What's more efficient, a piston or a turbine?

Discussion in 'Physics' started by strantor, May 7, 2012.

  1. strantor

    strantor Thread Starter Well-Known Member

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    I've entertained the thought of building a radial piston steam engine for a few years. I just happened to acquire 5 pneumatic actuators that are pretty much what I envisioned making the thing with. So the idea is rekindled and I've been devoting more than average thought to it. A few questions have come up - why are steam piston engines no longer used? All the nuke stuff that I'm aware of, that runs on steam, uses turbines. Are turbines more efficient than pistons? Seems to me like pistons would be more efficient since they capture the expanding gas and use it over a period of time instead of just letting it blow straight through; pneumatic power tools seem awfully wasteful to me.
  2. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    My understanding (that and 5ยข will get a cup of coffee) is turbines are much more efficient.

    They are much more precision machines too, much higher tolerances and requiring much more exotic materials, though they can be amazingly small. I remember a prototype that weighed only a few pounds several years ago, for micro drones. It produced several pounds of thrust.
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  3. strantor

    strantor Thread Starter Well-Known Member

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    I get a feeling that this is one of those questions where if I were told X and I asked why, I would be ill equipped to understand the answer.
  4. mlog

    mlog Member

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    It's not a simple answer to a simple question. However, to oversimplify the answer, the thermal efficiency of an Otto cycle piston engine is a function of the compression ratio. The thermal efficiency of a Brayton cycle turbine engine is a function of the pressure ratio. Generally speaking, piston engines are more efficient, but turbines are lighter and smoother running because of the rotation vs. piston movement.
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  5. strantor

    strantor Thread Starter Well-Known Member

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    do you think the same would be true of a radial piston engine? Imagine, all 5 pistons are acting on the crank simultaneously from different angles. I suspect it would be smooth, but I have no evidence or experience behind that suspicion.
  6. shortbus

    shortbus Senior Member

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    A piston steam engine is a "low" pressure,slow speed machine. A turbine is a "high" pressure, high speed machine. Thats the simple answer.

    For work to be done like in a generator, helicopter, or wheeled vehicle you need to add into the equation the gearing needed to take the high speed of the turbine down to a lower "usable" speed. A jet airplane is using the high speed of the turbine to create thrust so no gearing needed.

    The two different types of engines fill two different type of applications.
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  7. THE_RB

    THE_RB AAC Fanatic!

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    Ok I'll bite. ;) I don't think your efficiency will be significantly better from multi cylinder as for the same displacement engine the multicyl has more seal (piston ring) area and more drag. I would consider a single cyl engine (or maybe a 2 cyl?) and large flywheel.

    In large scale steam the turbines are very efficient but that drops a lot as the size comes down. Large turbines have many blade sets, maybe as many as 20 and each set harnesses some of the steam energy.

    Small turbines get inefficient as the leakage gaps etc are much larger compared to the blade diameter, and you get less blade sets, then you lose efficiency as the RPM goes right up and you get higher bearing losses and add in reduction gearing losses etc before you can get power out.

    I would think on a home made setup you could get the most efficient conversion of your steam resource by a piston engine if carefully designed and careful heat insulation.

    And really for efficiency you need cold water for cooling to get that max temp differential, which is why they build power stations next to deep lakes and oceans.
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  8. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Classic jet turbines have no cooling, it is fundamental to the design. It can be a weakness for things like cars, but they also can not burn up. While their materials tech tends to be much more advanced, they are much simpler mechanically.

    I don't know much about power stations, but being an air force brat taught me the basics of jet engines for aircraft.

    You want some interesting reading, google

    tiny jet engines

    It appears there is a large tech culture making jet engines for small applications, some of them will surprise you.
    Last edited: May 8, 2012
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  9. Brownout

    Brownout Well-Known Member

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    If you think you might want to experiment with a turbin engine, the WWW if full of video of projects that use a trubo-charger to make an engine. It's a pretty remarkable thing to see how these things are repurposed.

    Sorry this is a little O/T. I thought it might be of interest.
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  10. atferrari

    atferrari AAC Fanatic!

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    Navigation in a vessel turbine propelled is remarkably smooth. And much less noisy!

    Usually, only when giving half or full astern you can say what the engine is doing.

    Good for sleeping.
  11. nsaspook

    nsaspook Senior Member

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    Last edited: May 9, 2012
  12. strantor

    strantor Thread Starter Well-Known Member

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    I served on a nuclear submarine. Yes, it is smooth, but I don't have much other experience in the way of vessels to compare it to. When we would go all ahead emergency, the boat would do what I can only describe as "peeling out in the water" - it would shake and vibrate and groan and moan. It was fun. Super quiet too. over a billion dollars worth of technology, I am aiming my sights a little lower. I have a feeling that any type of steam turbine (other than possibilities with hobby jet engines and car turbos) is going to require significant mechanical engineering and access to high level tools. The piston engine however is doable with what I have in the garage,
  13. shortbus

    shortbus Senior Member

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    Have you looked into "Tesla turbines"?

    But the piston type engine is the way to go for a vehicle, no separate gear box/transmission required. A turbine needs to spin at much too high a RPM to be used without a high ratio gear box to slow it down to usable wheel speed. Even the ships use a final drive before the prop, right?

    The biggest reason gasoline engines survived and steam died in cars was the time it took to get a head of steam up to start to move. Gas was instant and took over the car market. With the new flash steam generators and electronic controls it could be a whole new ball game.
  14. nsaspook

    nsaspook Senior Member

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    On US Navy gas turbine ships a double reduction gear shifts it down from about 14000 at the turbine to about 700 rpm on the main shaft with controllable pitch propellers that can adjust from full forward to full backward in seconds.

    http://www.youtube.com/watch?v=lLSuWHvZzdU
  15. russ_hensel

    russ_hensel Well-Known Member

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    Efficiency in any heat engine is fundamentally limited by thermodynamics. For all cycles the higher the input T ( for a given output T ) the higher the efficiency. The Carnot cycle is the most efficient.

    In practical situations the turbine often wins in large part because it can with stand higher inlet temperature.
  16. Brownout

    Brownout Well-Known Member

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    Years ago, I read an article from a guy who built a steam trubo engine in his garage, using only good mechanical fabrication skills. He hammered out the "fins" -- for lack of knowledge of the proper term -- by hand. It was a single row of fins attached to a disk. He used it to power his DIY steam cleaner machine.

    Maybe if you poke around a little, you might find the article archive or something similar. I forget where I read it; might have been Popular Mechanics or something like that.
    Last edited: May 10, 2012
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