compression ratio...
 

alright so i was reading over my A1 ase study guide
one of the questions:

If you were to bore an engine and did not modify any other parts except for larger pistons for the increased cylinder size; the displacement will ______ and the compression will _______.

a. increase, increase
b. increase, decrease
c. decrease, increase
d. decrease, decrease


at first i thought it was increase and increase
of course the displacement will increase

but if i think about it more i think it would raise the compression also
since we did not do anything to the head the combustion chamber size is the same
more volume in the cylinder + same volume of combustion chamber should = increase in compression.

can i get some peoples thoughts?

alot of my friends thought it was decrease in compression
i dunno im still in between

letter B IMO

its an ez b, think bout it, you have lets say a coke bottle filled with air. now you just made ur bottle bigger with the same air inside. Not sure if im painting the right picture but that same air has more space now thus decressing compression. Now press in the coke bottle, you are decressing the volume in the bottle, and making the air inside more compressed. Blah im just confusing my self, go squeeze on some coke bottles :P

Cylinder Swept Volume

The swept volume of the cylinder indicates how much air the piston displaces as it moves from BDC to TDC. Increasing the cylinder volume without making any other changes will increase the compression ratio because it enlarges the cylinder volume without increasing the combustion chamber volume. In other words, the piston will have to cram more air into the same amount of space. Cylinder volume is calculated using the bore and stroke of the engine with this formula:



On a standard 350 Chevy, the bore is 4.00 inches and the stroke is 3.48. Apply the formula, and you’ll find that one cylinder is 43.730 ci (multiply this times eight cylinders and you get 349.84, which is rounded to 350 for total engine displacement).

If you overbore our sample 350 from 4.00 inches to 4.020 inches and make no other changes, the compression ratio will increase from 8.84:1 to 8.90:1 because the volume of the cylinder has increased. When overboring an engine, the percentage of gain in compression ratio decreases as you add clearance volume and increases as you remove clearance volume, as we’ll describe next.

Bore it...larger displacement...decrease in compression...assuming the piston surface remained the same because if i am not mistaken, the piston surface is a factor when changing around the compression ratio...IMO, same force on the compression stroke with a larger amount of mixture to compress should result in a decrease in compression...I THINK

Cylinder volume and combustion chamber volume are both affected equally when boreing the cylinders...and the stroke of the piston is unaffected so thats confusing....

ask the teacher and get back to us

yea thats what im thinking

source:
http://www.hotrod.com/techarticles/54258/

combustion chamber is in the head not the block so there is no change in combustion chamber volume
only the cylinder changed

i will ask my teacher tomorrow
he said its probably increased but will find out

after reading that article i think it depends on how much you bore out and the clearance volume

the clearance volume is still technically part of the combustion "chamber" but it is different in all engines

Well its the ratio of BDC to TDC.
So you change the diameter of the cylinder, there's no question that the displacement increases.
So the question really is the change in cylinder volume and whether or not its linearly proportional or not.... and it isn't.
the bore is a function of the radius squared. So keeping the stroke the same, TDC and BDC volumes will not change on a linear function. Multiply a bigger bore radius squared and then by the cyl. height at the bottom you get a big number. Multiply the radius squared by a small number, you get a smaller number, but its closer to the volume that you had with the original bore because of the height. Imagine an extra ring of volume created by the new cylinder area outside the original bore, if I was home, I'd draw up the explaination, but I'm at work. Compression INCREASES with INCREASE in bore only.

Displacement increase, compression has to decrease. pressure increase as vol decrease, vice versa, simple as that.

for those that do not know what displacement is:
Dis Vol=Final vol - Initial vol
OR
Dis vol= BDC vol -TDC vol.

as far as linear function or not, you know that the function of a Vol w/ a FIXED height is a parabolic and not linear, Vol(R)=Pi*R^2 *Height, but this is irrelevant.

Vol inc, Press decr. <==a simple thermodynamic law.

Too bad we're not comparing pressure. You either failed thermo or slept through the combustion lecture, Compression is a VOLUME ratio, not a pressure ratio.

you're an idiot.

yes, with the same amount of a gas contained in a larger volume, the pressure will decrease, but that's not the case here. we're talking COMpression (which is a ratio of volumes, it doesn't have units with it! pressure definitely has units such as PSI), and the amount of gas involved changes.

******* before explaining, one point: this problem clearly assumes the air system can supply the extra air to the larger cylinder. *******

boilermaker is right, along with the others above. i'm leaving out constants such as PI to make this simpler, as they don't affect anything. The '=~' sign means it's proportional.

volume(BDC) =~ (bore^2 * stroke) - cylinder volume (A)
+ head volume - combustion chamber (B)

volume(TDC) =~ head volume -(B)


so compression is v(BDC)/v(TDC) i.e. the ratio of wide open cylinder volume to fully compressed: A+B/B (from above)

a little math turns this into: compression =~ 1 + (A/B)

remembering 'A' is cylinder displacement, you'll see that this is proportional, although not directly. increase in displacement MUST cause increase in compression.

if you don't know enough math/physics to understand this, i'm sorry. but please don't argue things you know 'not enough' about.

hehe, i see you beat me to it boiler...i'll post anyway :_doh:

you have to make sure you treat the combustion chamber and cylinder volumes differently.

for example
lets say the combustion chamber vol is 2 and the cylinder volume is 20
the compression ratio is 10 to 1

if you raised JUST the cylinder volume lets say to 22 the compression ratio is now 11:1

even if part of the cylinder is the combustion chamber, lets say 1 for cylinder and 1 from the head the comp ratio still goes up.