Piston Valve Vs Slide Valve

Discussion in 'General Engine Discussion' started by Dom, Apr 1, 2013.

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  1. Jun 8, 2016 #21

    Blogwitch

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    Dom,

    There are two main differences between piston valve and slide valve.

    A piston valve is no more efficient than a normal oscillating engine, using the same principles for steam use and reversible by just swapping over the steam exhaust/inlet lines by use of a normal fwd/rev/speed valve. But a slide valve does look nicer than an oscillator because of the eccentric system rather than wobbling cylinders.
    If I was going to use an engine in say a model boat, I would choose an oscillator any day, less messing about and super reliable, less things to go wrong. Just requiring one servo to control fwd/rev and speed.

    A slide valve is usually more steam efficient, especially if more than one cylinder and expansion cylinders are used, the down side is that it requires mechanical means to swap the eccentric timing to make it run in reverse.
    If you want it to power something remotely, it would require two servos, one to control the reversing linkage and another to control the speed.

    John
     
  2. Jun 8, 2016 #22

    Charles Lamont

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    The OP was three and a half years ago so it does not matter much, but John, I am puzzled by your claims here. You say a slide valve is more efficient than a piston valve. Why?
     
  3. Jun 9, 2016 #23

    Blogwitch

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    Norman,

    We are talking about MODEL engines here, whereas in full size the opposite should be the case.

    In full sized, slide valves (and oscillators) came first, and stood the test of time, but as boiler pressures got higher slide valves became inefficient and wore away quite quickly, so a few of the old loco engines got refitted with piston valves and outside operating systems (Walchaerts) whereas new engines got built with piston valves from the start.
    They can do this in full sized as it is fairly easy to put piston rings onto the piston valve to prevent blow-by, whereas in model size, it boils down to high precision machining to do the same thing (you can't use rubber 'o' rings to seal them as the sharp edges cut them up, so invariably, in model size, after a very short while, piston valves start to blow steam out of the top and bottom because of the wear between the piston and it's sleeve (if it isn't leaking steam already because of not a tight enough fit between the piston and sleeve) and it doesn't really help by fitting a stuffing gland at either end of the piston rod, you get internal leakage then between top and bottom ports, which causes all sorts of problems at times.

    You can also bring into the equation on a slide valve engine that of steam injection timing, where, as the engine reaches higher speeds, a smaller dose of steam can be injected into the cylinder at a later timing and still have the same speed and power output. This would be used on model locos rather than on an engine designed for a model boat as there would be an operator on board of a loco to do it rather than on a model boat where the operator wouldn't be in intimate contact with the model and so couldn't control it correctly.

    In the model fraternity, we are blessed with not having to resort to high pressures to get our steam engines to work, so you will find that in our sizes, what I stated before holds perfectly true, you will get more efficient running from a slide valve than you will from a piston valve, purely because in the sizes we are talking about, the slide valve will always have less steam blow-by loss.

    I didn't want to say anything on his post about the 'Borkum' build as it wouldn't be right to criticize it there, but that has the most inefficient piston slide valve engine available, and if you were to ask him to show the model on the water after say an hour running it in, you wouldn't be able to see the boat for the steam coming out of either end of the piston valves. I know, I have thrown many of those engines away for people and fitted an easy to operate wobbler instead.
    I designed the 'Paddleducks' engine, not for use in anything, although it could be used if it was made to very tight tolerances and kept that way, but as an engine to give experience in making one.
    If it wasn't to be seen, in fact, even if it could, I, as before, would recommend a wobbler every day of the week over any others.

    John
     
  4. Jun 9, 2016 #24

    Charles Lamont

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    I don't know wkat you mean by a later timing, but I think you are talking about using lap on a slide valve and eccentric 'advance' to provide an earlier cut-off and consequently a degree of expansive working. This is generally done to some extent even in engines without reversing gear (but is, of course, impossible with the simple form of model oscillating engine). And it also applies to piston valves, which are exactly the same as slide valves in this respect. If a non-expansive engine is altered to work expansively, the power output, at a the same pressure and speed, would be less. However, because the engine is working more efficiently, it can give more shaft power (for example by running faster) for the same boiler output.
     
  5. Jun 13, 2016 #25

    goldstar31

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    Well, not this one! I positively hate 'steam' and I think that my last incursion was trimming and stoking a small but full size thingy which was about the same size as a sea going tug. Before that, my great uncle had been buckled and arthritic from driving the Royals on the old LMS line out of Carlisle and a family who had been 'in steam' since Timothy Hackworth who presumably employed my great or was it great great grandfather at Shildon? Somewhere 'Blackadder' Rowan appears as well. Whatever but we all drifted away.

    For such, probably the best is dear old Don, writing as D.L.Ashton and his epic Walshaert's Gear and Stephenson's Gear stuff. Last heard of was building a fullsize steam loco as a world authority on tug boats and the saxophone.

    I jest not- he used to write and dedicate music to my late wife! The last real involvement really was something called 'acid corrosion' in engines.

    Heady stuff- indeed.:wall:
     
  6. Jun 14, 2016 #26

    cfellows

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    Hey, John, good to see you out and about. I have to agree with you, it's near impossible to get piston valves to seal and even if successful, they will wear and start leaking.

    Oscillator engines might not look as sexy or sophisticated, but as prime mover model engines go, they get the job done with far less work, they take up less room in your model, and they are quite reliable.

    On the other hand, as a running scale model, double acting, reciprocating steam engines with slide valves are among the most beautiful and fascinating to watch.

    Chuck
     
  7. Jun 14, 2016 #27

    Blogwitch

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    Hi Chuck,

    Still trying to get over my driving holiday, 2,200 miles of bad roads on the way back (including lots of goat tracks in Romania and Bulgaria) over 5 days really got to me, about a week later. Ended up laying on my stomach for nearly a week, and still a bit out of it now. Time to think about retiring from doing things like that, but on the other hand, I am off on another one, just a short one this time, to southern Germany, early September.

    Really glad to hear that you are almost in the clear, you had me very worried for a while.

    With regards to oscillators, I still have people from the model boat fraternity contacting me to see if I am going to make any more batches, the last batch I made (qty 6 upright and 2 horizontal) a few years ago turned out perfect and I haven't had a single complaint about them, they seem to be lasting forever. Fully stainless ballraced with all stainless rods, shafts and fasteners plus brass or bronze almost everything else.

    [​IMG]

    I do agree with you about slide valve engines and their control gear, they look so majestic in operation.

    I have a very rare set of laser cut control gear and main plates for a 'Steamech' engine.

    [ame]https://www.youtube.com/watch?v=Fc3qxbap1qY[/ame]

    [​IMG]

    and a very full casting set for 'Borderer' by John Bertinat, in either slide valve or piston valve configuration, but as things with me are going, they will never be made.

    John
     
    Last edited: Jun 14, 2016
  8. Mar 10, 2017 #28

    larrydoucet1946

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    Could you tell me where you got your casting for this engine?
     
  9. Mar 10, 2017 #29

    Blogwitch

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    The two at the top are pure barstock, a modified design of a French engine to make it easier to produce and they run much better as well.

    The video is a Steamech engine, again made from bar stock, so no castings, but at a certain point in time a few years ago, the laser cut linkages shown in the last picture were made available for a very short length of time (definitely no longer available).

    John
     
  10. Apr 23, 2018 #30

    Anatol

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    Hi John
    I know this is an old thread but hopefully you'll see this note. Speaking as someone who has do quite a lot of research hang thinking but no actual making yet -

    1.Wobblers. I'm attracted to idea due to the minimizing of parts, but I wonder about two issues. A. port timing. How optimal can you get it? B. Seal. How do you make the best seal on those plates that slide over each other without overly increasing friction from the bolt use to force them together?

    2. Piston valves. Interesting conversation. At what size do you think piston valves become viable? Are there any new materials which might be applicable? (self lubricating composites with minimal thermal expansion...?

    3. Paddleduck. Might just have to be my first build :)
     
  11. Apr 24, 2018 #31

    Blogwitch

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    Anatol,

    To cover your questions in the same order.

    Wobblers - they can be got to very efficient engines. In fact, some of the early steam ships had oscillating engines, with something like 7 foot bores or even larger, so they weren't just used for models.
    Having made piston valve and slide valve engines as well, I always liked oscillators purely because of their simplicity, and if made well, are just as efficient as piston valve engines. My motto is that what isn't there, can't go wrong.
    The more precise you can get the port timing, the more efficient and well the engine will run, if you can get them spot on, you can get a great engine. You can easily get rid of a lot of friction between the port faces by machining away most of it (just about 0.005" deep), leaving where just the port faces sit plus a little more will remove most of the friction. In the two pictures below, you will see an engine that was sent to me to get running, the ports were in the wrong position, the second one shows the holes repositioned perfectly and the friction area cut back between the two sets of ports.
    One thing you most do, after the engine is run in, is to slacken off the spring loading until just before the faces start to lift off each other and start to lose steam. It pays to have two steam control valves, one to control the max amount of steam out of the boiler and the other to control the amount of steam sent to the engine, the second one should be locked up solid to prevent the engine getting too much steam and lifting the port faces. The first control valve can be used to shut steam off from the boiler so that you can top up your lubricator without resetting the steam eventually going to the engine.

    Piston valves - in the sizes we make them we are limited to what we can do to eliminate steam leakage, plus sometimes you have to get very close tolerances onto the spool to allow them to work, so as such, not much you can do as sharp edges tend to chop up any seals you try to introduce. The correct materials can make the engine last a lot longer, do not try to use silver steel as the spools because of it's good fit and finish, it will rust up in no time and ruin everything you have done. I tend to use ground stainless in either a brass or bronze valve block. If you can't get stainless, I have seen used either bronze or brass for the spool to fairly good effect to reduce steam leakage.
    BTW, both wobblers and piston valves can be controlled by just two channels on a model boat whereas a slide valve requires a third channel to operate fwds/reverse.

    The 'Paddleducks' was made on the fly, making it out of bits from my scrap box and designed as it went along from the junk I had, but even so, it will give you a good understanding about how things are made and held while machining to get to a finished product, and if you do build it, you will be a lot further along in you education than you were before.
    Although a little large for what it is (because of it being a slow revving long stroke) in say a 4 to 5ft boat and a reliable boiler, it should do well. Plus if you have a little spare time you can change it from the basic first picture to the fully blinged second one.

    I hope this helps a little

    John

    Stuart Turner refurb.JPG

    twinfix 17.JPG

    custom a.JPG

    custom 1.JPG
     
  12. Apr 25, 2018 #32

    Anatol

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    John
    Thankyou, I really appreciate your reply.

    "I always liked oscillators purely because of their simplicity, and if made well, are just as efficient as piston valve engines."
    I agree entirely about the wobbler having less to go wrong. Your wobbler with the gear output is similar to my thinking, though I'd use a belt or chain. Are plans available, or do you recommend a particular design (some of the 'famous' designers - Elmer, ET Westbury, etc had wobblers.)

    My general design goal is to find ways of reducing precision and moving parts, so I'm looking at wobblers, uniflow exhaust, and rotary valves, and overhung crank geometries to avoid making crankshafts. I currently think a double acting two cylinder V on overhung cranks might be a good way to go, with eccentric operated piston valve intake and uniflow exhaust. But I'm looking at all kinds of layouts. - triradial is nice. I've thought about a single acting 4 cylinder boxer too.

    "the ports were in the wrong position"
    so what is the right position? Can you say more about port geometry on the wobbler. As I understand it, the intake opens before TDC, and the exhaust is only open around BDC. So there seems to be lots of negatives - inlet too early, incomplete exhausting. Do you think these concerns are significant? I can see making the exhaust port elliptical for longer exhaust, what else can you do to improve timing?


    "I tend to use ground stainless in either a brass or bronze valve block".
    noted. so you're saying that with enough precision and the right materials, piston valve are viable even at small scale? What about delrin an that family of plastics?I'm thinking a 'self lubricating' composite with minimal thermal expansion would be a good thing, but don't know how they hold up in steam.

    "The 'Paddleducks' was made on the fly,"
    I know, and as I recall you used hollow rotating valves, which interests me, , but I need to review the files.
    more later, gotta go.
     
  13. May 16, 2018 #33

    pipemakermike

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    I have produced all the drawings for a 2ft narrow gauge loco that is now running on the Lynton to Barnstaple railway in Devon. This loco is a recreation of an 1898 Baldwin for which there are no drawings but a lot of photos. The design was significantly modernised to improve the power and drivability of the loco and part of that was to replace the Baldwin slide valve design with a piston valve design that fitted within the external shape of the original slide valve.
    You will find pictures of the redesigned valve here
    you will need to scroll down a bit.

    Mike
    [​IMG]
     

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