Compression ratios? I don't get it....

[HardDrive]

I don't understand how you can increase the compression ration in an engine by changing the piston heads. If you increased the 'height' of the piston (the distance from the axis of the connecting pin to the piston surface), doesn't this decrease the overall volume of the engine?

I don't get how you can effect the compression ratio without changing the stroke of the crank.

[cgarr]

Its all about the Combustion Chamber Volume in a NA engine. I just went from a 66cc volume to a 59cc or a 9.5 to a 10.5 cr Here is something you can play with: http://www.csgnetwork.com/compcalc.html

[Jeff Higgins]

When the piston is all the way down at bottom dead center, any increase in dome height will in fact reduce the volume above the piston. The trick is that it reduces that volume by a lesser percentage (because it is a far greater volume) than it does at top dead center.

Let's say a flat top piston when at TDC still has a combustion chamber above it that measures 100cc's. Put a dome on that piston that reduces that combustion chamber to 50cc's, or half. The dome must be 50cc's. Now drop that piston to BDC and let's say it's a 500cc cylinder. The downward motion of the piston "displaces" 500 cc's in addition to that 100cc combustion chamber - that's what we mean by "displacement". So with a flat top you have 600cc's of airspace above that piston. Run that piston all the way back up to TDC and you have crammed 600cc's of air into 100cc's - a 6:1 "compression ratio".

Now put our 50cc dome on that piston. At BDC, you still have 550cc's of air space above it. Now run it up to TDC, where there is only 50cc's of space because of that dome. Ta-da: 11:1 compression. Pretty dramatic change. Of course going from 6:1 to 11:1 is rarely done in real life, but the numbers serve to illustrate what happens here. Make sense?

[Gordo2]

Your engine has a gas and air mixture in the cylindar that it compresses just prior to ignition by the spark on the combustion stroke.

The higher your piston raises in it's stroke, the more the gas/air mixture is compressed. The more it is compressed the greater the pressure of expansion will be when it is ignited. The greater pressure of expansion/pressure against the piston the greater the power.

A raised dome piston/higher compression ratio essentially compresses the gas/air mixture to a greater extent (it leaves less space/volume between the piston and the head pocket when combustion/spark occurs).

Make sense?

Gordo

[wholberg]

Here is a crude (an artist I am not!) drawing of a cylinder and piston at TDC. The cylinder on the left has a flat top piston and the one on the right has a domed piston. Assuming that the bore and stroke are identical, you can see that the displaced volume containing the air-fuel mixture is compressed into a much smaller space on the right side. This is how a a "high compression" piston works.

[ianc]

Not much to be said after wholberg's piece de resistance, except: you can see how the shape of the piston influences the final size of the compression chamber radically at TDC. But at the bottom of the stroke, percentage wise, the change is actually very small. Or did someone already say that?

ianc

[HardDrive]

Wow. Thanks guys! Clear as bell.

I was missing the point that the increased ratio at the top of the stroke will be much more dramatic than the corresponding decrease in volume at the bottom of the stroke.

This board rocks.

[wholberg]

Originally posted by HardDrive
Wow. Thanks guys! Clear as bell.

I was missing the point that the increased ratio at the top of the stroke will be much more dramatic than the corresponding decrease in volume at the bottom of the stroke.

This board rocks.

There actually is no decrease in volume at the bottom of the stroke. Total displacement is calculated using only the bore and stroke. It does not matter if the combustion chamber in the head is 50 cc or 500 cc, or what the shape of the piston top is, the amount of air the piston displaces does not change

[Wil Ferch]

Correct.... I don't think you're "seeing" the right picture....

Try it this way...look at the picture shown and focus on the piston wrist pin ( the piston "hinge" on top)...

The "displacement" or size of an engine is a combo of the piston diameter ( bore) and the amount it moves up ( or down.. if you prefer)....caled the "stroke". Total length travelled in one direction is the stroke.....

The wrist pin will help you see that the total travel distance up (or down) is the same for both cases..the case where the top is "squished" more and the case where the top is not so squished.

- Wil

[randywebb]

and here's another try...

- the shape or size of the piston is the same no matter where it is -- at the top or at the bottom. You are going to subtract the two so that would drop out if you do it mathematically...

[Huh]

Try reading this short article. Does this help?

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

Basically says what the other guys have been saying.