General engine questions: Compression ratio and Displacement

[tberk@mindspring.com (T Berk)]

Eric Anastas wrote:
> 
>         I've never really understood what compression ratio is, but I think I get
> it now. Is compression ratio just the ratio between the maximum and minimum
> volume in the cylinder chamber? Like 10:1 would mean there is 10 times as
> much area when then piston is at the bottom then at the top. Is this right?
> 


Pretty much.



> Is displacement the difference in volume between when the piston is at the
> top and when it is at the bottom?
> 

I understand Displacement to be the total amount of fluid (air or water,
etc.) that the chamber will hold at it's largest position: full down
dead center. The difference between TDC and it's opposite isn't used
when calculating displacement. It's just how much air the cylinders
hold.


> With all that: Is this math right?
> 
> A = displacement
> B = volume when cylinder is at the top
> C = Compression Ratio
> 
> C = (A+B)/B = A/B + 1
> 
> Compression ratio is increased by increasing displacement,

Eeeerrrrrrt! Stop right there pardner: You can increase the cylinder
diameter and keep the same compression ratio. If you increase the STOKE
making it's lower position deeper you will have a corresponding higher
upper position increasing compression. If on the other hand you keep the
same stoke but change the height of the top of the piston you have
increaed displacement but not compression. (This is very simplified
btw.) 


> or decreasing the
> area of the cylinder chamber when the piston is at the top (shaving the
> head). Correct? Completely wrong?

Not completely, but the two examples don't act on the same dimensions,
thereby creating two differing results.

> 
> Increasing the compression makes your engine more efficient as it then
> squishes the fuel and oxegen closer together before it is ignited. So more
> fuel is burned. Am I right about this as well?
> 
> eric
> 

Well, I think of it as holding a increased amount of fuel (and of course
air too) in the 'same' area as a low compression motor leading to
greater power. If the increased compression results in a smaller space
than the lower compression motor _then_ you are getting the same power
from less air/fuel, and therefore more efficiency.


mymakinsence?

TBerk