Gallery, Projects and General > How do I??

Dimensions for slow runner

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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?  :scratch: :scratch:

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

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