Compression to compression ratio

[mustangsilly]

Is there a mathematical way of converting a cylinder compression reading (say 125 lb) to compression ratio? Any scholars out there that can clue me in? Seems like it should be possible.

Thanks

 

[addo]

You need a whole bunch of specs to get that right.

It's easier to measure the cylinder fill volume at BDC, and at TDC. You can do this with a foot-long clear piece of poly tubing jammed into the spark plug hole, and raised above the motor. The tube must go in exactly to where the spark plug threads end inside the combustion chamber.

Start with the motor at #1 TDC, and pour in a measured amount - say 100CC of auto fluid. Mark the clear tube with a sharpie at the fluid level.

Now turn the crank until it's at #1 BDC. Add more trans fluid - measuring as you go - to reach your original mark.

Amount you just added is the swept volume.

Turn the motor around to TDC again and catch as much expelled fluid as you can. Remove the tube and with a helper holding a rag over the spark plug hole, crank the engine a few times with the coil lead grounded - this will expel much remaining fluid onto the rag. Put the plug back in and hook it all up.

Remove the tubing, close off the end with duct tape and measure how much fluid stays in the tube when filled to the Sharpie mark. Subtract this from your original amount added (where I suggested 100CC) and you have the static volume. Expect it to be between 75CC and 50CC.

To get compression ratio, you next add the swept volume and static volume. Divide this figure by the static volume alone.

 

[66 Fastback]

The answer is yes & no. In engine building terms, the compression ratio is a volumetric ratio. In compressor terms, the compression ratio is a pressure ratio. They are not the same. But you might get an idea of what the engine's compression ratio is or what the cranking pressure could be.

You can use the gauge pressure (psig) from the compression tester, but in order to do so you have to work in absolute pressure (psia). You need to know the atmospheric pressure. At sea level, the atmospheric pressure is about 14.7 psia. So your 125 psig is actually 139.7 psia.
Now taking the ratio of 139.7 divided by 14.7 psia yields a compression ratio of 9.5.

I looked up an atmospheric pressure for Denver (5280 ft elevation) and it is about 12.1 psia. So your 125 psig is about 137 psia. The same math yields a compression ratio 11.3. So you can see that it is important to know the atmospheric pressure.

Typically, an engine will not achieve a cranking pressure equal to the atmospheric pressure times the volumetric compression ratio. At low cranking speeds, you have valve timing and overlap issues as well as reversion keeping you from achieving cylinder fillage. Also the rings and valves may be leaking which would leak off a bit of pressure.
Doug

 

[Bort62]

The answer is yes & no. In engine building terms, the compression ratio is a volumetric ratio. In compressor terms, the compression ratio is a pressure ratio. They are not the same. But you might get an idea of what the engine's compression ratio is or what the cranking pressure could be.

You can use the gauge pressure (psig) from the compression tester, but in order to do so you have to work in absolute pressure (psia). You need to know the atmospheric pressure. At sea level, the atmospheric pressure is about 14.7 psia. So your 125 psig is actually 139.7 psia.
Now taking the ratio of 139.7 divided by 14.7 psia yields a compression ratio of 9.5.

I looked up an atmospheric pressure for Denver (5280 ft elevation) and it is about 12.1 psia. So your 125 psig is about 137 psia. The same math yields a compression ratio 11.3. So you can see that it is important to know the atmospheric pressure.

Typically, an engine will not achieve a cranking pressure equal to the atmospheric pressure times the volumetric compression ratio. At low cranking speeds, you have valve timing and overlap issues as well as reversion keeping you from achieving cylinder fillage. Also the rings and valves may be leaking which would leak off a bit of pressure.
Doug

I think you are making some invalid assumptions here. By your math, My engine would be (160 + 14.7) / 14.7 or 11.8:1

Amazing it runs on 87...

 

[66 Fastback]

You are right, there are some assumptions. But I did state they were not the same. An 11.8 compression ratio is not the same as an 11.8 volumetric ratio. You are assuming that the 11.8 is the same as a volumetric compression ratio which would not run on low octane fuel.

One assumption may be the accuracy of any particular compression gauge. Those gauges may not be the most accurate things and I imagine they may not be reading a true pressure but some sort of surge that gets the spring moving more than it should. Kind of like a water hammer effect. There are a lot of things going on in a compression cycle that are not accounted for such as compressibility factor, gas density of fuel as opposed to air. There are temp changes and phase changes. The gas enters a hot cylinder, so right there, it expands and it is at a temp that is different than atmospheric temp and pressure. After thinking about it some more, one overlooked fact is that the engine suction is operating below atmospheric pressure. I would think that the multiple cylinders are helping generate lower manifold pressure than would not be possible with a single cylinder engine. That would help cylinder fillage.
I know the math does not work out. I have some engines that register 160 psig too. But for gas compressor sizing, that is the compression ratio that this old oil patch engineer works with. You are the rocket engineer, maybe you can tell me what I missed.
Doug

 

[66 Fastback]

OK, I could not resist playing with the gas law PV= znRT
If you assumed that the change in z is small at these pressures and that R is a constant, you can come up with an equation of
P2/P1 = (V1/V2)*(T2/T1)

V1/V2 would be your volumetric compression ratio. You can see that it differs from the gas compression ratio (P2/P1) by the ratio of the temperatures (T2/T1). Again, you have to work in absolute units. Ambient temperature of 60 deg F is about 520 deg R (60+460). When you do a compression test on an engine they say to do it with the engine warmed up. So I'm gonna assume a temp of 180 deg F or 640 deg.

Using as an example, an engine with a volumetric compression ratio of 9.5. Then solving for the gas compression ratio (P2/P1)
= 9.5*(640/520) = 11.7
At sea level, 11.7 multiplied by the atmospheric pressure of 14.7 psia would give you 172 psia. The compression gauge would read about 157.3 psig which would come closer to Ian's engine.

So in order to convert the compression reading to engine volumetric compression ratio, you need to know the cylinder temp.
To find the volumetric ratio given the compression test reading:
V1/V2 = (P2/P1)*(T1/T2)

In the Denver Example, P2 = 125 psig. The atmospheric was 12.1 psia.
V1/V2 = [(125+12.1)/(12.1)]*[(60+460)/(180+460)]

Volumetric Compression ratio = 9.2

Now the formulas passes the reasonableness test.

But you might get an idea of what the engine's compression ratio is or what the cranking pressure could be.

I should have been more clear that you can easily calculate the gas compression ratio, but that it would require a adjustment to convert to a volumetric compression ratio.
Doug

 

[mustangsilly]

Great information! I knew some of the readers on this forum were smart, but I didn't know you were physicists! Thanks for the replies.

 

[Anonymous]

Holy smokes, is this Physics 101? Did I come in late?

I need a beer .......