Entropy455
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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 theres a real energy return to be had if you can heat the CO2 upstream of the engine early on.
Heres 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, its equal to 0.25 extra horsepower which is a boatload for a model submarine (pun indented).
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 theres a real energy return to be had if you can heat the CO2 upstream of the engine early on.
Heres 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, its equal to 0.25 extra horsepower which is a boatload for a model submarine (pun indented).
