General engine questions: Compression ratio and Displacement

[jdbubb@ix.netcom.com (Dan Bubb)]

T Berk wrote:
> 
> 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.

Aside from the minor point that it's volume and not area this statement
is correct.

> 
> > 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.

Eric's statement is correct. Displacement is "swept volume". ie. it's
the amount of volume swept by the piston and does not include the volume
remaining when the piston is at TDC.
(Bore/2)^2 X PI X Stroke X # of Cylinders=displacement. 

> 
> > 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


Yes! the math is correct.

> >
> > Compression ratio is increased by increasing displacement,
> 
> Eeeerrrrrrt! Stop right there pardner: You can increase the cylinder
> diameter and keep the same compression ratio.

This statement is not correct. If you increase the bore and make no
changes to the combustion chamber (either in the head or the piston
crown) then the CR will increase as a quick look at the equation will
show. If A is bigger and B doesn't change then C will be bigger also.

> If you increase the STOKE
> making it's lower position deeper you will have a corresponding higher
> upper position increasing compression. 

Once again it needs to be pointed out that increasing the stroke without
changes to rod length or piston pin height such that the piston runs
higher in the bore will cause the piston to HIT THE HEAD!
Increasing the stroke, and making the correct changes to rod or piston
to maintain piston to head clearance will increase CR. Again increasing
A with the same B increases C. Note that the CR of a 2.0 ABA is not
necessarily higher than a 1.8 RD because while A is larger B is also
larger (bigger dish in the piston) to maintain the same C.

>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.)

Again, this is not feasible due to the fact the piston is ~1mm from the
head from the factory and reducing this clearance will cause the piston
to hit the head.

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

Yes, this is correct.

> 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?

It's not that more fuel is burned since no matter the compression the
engine should burn all the fuel it inhales. Compressing the mixture more
before ignition causes coorespondingly higher pressures after ignition
which increases the force on the piston and therefore the torque and
power of the engine.

> >
> > 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?

No! Changing CR will have only a small impact on the volume or mass of
fuel and air the engine breathes, so this statement is not correct.

Dan
Wondering if I've pissed off anybody this time.
> 
> TBerk
> 
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