How to Scale Up An Engine

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CFLBob

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I'm looking for my next engine project, and I'm interested in a hit and miss.

I like the water cooled PM Research Red Wing model, but I'm looking for something made from bar stock rather than castings.

I have the drawings for David Kerzel's Horizontal Hit&Miss engine but it's a smaller bore and stroke than my Webster and I'd like it to be more like the size of the PM Research model.

My thought was scaling up the Kerzel engine. I don't know how much, but I'll say double the bore. The Kerzel bore is 0.75" and the PM Research is 1.25" so that's an weirder ratio - 5/3 size, instead of twice.. From the reading I've done here, I'm guessing the starting point is to double the size of every component and then look at specific components, like the piston connecting rod might be bigger and heavier than needed.

Am I on the right track or do I need to start over? I can also keep looking for plans for an engine a bit bigger than the Kerzel.


Bob
 

Eccentric

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Bob,

You can certainly scale plans and it is often done. It would be best to scale by a nice even number like 2 becasue you will have to scale EVERY dimension and if you scale by 5/3 (1.667) you are more likely to make a mistake. You can scale by odd ball amounts if you are diciplined and taking the print and changing every dimension and if you are OK working with wacky three decimal dimensions .458 instead of some number of 16ths, for example.

The second part of your question is bit more tricky. There probably would be cases where you could tweak the scaled design and reduce some of the dimensions. A small model may have some of the thicknesses larger than they need to be becasue it is easier to fabricate, and when you scale up they could be reduced, for example. But now you are really talking about redesigning the model and when you change one dimension you need to consider everything it interfaces with and how that one change will impact other parts. This would best be done in CAD and will require several design iterations and careful checking of the final result (depending on how carried away you get).

You definetly are on the right track and if the Kerzel is well documented, you like the looks of it, and you are comfortable with CAD, then go for it. I certainly would not let having to scale a model prevent me from making one I really like.
 

Eccentric

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You know Bob, I just had another thought.

I have a set of reamers of a standard size. I think that if I scaled an engine I would like to adjust the design so I could still use my set of standard reamers( if your engine requires these). There may be other tools that you have of standard sizes that you would like to use, taps and dies are obvious examples.
 

CFLBob

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Bob,

You can certainly scale plans and it is often done. It would be best to scale by a nice even number like 2 becasue you will have to scale EVERY dimension and if you scale by 5/3 (1.667) you are more likely to make a mistake. You can scale by odd ball amounts if you are diciplined and taking the print and changing every dimension and if you are OK working with wacky three decimal dimensions .458 instead of some number of 16ths, for example.

The second part of your question is bit more tricky. There probably would be cases where you could tweak the scaled design and reduce some of the dimensions. A small model may have some of the thicknesses larger than they need to be becasue it is easier to fabricate, and when you scale up they could be reduced, for example. But now you are really talking about redesigning the model and when you change one dimension you need to consider everything it interfaces with and how that one change will impact other parts. This would best be done in CAD and will require several design iterations and careful checking of the final result (depending on how carried away you get).

You definetly are on the right track and if the Kerzel is well documented, you like the looks of it, and you are comfortable with CAD, then go for it. I certainly would not let having to scale a model prevent me from making one I really like.
Thanks, Eccentric,

Looking over some notes, I think I conclude that the biggest issue in scaling something up is that parts may end up heavier than they need be. That's because things that are under a bending load get stronger as radius to the 4th power, so if double the radius, they get 16x stronger. I could change the design to make that part lighter, but this is a model engine that will live an easy life, and being too heavy isn't much of a concern. I think.

The thing about the Kerzel is that all I have is a PDF of the plans, which means pretty much every part has to be put into CAD and redrawn anyway. It's relatively easy to scale it at that point.

You know Bob, I just had another thought.

I have a set of reamers of a standard size. I think that if I scaled an engine I would like to adjust the design so I could still use my set of standard reamers( if your engine requires these). There may be other tools that you have of standard sizes that you would like to use, taps and dies are obvious examples.
Good point! I have a set of fractional reamers but by 16ths from 1/8 to 1/2", not by 32nds or 64ths, and I have a set .001 over/under reamers. There are a few others not part of those two sets that I bought one of. I'll have to settle on a nearest reamer no matter what I do.
 

Jasonb

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I often scale engines up or down and just use a conversion factor for all parts, leave critical dimensions like stroke and bore to what that gives but for shafts, reamed holes etc round up or down as I feel appropriate so stock sizes and tools can be used.

If you want barstock plans than take a look at Dario's "zero-six" as he previously built Redwing quite a few of the mechanicals are similar but it's a barstock design Gallery

Digial back issues of Model Engine Builder mag are quite reasonably priced as it was published in two of those #6 & #7 Subscription to Model Engine Builder Magazine - 4 Issues

You can play about with that design to make it look how you want this is my version, all barstock except flywheels which are bought in castings
 

CFLBob

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Thanks, JasonB. That's a great looking engine. It's much better looking than the '06.

This engine is completely new to me, but it seems the major advantage of this engine over scaling the Kerzel is knowing that it can work. Either way, I copy drawings into CAD, no scaling with this one.

The advantage of the PM Research is one (bigger) out of pocket expense and then I'd have all the resources I'd need.
 

Engine maker

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I built Bob Shores Little Angel double sized. The original Little Angel is about the same size as the Kerzel. I just doubled all the sizes. There were no problems and it runs great.

I actually built the double sized one first!

Jim G
 

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Sprocket

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The picture to the left is my version of the "Zero-6". Mechanically, it is the same , but I didn't like the way his looked. He also said in the article that he would have used bigger fly wheels. So I did.
IMG_0813.JPG IMG_0817.JPG IMG_0818.JPG IMG_0815.JPG

The gears were purchased, The flywheels were "mill-whittled" from cast iron discs, the lever inside the right flywheel allows you to change the spark timing. I always thought a hit and miss engine with electronic ignition was an interesting mix of technologies.
I used a piece of rectangular tubing for the tank, and since I never fill the water tank, leaks are not a problem.
Not as pretty as Jasonb's but it runs well.
Doug
 

CFLBob

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Not as pretty as Jasonb's but it runs well.
Doug
Don't sell it short - I think it's really pretty. A couple of questions, though. First, is it the designed size, 1-1/8 bore, by 2" stroke? Second - what's the diameter of those flywheels? I don't have a feel for the size I'm looking at.
 

Sprocket

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The flywheels are 6” by 1” more or less, the 1 1/8” bore sounds right, stroke is long, can’t remember exactly what it was. It was made to the plans, mechanically,
Cosmetically, it’s my own. I’ll look tomorrow. Plans are out in the shop.
Doug
 

CFLBob

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The flywheels are 6” by 1” more or less, the 1 1/8” bore sounds right, stroke is long, can’t remember exactly what it was. It was made to the plans, mechanically,
Cosmetically, it’s my own. I’ll look tomorrow. Plans are out in the shop.
Doug
That Gallery page that Jasonb linked to said 2" stroke. It's where I got those numbers.

 

Jasonb

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Mine is also mechanically the same with the 1 1/8" bore x 2" stroke. I used 7" Stuart Turner Flywheels.

This shows it in the flesh so you can get an idea of materials used



And all the parts

 

cds4byu

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The thing about the Kerzel is that all I have is a PDF of the plans, which means pretty much every part has to be put into CAD and redrawn anyway. It's relatively easy to scale it at that
I have copy of the Kerzel in Onshape, if you are interested.

Carl
 

cds4byu

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Onshape is a free, web-based 3D modeling system.


It can import and export STEP files, but I haven't played much with the STEP file exchange.

Carl
 

cds4byu

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There are also free plans available for the J&E Junior hit and miss engine, which has a 2 inch bore and a 2.5 inch stroke.


I have taken the multiple individual files that were downloaded, and will be happy to share a combined pdf file, if anyone is interested.

Carl
 

CFLBob

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Onshape is a free, web-based 3D modeling system.


It can import and export STEP files, but I haven't played much with the STEP file exchange.

Carl
Sorry for the delay answering, Carl. Rhino reads STEP files, both *.stp and *.step. The only potential snag is that I'm using an old version of Rhino, so it will only read files compatible with the format back then. All I can think of is that I can test a file to see if it comes across.
 

cds4byu

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Sorry for the delay answering, Carl. Rhino reads STEP files, both *.stp and *.step. The only potential snag is that I'm using an old version of Rhino, so it will only read files compatible with the format back then. All I can think of is that I can test a file to see if it comes across.
Here's a step file of the Kerzel:
 

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