Scale a Model from a Full Size Engine, for example

When scaling down from a full size engine, or anything else for that matter, not everything can scale down exactly, I appreciate that. My question is, are there any rules or guidelines to follow to get round the bit/s that don't scale reasonably.

For example, if one is want to make a model engine at 1/6th the scale of the full size beast, the bore and stroke would scale down well enough. But if the inlet valve tappet clearance was just 0.006" in real life, at 1/6th it would be just 0.001" (just one thou) and the model would probably not breathe at all at that clearance, 0.004" say would probably be more realistic for the model but is no way 1/6th scale.

So what's the way to deal with it - the bits that don't work? Are there any rules or guidelines to try and ensure the model is as faithful to real life as possible, or does one just use what one thinks would work and hope the result is a suitably realistic scale model?

Chris

Edited By ChrisH on 29/10/2016 14:44:40

By the sound of it you are wanting to make model I.C. Engines. I would suggest you find a book by ET Westbury (I think) entitled something like Model Petrol Engines. In it he details the figures he used to derive valve sizes etc. In general he relates the sizes of various components as a proportion of the bore of the engine, EG valve lift being 1/8" of the bore.

With model cams that may only have 0.040" of lift you need to be carful about clearance otherwise you can loose a lot of the cams duration, have aread of this on Model Engine News

Bob - thanks, will look out for that book, but whilst I highlighted a model engine the same questions applies to all modelling down scaling.

Jason - thanks too, and yes, I did appreciate the possibility of losing a fair bit of the cam's duration with too great a tappet clearance and I know you can allow for it, but the tappet clearance example was just being used to illustrate as clearly as I could how taking scaling too much to the letter can have unwanted results, hence the query how to work around.  I have used that cam calc programme that contains the article you linked to and very good it is too.

Chris

Edited By ChrisH on 29/10/2016 17:15:20

Bob - I have purchased a copy of Model Petrol Engines by ET Westbury, and very good interesting read it is too, still relevant despite coming from an age when pipe smoking model engines went about playing with model engines in sports jackets, flannels, shirts and ties, knitted waistcoats and even double breasted suits!

The book it was only a tenner off Amazon too, so a bonus.

But whilst there is an awful lot of info there I haven't seen anything about sizing relative to bore yet. Maybe I haven't come across that yet but I wonder, could that be in another book? I am still none the wiser as to if any rules or conventions re sizing down exist or whether one just sizes down to a degree and then goes with that engineering standard that says "if it looks right it probably is right" for the rest of the engine.

Someone out there must know the form........

Chris

If your brain is up to it, the anal / professional answer would be dimensional analysis but that's a bit OTT for model engineering. I think the pragmatic answer is that scaling everything precisely isn't going to work on an engine that needs to function.

A good starting point is to examine the units of the critical parameters, reduced to meters, kilograms and seconds (ALL properties can be expressed in terms of these three basic units). IIRC, units that contain differing powers of the basic units don't scale in a pain free fashion. Carburettors must be a good example where the density and viscosity of the fuel don't scale with the linear dimension - and dimensional analysis would probably show you why if you could be arsed to see it through. I'm not claiming to be an expert in this - it's no accident that this stuff is an ancient memory for me now....

The example of the valve clearance is surely an instance where the dimension DOES scale. The coefficient of thermal expansion is constant regardless of size (it's "dimensionless", so the clearance should be in the same proportion as the length of the valve.

Murray

Edited By Muzzer on 03/11/2016 11:20:39