Scaling engines?

[Alex]

When scaling down engines, is it essential to make the ports a little bit larger (in proportion to the other dimensions)? I read in a steam book that you can't just take plans from a full size locomotive and decrease every dimensions by the same amount. That's because you don't scale down the molecules in the steam.

I have a plan of a little oscillator (½" bore) that I would like to make 2,5 times smaller to suit the metric standards better. Do you think it would be ok to make the ports 2,5 smaller aswell?

 

[Alex]

Well, what I was trying to ask was if small passages can be scaled by the same amount as the for example bore, stroke etc. Maybe it doesn't matter at all when we are talking only 2,5 times smaller.

Anyway, thanks for the tip!

 

[Tin Falcon]

Alex:
I assume you are talking about the passages that feed steam from the engine frame to the piston?
For an engine that size I think you will be fine not changing the port size.
my concern with changing the sizes in the ports of an oscillator is that they act as the valving. the hole sizes are determined by the stroke of the engine in proportion to the distance from the pivot to the valve ports. Enlarging the valve port holes in proportion to the rest of the engine will likely cause blow by and therefore cause the engine to run poorly, or possibly not run at all.
Tin

 

[chrsbrbnk]

Smaller ports or pipes pass a lot less than scale due to drag and stuff in the pipe ect. so that say every square inch in a 6" pipe would be passing much more air than a 2" pipe at the same pressure . Also fly wheels and steam whistles don't scale that well

 

[Alex]

Thanks guys for your inputs. Tin, 1"= 25,4 mm. Just in case so you don't end up with a wierd engine if you decide to make one from a metric plan. :wink:

 

[Tin Falcon]

Alex:
Yes I know, did not catch the mistake at the time. I was thinking about it in the shower last night. the number was transposed and is now corrected. . The point of what I mentioned is not a true unit conversion but it allows an engine to be built with a set of prints of one system with a pile of stock from the other. True conversion is not a real problem as far as crunching numbers. Calculators , modern digital calipers, and spread sheet programs as well as published charts, make true conversion real easy. the problem is the numbers do not come out even.
As far as the port size in a scaled down engine the area of the port is not reduced as much as the volume of the cylinder. For example if you are filling a 55 gallon drum though a 6 " pipe. then scale the system so you are using 2 inch pipe. The pipe diameter is 1/3 the original size the drum would only hold 2.4 gallons the volume is now 1/27th of the original and the area cross section of the pipe is 1/9 of the original. So doesn't logic dictate that the drum would fill faster?

Tin