Up-scaling model engines

[Brass_Machine]

Thinking about up-scaling a model steam engine for grins. May want to make it it 50% bigger. What do people do to upscale it? Say the bore is .5"... does it automatically become .75" for a 50% increase? Or is there a good equation to figure it out.

Help?

Eric

[Bluechip]

Hi Eric

50% increase in what ?

Bore will increase by 50% for sure. But the cylinder capacity will be 2.25 times greater I think ( ?? ) if the stroke is kept the same.

ie Vol = pi * r² * h  will become

pi * 1.5r² * h

Say the original is 0.5" bore by 1" stroke the volume is      pi * 0.25² * 1 = 0.196 cu. ins.

Now 0.75" bore is                                                       p1 * 0.375² * 1 = 0.442 cu. ins.

Which is about 2.25 times larger. ( Should be exactly 2.25, but I've used truncated decimals ..   )

If you want to scale the volume only, keeping the same 1" stroke, I get this rather clumsy equation

New bore RADIUS = sqrt ( ( Scale factor * Original Vol ) / pi )

                        = sqrt ( (1.5 * 0.196) / pi )

                        = sqrt ( 0.294 / pi )

                        = sqrt ( 0.094 )

                      r  = 0.306

So, the new bore would be 0.612 ins. to increase the vol. by 50%.

Which seems to work out ... unusually for me  

Good Heavens ... 23:02 hs ... me bedtime bevvy long overdue .. I'm off ..  


Where's Marv, when you need him most ??????              








Dave BC

[cidrontmg]

Scaling up double: the linear measures will be 2-fold, areas will be 4-fold, and volumes 8-fold. The weight will go 8 times higher, so will swarf production, and the cost of materials (if bought). Machining time, to some surprise, will hardly increase at all, and if the original was very small, might actually decrease. Some structural elements, shafts, bearings, air and oil passages, screws, etc. would be unnecessarily large, to the point of not functioning as intended. Also, tolerances would be best left as is, no scaling there     Scaling up with some fixed ratio might be best, that way the raw materials would be better obtainable. I´ve done some builds that way, when the original seemed like watchmaking.
 

[John-Som]

Hi Eric

All my recent builds have been upped by 50%. It really is as simple as increasing all measurements by 50%. Two engines in particular, Elmer's mill engines #32 and 33 were built in this way and they both run beautifully. Obviously material costs increase but other than that I have not encountered any problems.

John-Som

[Brass_Machine]

Hi Eric

All my recent builds have been upped by 50%. It really is as simple as increasing all measurements by 50%. Two engines in particular, Elmer's mill engines #32 and 33 were built in this way and they both run beautifully. Obviously material costs increase but other than that I have not encountered any problems.

John-Som

So how did you determine the piston bore/stroke?

Eric

[John-Som]

Eric
Sorry to be so long in responding to your question Eric but I stand by my original statement that it really is as simple as increasing all measurements by 50% and this applies just as well to bore and stroke dimensions. Like you I was sceptical about the simplicity of it all but I have now built two of Elmer's finest working om this simple principal.

John S