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Compression ratio versus measured compression 19 Jan 2019 07:31 #797075

  • TexasKZ
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In the interest of learning something new and in not hijacking another thread, I want to post my questions here in the hope that some of you with more engineering knowledge than I have can sort me out. The thread that I am trying not to derail is this one - www.kzrider.com/forum/2-engine/609912-19...in-blown-head-gasket
The discussion concerning wet versus dry compression tests has been a fairly common one that leaves me with more questions than answers. To wit-
Typically, when we discuss compression ratios, we are talking about the calculated (rarely actually measured) ratio between combustion chamber volume(with the piston at TDC) and cylinder volume plus combustion chamber volume. Expressed as a ratio, the combustion chamber volume is 1 and the other side of the expression is the total volume of the cylinder and combustion chamber when the piston is at BDC. As 650Ed has so clearly demonstrated, removing volume from the whole has a linear relationship to the calculated ratio.
However, what most discussions here are about is measuring combustion chamber pressure. This, in my mind, is a wholly different kettle of fish.Does removing volume from the combustion chamber have a linear effect on measured pressure? Is there any reliable relationship between measured volume and measured pressure?
Put another way, If we measure 120psi in a cylinder, we would have a pressure ratio of 0:120. To convert this to a more useful expression, we need to include ambient pressure, which our gauges show as 0. If we add 14.7psi (typical sea-level pressure) to each side, then divide by that same amount, we get 9.16:1. Does that mean that every cylinder, no matter its volume or design, will have the same volume ratio and pressure ratio? I am pretty sure that this is not the case, as changing the cam profile will change the pressure readings. Is there a dependable relationship between the two?
I am so befuddled.
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Last edit: by TexasKZ.

Compression ratio versus measured compression 19 Jan 2019 10:43 #797083

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For a given amount of gas there is a direct and linear relationship between pressure, volume and temperature. (Pressure times volume) divided by temperature equals a constant. In math terms PV/T = Constant. Cutting the volume in half will double the pressure. Cutting the volume by 10/11 (In a 10/1 compression ratio cylinder) will increase the pressure to 10 times the original pressure.

Notice also if you have a cylinder containing a volume behind a piston and you increase the temperature one of two things will happen. Either the pressure will increase or the gas behind the piston will expand. The pressure increase is because hotter gas molecules are flying around more energetically and striking the inside of the piston with more force (and more often) causing it to move outwards. If the piston is not allowed to move then the more energetic gas molecules will increase the pressure inside the cylinder.

In an operating internal combustion engine the temperature is more or less constant but it's hard for me to guess what pressure one would measure in a moving piston at bottom dead center. I'm a mechanical engineer but not a combustion expert. Perhaps someone who knows combustion dynamics can provide input.
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Compression ratio versus measured compression 19 Jan 2019 12:17 #797093

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I realize that combustion is going to introduce a bucket-load of variables. At this point, I am still trying to grasp the mechanical side.

I see that once a baseline constant is determined, the effects of changing any one variable can be calculated.

Let's say that we have a cylinder with a total volume of 100cc. Ten are the combustion chamber and 90 in the cylinder itself. We would say that this has a 10:1 compression ratio (100:10). Would we see a cylinder pressure of ten atmospheres (147psi)?

In the garage, I suspect that would not be possible, since pressure does not begin to build until the intake valve closes. Given the same dimensions, a longer duration cam will allow for less pressure build up than will a short duration cam, right?

Looking at it another way, I could take a 10:1 cylinder and install the piston with no rings and have virtually no psi, though it is on paper a 10:1 engine. (Sorry for the tangent.)

Meanwhile back at the topic, I put some numbers to the formula and got these results.

(120psi * 100cc) / 80F = 150

If I increase V by 10% I get a drop in pressure, but only a skosh over 9%.
(N * 110) / 80 = 150
or
150 * 80 = 110N N = 109.09 (9.09%)

Going the other way, a decrease of 10% in chamber volume--

(N * 90) / 80 = 150
or
150 * 80 = 90N N = 133.33 or an 11.1% increase.

Do I have the basic math correct?
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Last edit: by TexasKZ. Reason: bad math

Compression ratio versus measured compression 19 Jan 2019 12:48 #797096

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Volume pressure temperature formula
Boyle's Law, Charles's law, and Gay-Lussac's Law
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Compression ratio versus measured compression 19 Jan 2019 14:37 #797105

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You're right that there are a lot of operational issues in an actual combustion process, and for discussion purposes let's assume temperature is constant, which is probably a realistic assumption for an internal combustion engine operating at normal temperature. In any case this is a first pass assumption just to see if we're in the ballpark. If there truly is a temperature difference then the result would be different but hopefully not far off.

As an aside, temperature in the ideal gas formula is measured in degrees kelvin. This scale has the same increments as celsius but it starts at absolute zero. Zero Celsius is 273 deg Kelvin.

Back to the pressure/volume question. Lets call bottom dead center 1 and top dead center 2. In this case P1*V1 = P2*V2. If you want to know what P2 is you need to know P1, V1 and V2. In your example P1 is 14.7 psi. V1 is 100 cc and V2 is 10 cc. Solving for P2 gives us 14.7*100/10, or 147 psi. This is the mathematical solution that in an operating engine is subject to weak rings, bad valves and carbon buildup on the piston or combustion chamber among other things.

Note that you have compressed 100 cc down to 10 cc, so the volume reduction is 90 cc and the compression ratio as it's normally defined is 9/1, not 10/1.
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Last edit: by hardrockminer.

Compression ratio versus measured compression 20 Jan 2019 06:46 #797133

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hardrockminer wrote: degrees kelvin.

:blink:

Not to derail, but to keep the perfered nomenclature Kelvin is an absolute scale and not stated in degrees.

Thanks for the posting and discussion, brings back some memories of physics and engineering classes. On second thought, those memories were better left suppressed. :pinch:
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Compression ratio versus measured compression 20 Jan 2019 07:04 #797134

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hardrockminer wrote:
Note that you have compressed 100 cc down to 10 cc, so the volume reduction is 90 cc and the compression ratio as it's normally defined is 9/1, not 10/1.


Sorry! but that is wrong.
100cc compressed down to 10cc IS a 10 to 1 compression ratio.
The volume is decreased by 90cc but it is all compressed in to 1 tenth of the original volume.
So ten into one = 10 to 1 compression.
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Compression ratio versus measured compression 20 Jan 2019 10:28 #797145

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Yikes! What was I thinking!
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Compression ratio versus measured compression 21 Jan 2019 10:30 #797195

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Ok, so there does not seem to be a direct correlation between compression (volume) ratio and measured combustion chamber pressure due to differences in cam timing and chamber size, oh, and temperature.

If we wanted to have a measurement of the actual compression ratio, we would need to divide the measured maximum pressure by atmospheric pressure, right?

Or is compression always a measure of volume rather than pressure?
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Compression ratio versus measured compression 21 Jan 2019 10:49 #797197

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TexasKZ wrote: Ok, so there does not seem to be a direct correlation between compression (volume) ratio and measured combustion chamber pressure due to differences in cam timing and chamber size, oh, and temperature.

If we wanted to have a measurement of the actual compression ratio, we would need to divide the measured maximum pressure by atmospheric pressure, right?

Or is compression always a measure of volume rather than pressure?[/quote}

For a given compression ratio there is a direct corresponding PSI measurement range regardless of cylinder capacity.
The PSI measurement should be taken at as high a cranking speed as possible or while the engine is running (on a multi cylinder) to offset the effects of cam timing.

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Compression ratio versus measured compression 21 Jan 2019 10:54 #797198

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Scirocco wrote: Volume pressure temperature formula
Boyle's Law, Charles's law, and Gay-Lussac's Law
I have to learn the laws for my diver's license

en.wikipedia.org/wiki/Gas_laws


Interesting information, but I am not sure how important it is to safely operating a motor vehicle.
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Compression ratio versus measured compression 21 Jan 2019 10:59 #797199

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zed1015 wrote:

TexasKZ wrote: Ok, so there does not seem to be a direct correlation between compression (volume) ratio and measured combustion chamber pressure due to differences in cam timing and chamber size, oh, and temperature.

If we wanted to have a measurement of the actual compression ratio, we would need to divide the measured maximum pressure by atmospheric pressure, right?

Or is compression always a measure of volume rather than pressure?[/quote}

For a given compression ratio there is a direct corresponding PSI measurement range regardless of cylinder capacity.
The PSI measurement should be taken at as high a cranking speed as possible or while the engine is running (on a multi cylinder) to offset the effects of cam timing.


Clearly I am missing something here. If I take an engine with a very short duration camshaft and then install a camshaft with significantly more duration, will I not get a much lower pressure reading the second time as the volume of gas being compressed will be measurably less?

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