed air tanks
BigBad, Sat Mar 28 2015, 08:55PM

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.

Re: Running a petrol generator underwater with compressed air tanks
nzoomed, Sat Mar 28 2015, 10:49PM


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.

Re: Running a petrol generator underwater with compressed air tanks
Andy, Sun Mar 29 2015, 04:09AM

Dont know weather this should be in the chemistry part, but some numbers on a idea.
NaClO4 makes O2
Diesel = C12H23

168 grams of diesel burnt in a serperate step with oxygen from above make 8.7kg of Sodium chloride
if the specific gravity equals one, one liter at 4kwh for a Lister engine needs 51.8 kg of sodium chloride.

The boat/sub as 0.81m2 area(to water line) so at 3m/sec is 6014Watts, with losses is about double the input fuel and genny.

The thing mentioned might work Bjorn, what would the Lister engine behave like, they normal can take a hit well.

Edit

Re: Running a petrol generator underwater with compressed air tanks
Dr. Slack, Sun Mar 29 2015, 08:44AM




big air tanks, big boat, so lots of space for them, short submerged duration, and low submerged speed requiring lower power, some used liquid oxygen to store more oxidiser in a smaller volume. At low depths, exhaust could be vented into the sea directly. At larger depths, or for stealth diesels (something of a contradiction in terms) sea water was brought into the boat through a turbine to reduce the pressure, exhaust CO2 was dissolved into it, then the sea water pumped back out, using the incoming turbine to provide most of the power for the pump.

I think the lesson of the 168g diesel versus 5.4kg sodium chloride could be better expressed as diesel requires two orders of magnitude more mass of oxidiser if the oxidiser is sodium chlorate. It would be interesting to see how much oxygen can be obtained from 10kg of sodium chlorate, or 10kg of diving cylinder charged to 300 bar. With a snorkel, it requires zero times the mass of fuel. That zero multiplier could be very significant.

While in principle there's no reason that an engine should stall even if working into considerable back pressure, in practice, COTS engines are designed to work into one bar back pressure. Working into higher back pressures may well upset gas flows and nice tweaks made tot he engines over years of development, that a COTS engine will probably take a huge hit in efficiency if working into much more than one bar. It should be possible with development to run an engine into higher pressures, but that's not a readily available engine.

If you already have an air input snorkel, then supplying an exhaust output snorkel would allow the engine to run at one bar input and output presures. Just a thought.

Re: Running a petrol generator underwater with compressed air tanks
hen918, Sun Mar 29 2015, 06:49PM

From what I've read, submarines didn't have air tanks at all, except for the crew's breathing supply. Whilst the sub was surfaced, it used a diesel engine attached to the prop, as well as a generator for charging MASSIVE lead acid batteries, (the sort found in electric fork lifts). The batteries doubled as the ballast for stability and were used for driving the prop via a DC motor whilst the sub was submerged. The engine had snorkels for the exhaust as well as the intake.

Re: Running a petrol generator underwater with compressed air tanks
GrantX, Mon Mar 30 2015, 05:26AM

Yeah, the Australian Collins-class subs use a 5.4MW DC motor for propulsion, with battery banks and diesel motor/generators.

Re: Running a petrol generator underwater with compressed air tanks
Andy, Fri Apr 03 2015, 03:05AM

The goal is to trying and go without a snokel, thought about a themite reaction, in little pellets which heat up oil, and themo piles, convert to electricty.
It should have the energy density.
Does anyone have thoughts down that path or another idea, for more freedom from cables.

Re: Running a petrol generator underwater with compressed air tanks
Sulaiman, Fri Apr 03 2015, 05:24AM

ultimately you have the same requirement as 'spaceships' ... energy storage density.
I think liquid O2 and H2 are current 'best'.
Fuel cells for electricity/propulsion and fresh water like in space?

how much the crew may enjoy the idea of huge tanks of liquid O2 and H2 I don't know,
some seem comfortable with a nuclear reactor on board.

Re: Running a petrol generator underwater with compressed air tanks
Patrick, Fri Apr 03 2015, 06:14AM

well i guess a 17,000$, 6 pound fuel cell, at 1kw is out of the question. Though it solves most (or all) of your problems. Sodium Borohydride is becoming available by some of the larger makers.

There are nitro / alcohol / glow engines that could make a generator as -- Im currently trying out. The problem is there easy to overwhelm and just quit. They have such light but fast RPM, those little pistons just stall with load. Nitro adds way more oxidizer once decomposed in the cylinder. however you have big problems in so many directions.

What dimensions, duration and mass is your machine expected to come in at? I might be able to make some fiberglass parts. though high pressure containers could be killing devices... the scuba tank might be better. but custom fits to carry them i could do, as well as the body parts. The cylinder with a this 24 ounce FG cloth, would make your body the same diameter as a scuba cylinder, maybe a wire tunnel too.

Once i start my robotics company, ill be doing more than just aerial drone work. Uzzors2k here on the forum, has some umbilical subs on his personal page. Not sure what your planning.

Re: Running a petrol generator underwater with compressed air tanks
Andy, Fri Apr 03 2015, 07:19AM

The thing at the moment will be 5 meters by 2 meters by 1.8 meters high, single seater at present, but would like a two seater later.
Duration hopefully over one hour, I havnt run the structal weight or strength at present, assumed a rought esstimate at 4.5 tonnes, but can get it increassed to around 6-7 tonne, with a different door postion.
Will look into Nos and alochol, at present running some numbers on copper sulphate, magnesium and aluminium, with sulfuric acid to start the reaction, and a pallet load into a copper heat sink, submissered in oil as the heat storage.
Pretty much, if I cant get a decent underwater time with a power source it will put a break on the thing.
Dont mind having a $700,1000 refuel if its at one hour, but $70,000 for batterys is out.
Thanks for the offer about parts, but need tests proformed for structual strength, for latter.
Interested in the underwater berings, have you found a way to keep the motors cool without airflow?

Re: Running a petrol generator underwater with compressed air tanks
Carbon_Rod, Fri Apr 03 2015, 07:20AM

Diesel electric hybrids have driven massive trains, and some of the earliest submarines.
This method is still the most practical design for economic transportation models.

There have also been mono-propellants around for 60 years, but they are dangerous to handle given the chemical volatility. Every few years one will learn of a forgotten torpedo undergoing catastrophic rupture inside a submarine. IIRC, the last group unlucky enough to have this happen was a Russian crew in the 1990s, and there were no survivors.

@Patrick
Robotics do not sell very well, are highly prized targets for "problem entities", and are often cloned within months. You will start to notice supply shipment boxes sporting yellow-tape from Asia, TSA tape from the USA, and items exempt from duty thoroughly inspected by customs anyway. Even if you are a legitimate tax paying company, the random crushing and theft of parts will continue as part of the business model.
I'd recommend against starting a company like WowWee, as they were a rare success in a hostile market sector.
Watch out for export restrictions, as Elon Musk proved that not filing patents is sometimes more effective at containing trade secrets.

Re: Running a petrol generator underwater with compressed air tanks
Dr. Slack, Fri Apr 03 2015, 08:57AM



That's pretty much the entire argument in those two quotes. You know just as well as the next man that there aren't many options for energy storage. The question is, which system's drawbacks are you comfortable living with. If I am going to put myself in a box, and then cover myself with 20m of water, I'm not sure any of the combustion options would make me feel all that comfortable, especially if to get the oxygen for it required some interesting exothermic chemistry.

But it's not quite energy density that's the issue as for spacecraft, in which lift mass is expensive. In subs, mass carrying ability is cheap, so you need good energy storage per volume, or per dollar.

Do the sums for shaft kWh out per cubic metre of volume, or $1000 expenditure, whichever you feel is the more important. Amongst the options are lead acid, LiPO or NimH driving PM or more advanced motors. Another to consider is a 300 bar scuba cylinder driving an air turbine or reciprocating expansion engine of some type, no combustion involved. That would be very clean and relatively safe. I reckon you could have fun designing and building a water-payload potato-cannon sort of thing to fire water out of the back, using the high pressure compressed air directly. With a multi-barrel design appropriately phased, you could have reasonably steady thrust. I wonder if you could have different sized barrels and operate the set at different pressures, so making best use of the high pressure available directly from the cylinder.

I was astonished when I did the sums for specific energy density for a hammer system for my robot, that rubber beat out air and steel for springs by a long way. Do the sums for 100kg of aeroplane elastic just for the sake of completeness, though I doubt that with your constraints it will figure very highly. My key constraint was very high specific output power with a delivery time of 50ms, which is rather shorter than your (I guess) 5ks.

And then finally do the sums for oxidiser/fuel systems, but do remember to figure in the containers and the safety systems for handling the oxidiser, and the asphyxiant gas the engine produces.

Don't forget to trim your power requirement down by having your top speed target as low as practical, and your energy requirement by similarly limiting your cruise speed. It's worth considering top and cruise speeds seperately, it allows you another degree of freedom, you can fight a strong current for a few minutes, and still cruise out to the fishing grounds and back.

Re: Running a petrol generator underwater with compressed air tanks
Ash Small, Fri Apr 03 2015, 10:20AM

I think the 'obvious' solution here is a diesel engine running a generator to charge lead acid batteries on the surface.

This should easily give dive durations of well over an hour with a craft of the dimensions that you describe.

Keep it simple

Re: Running a petrol generator underwater with compressed air tanks
Patrick, Sat Apr 04 2015, 05:09PM

Never mind about what i previously said, I thought this was supposed to be a unmanned sub of some-sort. (which I will be doing at Lake Shasta as part of my business.) Uzzor2k has some on his sight as i said.

20 meters of water, with a petrol engine under my seat is to far for me to go, with out some serious $500k and personal investment of time to make sure were not getting killed or hurt. Though this is a cool idea, and here at 4HV, you'll get better answers, I think, than at other forums with radicals.

I was thinking this was a "bue-fin 21" like idea.

Carbon Rod, it looks like WeeWow is a toy maker? is that all they do?

Re: Running a petrol generator underwater with compressed air tanks
BigBad, Sat Apr 04 2015, 11:24PM

No, the energy storage density of liquid hydrogen is fairly awful. Rockets use it because the exhaust velocity is very high, but that's not going to be useful in a submarine.

If you want high energy density LOX/Kero is much better. Actually, I think High Test Peroxide/Kero may be better still because HTP has a higher density, but I might be wrong, I know it has a better ISP density, but that's not quite the same thing.

In the context of a submarine, LOX/Kero might burn a bit hot. It might be possible to recirculate the exhaust gases around, cooling them with sea water and to remove some but not all of the carbon dioxide, dumping water overboard, and then inject more LOX and send it back around again, but it's quite complicated.

A simpler alternative is to use liquid air, but that has much lower energy density.

Re: Running a petrol generator underwater with compressed air tanks
Andy, Sun Apr 05 2015, 11:00PM

Planning on have batterys for five min run at max, the motors will be induction 10kw with three, and two power settings. A Vfd drive controls one from zero to hunderd, the three from zero to hunderd percent.
About the batterys, ive seen lead acid with 20C, which should give about 500mA at 12volt,2Ah,
Im abit lost on what setup would work, I would like 30kw of drain for say five mins, with a 10kw genny, and have roughtly 5kw of general thrust.

Supercapactors,Sla,Lipoly,Nimh
For the genny, a lister desiol engine, a custom themopile setup, a car air engine, batterys by th err m selves(no genny on board).

Re: Running a petrol generator underwater with compressed air tanks
BigBad, Mon Apr 06 2015, 02:13AM

IRC peroxide and a fuel is a traditional choice in torpedoes- it burns fairly cold(ish):



Of course rockets are the most amazing propulsion for underwater use!

Re: Running a petrol generator underwater with compressed air tanks
Bored Chemist, Wed Apr 08 2015, 08:06AM

Liquid air is abrely a viable option.
The nitrogen boils off first (which is useless for combustion) then the liquid oxygen that's left boils off and feeds nearly pure oxygen into the engine which also isn't good...
A stack of car or truck batteries still looks to me like a good idea.

Re: Running a petrol generator underwater with compressed air tanks
Andy, Wed Apr 08 2015, 10:11AM

Got two battery choices, the lead acid has more storage for price, but the NiMh with the same price proable can discharge more juice at once.

For 15mins run time at 20kw, what one do you lot thing would be better, andtake up less room with wireing.

Would it be a good idea to use the metal structure as the negtive earth connection for the batterys, aswell as ground for the mains ac source, would it be better for the people inside aswell is releabile for the electronics?



HR-DUX(10.0AH)/SY148-ND-1202991

MSB200FR/ MSB200FR-ND/4484928

Cheers

Re: Running a petrol generator underwater with compressed air tanks
Dr. Slack, Wed Apr 08 2015, 02:06PM

Even if the lead acid has more rated capacity for the price, that will be at the C/10 or C/20 rate. For a 15 min run time, that's a 4C rate. Lead acid gets much less efficient as the rate goes up. If you want to compare two battery chemistries for cost, compare them at the rate you will be using.

Cycle life. Lead doesn't like deep discharge, more than 50% shortens its life dramatically. So you would want to install 2x rated capacity with lead acid for a similar lifetime. NimH OTOH will discharge to full depth.