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aostling
11-01-2016, 05:23 PM
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?

Paul Alciatore
11-01-2016, 06:10 PM
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:

http://www.csgnetwork.com/multirpmcalc.html
http://www.propline.com/Propeller-General-Information/Determining_RPM.htm
http://www.hipermath.com//engines/max_rpm
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.

BCRider
11-01-2016, 06:22 PM
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.

ammcoman2
11-01-2016, 06:36 PM
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.

Geoff

SGW
11-01-2016, 06:53 PM
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.

aostling
11-01-2016, 07:07 PM
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.

enl
11-01-2016, 07:22 PM
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)

aostling
11-01-2016, 07:24 PM
... 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).

David Powell
11-01-2016, 07:30 PM
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.

BCRider
11-01-2016, 07:43 PM
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.

Howzatt?

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.

J Tiers
11-01-2016, 07:59 PM
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.

Weston Bye
11-01-2016, 08:03 PM
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?)

brian Rupnow
11-01-2016, 08:26 PM
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.

Mcgyver
11-01-2016, 10:08 PM
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

BCRider
11-01-2016, 11:44 PM
.....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.

Evan
11-02-2016, 01:36 AM
I didn't read all of this but I would expect the scale to be a cube since the mass is a cube factor while the friction is a square by area, good old cube square ratio. Make it bigger and the friction goes up a little and the mass far more. Friction won't slow down that much greater mass so easily so it can run much slower (and the inverse). The only way to beat the friction at small scale is to raise the energy which means higher velocity of moving parts.

David Powell
11-02-2016, 08:10 AM
I didn't read all of this but I would expect the scale to be a cube since the mass is a cube factor while the friction is a square by area, good old cube square ratio. Make it bigger and the friction goes up a little and the mass far more. Friction won't slow down that much greater mass so easily so it can run much slower (and the inverse). The only way to beat the friction at small scale is to raise the energy which means higher velocity of moving parts.

I have seen that far too many model steam engines that are simply " too tight". The first essential is that the parts must be square to each other, the second is that each interface, revolving, or sliding motion, must work smoothly without any binding or hesitation throughout its action.( One can, of course make everything " sloppy" and it will, after a fashion " work" and even run slowly at times) thirdly the valve setting must be the best possible. Spare no pains to find out what is causing " That niggling slight tight spot, or hesitation" and cure it. Then you will have an engine which will run smoothly slowly on a very low pressure supply and accelerate quickly and rev like the devil when you increase the pressure. Small engines, like the Stuart 10 should turn with just light finger force on the flywheel, indeed , should run freely when blown round by lung pressure. Back to friction, give enough theoretical clearance to allow for a sensible oil film, two thous clearance on a pin joint is fine, a tenth or two is likely too tight, five thous is likely too slack. Regards David Powell.

Evan
11-02-2016, 05:07 PM
There are certainly ways to work around the problem but the essential problem is plain old physics. Anything that can be done both to reduce friction and to increase mass will help. In the case of a piston engine make the piston as heavy as possible along with a heavy flywheel. It is the cube-square ratio that you cannot beat but you can at least cut it down as much as possible. Lead can be handy in that regard.

To give a graphical idea of what that means, on this graph the upper line is how much the friction goes down with size compared to the lower line which is how much mass goes down with size. As the mass drops so much more than the friction the momentum of moving parts if far more affected by friction. Keep the weight as high as physics allows and the parts will coast past the friction better.

http://ixian.ca/pics12/cubesquare.png

J Tiers
11-02-2016, 05:28 PM
Yes.

If that is not clear to all, remember that a 1/12 scale model of a 12" cube of material is a 1" cube. It is 1/12 as high, 1/12 as wide, and 1/12 as thick. So its volume (and weight/mass) is only 1/1728 of the original.

AND, a flywheel that is to-scale also has a diameter of 1/12 the original, so the rim of the flywheel has a velocity of 1/12 the original size flywheel at same rpm. That reduces the stored energy by 144x right there. The stored energy is then 1/(144 x 1728) that of the original, or about 1/250,000. Some of it is balanced out by reduced sizes of parts.

But, looking at the piston, the diameter is 1/12, and the stroke is 1/12, so the swept area is only reduced by 144x, and the circumference of piston ring that is rubbing is only reduced by 12x.

Heat losses are more, because area went down 144x at 1/12 scale, but volume went down by 1728x. So area per unit volume is larger.

Basically, most everything is against you in a model.

darryl
11-02-2016, 05:31 PM
If you do go with planetary gearing, cordless drills are an excellent source. Pretty much anyone can find a cordless drill for free since usually the demise of the battery pack means the drill is scrapped.

A two speed drill will have two planetary sections. If you use the first section you will have to use the pinion that's on the motor, but if you use the second section you will have an input gear with a flange attached. It might be easier to mate to a crankshaft. But do heed the warnings about backlash in the gearing.

J Tiers
11-02-2016, 06:07 PM
If you do go with planetary gearing, cordless drills are an excellent source. Pretty much anyone can find a cordless drill for free since usually the demise of the battery pack means the drill is scrapped.

.....

Unless you take it to Batteries Plus (now "Batteries and bulbs" I believe), and have it rebuilt. I've had some done twice, and it was cheaper than a new drill. I DID buy a new drill once to get the batteries, but I'm still using the old one, actually. Don't like the chuck on the new version.

The new batteries are screw-closed, not glued like the originals, so they are easy to rebuild.

Thare is also the article by Paul A in the magazine concerning rebuilding the packs yourself based on cheap HF battery packs as donor.

small.planes
11-02-2016, 06:25 PM
My little traction engine will tick over at a satisfying slow speed. It's steam, so throttle it back, and notch it up.
Then the majority of the push is expansion of the steam.
Its a D R Mercer like this one http://i493.photobucket.com/albums/rr296/ozsteamdemon/DR%20Mercer%20type%202/DRMercer002.jpg
About mamod size, give or take.
Lagging the boiler and pipes to keep the steam hot helped a lot.

Dave

Rich Carlstedt
11-02-2016, 06:50 PM
I have to respectfully disagree with those that consider the flywheel as paramount to speed control in a steam engine
My Monitor can run slow and has no flywheel or propeller. I can run it even slower than shown here in the video
The engine does have a lope , but that is due to the design of timing at 80/100 degrees versus the 90/90 difference in normal 2 cylinder engines
https://www.youtube.com/watch?v=VWn8gQ9Ykpk
If yo desire slow running , all moving parts must not have any binding or friction changes.
Smooth running means slow running ......and proper valve timing and seating ( !) are critical

I have to agree with Wes, I built a exhaust throttle for my Cameron 09 back in the 1950's that looked like a intake throttle butterfly but restricted the exhaust for slow speed and yet it maintained heat in the all important cylinder for smoothness.

Rich

Mcgyver
11-02-2016, 07:07 PM
I have seen that far too many model steam engines that are simply " too tight". The first essential is that the parts must be square to each other, the second is that each interface, revolving, or sliding motion, must work smoothly without any binding or hesitation throughout its action.( One can, of course make everything " sloppy" and it will, after a fashion " work" and even run slowly at times)

agreed on the right amount of clearance, things square and no binding.....but a properly done piston ring exerts a force against the cylinder...times however many rings which creats some drag

I always thought this engine by John McDivitt looked about right....except for the governor balls

https://www.youtube.com/watch?v=mE9dZmqBF6c

If you haven't seen it, the following is a great video of the Kempton Park triple expansion engine. Perhaps I'm prejudiced in my views on it, when he opens the small motor to get it turning (around 2:50) its certainly whipping around at high speed......but I prefer the pace of the big engine when its going

https://www.youtube.com/watch?v=KhlJp1VZMB8

J Tiers
11-02-2016, 07:11 PM
It is NOT "paramount" on a steam unit, aside fro somewhat controlling the acceleration of the piston/rod/crankshaft due to its mass. Speed is really the throttling. With steam, the force on the piston is not dependent on overcoming compression, etc. if not double-acting, it does need to get through more movement, though.

For an IC engine yes, it has to do the compression, so the mass and rpm are important..

aostling
11-02-2016, 08:12 PM
http://ixian.ca/pics12/cubesquare.png

What kind of a graph is this?

If the quantity squared equals 4, then the quantity cubed equals 8.

gbritnell
11-02-2016, 11:25 PM
This is my double acting single cylinder 'Mary' beam engine running on air, about 8lbs. I would say it runs a tick faster than the prototype but still has the elegance of running slow so all the motions can be seen. The only draw-back is that there isn't enough centrifugal force to make the governor balls fly out. A multi-cylinder steam engine will run quite slowly due to the fact that there is always pressure on one of the pistons, that is if there isn't too much friction.

https://youtu.be/bmBgGAN-7aE

As far as 4 cycle internal combustion engines go there are three factors that regulate how slow they will revolve, first is the flywheel mass. As someone mentioned there needs to be enough inertia to get a cylinder over the compression stroke, which leads to number 2. This is the compression ratio, the greater the compression ratio (along with cam timing) the more inertia is needed. The third factor is the ignition timing. You can retard the time so that when the cylinder fires there is no negative pressure on the piston. That will get the engine to run slow but it won't rev up because there's not enough timing.
My Holt will run down to 550 rpm. I can get it to run a little slower but then that's all it will do, run slow.
You have to realize these are miniatures and the physics of running them isn't the same as their full sized counterparts.

https://youtu.be/tl0nD1smbAg

gbritnell

Evan
11-02-2016, 11:54 PM
Yes, you are right Allan. I was thinking 2 squared and then squared again I guess. Working a little too hard right now. I have been fighting a bug that turns out to be a real bug in Ruby, not my code. I need my laptop working by Friday morning when I meet with the prof and I couldn't get my code to work on Windows 10. Driving me crazy it was until I finally figured out what is wrong. I just now solved it and can work around it fortunately. I have wasted much of this day when I was hoping to improve the code. The bug wouldn't let me show more than 16 channels even though it works just fine on Win 7. On Win 10 it complains about a bit of code that doesn't even exist in my program. A certain tiny bit of Ruby can't count higher than 16 when run in Win 10. That is most likely the reason I was thinking of sixteen and not eight.

aostling
11-03-2016, 12:48 AM
I hope that by Friday you will have this all sorted out. Judging by his name, your prof might understand if you talk to him in Danish!

Ian B
11-03-2016, 04:19 AM
Back on the 'slowing a model down'. Is there any way of mounting the model on a base of some kind, and then hiding a decent sized flywheel inside it? Drive this flywheel from the model, and you should be there. A right angled bevel gear on the crankshaft driving a vertical shaft down into the base shouldn't be too noticeable.

Ian

Evan
11-03-2016, 04:36 AM
Krigolson is Swedish while Krigolsen is Danish... And yes, it is sorted out well enough. With even just 16 channels that is all that is normally visible in the front half in the 10-20 EEG standard electrode system with 64 channel setups. They can and do do more but it is plenty enough to show how it is working now. It looks amazing, just like the prof says. Seeing it the way it is now it makes me wonder how they ever made any sense out of it. I can't take credit for this though, it was the prof's idea. I am just a programmer that happened to be in the right place at the right time.