smart51
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| posted on 1/11/06 at 03:16 PM |
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compression ratio vs compression pressure
I'm confused. The manual states that my engine has an 11.8:1 compression ratio. What I take this to mean is that the volume of the combustion
chamber and cylinder when the piston is at BDC is 11.8 time greater than when it is at TDC. at 14.5 PSI ambient air pressure then maximum compression
pressure should be 172 PSI.
The manual also states that a compression test should read 206 PSI nominally with 192 - 220 PSI being the acceptable range.
Where does the extra pressure come from?
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02GF74
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| posted on 1/11/06 at 03:30 PM |
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if fuel is getting in, it is incompressible (small droplets) so that will in crease CR.
Also engines can be designed with ram air effect (damn it, cant remember what the correct term is) but I doubt you will see mcuh of that when
cranking the engine over. Dunno.
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JAG
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| posted on 1/11/06 at 03:32 PM |
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It's all to do with the valve timing and the fact that air moving at speed will increase in pressure when it slows down or stops moving - called
stagnation pressure and it's how superchargers and turbochargers generate a lot of their pressure.
So the speed of the airflow into the engine and the valve timing CAN generate extra pressure when the engine is spinning over at cranking speed - say
during a compression test 
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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smart51
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| posted on 1/11/06 at 03:38 PM |
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With a 14:1 air fuel mixture by weight, the 0.028cc of fuel per cylinder would increase the pressure from 171.8 PSI to 171.97. It isn't
that.
[Edited on 1-11-2006 by smart51]
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mookaloid
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| posted on 1/11/06 at 03:40 PM |
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Heat also I guess
If you compress the air rapidly it heats up causing extra pressure?
Isn't this how diesels work?
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smart51
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| posted on 1/11/06 at 03:43 PM |
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Justin, There'd have to be 20% more air+fuel in the cylinder when the valves close than there is volume to get up to 206 PSI. That would be
quite a trick if you could do it.
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smart51
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| posted on 1/11/06 at 03:56 PM |
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quote:
Originally posted by mookaloidIf you compress the air rapidly it heats up causing extra pressure?
Boyle's law
If the temperature went up by 56°C then the pressure would read 206 PSI. I guess that's part of why you should do the test with a hot engine -
so the cold cylinder doesn't suck temperature out of your mixture.
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Confused but excited.
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| posted on 1/11/06 at 04:05 PM |
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The difference is due to the thermal effects of fast compression.
Using the straight mathematical calculation using the swept volume of the cylinder ( neglecting losses past the rings) igives a theoretical
compression and assumes no heating effect and is referred to as isothermal compression.
In reality the compressed gas is heated (work generates heat) by the act of compression. This is referred to as adiabetic compression . To get the
true figure, you multiply the mathematical derived value by a frig factor. The value of which I am buggered if I can remember. I think it is 1.14 or
1.4, some thing like that anyway. Further research will confirm.
As stated previously, engine temp will make a major difference also.
[Edited on 1/11/06 by Confused but excited.]
Tell them about the bent treacle edges!
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snapper
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| posted on 1/11/06 at 04:13 PM |
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I think i have it, but this is just a wild guess.
Combustion chamber in Cubic centimeters, lets say 20ccm for a 1000cc, times by 0.06 to get cubic inches, 1.2 in this case, multiplied by 14.5 (also
quoted at 14.7) ambient air pressure multiplied by compression ratio 11.8 equals 205.32.
the cubic inch factor of 1.2 accounts for the 20% error
Oh i am so clever for an uneducated
secondry school boy.
Or am I
I open it up to the more qualified
[Edited on 1/11/06 by snapper]
[Edited on 1/11/06 by snapper]
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Alan_Thomas
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| posted on 1/11/06 at 04:38 PM |
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quote:
Originally posted by mookaloid
Heat also I guess
If you compress the air rapidly it heats up causing extra pressure?
Not unless they changed Physics in the last 30 years
P1xV1 / T1 = P2xV2 /T2
So if the volume decreases the temp goes up and the pressure increases but not as much as if the temp did not increase.
- Alan
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smart51
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| posted on 1/11/06 at 05:52 PM |
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No, that's not right. using boyles law (P1 V1 / T1...) If the volume decreases from 250 to 25 (say), then the pressure would multiply by 10,
if the temperature stayed the same. If the temperature went up by 20% at the same time, to balance boyles law, the pressure would have to go up by 10
x AND 20% as well. The 10x is just due to the volume and the 20% must be due to the work done by compressing it quickly.
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MikeRJ
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| posted on 1/11/06 at 06:38 PM |
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quote:
Originally posted by Alan_Thomas
quote:
Originally posted by mookaloid
Heat also I guess
If you compress the air rapidly it heats up causing extra pressure?
Not unless they changed Physics in the last 30 years
P1xV1 / T1 = P2xV2 /T2
So if the volume decreases the temp goes up and the pressure increases but not as much as if the temp did not increase.
- Alan
Schoolboy error alert!
If the temperature increases then the pressure has to increase more in order to balance the equation!
The ideal gas law states that for a fixed quantity of gas, pressure is proportional to T/V, i.e. is temperature goes up or volume decreases pressure
is increased. If both happen, pressure is increased more.
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liam.mccaffrey
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| posted on 1/11/06 at 06:51 PM |
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MikeRJ is correct
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britishtrident
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| posted on 1/11/06 at 09:01 PM |
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As already pointed out the compression process isn't an ideal process and just to add to the confusion Air is far from an ideal gas it is a
mixture of gases and vapour.
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ecsjwhi2
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| posted on 1/11/06 at 10:07 PM |
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Just to cloudy the waters a little.
Interesting discussion this one but (IMHO) points are being missed.
The initial volume (or mass) of air which is being compressed is unknown.
The volume of air is a function of swept volume, port efficiency, air velocity, air compressibility, the closing of the intake valve. Valves do not
close at BDC because air continues to flow into the cylinder whilst the cylinder is just beginning the upstroke.
Put simply, a 500cc cylinder does not compress 500cc of air
In order to do the calculations you need to know the mass of air in the cylinder and the dynamic compression ratio of the engine neither of which are
being discussed.
Just my two penn'orth.
Cheers
JohnW
[Edited on 1/11/06 by ecsjwhi2]
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MikeRJ
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| posted on 2/11/06 at 09:54 AM |
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quote:
Originally posted by ecsjwhi2
Put simply, a 500cc cylinder does not compress 500cc of air
Not at cranking speeds anyway. Volumetric efficiencies of greater than 100% on a normaly aspirated engine are possible though!
However, the lower volumetric efficiency during cranking does not explain why the OP's reading were higher than expected, a smaller amount of
air would give a lower pressure. I do agree that working out CR simply from cylinder pressure is a most inexact procedure. You can get a rough idea
but there are far too many variables to make it accurate.
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dnmalc
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| posted on 4/11/06 at 10:12 PM |
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The equations that you are quoting PV/t are not quite correct they are for a perfect gas. For a normal gas you get a different equation this is what
"confused but excited" refers to. From memory its
(P1/P2)=(T1/T2)**1.4
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