Most efficient steam engine design?

Discussion in 'The Break Room' started by cfellows, Sep 27, 2011.

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  1. Sep 27, 2011 #1

    cfellows

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    I know this subject has been covered some already, but I'm interested in peoples thoughts on which engine will provide the most power for a given steam source: A 2 cylinder, double acting, reciprocating engine of the vertical marine type, a 2 cylinder, double acting oscillator, or a V-4 single acting engine like the saito V4PR?

    The 2 cylinder, double acting, reciprocating engine used in marine designs comes to mind first. Four power strokes per revolution, on 2 pistons with relatively small contact area, D valves which can be made pretty leak proof. But, on the downside, you have a smaller piston area on the back side because of the piston rod, potential problems with the piston rod seal, piston rod and crosshead friction.

    A 2 cylinder, double acting, oscillator doesn't have problems with the crosshead friction, but shares the piston rod friction and seal issues. And it has the added problems of potentially leaks around the cylinder ports. I'm assumming the added friction of the cylinder against the standard would be similar to the added friction of the D-valves in the reciprocating engine.

    A 4 cylinder single acting engine like the Saito V4 seems like it has a lot of advantages and not many disadvantages. It's true you have the added friction of 2 more pistons, 2 more connecting rod journals, and 2 more valves, so I don't know how this design would compare to the other two.

    Anybody have some insight into this subject? :)

    Chuck
     
  2. Sep 27, 2011 #2

    Captain Jerry

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    Chuck

    I think you are right on all counts but don't forget piston valves with inside admission which may be more efficient and does not need a gland on the valve rod.

    Jerry
     
  3. Sep 27, 2011 #3

    Jasonb

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    If you want to use the steam efficiently you need to start looking at compounding to make full use of the expansion of the steam. Also think about using superheated stem.

    J
     
  4. Sep 27, 2011 #4

    picclock

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    Hi Chuck

    Can you just clarify the meaning of 'piston rod and crosshead friction' (OK got it now, wikipedia lists it as slidebar for us UK based people). I would think the friction could be kept to a minimum by using two ball races, or just some rollers for the piston rod to bare against.

    Oscillator's lose a large amount of their energy moving the cylindrical mass.

    Are you going to use this with steam or compressed air. If steam and bulk/weight no object compound engine is likely best reciprocating kind.

    If you want best efficiency have you considered turbines ? At small sizes they do not scale well due to the blade/housing gap remaining constant.

    Don't know if this helps - perhaps more details such as power output/size weight restrictions/application would generate more ideas.

    Have done various drawings for a two cylinder double acting engine which I may get round to building using the boxer crankshaft idea below. Because of the configuration the engine is perfectly balanced so no vibration (well less vibration ;D). See picture below. Although this shows single acting its the crankshaft/conrod idea which makes it interesting.

    Best Regards

    picclock



    boxer.gif
     
  5. Sep 27, 2011 #5

    Maryak

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    Plus a vacuum, some 25% of the work done by steam is below NTP IIRC the most efficient steam engine is a turbine with reaction blading supplied by a super critical boiler, ( because there is no latent heat), + superheat + condensing and this mass of bits is around 33% efficient. Your average recip engine of whatever type is between very poor to around 8% efficient. :mad:

    IMHO just pick the one that has the most eye candy for you and enjoy ;D

    Best Regards
    Bob
     
  6. Sep 27, 2011 #6

    Ken I

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    Agree with Maryak - if history and thermodynamics teach us anything - a compound action triple expansion is the most efficient use of steam on a reciprocating engine. Almost all marine applictions went this way before the switch to turbines.

    Why the interest in efficiency ?

    Ken
     
  7. Sep 27, 2011 #7

    steamer

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    Compounding splits the expansions over two different and smaller temperature ranges. That minimizes condensation losses......there's a problem though

    Fully agree with Bob et al regarding compounding, turbines and such......however.....

    In small sizes....such as small steamboat sizes on down... the surface area of the engine completely overwhelms this benefit, and in general in small scale stuff the biggest loss is through condensation loss regardless.

    So IMHO....for small sized stuff.....

    Double acting twins with appropriately sized piston rods and some superheat, say 100F worth, to avoid the condensation losses as much as possible are much more efficient for these applications because you can raise your steam pressure above 40 psi which you can't do with a wobbler. Additionally, a wobbler has quite a bit of friction area.

    This becomes a problem if you run condensing as superheat REQUIRES internal lubrication which is hard to get back out of the condensate which can result in oil in the boiler feed water. Boilers don't like that much!

    So for small scale stuff......blow the exhaust up the stack.....

    Just my opinion ....worth exactly what you paid for it.....

    and for the record...my boat engine.....all 2 HP ....is a compound......cause I like them... ;D

    Dave
     
  8. Sep 27, 2011 #8

    Harold Lee

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    I know this thread is about efficiency, but one of the drawbacks to a compound engine is since the cylinders are in series, they are not necessarily self starting and have the same "dead spots" as a single cylinder engine. On a model that is static run on a display this is not a problem but if one wants to put an engine in a radio controlled model boat or a model train, this can create problems. The real world solution was to put a "simpling valve" in the low pressure line which would allow boiler steam to directly enter the low pressure cylinder and effectively start as a simple two cylinder engine. After the engine was running the valve would be closed and the engine would continue to run as a compound. I built this compound a few years back based on Rudy Kouhoupt's design. I played with building a simpling valve for it and finally moved on since I did not have any plans on putting it in a boat.

    [​IMG]

    Just my $.02

    Harold



     
  9. Sep 27, 2011 #9

    Wrist Pin

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    Since we are on the subject of efficiency...
    Years ago, I got into a discussion on steam mill engines. A lot of steam /energy loss was in the slide valve(s). The prevailing solution was to incorporate solenoid valves instead of mechanical. The theory being that having sharp opening and closing valves would capture more of the expansion of the steam, increasing efficiency and horsepower.
    Has anyone gone down this road or would like to discuss the concept?
     
  10. Sep 27, 2011 #10

    Dan Rowe

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  11. Sep 27, 2011 #11

    Wrist Pin

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    Fascinating stuff, Dan.
    But one still has the drag of the associated mechanical parts to contend with. In the discussion years ago we thought of using electric solenoid valves fired by relays that were triggered by a Hall Effect sensor or some such.
     
  12. Sep 27, 2011 #12

    Dan Rowe

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  13. Sep 27, 2011 #13

    Wrist Pin

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    More fascinating stuff!
    Just goes to prove nothing is new under the sun! It also proves one can find anything on line if one knows where to look. Thanks Dan, for enlightening me!
     
  14. Sep 27, 2011 #14

    lescad

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    Hi Chuck
    Another loss of efficiency with conventional valve arrangements is the relatively cool exhaust steam taking heat from the cylinder and valve.

    Maybe a single acting V4 uniflow with piston or poppet inlet valves ??

    Les
     
  15. Sep 27, 2011 #15
    Chuck-

    My slant on the answer to your question is as follows, and is just my hunch only, not necessarily based on hard evidence.

    The oscillators used after the big steam boat paddlewheels were generally small units, and were known to be steam hogs, but were simple and reliable since they had so few parts.
    The oscillators were generally made for intermittant loads, and efficiency was not a concern, just simplicity, reliability and low cost. The steam ports were probably designed for no cutoff, so the steam would not see any serious expansion, but again efficiency was not necessarily a concern with an oscillator.

    For other type engines, the critical factor will be the cutoff, and how much of the power of the steam can be extracted during the expansion phase. Any steam engine will be highly inefficient if it does not cut off early in the stroke.
    I am assuming that the ports, valve and valve travel have beeen correctly designed, but more often than not, in modern model engines these items are anything but correctly designed.

    So the bottom line is that any engine with a good valve/port/valve travel design will be more efficient than a poor design, regardless of whether the engine is an oscillator, a compound, single, double acting, etc.

    The effect of the rod is minor. You can even compensate for that in valve timing.

    A single acting engine will be more effective if it has to operate at higher rpms.
    For low rpms, the double acting engine probably will be more efficient than a single acting engine and an oscillator just because an oscillator cannot control cutoff, and the single acting will have more pistons and friction.
    A well designed single acting steam engine can be very efficient though.

    A compound willl be more efficient than a non-compound, but if simplicity and easy starting are important, thena multi-cylinder non-compound is the way to go.
    If you are going to steam across the ocean, then a compound will very quickly pay for itself.

    A compound will also allow a much smaller boiler for the same horsepower, so that may be a good reason to go compound.

    Pat J
     
  16. Sep 28, 2011 #16

    steamer

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    Again...be careful

    I small sizes ALL steam engines are very steam hungry due to condensation.

    Assuming a well designed a constructed engine....superheat goes a long way, but as Dan said...there is no free lunch.

    Dave

     
  17. Sep 28, 2011 #17

    kf2qd

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    For a small engine you are going to have rather large thermal losses. To make the engine more efficient you should look at ways of thermally isolating the cylinder liner and the piston from other metal parts. Keep the steam hot until it comes out the exhaust and you will be more efficient. You want the heat to move the piston, not heat the air.
     
  18. Sep 28, 2011 #18

    Ken I

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    Steamer is correct and my earlier comments about triple expansion should have carried the caveat that these principals do not scale well.

    As an aside, about 20 years ago I spent a pleasant day in the engine room of South Africa's last working steam tug the Alwyn Vincent - this had a tripple expansion engine.

    This had a "big switch" simpling valve arrangement that induced live steam into all three cylinders - for starting or for emergency power if the helm rang for emergency full ahead or astern - literally quadrupling tha output in short bursts.

    Sadly I took no picture but the vessel is being restored.

    http://alwynvincent.wetpaint.com/page/Triple+Expansion+Engines

    Ken
     
  19. Sep 28, 2011 #19

    Maryak

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    Just for you Ken ::)

    [​IMG]

    Best Regards
    Bob
     
  20. Sep 28, 2011 #20

    Captain Jerry

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    I know you said it was just for Ken, but I watched anyway. Great stuff.

    Jerry
     

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