Author Topic: Expected Cylinder Pressure  (Read 1516 times)

Offline SmithSwede

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Expected Cylinder Pressure
« on: December 17, 2021, 06:45:01 PM »
Is it true, or at least roughly correct, that expected cylinder pressure is just the product of the compression ratio times the atmospheric pressure? 

For example.  Assume atmospheric pressure is 14.7 psi.  Your engine has a 9:1 compression ratio.  Do those facts imply that a perfect condition engine would generate 132 psi on a compression test?

Maybe there’s a fudge factor where you deduct 5 to 10%?
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Online Huzo

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Re: Expected Cylinder Pressure
« Reply #1 on: December 17, 2021, 07:09:18 PM »
I will await possible correction on this, but here’s what I would think.
As your cylinder is being fed during the intake stroke, it is 14.7 psi (as per example) that’s stuffing the atmosphere in.
But the air will only travel down the tract if there is a differential, so I’d guess that the pressure in the cylinder at the time of closing of the valve, would be significantly less than atmospheric.

However, I’d think that the pressure at full compression would be expressed as the instantaneous pressure at BDC x ratio of the volume of the chamber at BDC to the volume at TDC.
If in a 500 cc swept volume cylinder, you had a combustion chamber volume of say 50 cc, you’d have 550 cc compressing into 50 cc.
So that’d be 11:1

If your instantaneous pressure at BDC was say 12 psi, then your max compression figure would be 132 psi..

Anyway, that’s my guess... :popcorn: :popcorn: :popcorn:

Offline guzzisteve

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Re: Expected Cylinder Pressure
« Reply #2 on: December 17, 2021, 07:20:18 PM »
WOW, you freeking LOST me!

No, it sucks and goes bang.
« Last Edit: December 17, 2021, 07:21:37 PM by guzzisteve »
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Offline SmithSwede

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Re: Expected Cylinder Pressure
« Reply #3 on: December 17, 2021, 08:10:17 PM »
I think we agree. 

Putting aside the obvious Van de Graaff effects and adjusting for Köppen climate change precessional perturbations.

Expected psi is just the product of resident air pressure and mechanical compression ratio. 
Accentuate the positive;
Eliminate the negative;
Latch on to the affirmative;
Don't mess with Mister In-Between.

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Re: Expected Cylinder Pressure
« Reply #3 on: December 17, 2021, 08:10:17 PM »

Online Huzo

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Re: Expected Cylinder Pressure
« Reply #4 on: December 17, 2021, 08:32:30 PM »
WOW, you freeking LOST me!

No, it sucks and goes bang.
Oh stop it now Steve....
You’re just being characteristically modest... :kiss:
BTW..
It’s freaking....
« Last Edit: December 17, 2021, 08:48:25 PM by Huzo »

Online Huzo

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Re: Expected Cylinder Pressure
« Reply #5 on: December 17, 2021, 08:59:10 PM »
At the RPMs normally used for a compression readings , an engine with a 10 to 1 CR and a short duration low lift cam will normally show readings higher than said engine with a high lift long duration cam . The opposite is also true at high RPMs , a camshaft with bumpier specs will produce higher readings , or greater BMEP . It's what we used to call "coming on the cam" . The static pressure at BDC in an IC engine is almost never atmospheric pressure, it is normally higher , although with extreme camshaft specs it can actually be lower . A static CR of 10 to 1 in a well set up IC engine will normally produce static high pressures in the 170 PSI range , not the 147 predicted by multiplying static atmospheric pressure by 10 . And no we aren't breaking the laws of physics , only applying them in the conditions existing in the IC engine .

 Dusty
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Offline lucky phil

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Re: Expected Cylinder Pressure
« Reply #6 on: December 17, 2021, 09:00:10 PM »
Is it true, or at least roughly correct, that expected cylinder pressure is just the product of the compression ratio times the atmospheric pressure? 

For example.  Assume atmospheric pressure is 14.7 psi.  Your engine has a 9:1 compression ratio.  Do those facts imply that a perfect condition engine would generate 132 psi on a compression test?

Maybe there’s a fudge factor where you deduct 5 to 10%?

There are variables including cam timing as someone mentioned ( noticeable if you change from a street to a race cam in general terms and why "hot rodded" engines to use another loose term have that lumpy idle because they now have lower cranking and idle speed compression) but there is also the pressure increase you get from the heating of the air charge due to compression. The Adiabatic compression temperature rise is also a factor that needs to be included. Thats why the figure is often in excess of the calculated.

Ciao
« Last Edit: December 17, 2021, 09:03:25 PM by lucky phil »
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Offline guzzisteve

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Re: Expected Cylinder Pressure
« Reply #7 on: December 17, 2021, 09:29:05 PM »
Oh stop it now Steve....
You’re just being characteristically modest... :kiss:
BTW..
It’s freaking....
Does this mean if there low pressure in forecast you can have less CR? Wow, who knew.
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Offline SSGG Geezer

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Re: Expected Cylinder Pressure
« Reply #8 on: December 17, 2021, 10:22:04 PM »
Does this mean if there low pressure in forecast you can have less CR? Wow, who knew.
Nope, not really. Just denser air for more potential power per stroke.

Offline Kiwi_Roy

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Re: Expected Cylinder Pressure
« Reply #9 on: December 17, 2021, 10:44:41 PM »
I'm more use to seeing about 160 PSI
Don't forget when you compress the air you also heat it up resulting in a higher pressure.
And don't forget to open the throttle wide so it has a full cylinder to compress.
I dont think its quite as simple as atmospheric pressure x compression ratio but I don't know what
To get my 160# that would be 160 / 14.7 or 10.8:1, I don't think any of my bikes are that high. Doesn't the pressure go up in several steps making me think some of the pressure is remaining from the previous compression stroke?
« Last Edit: December 17, 2021, 10:51:56 PM by Kiwi_Roy »
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Online Huzo

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Re: Expected Cylinder Pressure
« Reply #10 on: December 18, 2021, 12:36:19 AM »
Does this mean if there low pressure in forecast you can have less CR? Wow, who knew.
Well I guess the ratio won’t change, but the mass of air intake will.

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Re: Expected Cylinder Pressure
« Reply #11 on: December 18, 2021, 07:19:58 AM »
 To my way of thinking, compression readings done with a gauge at engine cranking speed is a diagnostic tool only ...It may not reflect what goes on at engine running rpm..Most guys using a gauge will crank the engine until the gauge shows no more pressure rise...When the engine is running the piston only get one shot at compression that happens at what, 30 times a second at normal highway speeds?
  My 650 Triumph LSR bike crowds 60 hp at the rear wheel, it has moderate cams and compression, 10.5 . With plugs removed, kick the engine until the pressure stops rising shows only 130 psi...It leaks a bit but at higher speeds there is far less time for a leak to lower pressure..





 

Offline guzzisteve

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Re: Expected Cylinder Pressure
« Reply #12 on: December 18, 2021, 08:41:18 AM »
Well I guess the ratio won’t change, but the mass of air intake will.
Thanks, I never thought about this stuff, AND I was lit last night. Could you tell? I just make sure valves have clearance.
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Offline dxhall

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Re: Expected Cylinder Pressure
« Reply #13 on: December 18, 2021, 12:56:47 PM »
A compression gauge is a diagnostic tool only because absolute readings will vary depending on cranking speed and temperature.

Compression numbers are significant only if (1) there’s a big difference between cylinders, or (2) the numbers are really low, like 50psi or something.  Is a motor which shows 150psi on one day in better shape than another motor which shows 130psi on a different day?  I don’t think you can say that.

Offline SmithSwede

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Re: Expected Cylinder Pressure
« Reply #14 on: December 18, 2021, 01:14:58 PM »
  It would be better if there was any money in this knowledge , sadly there isn't .

 TS

Dusty, how’s about I buy you a cup of coffee and a donut of your choice?   Totally in compensation for you sharing such arcane knowledge
« Last Edit: December 18, 2021, 05:20:54 PM by SmithSwede »
Accentuate the positive;
Eliminate the negative;
Latch on to the affirmative;
Don't mess with Mister In-Between.

Offline lucky phil

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Re: Expected Cylinder Pressure
« Reply #15 on: December 18, 2021, 03:54:43 PM »
A compression gauge is a diagnostic tool only because absolute readings will vary depending on cranking speed and temperature.

Compression numbers are significant only if (1) there’s a big difference between cylinders, or (2) the numbers are really low, like 50psi or something.  Is a motor which shows 150psi on one day in better shape than another motor which shows 130psi on a different day?  I don’t think you can say that.

There's another significant and very important point of using a compression tester. The observed pressure should be achieved in 2 or maximum 3 compression cycles. Any more and it's a somewhat false reading.

Ciao
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Offline SmithSwede

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Re: Expected Cylinder Pressure
« Reply #16 on: December 18, 2021, 05:21:54 PM »
That’s interesting Phil.  But why is that the case?
Accentuate the positive;
Eliminate the negative;
Latch on to the affirmative;
Don't mess with Mister In-Between.

Offline lucky phil

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Re: Expected Cylinder Pressure
« Reply #17 on: December 18, 2021, 05:47:26 PM »
That’s interesting Phil.  But why is that the case?

It's a indicator of the real efficiency and integrity of the sealing. As you continue to spin the engine on the starter heat generated by friction and repetitive compression heating events distorts the true reading. Holding it on the starter to try and eek out the last 5 or 6 psi doesn't give you a true accurate indication of sealing efficiency. It's a bit like pressure leak testing something in reverse. You apply the pressure, allow a few minutes for it to shed the heat of compression and for the pressure indication to stabilise and then start your leak down assessment.

Ciao
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Online bmc5733946

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Re: Expected Cylinder Pressure
« Reply #18 on: December 18, 2021, 06:44:05 PM »
I was taught to stay on the starter for seven hits, no more, no less. I don't recall the reasoning but it's what I've always used.

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