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Cylinder volume

Displacement volume doesn't change with different compressions. It is a simple function of bore and stroke.

If you are asking about the cylinder head chamber volume, it does change.

To figure that you need to know the cylinder volume, which is 90 cuin for 360 engines, and 80 cuin for 320 engines.

There are online calculators that can be used to figure the head chamber volume from there. Here is one example http://www.wiseco.com/Calculators.aspx
 
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Displacement is how much volume is compressed. C/R relates to the total volume after displacement / compression. Some of the total cyl volume is not displaced. C/R is a ratio of total volume relative to displaced volume.
 
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Sorry but I disagree.

Displacement is how much volume is compressed./QUOTE]

Displacement is bore times stroke. Also known as "Swept Volume". For multi cylinder engines, it is also times the number of cylinders for total displacement. But, each cylinder/head unit is nominally equal, so it is easier to only talk about a single cylinder. The volume compressed is displacement plus cylinder head volume plus any cylinder volume above the piston at TDC plus head gasket thickness times bore. I am leaving out piston dome or relief to make things simpler.

C/R relates to the total volume after displacement / compression.
???? No idea what you are trying to say.

Some of the total cyl volume is not displaced. C/R is a ratio of total volume relative to displaced volume.
Yep, this.

Good explanation here. https://www.youtube.com/watch?v=8Q83erQRAcU
 
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It's simpler than that

I'm not starting an argument, just giving a different way to think about it/help visualize it for the OP. If somebody wants to argue, great. You're right and I'm wrong.

A piston has a fixed geometry. If it is "taller" high compression piston, that delta height exists at the top and bottom of the piston stroke. Regardless of definition of displacement, swept volume, cylinder volume (the OPs term), etc. the new geometry isn't changing the the fuel/air charge volume.

Want to make this more fun? Stroking an engine will change this and increase torque as well. HP tends to get too much credit around here. Enjoy.
 
I am going to assume the OPs question is actually different than stated. Swept volumes have already been answered. It is possible to calculate the starting and ending volumes with the CR and the swept volume. The relative piston volumes are then trivial.
Assuming my physics molecule still works..:rolleyes:.

Compression ratio (CR) = Starting volume/ending volume
Lets call the compressed ending volume = X
The starting volume will be = Swept volume + X
Lets calculate the ending volume on a 4 cyl 360 @ 7:1

7= (360+X)/X
7X=360+X
6X=360
X=60
Divide by 4 cylinders to get 15 in^3 per cylinder.

8.5 CR gives 12 in^3 per cylinder

9 CR gives 11.25 in^3 per cylinder

The differences in these values is how much volume difference is created by the different pistons. Without knowing the detail piston design, one cannot simply say the 9 CR piston is 3.75 in^3 of aluminum heavier than the 7 CR. Can only say that the surface presented to the cylinder is effectively 3.75 in^3 taller from the wrist pin.
 
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The ratio of the swept volume (top of piston, down) that is crammed into the fixed volume (top of piston, up) defines compression ratio. Higher or lower compression does not change the swept volume. Swept volume is the displacement of that cylinder.
 
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I think you meant to say ‘3.75 DIVIDED BY the cross sectional area of the cylinder taller...’

Not really, but it is implied. It is true if comparing flat topped pistons or piston with identical top shape. Not true if comparing piston tops of different shape. I left the comparison at volume on purpose because that it true in all cases.

Lycomings piston may all be flat. I have not seen 9:1 versions so they are unknown to me.
 
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Can't you just do something like this.

8 to 1
(90÷7)8=102.86(beginning cylinder volume)

9 to 1
(90÷8)9=101.25(beginning cylinder volume)

Not sure just curious.

Brandon
 
One needs the volume of the cylinder at TDC. This is typically done by putting water into the cavity and measuring it. Once that is known, one can easily compute C/R if the bore and stroke are known. Most companies modify CR by raising or lowering the location of the piston pin, relative to the piston top.

If you know the CR, the bore and the stroke, you can calculate the un-compressed volume and therefore deduce the compressed volume at other CR's. However, you can't always assume that modifications to the compressed volume are done completely through changes in pin height without confirming that other changes to compressed volume aren't produced through cavities or protrusions on the piston head. Dishes and domes are common in the auto world. Have no idea if they are used with Lyc's.

Larry
 
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If instead, the OP is looking for clearance volume, the volume above the piston at TDC, the arithmetic is easy. ( Aero Mechanics High School, 1952)

Its Displacement / (CR-1).

So, for an O-360 lyc, 90 cu in / cylinder, CR = 8.5, the clearance volume of each jug would be :
90 / 7.5 = 12 cu in per cylinder.

Ron
 
If instead, the OP is looking for clearance volume, the volume above the piston at TDC, the arithmetic is easy. ( Aero Mechanics High School, 1952)

Its Displacement / (CR-1).

So, for an O-360 lyc, 90 cu in / cylinder, CR = 8.5, the clearance volume of each jug would be :
90 / 7.5 = 12 cu in per cylinder.

Ron

Yes, that's what it boils down to. I was trying to avoid simply saying "CR-1" but rather show a path from root principle.

CR = (Displacement+clearance volume)/clearance volume.
 
Can't you just do something like this.

8 to 1
(90÷7)8=102.86(beginning cylinder volume)

9 to 1
(90÷8)9=101.25(beginning cylinder volume)

Not sure just curious.

Brandon

Yes, if you want beginning volumes.
There are many ways to do the algebra.
 
Some of you are over thinking this. Bore and stroke determines swept volume. Swept volume is displacement. This swept volume does not change with pin height or dome shape. The piston (of whatever shape) clears a path in it's journey from the bottom of the stroke to the top. The volume of the fluid or gas the piston "displaces" in the trip from the bottom of the bore to the top does not change in any way with the size of the combustion chamber. Take the cylinder head off a 250cc lawn mower engine, put the piston on bottom dead center, fill with water, turn the crank until the piston is at top dead center... Collect the spilled water and you will find that it measures 250cc's. The size of the chamber you cram that 250ccs into determines CR, but not displacement.
 
There is a relationship between bore & stroke (displacement), cylinder head volume (combustion chamber), and piston shape in determining CR.

By definition.

BORE X STROKE = DISPLACEMENT

* You can have HIGH or LOW compression with any displacement
* You change displacement by changing stroke or bore.
* You can change CR by changing Bore and/or Stroke
* You can change CR by changing head combustion chamber volume and/or top of piston shape (dome/raised or flat/recessed)
 
Displacement (i.e. piston swept volume) Vs. CR

I think some people are mislead by the meaning of displacement due to the terminology used which is typically "cylinder displacement". It would be more correct to call it "piston displacement" because it is the volume "displaced" by the piston(s) as they move from bottom dead center (BDC) to top dead center (TDC).

Here is a nice diagram showing displacement and cylinder volume:

Basic-piston-and-cylinder-geometry-parameters-of-internal-combustion-engines.jpg


Vd represents the displacement or swept volume and is represented by the cyan colored area between BDC and TDC. Vc is the "clearance volume and represented by the pink colored area. Compression ratio is (Vd+Vc)/Vc.

Skylor
 
Recipe for CR

Lotsa good Engine 101 here, but this post is more to the point of the OP's original query, garbled or not, for a chart relating compression ratio (CR) to some measurement.

Typically our aircraft pistons are flat topped, so one piston can be compared directly with another for effect on CR with one simple dimension -- crown height. That is the distance from the piston pin to the top of the piston.

A difference in crown height for a given application will result in a different CR. We don't need the actual piston dimension to start with since we are only after the changes of dimension verses changes in CR.

For an example, I'll use the familiar Lyc O-360 which has a stroke of 4 3/8"(4.375") and see how much must be added to the crown height to get from, say, stock 8.5 CR to 10.5 CR. Here is an easy formula, or should I say recipe - since in this thread, pistons and biscuits, it seems, are being confused:

clearance = stroke/(CR-1) - This is the formula - simple.

First we will generate this fundamental dimension clearance height or simply clearance for the base engine using the above formula.

Plugging in the numbers from the O-360 engine spec sheet (all inches):
clearance = 4.375/(8.5-1) = 4.375/7.5 = .583"

Next we will do the same for the proposed 10.5 CR engine.
clearance = 4.375/(10.5-1) = 4.375/9.5 = .461"

As you see, the clearance has been reduced by .122". This is the additional piston crown height needed to get from 8.5 to 10.5 CR.

Now you can make your own chart.

Ron
 
Good for you, Bill !!

I am going to assume the OPs question is actually different than stated. Swept volumes have already been answered. It is possible to calculate the starting and ending volumes with the CR and the swept volume. The relative piston volumes are then trivial.
Assuming my physics molecule still works..:rolleyes:.

Compression ratio (CR) = Starting volume/ending volume
Lets call the compressed ending volume = X
The starting volume will be = Swept volume + X
Lets calculate the ending volume on a 4 cyl 360 @ 7:1

7= (360+X)/X
7X=360+X
6X=360
X=60
Divide by 4 cylinders to get 15 in^3 per cylinder.

8.5 CR gives 12 in^3 per cylinder

9 CR gives 11.25 in^3 per cylinder

The differences in these values is how much volume difference is created by the different pistons. Without knowing the detail piston design, one cannot simply say the 9 CR piston is 3.75 in^3 of aluminum heavier than the 7 CR. Can only say that the surface presented to the cylinder is effectively 3.75 in^3 taller from the wrist pin.

A+

Now, lets talk about expansion ratio and why it is directly related to Cr and efficiency on the PV diagram. :D

Next, we will cover variable valve timing to yield an effectively longer power stroke than the compression stroke.
 
Maybe, but why is an ounce of GOLD have more mass (heavier on earth) than an ounce of LEAD?

The answer to this is buoyancy. The denser material is smaller for the same mass, so it displaces less air, so there is less buoyant force, so it will weigh out heavier on earth.
 
The answer to this is buoyancy. The denser material is smaller for the same mass, so it displaces less air, so there is less buoyant force, so it will weigh out heavier on earth.

True. But a much larger effect is that gold is nearly always measured in ‘Troy Ounces’ (12 to a pound) while nearly everything else is measured in traditional ounces (16 to a pound).
Just love the English measurement system.
 
The answer to this is buoyancy. The denser material is smaller for the same mass, so it displaces less air, so there is less buoyant force, so it will weigh out heavier on earth.

Yes, that is certainly at least part of the answer. However, the major difference is actually proof that our process of compressing lead into gold is polytropic. At least, that is what my maintenance foreman, Mr. Troy, here at the mill tells me.

Ron

BTW, what are the symptoms of covid isolation?
 
True. But a much larger effect is that gold is nearly always measured in ‘Troy Ounces’ (12 to a pound) while nearly everything else is measured in traditional ounces (16 to a pound).
Just love the English measurement system.

OK,
troy ounce = 31.1034768 grams
av. ounce = 28.3494 gram
pound = 453.592 grams

Yes, good discussion on this . . . precious metal companies use ounce, troy for all transactions and discussions.

Now, I wonder what the OP really wanted?
 
My Chart

O360 8:5-1 O360 9:1 O360 10:1
4.375/(8:5-1) 0.583 4.375/9-1) 0.547 4.375/(10-1) 0.486


O320 7:1 O320 8.5:1 O320 9:1 O320 10:1
3.875/(7-1)0.646 3.875/7.5 0.517 3.875/8 0.484 3.875/9 0.431
 
Let me add another irrelevant but related comment. Stick with 8.5 to 1.... 9 to 1 max.

Personally I prefer 8.5 to 1 CR.... It has higher detonation margins than 9.0 to 1 CR, and another 10 HP is not going to make my day. I like going faster by reducing drag, the gift that keeps giving.

Going to higher than 9 to 1 CR you are getting into an area where strict operational limits and Av fuel must be adhere to. Add Elect Ign to high CR and you will reduce your detonation margins considerably. I had a piston burn due to detonation on a modified high CR aircraft engine. I respect it. Thus 8.5 to 1 stock O-360 (180 HP) for my daily flyer... Race plane do whatever you want.
 
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