Dimensions for slow runner

[rleete]

I'm starting on plans for a small horizontal mill engine.  Double acting, single cylinder with a spool type valve.  I'd like it to tick over nice and slowly.  Something that runs on just a few PSI.  Obviously, good fits are critical for reduced friction, and a large (somewhat oversized?) flywheel will help.

My question is, what is better for a slow running engine?  Long or short stroke, large or narrow bore?  Is is a ratio?  I'd like to keep the bore down under 1", if that helps.

[Darren]

I believe low RPM like a long stroke, nice long burn to make it efficient....

What do ya mean you want steam....

[Bernd]

Roger,

I'd think about incorporating ball bearings in the moving parts. Just for a challenge of course. 

I also think long stroke and large bore. Think of the big diesel engines that run at low rpm.

Bernd

[rleete]

Darren, is there any rule of thumb on that?  i.e. is "long stroke" double the bore?  Tripple?  More?

Bernd, already planned on bearings, just because i happen to have some I rescued from the scrap bin.

[John Hill]

I do not know if low RPM likes a long stroke but it is certainly true that high RPM calls for a short stroke!

Everyday internal combustion engines for cars etc became 'square' (stroke=bore) about 60 years ago most nowadays would be stroke less than bore (I think) however I recently read that stationery steam engines were typically stroke=bore x 3.

[bogstandard]

Roger,

If I was going to design a long stroke engine, I would give a ratio of stroke = double the bore. If you start to go for longer strokes, the cylinders can get inordinately long. That is the ratio of my long stroke oscillators, and I think my paddleducks engine.

When you see say model beam engines running, you will notice that the flywheel speeds up and slows down on each revolution. It is more noticeable when the engine is running slowly. This is because most of them don't have a balanced cylinder, so as the piston rod is pushed out, it goes out faster than when it is going in. This is caused by differential piston areas. The side of the piston with the rod on, has less surface area for the steam to act upon than the other side of the piston without the rod. To balance it out, you would make a cylinder with a rod and gland at both ends, but only connect one end to the flywheel. That will really make the engine run a lot smoother.

John

[John Hill]

If I was going to design a long stroke engine, I would give a ratio of stroke = double the bore. If you start to go for longer strokes, the cylinders can get inordinately long.

Wait till the nutating engine is complete, the piston and cylinder assembly is almost twice as long as would be expected due to the valve sleeve needing to be as long as the stroke.

[rleete]

If I was going to design a long stroke engine, I would give a ratio of stroke = double the bore.

Thanks.  I'll start there.

To balance it out, you would make a cylinder with a rod and gland at both ends, but only connect one end to the flywheel. That will really make the engine run a lot smoother.

That makes sense.  I've seen the same effect in pneumatic cylinders for automation I've designed.  It's usually compensated for by installing individually adjustable restrictors on the exhaust ports.  The "extra" rod would add more visual appeal, too.

[bogstandard]

It is also the way the gas struts work on your boot (trunk) lid. Charge up the whole cylinder with pressure, and because of differential areas, the strut will always extend by itself. More pressure for a heavier lid, light pressure for a lightweight lid.


Depending on the design of your valve gear, you can sometimes get away with throttling the exhaust to get a real slow runner, it is much more controllable than throttling the inlet.
I don't know why, but sometimes it will work with slide valves, sometimes not. You would need to try it after it is made.

The other way would be to put an adjustable friction device on the output side, say on the flywheel rim. It would take very little friction pressure to make it go as slow as you want.

I've just had a thought about using a double ended cylinder.

For a very quirky effect, have both rod ends anchored, and have the cylinder running up and down the rods. You would need flexible pipework, but that wouldn't pose much of a problem.

Or

Again a double ended cylinder, laying horizontal, but driving two flywheels, one at either end of the engine, but it could still be controlled by just one set of linkages.


I'll keep all the other stupid ideas to myself.

Bogs

[Bernd]

The stern wheeled paddle boats used long throw pistons. There is one called the California Cutoff engine. It can be seen here: http://www.agelessengines.com/s-boat.htm

Might give you some insight to long throw pistons.

Bernd

[bogstandard]

I think the problem with the very long stroke engines Bernd, is that they are limited to the maximum speed they can run at, and the size of machine you would need to fit one onto it.

Unless you wanted to make a diorama of say a factory installation or as you have shown, a sternwheeler system, then you are very limited in how to use one at that sort of bore/stroke ratio.

John

[rleete]

The other way would be to put an adjustable friction device on the output side, say on the flywheel rim. It would take very little friction pressure to make it go as slow as you want.

That's a good idea, but kinda "cheating", if you know what I mean.  I'd rather it just ran slow because of the way I made it.  If it doesn't go as slow as I want, well, that's just another engine to be made.

For a very quirky effect, have both rod ends anchored, and have the cylinder running up and down the rods. You would need flexible pipework, but that wouldn't pose much of a problem.

Might be difficult to control a slide valve unless it was separate, and not moving with the cylinder.  Interesting concept, though.  I'll have to think on it, and see what I can come up with.  Might be fun to see something like that run.

I'll keep all the other stupid ideas to myself.

No!  It's the custom design stuff that interests me.  After all, we aren't building anything new, just different ways of putting it all together.  Tossing around different ideas is good; gets the creative process started.  As long as we can think it up, someone will figure out a way to make it.  Might be some really neat engines made from a passing comment here.

[rleete]

The stern wheeled paddle boats used long throw pistons.

Yeah, I saw a model of one posted somewhere on the web, just slowly chugging along.  Really long throw, complete with paddlewheel.  It might be where I get the idea.  I just like to watch all the elements move, and when they're running at a blur, you don't get the same effect. 

[John Hill]

I think if the engine was designed with extremely short inlet valve opening it could be made to run slowly.  It would take just a puff of air/steam at the start of the power stroke which would continue to give decreasing power until the piston reached bottom at which time the exhaust would open and stay open for all of the up stroke.

[bogstandard]

Quote from: bogstandard on 22-07-2009, 23:16:01
For a very quirky effect, have both rod ends anchored, and have the cylinder running up and down the rods. You would need flexible pipework, but that wouldn't pose much of a problem.

Might be difficult to control a slide valve unless it was separate, and not moving with the cylinder.  Interesting concept, though.  I'll have to think on it, and see what I can come up with.  Might be fun to see something like that run.

That is where the quirkiness would come in, there would be no valve gear on the cylinder, it would have a remote valve block that is operated by the cylinder reaching either end of it's stroke. Just a simple flip-flop valve connected to the cylinder with flexi pipes.

John

[John Hill]

Bogs,  you would not need the flexible pipework if you could use a hollow pushrod for input, for the output you could use some sort of 'inertia' valve, imagine a bobbin free to bounce back and forth in a valve cylinder.

[NickG]

I agree a bore to stroke ration of 1:2 should be ok for a slow running engine and it might start to look a bit silly if you go to 1:3. I can't quite figure out in my head whether the ratio would effect the running speed or just the actual stroke? Would a 0.5" bore 2" stroke run at the same speed as a 1" bore 2" stroke with the same pressure applied? 

I just made a quick spreadsheet up to satisfy my own mind before writing any more on this, and initially it seems they would both run at the same speed if they had the same stroke and pressure. However, the engine with the 1" bore would have 4 x the torque at the same pressure, therefore, you could run it at 1/4 of the pressure which would give you 1/2 of the speed.

So what does my little spreadsheet show, well, it shows that the bore can have as much or more effect on the potential speed an engine can run at as the stroke does.

At the moment I'm just basing results on the peak acceleration of the piston, however, it is not constant so not quite sure how it will work out in practice, obviously the speed of the piston is sinusoidal so my spreadsheet needs more work to predict more accurately. And of course it doesn't take things like increased friction due to increased bore into account either or any other larger masses as a result of having a larger bore, this simply works out the mass of the piston assuming they are the same length.

What was my original question?! Oh yeah, is it the bore to stroke ratio that determines how slowly an engine can run? Well I can definitely answer that one - no! For example, you should be able to run a 2" bore 2" stroke engine more slowly than a 1" bore 2" stroke. You would, in theory neglecting extra frictional forces, be able to run it at 1/4 of the pressure to give the same torque at the flywheel, hence running at a much slower speed. Say I ran the 1" bore at 40 psi, that would give a speed of 252rpm by my rough ass calculations, running the 2" bore version at 10psi would give a speed of 126rpm!
I don't suppose for a minute these figures are correct, but they should give a good comparrison.

Purely because of the way the area of a circle is worked out, there is going to be more effect on the potential for an engine to run more slowly by adding an inch to the bore than there is by adding an inch to the stroke. At the same time it will give you the potential for more power and torque.

So I haven't exactly come up with a new theory of relativity here, it basically means a larger engine will potentially run more slowly - most of us probably knew or guessed that! But it just goes to show that simply increasing the stroke of an engine isn't necessarily the best way of getting it to run more slowly, you might have better results by increasing the bore!

Sorry if you think that was a load of b0110cks, I realise it's probably hard to follow, might go around in circles a bit and not make a great deal of sense. It's difficult to explain but I wanted to get it down here anyway for my own peace of mind.

If anybody wants a copy of the spreadsheet, PM me, it might be easier to visualise what I have said.

Nick

[bogstandard]

Nick,

That is why I design my engines as long stroke. The normal commercial ones usually have square or very close bore/stroke relationship. I make mine long stroke because more torque is produced for only a slight height gain. Because more torque is produced, the engine can run at lower speeds with a larger propellor, so giving a large increase in steam economy to have the boat moving at the same speed.

Hope that hasn't confused everyone even further.


John

[Bernd]

I would think that the size of the flywheel would also have an influence of how slow you can run an engine. The more mass revolving the the slower you can make it go. You can't scale down inertia.

Bernd

[NickG]

John, that makes sense for the boat application as increasing the bore the same amount would give you more torque but consume far more steam.

Bernd, I was supposed to mention that in my post but got too tied up with all the calcs! Yes, a larger flywheel with a higher moment of inertia should allow slower running, I guess it also would need higher starting torque.

Nick

[bogstandard]

Nick,

If you had to run the engine at the higher speed, then yes, it would use more steam. But in model boat situations, you can in fact run the longer stroke engines a lot slower, so you really don't use as much steam if using a shorter stroke engine.

I have people coming back to me saying, in some cases, they have had 50% longer sailing times using the same amount of water and gas, and the boat is performing just as well as before.

John

[CrewCab]

If anybody wants a copy of the spreadsheet, PM me, it might be easier to visualise what I have said.

Yes please Nick, whilst I'm quite enjoying this thread and think I'm understanding most of it, a visual aid would be useful .............. Cheers

Dave

[foozer]

I agree a bore to stroke ration of 1:2 should be ok for a slow running engine and it might start to look a bit silly if you go to 1:3. I can't quite figure out in my head whether the ratio would effect the running speed or just the actual stroke? Would a 0.5" bore 2" stroke run at the same speed as a 1" bore 2" stroke with the same pressure applied? 

General long stroke engines produce more torque but are run slower. Slower, not so much as the increase stroke makes it so but to keep the piston speed down. It does have to travel farther per cycle as same engine with shorter stroke both running same RPM. Heat build up from that can kill a long stroke running at elevated revs. Connecting rod length also effects performance, longer rod reduces the piston side loads but increases the rotating mass.

I need an aspirin now ..

Robert