Engine Specs

[Kpar]

Hi All, regarding engine specs, is it just a matter of changing specs to a half size or are there other issues to consider.
The Henry Ford's Plumbing engine I built runs well and now I'd like to have a go at making a smaller size one and change to a hit & miss.
Any comments/advice would be much appreciated
Kpar

 

[gbritnell]

Hi Kpar,
Yes there are other things to consider. If the original design had heavy enough wall thicknesses then scaling it down isn't an issue but if they were on the thinner side then you would have to compensate otherwise they would be too thin. As far as nuts and bolts then don't always scale exactly. For example a 10-24 is .190 diameter, half of that would be .095. A 3-48 is .099 which is close but some others might not be so you would have to pick the next closest size.

 

[stevehuckss396]

Gear mesh is another thing to consider. Depending on the percentage of reduction, you may need to shift things around to get the new gears to work.

 

[methuselah1]

When you scale an engine, power diminishes in proportion to the CUBE of the scaling factor. However, nature cannot be scaled, and builders of perfect scale engines start to have trouble with camshafts twisting, 'cos of the springs!

For any other aspect of a small engine, if the prototype was strong enough, the model will be more so.

 

[Kpar]

Thanks for the advice. As I said I'm going to scale my Henry Ford Engine down to about half. I have watched a work in progress from a late member, Chuck Fellows and will be following some of his build and hopefully changing it to a hit & miss.
One thing that I'm concerned about is the governor build which will be new to me.
Kpar

 

[GailInNM]

George and Steve both make good points and both are very accomplished builders of small IC engines.
From an engineering standpoint there are three main types of things to consider when scaling down.

First there are things that do not change. Atmospheric pressure, compression ratio and voltage breakdown for example. Atmospheric pressure and compression ratio combine to give about the same cylinder pressure on both the compression stroke and power stroke. If you scale the spark plug the voltage breakdown of the insulator may become and issue. This will be compensated by the reduced spark gap, but the gap needs to be at least 0.020 inch to keep plug fowling down and to allow proper ignition.

Second on the list are things that mostly vary with the square of the scale factor. That is for a scale of 1/2 the factor is 1/2 X 1/2 or 1/4. High on the list is drag on the pistor in the cylinder. If the original engine uses an O-Ring for sealing the piston then the wall pressure will need to stay the same as the cylinder pressure remains the same. The contact area will reduce by 1/2 as the circumference is 1/2 and the friction is reduced by 1/2 again as the stroke is also 1/2. By replacing the O-ring seal with a lapped cast iron piston and cylinder the friction can be reduced in a major way. Plain sleeve bearings also follow the square rules as the circumference is also 1/2 and the length is 1/2. As the loading is less it is a bit better than this, but this is negated quite a bit as the RPM of small engines is usually higher. Replacing any sleeve bearings on rotating parts with ball bearings. These include the main crankshaft and cam bearings. Since the Ford engine has a single overhung crank it would be relatively easy to put a ball bearing in the connecting rod big end.
So the good news is most of the friction losses decrease by a factor of 4 if we make no improvements. But the bad news is that the available power is decreased by a factor of 8, as methuselah1 mentioned, as the displacement follows the cube rule. Not much can be done about this unless the scale factor is modified to slightly increase the cylinder bore or stroke. Changes in the TRIM
he bore is more effective as it follows the square rule while the stroke is linear as it applies to the displacement. Also it is does not change the visual appearance as much as changing the stroke.

All of the above are approximations as there are many other factors that will apply in as least a minor way, but it will give you some guide lines for areas that can help you be more successful.

 

[Kpar]

Wow, will have to sit back and read it a few times for it to sink into my old head. But all good advice and I have been thinking on using bearings instead of bushes. I'm not much for designing or reading complicated plans but so far pleased with what I have done in the past.
Kpar

 

[Vietti]

Not sure I buy the 0 ring vs cast iron ring difference.

No offence.

John

 

[GailInNM]

No offense taken John. I feel that open discussion of differing opinions is one of the best ways for all of us to learn.

I have used O-rings quite successfully on engines with larger bores of 3/4 inch and up. But, for the 3/8 inch bore engines that I have been building for the last 10 years or so I find the lapped piston and cylinders are much superior. I can detail my experiments and results a little bit later but am leaving for family gathering now.

Kpar: Since you are planing on Hit & Miss operation here is a short video of one of my 3/8 bore X 1/2 stroke H&M engines in a Gauge 1 (45mm) locomotive. It is running at 3000 RPM and httting about 1 out of 10 or 12 revolutions. Video is about 9 years old.


[ame]

Details of the H&M version can be seen here:

https://www.homemodelenginemachinist.com/threads/tiny-i-c-engine.7397/page-16
Although it is a long thread there is a lot of useful information about 3/8 X 1/2 IC engines and their construction in it so it might be a useful read in your spare time.
Gail

 

[Vietti]

Gail,

Had no idea you were talking about such a small bore! I'm going to read your entire thread and maybe try to build a small one it. Smallest bore I've built is 3/4" and had more success with 0 rings although it ran on CI rings. Better oil control and compression.

As I age it's more pleasant to build smaller engines esp when going to shows.

John