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Old Jun 17th 2014, 06:22 PM   #1
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Energy conversion query

I write to inquire if there can be any meaningful transfer of the energy from... say the thrust of a rocket engine to mechanical energy. If so, are there any equations that would account for such.
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Old Jun 18th 2014, 04:52 AM   #2
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The thrust of a rocket is mechanical energy. You're going to have to be more specific with your question. There are many types of energy: mechanical energy includes kinetic energy (the energy of a mass with velocity) and potential energy (energy "stored" by going up hill against gravity, or by stretching a spring). Other types of energy include electrical, chemical, magnetic, nuclear, and heat. In theory these can be converted from one to another, although basic laws of thermodynamics limit the amount of heat energy that can be converted to the other forms.
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Old Jun 18th 2014, 07:24 AM   #3
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Meaningful

Any transfer of energy creates waste in human terms, there are as many ways as you can imagine to convert the enrgy of a jet engine to another form... however from jet fuel (chemical energy) to the final energy form there will be be a great deal of waste.
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Old Jun 18th 2014, 12:32 PM   #4
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Query Correspondence Continued

So kind of you to reply. The question is far out but there is a legitimate reason for the post. For the purpose of this conversation consider the engine repurposed to become a helical turbine for air compression. If the mass of the moving parts, frictional coefficients, etc. Are there equations that could give an estimate, in ergs, of how much work could be done. It is my hope that this is given the benefit of a serious consideration. equations would factor in such a calculation. I await your reply
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Old Jun 18th 2014, 01:29 PM   #5
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Your question is very vague, so my answer must be as well. Yes, there are equations that govern the maximum efficiency of engines, if that's what you mean. The idealized Carnot Cycle sets an absolute maximum efficiency that can be achieved of

Efficiency = 1- (T_c)/(T_h)

where T_c is the temp of the engine exhauset (typically assumed to be ambient temp) and T_h is the hot temp of the burning fuel. In the case of an gasoline powered internal combustion engine, for example, where T_c = 294 K and T_h = 1089K, this yiekds an efficiency of 73%. But that's ony theoretical - in practice there are additional losses due to friction, inefficient burning of fuel, etc that make typical internal combustion engines less efficient than this, typically around 35%. In other words a typical engine converts about 35% of the chemical eneregy of gasoline to mechanical energy.

As for calculating the work than an engine can do - well, it depends on the engine. Typically we talk about work per unit time, or power, when talking about engines. The amount of work per second that the engine can do is dependent on the size of the engine (obviously). Do you have a specific question?

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Old Jun 29th 2014, 11:52 PM   #6
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Originally Posted by ChipB View Post
The thrust of a rocket is mechanical energy. You're going to have to be more specific with your question.
Let's be more precise here. Thrust is a force. When a force is applied to a body, such as an aircraft or a rocket, it does work. That work appears as a change in kinetic energy which is one form of mechanical energy.
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Old Jun 30th 2014, 09:32 AM   #7
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Energy Query [Continued]

Much appreciation for responding to the post. I am rethinking the original post... I am considering the repurposing of two fuels... Liquid Oxygen and Ethylene plus water which once fueled a missile for a terrestrial purpose. The true question should be " what engine componenents would be needed for an engine to consume these fie;s in such a way as to drive moving parts and not thrust. Calculations and other details are secondary at this point. Any guidance in this regard is appreciated. I await a reply.
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Old Jun 30th 2014, 10:28 AM   #8
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Ethylene plus water results in ethanol (alcohol). Combine with oxygen to burn, and it can be used to power conventional internal combustion engines. It's used in "gasahol" in the US, where up to 85% of the fuel is ethanol and the other 15% gasoline. In Brazil they have cars that run on 100% ethanol. I assume this meets your objective to "drive moving parts and not thrust." So really the only unique part of your inquiry is the idea of using LOX as the oxidizer rather than oxygen in the air - I don't know that anyone has ever tried to do this with a conventional car engine (why would they?), but I don't see why it wouldn't work.

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Old Jul 1st 2014, 12:57 PM   #9
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The mechanical piece

Much thanks for staying with this thread. Please know that end game is not a car.. Although a car engine seems to be closest item that will take combustion energy to power parts. So if this project is distilled to the elements what parts of a car engine would absolutely be necessary to get rotary motion. Cannot reveal all of the details but one goal would be to direct motion to a rotary air compressor capable of running for months at a time. Hoping for informed commentary on this part of it.
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Old Jul 1st 2014, 02:40 PM   #10
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What part of car engine is needed to get rotary motion? All of it. The essential elements of an internal combustion engine include the pistons, crankshaft, spark plugs, fuel and oxygen delivery system, control system for timing of spark plugs, lubrication system and cooling system and exhaust system - all of which are relevant. Plus of course you need a transmission that converts the rotation of the crankshaft to proper RPM for your mechanism, and some mechanism that properly meters LOX flow.

Last edited by ChipB; Jul 2nd 2014 at 11:59 AM.
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