Scaling Plans

[ShedBoy]

I am considering making a larger version of a two stroke engine I have a plan for. I want to make the bore and stroke double. Is it as easy as doubling all measurements?
Will the ports end up in the same/ right position?
Will the ports end up the right size?

I seen a picture of a super size Cox.049 which got me thinking.

Brock

 

[gbritnell]

Brock,
When an engine is designed the creator tries to make all the dimensions fit the engine that he is developing. By that I mean he uses bolts and screws that are large enough for strength but not so large as to look out of place. He makes metal thicknesses adequate for strength but not heavier than needed. When you get into port dimensions and compression ratios you're dealing with square inches and cubic inches which change exponentially when enlarged or reduced.
Can you scale and engine up or down? Sure you can but a lot of the features need to be altered to make it practical.
gbritnell

 

[Mainer]

As I understand it, you can always scale down and be safe from a strength point of view, but you can't scale up and be assured it will still be strong enough. This situation results from the area increasing by the square, but volume increasing by the cube, as mentioned by gbritnell.

 

[Ken I]

If you were to take a 2 litre engine and scale everything up 2:1 - assuming mean effective pressure and max. piston velocity remain the same.

Then you get a 16 litre engine which puts out 8 times the torque at half the revs so you get 4 times the power - the specific power per litre is thus halved.

(Hence the use of lots of smaller cylinders for performance - the specific output is higher.)

There are all manner of structural problems related to 3rd and even 4th powers (polar moments etc).

As Mainer stated scaling down generally doesn't present (too many) problems but on scaling up you need to be careful.

Ken

 

[ShedBoy]

Thanx all for the answers. I thought it would be a bit more involved than just doubling. I understand that fastener sizes and material types would need to be changed to suit the larger engine but I was more concerned about port sizes and timing at the moment. Lots more investigating to do. Doing lots of reading at the moment about two stroke design from the 1970s by Cesare Bossaglia. Mint book with lots of info which takes me a bit to get my head around at times.

Any one scaled any type of engine up before?
This is the Cox which got me thinking.
Brock

 

[nsfr1206]

Like that pic. Looks like a babe bee. I've got one of them somewhere along with lots more without tanks.

 

[ShedBoy]

Complete line up, beautiful work. Your right it is a Baby Bee
Brock