Compression Ratio, Cad Assist

[petertha]

I was validating my compression ratio tonight & took a few screen grabs along the way for your viewing pleasure. The numbers are only meaningful to this particular design, but just demonstrating my approach. Of potential interest is the relative CR impact of either shortcut omissions or by diddling the dimensions just a bit. I was never able to do this without a Cad tool before, but now I can. FYI the target min CR is +9:1 so mission accomplished, the balance can be head shimmed.
Here is the section view. The head has a boss that is slightly submerged in the liner & it has 16mm radius hemispherical combustion chamber. Valve cups are both 10mm OD & 0.25” glow plug, all at 22 deg. Piston deck clearance is 0.8mm.

 

[petertha]

So what I did was reverse engineered the volume above the piston. This pics shows the color coded chunks. 1= the basic hemi shape. 2&3 are the flats of intake/exhaust valves slightly indenting (reducing) volume. 4=the volume add where the glow plug bottom is sits above the hemi ceiling. 5= the squish volume between piston & head flange at TDC. This volume equals 1,058 mm3

 

[petertha]

Then just stick the numbers into simple spreadsheet. Piston sweep volume is 9953 mm3.
**edited**
CR = ( 9953+1058 ) / 1058 = 10.41 : 1.
**edited**
In the yellow bar section I did some messing around & you can see variations.

 

[Till]

Cr = ( 9953 + 1058 ) / 1058 = 10.4

 

[petertha]

Thanks for pointing out the error. I knew that! :wall: I've replaced the calculation. This should teach me not to bang out posts when I should be in bed.

One would think, one should read ones own posts!
http://www.homemodelenginemachinist.com/showthread.php?t=23512

 

[Brian Rupnow]

Petertha--I'm not sure what CAD system you are using, but I find it very easy with Solidworks to park the piston at top dead center, use the "Top down modelling" mode and insert a new part using the top of the piston as the plane for a sketch, then use the "extrude up to surface" or "extrude up to next" option to extrude the new part up to the internal surfaces of the combustion chamber. Then name the new part you have created some name you will recognize, open it in a new window and "break all references", then query the software as to the number of cubic inches in the part. This works really well for non uniform combustion chamber displacements.---brian

 

[petertha]

Thanks Brian, Cad=SW, same here. The engine section in first post is an assembly at TDC position just like you say. But now you have me wondering... Do you mean extrude a new, kind of sacrificial solid projected from piston top, past the ceiling of the head which contains valves & plug etc mated in place. Then some sort of solid body subtraction to yield the net volume remaining? That would be better yet! I've only done basic solid body modeling, but I wasn't sure I could accomplish that with assembled parts. If you have an example workflow, Id be happy to learn.