Physicists help needed!!! Current technology limitations.

Discussion in 'General Science & Technology' started by Tirstan, Nov 15, 2005.

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  1. Tirstan Registered Member

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    What are the limiting factors involved in engine technology?
    Specifically, I am wondering why we are still using big old engines. Just easier to work with what already works?

    Development: First came gas otto cycle, then came diesel. Diesel is more efficient since the combustion is more complete due to the higher ignition temperatures. The major difference between gas and diesel (besides one uses gas and the other diesel) is in the compressed mixture and the igniting of the mixture. Gas mixes fuel and air at the carb or intake. The mixed fuel and air is compressed then ignited to preform the work of moving the piston which transfers the power to the crankshaft driving the flywheel, transmission, then wheels. Diesel draws air into the cylinder. The air is compressed raising the temperature of the air to over 400. Fuel is then sprayed into the cylinder where it instantaneously combusts. This process makes it possible to run the engine off vegetable oil. Von Diesel originally designed the engine so farmers could grow their own fuel(soybean oil).
    Gas has won over diesel because until recently the diesel engine's higher compression ratios required a much thicker engine, making it much heavier, the power to weight ratio of the gas engine made it a much more attractive engine. However recent advances in materials has enabled light diesel engines(Golf TDI).

    Inefficiencies of the engine: the transfer of power to the crankshaft and other parts, and this is the one that interests me the most LOSS OF HEAT

    Why do we waste this heat? Can we not use the heat to create steam to drive turbines? Why use pistons at all? Why not just add fuel to an enclosed chamber (spherical) then ignite it. If the sphere was strong enough not to explode from the force would it not transfer the energy of the ignition into heat which could then be used to heat water driving a steam turbine. Locomotion would be accomplished by electric motors. What are the inefficiencies involed with electric motors?

    Any thoughts? I realize the sphere would have to be able to blow off extreme pressure to prevent a possible explosion. Why is steam a bad idea?
     
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  3. Light Registered Senior Member

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    First, just a couple of misconceptions to rectify. Diesel didn't design his engine to run on soybean oil, there was no such thing in 1893. It was peanut oil. And he didn't complete all the basic innovations of the device, that was a fellow named Charles Kettering.

    There have been attempts at making practical turbine engines for cars but without a great deal of success. But those were gas (or oil-fired as in aircraft engines) and not steam as you suggest. The problem with a steam turbine is that there is still a tremendous amount of heat that has to be somehow rejected. Unless you want to stop every 50 miles or so and refill with water. And carrying around all that heavy water is tough on fuel economy as well.

    You probably aren't aware of the inherent losses that occur every time you go through an energy conversion. Your steam-driven electric motors actually have several of them and each one costs. They are: Chemical > mechanical (boiler/turbine), mechanical > electrical (generator or alternator), and finally electrical > mechanical (electric motors).

    Even the new hybrid cars pay a penalty for the power conversions but they minimize the losses by having the gasoline engine operate all the time at the peak efficiency of it's power curve. That is impossible to do from a practical standpoint with any sort of turbine - unless it is very, very small. A very tiny one might make it practical but would lack the acceleration needed at certain times in driving.
     
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  5. Facial Valued Senior Member

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    A tiny turbine... hmmm now that sounds interesting.
     
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  7. CANGAS Registered Senior Member

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    During World War 2 the germans were developing a diesel which used the exhaust to drive an integral turbine. It was a mechanical nightmare. Many, many parts.

    Any machine suffers wear on all its moving parts. The designer/manufacturer must strike an acceptable balance: a simple machine might have less fuel economy but have fewer breakdowns, whereas a complex machine may gain economy but be unreliable.

    A large factor in diesel economy in vehicles is volumetric efficiency. They do not have throttle plates, therefore airflow is never restricted. Compared to gasoline engines, diesels waste less work drawing in air.
     
    Last edited: Nov 15, 2005
  8. kevinalm Registered Senior Member

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    A major consideration is the limitations of materials. How high a temp can the pistons/sleaves/cylinderheads, turbine/combustionchamber, etc., stand before the mechanical properties go to pot. This limits the Carnot efficeincy of almost all heat engines, as most materials with acceptable wear/strength characteristics can't stand more than 300 or 400 F.
     
  9. Tirstan Registered Member

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    Thank you for the clarification, it has been awhile since I looked it up. Kind of funny since the number one agricultural product in the state I live in, Georgia, is peanutsJ

    I was thinking of recycling the fluid and it would not have to be water. A fluid with a really low boiling point would be better. The condenser could use forced air from the front of the vehicle for any cooling necessary, since air temp is almost always below 100 Fahrenheit. However, the design would need to conserve as much heat as possible, perhaps using the steam after it has started to cool to start the heating of fluid before it is drawn into the engine fluid jackets encircling the block.

    Aware, but ignorant of how the loses in energy compare to each other, which is why I posted. Not sure how efficient electric motors are, but I know the current combustion engine is very inefficient, along the lines of twenty percent or so. Thinking of energy out compared to the potential energy of the fuel.

    The combustion energy seems to loose a lot of energy to friction between moving parts. The smaller the number of parts the greater the efficiency. I believe a majority of the loss is from the loss of heat energy.

    What I am really unsure about is if a small explosion can be harnessed and converted into heat efficiently.

    The turbine could operate at a constant power output. The turbine would be directly connected to the alternator/generator. A design problem would be what to do with the power you do not need. I say this because the turbine would have a maximum efficiency speed of operation and you would want it to only operate when necessary. Perhaps buffering the power to large capacitors for storage. They already do that in some hybrids don’t they?

    Using a turbine hybrid instead of gas combustion hybrid.

    The efficiency gained by the gas combustion hybrid by operating it at peak efficiency is achieved by eliminating the transmission. Shifting gears necessitated the disengagement of the engine, which decelerates the engine below optimum efficiency. I would not want a transmission either.

    Lack of acceleration, makes me think of the differences between common combustion engine and the Wankel rotary engine. The Wankel gains efficiency because it operates around a central axis eliminating the up and down power conversion in the common combustion engine. However, Wankel engines lack the torque of the common combustion engine because it does not take advantage of the elasticity of air, the compressed explosion makes more torque. Wankel makes more horsepower but less torque.
     
  10. CANGAS Registered Senior Member

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    Tirstan: in piston engines a "square" bore/stroke ratio is popular. This a where the bore and stroke are the same: such as 3" bore and 3" stroke. The corresponding area of a 3" tall cylinder and its 3" diameter head are the radiative areas which lose heat which could otherwise perform work, but which also protect the engine by transferring excess heat into coolant.

    Sketch a few cylinders of different size but with the same square proportion. Calculate the internal volume of each one. The volume is the amount of working fluid which moves the piston. Then calculate the radiative area of each one. Finally compare the volume to the area of each one. Now you can judge the way in which a larger or smaller motor will have advantage in keeping heat in the working fluid or losing it by radiation.

    Electric motors are often claimed by an optimistic manufacturer to be 90 % or higher. There are skeptics who claim that it is unusual to get over 40 % with varying loads.
     
    Last edited: Nov 16, 2005
  11. Light Registered Senior Member

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    Well, you're close.

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    According to the USDA, peanuts are a distant third with 7% of the agricultural income for the state. They follow broilers at 44% and cotton at 10%.

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    Cooling is absolutely essential. A heat engine cannot even come close to being efficent unless it is able to reject heat very, very quickly.

    The efficency of electric motors will average around 38% with varying loads, such as driving. Each of those stages of energy conversion I listed before will cost about 15 - 22% each, again, depending on loading. You've already mentioned the "cost" of the major one - converting fuel into mechanical motion. I'm pretty sure you're following but in case someone else doesn't quite catch it, you don't simply add all those losses together because they would quickly exceed 100%.

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    They are interstage losses and must be subtracted incrementally to get the real numbers.

    You are correct about heat losses. That's about 70% in a gasoline engine while friction is about 5% and inertial losses (such as reversing the direction of piston movement) is about another 5%.

    Not quite sure I follow this because the conversion to heat is close to 100%. I suspect you really mean converted to usable energy and that's what we're discussing.


    Direct-drive is certainly best. No, capacitors aren't useful for energy storage beyond a couple of minutes. For one thing, they have VERY limited capacity and are also prone to leakage, even with the best design. Battery storage is the only thing practical right now. Flywheel storage once looked promising but brings with it another whole new set of challenges.

    That's true but there is a little more to it. Every engine has an optimal speed for maximum output power. Any slower or faster costs efficency.

    Wankels can produce impressive acceleration. I believe that perhaps you are actually thinking of the Stirling engine? Very efficent (as engines go) but with horrible acceleration capabilities.
     
  12. Tirstan Registered Member

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    Georgia produces 42% of the entire peanut crop in the US.
    75% is converted into peanut butter.

    http://resources.caes.uga.edu/media/GAR/peanuts.htm


    kindof busy lately I'll write more soon
     
  13. Light Registered Senior Member

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    Yes, that's true but it doesn't match you original statement.

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    " ...since the number one agricultural product in the state I live in, Georgia, is peanutsJ " (Emphasis mine.)

    (See what I mean?)

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  14. MetaKron Registered Senior Member

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    I think that the Carnot equation only says that you can't get more heat out of an engine than you put into it.

    Stirling engines really need something like a flywheel and a starter motor. Then they can act as if resonant over a fair range of thrust and rpms and be kicked into their proper operating range instead of waiting for random chance to get them cycling. I think that Stirling engines, when used right, may be about the most efficient engines. I've read of models that can actually ice up, which tells me that they may be able to exceed Carnot efficiency by acting like a heat pump.
     
  15. Light Registered Senior Member

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    Wow! You'd better go back and do some more reading. The first part is just basic, simple thermodynamics and the last paragraph is totally wrong expert for the part about the efficency. (And that's still debatable since a turbine is the most efficient of all but that's a rather different class.)
     
  16. MetaKron Registered Senior Member

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    Stirling engines are low temperature differential engines and are assumed to be less efficient because they are labelled according to Carnot efficiency. I've done my reading. The Carnot equation reveals nothing more and nothing less than the percentage difference between two temperatures on either the Kelvin or the Rankine scale. It's like saying that if you have water at 300 degrees K and lower its temperature to 270 degrees K by extracting useful energy out of it, you are only 10 percent efficient. But if that 300 K water started out at 270 K, and you used something like a solar panel to raise it to 300 K, and you lower it back to 270 K, you have no way of knowing your efficiency unless you compare the useful energy you got out of the water with the the energy you put into it. There's thirty calories per gram of hot water there, how many watt-seconds did you get out of each gram?

    The turbine is assumed to be more efficient because it operates at a higher temperature. I don't believe in this assumption.

    I was thinking about the diaphragm type of Stirling engine and had to review a little because there are piston types. I don't think I said a thing that is wrong in that paragraph because at least some Stirling engines aren't very good at self-starting and even a lightweight flywheel helps the engine run at an even pace. Some of the more popular recent designs use diaphragms because they are easier to make and don't have to slide up and down cylinders. I am pretty certain that these designs can be improved for greater power and efficiency if they don't have to start themselves, because most improvements would require a certain amount of force to get the cycle started.
     
    Last edited: Nov 23, 2005
  17. Light Registered Senior Member

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    Yes, yes, MetaKron. I believe everyone realizes that it's temperature differential that makes any engine work. It's all about transforming chemical energy into heat and converting that into mechanical motion.

    As to turbines, they ARE more efficient - no assumptions as you claim. Their energy in vs power output is easily measured. It's just you making a silly assumption - not all the power engineers in the world. {heavy sigh!} You still need to do a LOT more serious study to prevent embarrassing yourself so badly. You do yourself a disservice by talking about things you know too little about.
     
  18. MetaKron Registered Senior Member

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    Light, you still need to grow up a little.
     
  19. Light Registered Senior Member

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    Sorry, but that actually applies to you. I've had considerably more experience with engines of many types and with all sorts of engineers as well. Exactly how much have you had at your young age? I'm guessing from the general tone of all your posts that you are probably still in school and are just going by what you think you know. A few more years of living will teach you a lot.
     
  20. MetaKron Registered Senior Member

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    Trouble is, Light, you are way too overbearing when you want to "explain" something to people. If a little courtesy would cost you your life or your job, by all means, take the chance.
     
  21. Light Registered Senior Member

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    Hmmmm....

    Here's the thing. I spent many years as a teacher (professor in college) and that's probably why you view me in the way you do. The biggest problem any teacher has is dealing with people who think they know something when actually they are totally incorrect. There's a lot more to teaching than just presenting facts. Trying to get people to unlearn their "facts" and look at things logically and accurately can often be a major task.

    And that's what I see a lot of here. People like myself trying to correct the mis-thinking of others - like your assumptions about turbines. You were right about a part of it but didn't even know why. They DO operate at higher temperatures - and all engines that operate at higher temps usually ARE more efficient than those that are working at lower temperatures.

    But the thing is that people who know a little about something (like you and the Carnot cycle) try to use fancy words and terms like that to fool others into thinking that they know more than they really do. But the end result is they wind up looking very foolish and loose what they were after in the first place - the respect of others. Anyone who tries to fake knowledge always winds up the looser.

    You probably think all I just said was overbearing as well. So be it - welcome to the real world of adults.
     
  22. CANGAS Registered Senior Member

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    To paraphrase a famous author: "The real world is a harsh mistress.".
     
  23. Light Registered Senior Member

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    Indeed!!

    (Well noted, CANGAS.)
     
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