NEW CAR $12,700. New Motor. I want one for my boat!

Having trouble coming to terms with the math.

From post 33, 1 gallon of gas = 193 compressed gallons of air at 4,300psi.

Applying a 1.6x efficiency factor from post 39, 193/1.6=120 compressed gallons

A pint = 1/8 gallon. 1 pint gas "safety margin" = 24 compressed gallons of air at 4,300psi.

Did I figure this all wrong?

 

Bol,

I am not sure exacally what you are asking... But o be clear, one gallon of gas has the same amount of energy in it as 193 gallons of air compressed to 4,300psi.

Fundamentally the real question about this system though is how much compressed air would it take to move a dingy say 1 mile. Of course this assumes some given design and speed, but let's just say a small 4 man rib.

If the amount of air is a reasonable amount to carry on board it would be fine for the intended purpose regardless of how it compares to gasoline. If not, then we keep looking.

 

I think calculating is a better way to compare.
Here are formulas and an actual 24 lb 4HP (3KW) motor and the links.

http://www.pulsair.com/info-sys/comp-air.htm

Compressed Air Calculations

Because PULSAIR uses compressed air or gas to operate, it is helpful to understand how the volume of compressed air is calculated.

A standard cubic foot of air is the amount of air in one cubic foot of space at 70 degrees F and 14.7 PSIA, i.e. standard atmospheric conditions. Since pressure gauges do not register atmospheric pressure, 14.7 PSIA equates to 0 PSIG. Because this is approximately the way air exists in nature, air at atmospheric conditions is also called "free air". Therefore, 1 Standard Cubic Foot of air is the same as I cubic foot of free air.

A tank with a volume of 4 cubic feet holds 4 Standard Cubic Feet of air at atmospheric pressure (0 PSIG). If we inject air into the tank until the internal pressure is 44.1 PSIG, we have 4 cubic feet of air at 44.1 PSIG. How many Standard Cubic Feet of air is this?

Answer: Divide 44.1 by 14.7 = 3. This means that at 44.1 PSIG each cubic foot will contain three atmospheres plus the original 1 atmosphere that existed before pressurizing, or a total of 4 atmospheres. Multiply 4 cubic feet of volume by 4 atmospheres = 16 Standard Cubic Feet of air.

If we inject more air and double the pressure to 88.2 PSIG, the same math tells us we have 28 Standard Cubic Feet of air in the 4 cubic foot tank (88.2/14.7 = 6 atmospheres + 1 atmosphere = 7 atmospheres x 4 cu. ft = 28 SCF).

By using this standard unit of measurement we greatly simplify discussions and calculations of air volumes.

Divide 4100 psi by 14.7 = 279 and add 1 = 280 atmospheres times cu ft capacity of tank. A 28 cu ft scuba tank would hold 7840 cu ft free air at this pressure.

Lubed air motor 130 cu ft min free air at 100 psi (7 bar) @ 3000rpm and 4 hp (3kw) $400 would run 60 minutes off this one scuba tank. This is a vaned turbine air motor. I believe a piston air motor would be more torque and better economy in air use, but lower rpm probably.

http://fergusonengineering.com/GastAirMotor_lubed_motors.html

Sounds feasible!

 

I am really, really thinking about buying one of those motors and mounting it to a prop and seeing what happens.... The only problem is the rpm is really high, so I would need a gear box, which I don't have available...... Anyone feel like experimenting

 

I was thinking finding an cheap old outboard motor 3 to 5 hp, with a ruined engine. use entire outboard but just replace motor with air motor.

 

Also I happen to have lower units of 2 seagull motors. 4:1 reduction and 5 bladed props. But my boat is 1500 miles away. I cant experiment right now.

 

also there are smaller motors, lower rpm motors, and a simple valve reduces air flow and rpm. tank last longer with less air flow too!

 

Here is a British Seagull 4.5 HP outboard with 4:1 reduction and 11 inch 5 bladed prop, 10 inch pitch on Ebay. Says not running. They want $300 to buy outright.

http://www.ebay.com/itm/British-Sea...298614&pt=Boat_Parts_Accessories_Gear&vxp=mtr

 

The Seagull 2 stroke outboard was a smokey, drippy motor. That's why they aren't made any more. The aluminum alloy used throughout is a company secret and nearly perfectly corrosion proof. They apparently last forever. Changing the motor to air might be just the ticket for an auxilary for a 25 footer and for the dinghy as well!

http://www.britishseagullshop.com/sailing.html

THE OCEAN SAILING YACHT
DONALD M STREET JR First edition 1973

This is a classic for the cruising sailor. 703 pages, about twenty loose pages at the end. Dust jacket included and showing some wear, cover in good condition hardbound in blue cloth. The book being large and detailed with excellent illustrations covers most of what you should know when considering a cruising boat. This is a good reference book to have on board.

If the book has any problems one might be that it is just too big and consequently trys to cover too much. 703 pages represents a lot of reading. There is an index and so the smart thing to do is use it and read what you need when you need to know it! There are a lot of good ideas.
*****On page 560-561 Street recommends using British Seagull engines and gives solid reasons which still apply today. Mind you he does get his gear ratios wrong. ( the big seagulls have a 1:4 reduction gear not a 1:2 gear.

This is his 40 ft heavy displacement 'Iolaire' that for many years, the only engines were two British Seagulls with the 5 blade props. Powerfull pushers!

http://www.street-iolaire.com/disc.htm

 

Was just reading this thread with great interest, so I looked up a few things, including the energy density of compressed air:

"Compressed air has relatively low energy density. Air at 30 MPa (4,500 psi) contains about 50 Wh of energy per liter. For comparison, a lead–acid battery contains 60-75 Wh/l. A lithium-ion battery contains about 250-620 Wh/l. Gasoline contains about 9411 Wh per liter.[1]; however, a typical gasoline engine with 18% efficiency can only recover the equivalent of 1694 Wh/l. "

 

Is this 60 scuba tanks to equal one gallon of gas?

 

I think people, very inteligent people, well educated people, get confused over different types of engines, motors, and fuels.

It seems to me they're trying to make a UNIFIED motor energy theory. IMO they aren't anymore sucessfull than the physicists with a UNIFIED field theory.

The realm of human knowledge isn't complete.

Perhaps this will make my point. How big of a diesel engine is needed on a space shuttle for a repower replacing the rocket engines? Silly huh?

Electric motors, steam turbines, reciprocating steam, air motors, internal combustion engines (various fuels), rockets, jets, ect all have advantages and disadvantages, and not the same ones.

It depends on application. Space shuttles don't want torque, just thrust.
Cars, trucks, trains, and propeller boats need torque. Trains use diesel electric because the start up torque of the electric is huge and ICEs is tiny. Steam works even better at low end torque.
Air pressure in some ways resembles steam.

Can't compare apples to oranges, except on paper. You can analyze and compare sugar content, vitamins, calories, ect.

But in application, there is the reality. How many oranges equates with how many apples to make an identical apple pie?

Sorry I can't explain any better. The motor link I posted earlier has PDF spec sheets showing torque, HP, RPM, air flow ect. Including power curves, except in an air motor the power isn't curved. It's a straight line. More pressure and air flow = more torque, HP, and speed.

I'm going to try it when I get the chance.

 

I tried to force air to gas comparison so I could grasp how much power there was.
I think in gas engines, the amount of gas I carry, etc. I don't know air power.

 

No problem. I'm trying to learn myself. I've been looking at scuba bottles for sale. Their rated capacity is different from other bottles. Some marketing tactic I guess. A bottle measuring 7.25 inches diameter and 28 inches long is 41.25 sq inches cross section, or less the wall thickness. Times 28 inches long = 1155 cu inches. A cu ft has 1728 cu inches. That works out to 2/3 of a cu ft capacity or .668 cu ft. At 3000 psi (normal load) = 204 bar or atmospheres. Plus one bar for air in bottle at atmosphere pressure, these bottles only hold 137 scf (standard cubic feet) of air. Scuba companys say 80 cu ft for this bottle. Maybe the ultimate 57 cu ft is at minimum pressure and unusable.
let's use 80 cu ft. for one tank air.
I'll be back with motor data.

 

Thank you. I did not see the pdf before.