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slowing down scale-model engines

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  • slowing down scale-model engines

    At the end of my first year of engineering school I spent a summer in Scandinavia. It was 1960, and the cheapest way to get across the Atlantic was by ocean liner, a ten-day voyage between New York and Oslo. The ship -- the SS Stavangerfjord -- had been making these crossings since 1918. Each of its twin screw propellers was driven by a quadruple-expansion reciprocating steam engine. On several occasions I got down in the engine room, and sat spellbound watching all that wonderful machinery in motion.

    I don't get the same thrill when I see model engines in action. They turn over too fast. All you can see is a blur.

    I don't know how the RPMs scale -- it might be a simple inverse, with the flywheel on a 1:10 scale model spinning over ten times faster. It seems to me that one way to slow the speed of a miniature engine is to use speed-increasing gearing between the crankshaft and the flywheel. The flywheel would spin faster, but so what? The crankshaft, the valve gearing, governor, and anything worth seeing would be slowed. You would not need to take a slow-motion video to appreciate the running of the engine.

    There should be no need for an intermediate gearbox. Perhaps a planetary speed increaser might be incorporated between the flywheel and its driven hub. In motion, this gear train would be as blurred as the flywheel, unnoticed.

    Has something like this been tried before?
    Allan Ostling

    Phoenix, Arizona

  • #2
    I too have seen large steam engines at work. I loved to watch the engines on the steamer President as it cruised up and down the Mississippi. Majestic!

    From my experience with model aircraft engines I can definitely say yes, the speed is inversely proportional to the size. The smaller engines were screamers running thousands of RPMs vs. around 4 to 8 RPM on that steamship. I do not know the math involved but I am sure you could get some idea from catalogs of model engines that would list their displacement, power, and RPM. A quick search for "engine RPM vs. size" did turn up some interesting results:
    and many, many more

    Actually, I would think it is a complicated matter with the displacement, the mass of the moving parts, the amount of work load, and probably other factors.

    And yes, much of the reason for this was the limited size of the flywheel/propeller. A smaller flywheel still must supply the energy needed to compress the fuel/air mixture on the next stroke and since it has limited mass, it must use speed to store that energy for the following stroke. When any IC engine is slowed down too much, it will stop. Steam engines use external combustion and are more forgiving. A steam RR locomotive can start from zero RPM and apply enough torque to start moving a heavy train. If you study the valving on those steam engines, perhaps you can devise a throttle that will allow an external combustion model engine to function at lower speeds. If the steam or compressed air is fed to the cylinder in a controlled manner, then it should run slower. And the flywheel will only need to carry the motion across the "top" and "bottom" dead center positions. Those RR locomotives used two cylinders (left and right sides) that were 90 degrees out of phase so one of them was always away from the dead center position and they could always start moving. With two cylinders, perhaps all you would need to keep the speed slower is some friction or another kind of load for the engine to work against.

    In theory, your gearing idea should work. Actually if you are talking about a steam engine I don't see why you could not just increase the size of the flywheel. A flywheel with a larger mass should be able to work fine at a slower speed. Or combine the two ideas: make the flywheel more massive and put some gearing between it and the cylinder(s).

    You might machine a hollow flywheel from steel and pour some lead into that cavity.

    Do post a video if you get this to work.
    Paul A.
    SE Texas

    And if you look REAL close at an analog signal,
    You will find that it has discrete steps.


    • #3
      I think you are on to something. But when you say "There should be no need for an intermediate gearbox. Perhaps a planetary speed increaser might be incorporated....." I'd say that the planetary arrangement would qualify as a "gearbox". That's a minor niggle though.

      It is an intriguing ideal though. Speeding up a "scale" size flywheel would certainly cause it to store more energy and keep things turning smoothly through the idle portions of the cycle while still having enough energy to punch through the compression stroke. So a slower idle speed should still be possible.

      How slow do you want it to run? I've seen some "bread loaf" size table top stationary engines that are running down at something like 120 to 150 RPM. It's slow enough to see the weights of the interrupter moving around.

      Another option is to use a flywheel that is somewhat larger than "scale" or that most of us would find visually "balanced" with the rest of the engine. For the same RPM the energy contained by a flywheel goes up by the square of any change in the radius. And for a flywheel of the same rim cross section the change in the radius is doubled and then multiplied by pi to give a factor of increase in the mass of the rim. So that's another gain.
      Chilliwack BC, Canada


      • #4
        I too, like to have model engines turning over at a leisurely rate. My experience to date has been that I can easily get model steam engines (running on air) to turn at 60rpm with a nice smooth action. The key is a large diameter flywheel which most of the old style engines did have. So a 1/12 scale engine would still have a 9" or 10" diam. flywheel, which is key.

        IC engines are much more difficult to run at full size rates. I have made the Bob Herder "Nanzy" 1/2 scale hit 'n miss engine. It can run as low as 400rpm but it has relatively big twin flywheels. The key IMO is to make the flywheel a larger diameter than scale and to put as much of the mass as possible on the periphery. Setting the cam timing to enhance low speed operation is also a must. I think Brian Rupnow has found success with this approach.



        • #5
          My old mentor, Dan Fay, wrote an article probably 20 or more years ago about scaling. According to Dan,to scale time one should consider the period of a pendulum. A pendulum has a period proportional to the square root of pendulum length. The period for a swing is

          T = 2*pi*SQRT(L/g)

          where g is local gravity.

          Ignoring all the constants and units and just choosing a couple of numbers for L, if a pendulum has a length of 16, its period would be 4. If another pendulum has a length of 8 (a half-scale model of the original) it will have a period of about 2.83.
          So....if on a prototype ship the propeller turns at 100 rpm, on a half-scale model the propeller ought to turn at about 141 rpm.

          At least that's how I remember it. I still have Dan's article, somewhere, but a brief search didn't come up with it.
          Try to make a living, not a killing. -- Utah Phillips
          Don't believe everything you know. -- Bumper sticker
          Everybody is ignorant, only on different subjects. -- Will Rogers
          There are lots of people who mistake their imagination for their memory. - Josh Billings
          Law of Logical Argument - Anything is possible if you don't know what you are talking about.
          Don't own anything you have to feed or paint. - Hood River Blackie


          • #6
            It occurred to me on my afternoon walk that model steam engines, if double-acting or two-cylinder, should have no problem running slow, simply by throttling the steam.

            That still leaves a large class of models, single-acting steam and I.C., which could benefit from the ideas so far discussed.
            Allan Ostling

            Phoenix, Arizona


            • #7
              Back in the day, there were a number of steam engine designs that, for a variety of reasons, had the flywheel running faster than the piston would imply. In some cases, it was the straight line motion, in others it was to allow a lighter or smaller flywheel. The primary thing to watch out for if building such, in my opinion, would be backlash in gearing that would lead to pounding. Pounding destroys bearings very, very fast.

              A double acting (or two or more cylinder) engine can be run pretty much as slowly as throttling and friction allow, but the efficiency drops to nil since at low speed, expansion can't be used. A high speed flywheel would allow for some use of expansion, even unloaded (only bearing and engine friction)


              • #8
                Originally posted by BCRider View Post
                ... I'd say that the planetary arrangement would qualify as a "gearbox". That's a minor niggle though.


                ...another option is to use a flywheel that is somewhat larger than "scale" or that most of us would find visually "balanced" with the rest of the engine.
                A planetary would be a gearbox. I was thinking in terms of sort of disguising its presence

                I like your idea of increasing the size and/or mass of the flywheel. That might be all that is necessary for some engines.

                A most direct way to reduce speed is to simply load the engine. That might stall most small models, so then the question becomes one of how to increase the torque (i.e. the BMEP).
                Allan Ostling

                Phoenix, Arizona


                • #9
                  if you want a model steamer to run slowly---

                  Originally posted by aostling View Post
                  It occurred to me on my afternoon walk that model steam engines, if double-acting or two-cylinder, should have no problem running slow, simply by throttling the steam.

                  That still leaves a large class of models, single-acting steam and I.C., which could benefit from the ideas so far discussed.
                  Simply throttling the steam will not do it, unless two other circumstances are occurring. Firstly there should be absolutely no unnecessary friction. yet no play or lost motion, especially in the valve operating mechanism, secondly the actual valve setting must be correct, if a single cylinder double acting engine steam admission must not be before dead centres, but should be as close to dead centres as possible, and as even as possible at each end. Multi cylinder jobs can run slower or have less flywheel weight, but they must be setup one cylinder at a time, just like singles, if you want really slow running. If you want performance rather than show case slow running multi cylinder engines can benefit from considerable advance in valve timing without much loss of ability to run fairly slowly, Singles become balky and difficult to drive if given any advance. Many single cylinder traction engines built by experienced railway modellers suffer to some extent from this. I aim to be able to blow any model steam engine round with no more than 5 lbs pressure of air or even less steam. Hope this helps David Powell.


                  • #10
                    With steam engines the valving can be set so the inlet doesn't open until TDC. Also a steam engine is more similar to a 2 stroke engine but with the valves in the head so the "exhaust" stroke runs all the way to or nearly to TDC and there's no significant compression stroke to deal with.

                    But any sort of IC engine involves a compression stroke. And that requires the flywheel energy from the rotating mass of flywheel and crankshaft. On model airplane engines the mass of the prop is the flywhee. A poor one for sure but it's still what keeps things running. Mind you on the high performance engines of up around .15cu inch and over the RPM's where high enough that more than once I've seen them "shaft run" after the prop was busted away completely and relying on the mass of the crankshaft and counter balance only. Those tended to be rather exciting moments for the owners of those engines as they ran to the models to pull of the fuel line or jam the nose of the crankshaft into the pavement to shut them down. One in particular was rather exciting for everyone when the blades of a Rossi or Nelson .15 on an FAI free flight model shed while being tuned for launch and the blades fluttered a high pitched warble as these carbon fiber "daggers" whistled through the folks nearby.

                    Anyway I'm waxing poetic now....

                    I like the idea of the planetary gearing up. The planetary cluster could be incorporated into a "solid" hub such that the fake spokes on the outer cap is the part connected to the crankshaft and turns at the shaft speed. The inner flange could run on bearings on the shaft. And in between are the planetary gears and inner and outer sun and ring gears. A smooth polished look would hide the true speed of the rim and inner flange while the fake "spokes" on the outer cap make it look like it's barely turning over.


                    Loading it down won't work. The flywheel still needs enough inertial energy to bump the piston past TDC even if the firing timing is set to be quite delayed so it doesn't have to fight the ignition compression as well. Loading it would rob the flywheel of energy before it reaches compression. And if we open the throttle enough to gain the power to handle the load the minimum speed that retains the energy needed to kick over TDC under compression is still tied to some specific RPM, loaded or not.
                    Chilliwack BC, Canada


                    • #11
                      You need a certain amount of energy to get the thing to turn over to the next power stroke, which with a double acting steam engine should be easy as long as there is more than one cylinder.

                      But, anyhow, the flywheel may have to have more mass.

                      Problem with models is the stored energy in any moving mass goes as the SQUARE of speed (rotation) and only directly as mass. You start out with limited mass just because the thing is smaller, and reducing size reduces mass as the cube of the reduction. It's a "model" and so much smaller. Proportionate flywheels will be lightweight compared to the engine, due to that scaling. To double the energy you need to have 2x the mass, OR turn 1.4 x faster, OR increase diameter. Obviously it is easier to go faster.

                      Brian Rupnow has gotten several engines to run at a pretty slow and "appropriate" speed. He does use brass flywheels, which are denser than steel. but he also uses rather small flywheels, and they have been on 4 stroke IC engines, with compression to contend with, as well as one idle turn per cycle. So it is do-able.

                      Reduce friction, I'd say is the first thing.

                      But that marine engine? You should be able to run a multi-expansion marine engine at nearly any speed, as it will be double-acting, and have multiple cylinders. In full scale, they can be run quite slow. You can, as with full scale, admit HP steam to the LP cylinders for starting.

                      For IC engines, maximizing flywheel diameter and mass, plus perhaps a lower compression ratio, would be one way. Stroke is another issue. A short stroke engine is typically fast, and a long stroke typically slower.

                      It would be possible to lead-load a flywheel, a groove on the unseen side filled with lead. I'd suggest depleted uranium, but that might be an issue in various ways.
                      Last edited by J Tiers; 11-01-2016, 08:04 PM.
                      CNC machines only go through the motions


                      • #12
                        Taking a lesson from pneumatic design, feeding full supply pressure and restricting or throttling the exhaust of a steam or air engine might be in order. Air cylinders customarily use flow controls to "meter out" the air in the exhaust side of the cylinder, relying on the compressibility of the air to smooth the motion and overcome "stiction" of the piston and related friction and load.

                        Similarly, I think I have seen small i.c. aero engines that used an adjustable exhaust port baffle maybe linked with the throttle for engine speed control. (Jacobs Brake?)
                        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                        ~Practitioner of the Electromechanical Arts~


                        • #13
                          For i.c. engines, the secret to relatively slow running is in the flywheel size and the valve timing and low friction. For slow running steam engines, it is a combination of low friction, large flywheel, and high pressure but metered flow steam or air. I have no problem running my steam engines at less than 100 rpm, while 600 rpm seems the lower limit for 4 cycle i.c. engines. I have thought myself of using a geared flywheel like the original poster has mentioned, but with any kind of backlash at all in the gearing the load reversals between power and compression strokes would beat the gears to death in a very short time. I have seen a couple of people try to build engines with this geared flywheel approach, but they faded away and weren't successful. I'm not saying it can't be done. It's just that I have never seen it done.
                          Brian Rupnow
                          Design engineer
                          Barrie, Ontario, Canada


                          • #14
                            I agree the speed makes most model engines look toy-ish - a big steam expansion engine should be slowly moving around like an elegant orbit not going like a bumble bee. Some guys really like running the engines, me, I put them on a shelf and rarely run them when they are complete. I've a triple marine expansion engine that I think will put an electric motor on (hidden of course) and a push button in the stand so the curious can see it slowly revolving....cheating compared to lighting a boiler, but imo it looks better

                            I've also a Corliss with a 16" flywheel....if I ever get it done it just might run slowly enough.....if I had a big enough boiler
                            in Toronto Ontario - where are you?


                            • #15
                              Originally posted by Weston Bye View Post
                              .....Similarly, I think I have seen small i.c. aero engines that used an adjustable exhaust port baffle maybe linked with the throttle for engine speed control. (Jacobs Brake?)
                              Back before modern glow plug ignition two stroke model engines universally came with mufflers they found it helped to hold the heat in the plug if the exhaust was set up with a baffle that closed the exhaust port down in connection with the throttle valve closing. It wasn't adjustable for anything other than running at low throttle to keep enough heat in the glow plug to avoid it cooling off too far to run. With a muffler the heat is held in sufficiently that this baffle was no longer needed. A couple of mufflers during the early transition from open exhaust to mufflers came with a baffle inside the connecting stack but the practice was soon dropped.

                              The baffle was linked to the throttle valve with a fixed wire. MOST of the throttling down action came from restricting the carb. Engines that are run using an exhaust baffle only simply don't have a very low idle. Likely because the simple baffle designs leaked like a seive.
                              Chilliwack BC, Canada