Calculating compression ratio from differences in volume

[digiex-chris]

3d CAD is amazing sometimes, but sometimes it makes me start from unreasonable assumptions which then makes me question my math. Right now, it's throwing off my estimation skills by being so far out of the ballpark I must have messed up with my calculator.

I can easily determine the differences in volume between the piston at TDC with the valves closed, and BDC with the valves closed. The volume of the air gap at TDC is 0.021 cubic inches, and at BDC it's .807 cubic inches.

0.807/0.021 = 38.42 as a ratio.

Is that a good definition of compression ratio in 4 stroke? If so, I wasted a lot of time trying to make things not hit other things while minimizing gap. I'm kinda happy about that because I would have spent 3 years making that piston ;D

 

[Cogsy]

Your math is right so if you figures are right then your compression ratio is also right. And yes, it does seem a little 'over optimised' .

 

[barnesrickw]

I'm not sure about small scale engines, but a car with what is considered high compression has a ratio of about 11:1. What you describe is in the diesel range. Large, twelve cylinder monster diesel. But like I said, I don't know much about the scaling of compression ratio.

 

[barnesrickw]

\mbox{CR} = \frac { \tfrac{\pi}{4} b^2 s + V_c } {V_c}, where
b\; = cylinder bore (diameter)
s\; = piston stroke length
V_c\; = clearance volume. It is the volume of the combustion chamber (including head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches top dead center (TDC). Because of the complex shape of this space, it is usually measured directly rather than calculated.

 

[digiex-chris]

Thanks everyone, back to the drawing board!

 

[Brian Rupnow]

Just make the combustion chamber slightly taller or shorten the con rod a bit. No other changes needed.

 

[Jyman]

Like Brian said, if you shorten the connecting rod it would decrease your compression a lot. And it's the easiest to modify


Sent from my iPhone using Model Engines

 

[digiex-chris]

In those case I worked hard on making a domed piston and cutouts for the valves. If I go with a flat top I think it solves the problem and avoids 4 more setups and operations.

 

[petertha]

maybe useful?
http://www.homemodelenginemachinist.com/showthread.php?t=23512
http://www.homemodelenginemachinist.com/showthread.php?t=23705

 

[digiex-chris]

Heh yeah that's exactly what I did. Subtracted the assembly from a slug at tdc and bdc. Worked well, but the results pegged my BS meter. Guess its accurate.

 

[Gedeon Spilett]

you are calculating a volumetric ratio, a true compression ratio requires some thermodynamics in it , depends on temperature etc... I did that by measuring the volume of light oil needed to fill the cylinder head, to cope with the complex shape of this part.

 

[digiex-chris]

And valve overlap and flow probably has something to do with true compression too, right?