Building a submarine

[Entropy455]

So the ultimate design goal is to rip though 10-meters of water as fast as possible.

Does the sub need to be fully submerged for the entire run? Are you disqualified if you bounce off the bottom, or broach the surface?

16 grams of CO2 is not a lot. One thing to remember is that a compressed CO2 engine is still a thermodynamic heat engine. There are things to can do to increase its efficiency.

The “power source” is the thermal expansion of compressed CO2. I say thermal expansion, because internal energy is converted into flow-work as the gas expands (see enthalpy). From the ideal gas law, if you expand CO2 without any external heat input, PV^gama is a content (polytrophic process) and the CO2 will be quite cold after it expands, with much lower pressures. You can obtain appreciably higher expansion rates (which means more crankshaft horsepower from your CO2 power source) if you introduce heat into the gas upstream of the engine. See the ideal gas law (PV=NRT). Both expansion pressure and expansion volume will be appreciably higher when heat is introduced into the expanding gas. Additional bonus: the heat capacity of CO2 increases significantly at elevated pressures – thus there’s a real energy return to be had if you can heat the CO2 upstream of the engine early on.

Here’s a bit of engineering:

16 grams of liquid CO2 at 77 degrees F, will have a pressure of 914 psig, and take up a volume of about 1.38 cubic inches. If you introduce 1.84 BTU of energy into the CO2 upstream of the engine during the initial enthalpy of vaporization (using the surrounding water as a heat source, or possibly some other self-contained heat source, perhaps DC electric) - the CO2 will flash into a gas while maintaining a pressure of 914 pisg, at a temperature of 77 degrees F, with a volume of 4.09 cubic inches. Without this introduction of heat, both the gas temperature and pressure would drop appreciably during initial vaporization.

1.84 BTU might not seem like a lot of energy, but over a ten second race, it’s equal to 0.25 extra horsepower – which is a boatload for a model submarine (pun indented).

 

[kf2qd]

A uniflow would probably work better for CO2. A schrader valve in the cylinder head and exhaust ports at BDC for exhaust. It would be more conservative on CO2 usage. A conventional stem engine design would have many more places for CO2 to leak out and any leakage would cripple performance.

And you will have to deal with CO2 usage. As the pressure inside the sub changes the motor will slow. How much CO2 and how much space. (Hope that doesn't spoil the fun for your teacher...) Though if you vent any CO2 the total weight of the sub will change.

 

[Tin Falcon]

If you go to the FAME site http://www.floridaame.org/ then click on the galley, then non combustion engines you will see several co2 engines. some of those photos are links to plans take a look at Dave kerzels modular engine a good explanation of how co2 engines work. It only makes sense to use an engine designed for CO2 if that is the required"fuel" or energy source.
The v-2 is nice little engine and should have plenty of power for a small sub. or use the modular design for aa mutiple cylinder engine.
Tin

 

[Entropy455]

basic power formula (P x L x A x N)/33,000

P= pressure
L= Lengths of stroke
Area of piston
number revolutions per minute
33,000 converts foot lbs per minute to horse power
dimensions are in feet.

I saw “number revolutions per minute” and incorrectly assumed you were talking about rotational power.

ft-lbs of torque is not the same as ft-lbs of energy. One is a force vector; the other is an energy magnitude.

Hp = (P x L x A xN)/33,000

Pressure in lbs per square foot (P), times bore area insquare feet (A), times cylinder stroke in feet (L), times the number of cylinder power-strokes per minute (N), divided by 33000, is a perfectly valid equation for horsepower.

My bad.

 

[OrangeAlpine]

How about a CO2 rocket powered sub?

Consider: No engine, which means no problems dealing with clearances as the engine cools, no lube problems, no prop to optimize, no prop shaft seal, no waste gases, just a tiny pressure regulator that can be adjusted to ensure the tank is empty at the end of the 10 meter course.

Bill

 

[TorontoBuilder]

More nice numbers, and why I thought a pressure regulator would be required, since I don't know any oscillating engine designed for 900 psi. Please someone correct me if I am wrong and point me to the design.

In fact that pressure is approaching the high end of the max pressure for annealed copper tubing... so you had better trust your supplier to send the right product on fittings and tubing.

If it could handle the pressure, 900 psi in a two cylinder single acting engine with a 1/2" bore and 5/8" stroke turning at 800 rpm will generate 0.446 HP versus 0.049 hp @ 100 psi.

Given the OPs originally stated criteria (oscillating engine) I don't think there would be a need to consider adding heat because their system likely couldn't handle the pressures.

Now if this was free form design challenge then maximizing the psi via heated tank and vapour lines would be advantage. I'd run my CO2 line to the engine thru the hull and around the hull exterior like a keel mounted condenser in a steam launch would be simple in the space allowed.

 

[TorontoBuilder]

How about a CO2 rocket powered sub?

Consider: No engine, which means no problems dealing with clearances as the engine cools, no lube problems, no prop to optimize, no prop shaft seal, no waste gases, just a tiny pressure regulator that can be adjusted to ensure the tank is empty at the end of the 10 meter course.

Bill

Ha, I had the same thought, rocket nozzle at stern... but what about icing of the nozzle? Especially if the tank is very close to the nozzle... doesn't CO2 expand into both vapour and some solids... and drop in temperature as expands?

 

[TorontoBuilder]

Hp = (P x L x A xN)/33,000

Pressure in lbs per square foot (P), times bore area in square feet (A), times cylinder stroke in feet (L), times the number of cylinder power-strokes per minute (N), divided by 33000, is a perfectly valid equation for horsepower.

I know most model engine builder dont work in square feet, so an easy correction is to add conversion for using inch measurements....

Hp = (P x L x A x N) / (12 x 33,000)

Multiply the denominator 33,000 by 12

Don't forget to also add a factor for multiple cylinder engines or less apparent, double acting steam engine designs.

Hp = (P x L x A x N x C) / (12 x 33,000)

where C = number of cylinders

 

[dman]

from what I understand of what entropy was saying the heat isn't to increase pressure. its to prevent liquid co2 and/or dry ice from entering the engine and increase total volume that moves through the engine allowing for either more rpm or a larger engine to produce more torque. thus needing a bigger prop or altered prop pitch. yet not run out of gas within the same distance.

 

[TorontoBuilder]

from what I understand of what entropy was saying the heat isn't to increase pressure. its to prevent liquid co2 and/or dry ice from entering the engine and increase total volume that moves through the engine allowing for either more rpm or a larger engine to produce more torque. thus needing a bigger prop or altered prop pitch. yet not run out of gas within the same distance.

I don't think you are correct. Temperature and pressure are related... temperature decreases resulting in pressure losses. As CO2 is released at 900 psi it drops in pressure and temperature due to the expansion out of the tank.

If you reheat the "fuel" lines prior to entering the engine you regain pressure lost due to the expansion and pressure drop leaving the tank...

That it may also melt any dry ice that forms is bonus.

It has been many years since I took my refrigeration course, and we didn't work with CO2 but pressure laws are same.

 

[OrangeAlpine]

Ha, I had the same thought, rocket nozzle at stern... but what about icing of the nozzle? Especially if the tank is very close to the nozzle... doesn't CO2 expand into both vapour and some solids... and drop in temperature as expands?

I figure that the nozzle could be designed to be water heated, which is more than can be said about the engine.

Bill

 

[TorontoBuilder]

Perhaps we could have a challenge... everyone design and build a model sub, open engine class... but with a 45 gram disposable CO2 cartridge as the "fuel" supply. 45g cartridges are a standard size and have threaded connection. I haven't ever seen 16g cartridges.

Fastest recorded time to cover 10M distance in an indoor pool (water temp between 70 -76 F) without breaching the surface wins. Times verified by video recording posted to youtube...

 

[Tin Falcon]

basic power formula (P x L x A x N)/33,000

P= pressure
L= Lengths of stroke
Area of piston
number revolutions per minute
33,000 converts foot lbs per minute to horse power
dimensions are in feet.

Sorry for the confusion I caused I was going by memory. I hve page 4 of the KN Harris engine book in front of me .

P Mean effective pressure at the piston. I assume PSI Have you everseen a gage read in pounds per square foot.
L stoke in feet
A piston area in square INCHES
N number of stokes per minute in a double acting engine.
Tin

 

[dman]

I don't think you are correct. Temperature and pressure are related... temperature decreases resulting in pressure losses. As CO2 is released at 900 psi it drops in pressure and temperature due to the expansion out of the tank.

If you reheat the "fuel" lines prior to entering the engine you regain pressure lost due to the expansion and pressure drop leaving the tank...

That it may also melt any dry ice that forms is bonus.

It has been many years since I took my refrigeration course, and we didn't work with CO2 but pressure laws are same.

I may have misunderstood the comment about the tubing being unable to handle the pressure. I thought the comment assumed the heat would increase pressure beyond the 914psi in the cartridge but now I see it could as easily mean a pressure drop could save it from failure.

 

[TorontoBuilder]

basic power formula (P x L x A x N)/33,000

P= pressure
L= Lengths of stroke
Area of piston
number revolutions per minute
33,000 converts foot lbs per minute to horse power
dimensions are in feet.

Sorry for the confusion I caused I was going by memory. I hve page 4 of the KN Harris engine book in front of me .

P Mean effective pressure at the piston. I assume PSI Have you everseen a gage read in pounds per square foot.
L stoke in feet
A piston area in square INCHES
N number of stokes per minute in a double acting engine.
Tin

Tin,

Does your reference say implicitly that the formula is for double acting engine?

I have same formula in regarding Greeley's half horse engine and the calculation requires factoring in the double action of the cylinder...

See here:

 

[TorontoBuilder]

sorry Tin I should have trusted you...

I checked Harris and it does state in double acting cylinder...



So now I am confused what Greeley did with his calculation...

 

[Swifty]

I tend to agree with "OrangeAlpine" regarding working with the pitch of the prop as well as the size of engine. You may want to make a temporary prop for a start, changing the pitch with a pair of pliers until you hit on the best combination of motor and pitch. When this is done a final prop can be made. You will only know what's best by running it in water, fully submerged as weight and resistance will all come into play.

Paul.

 

[Tin Falcon]

TB : as far as double action vs single and multiple cylinder I would think stokes per minute should cover all engines except for multiple expansion then one would have to calculate each cylinder seperately and add the totals.
I would think it would work for single action single cylinder as well.
As far as the trust thing I do try to post accurate and complete information. but I am human and I do sometime post in a hurry. So extra eyes to validate is appreciated. the reason I cited Harris is I wanted to quote a respected reference . The first attempts were somewhat from memory and not totally complete and accurate. sorry for the confusion guys.
Tin

 

[metal]

Nice challenge!
Another angle: when the speed of the vessel is limited by the hull friction, reduce the friction by discharging the used propellant along the hull, as done with torpedoes.
Good luck & keep us informed!

Mike

 

[akitene]

Hi FC4,

Your project sounds great. Here's a link to a French site dealing with model submarine propeller construction. The translation is not that good but the propeller is really nice.

Hope this helps!

Akitene