Compression Ratio Vs Cylinder Fill

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Re: Compression Ratio Vs Cylinder Fill

Post by gruntguru » Thu Oct 04, 2018 12:42 am

Stan Weiss wrote:
Wed Oct 03, 2018 11:47 pm
modok wrote:
Wed Oct 03, 2018 7:55 pm
Probably, very near atmospheric
I am sure someone will correct me if I got the calculations wrong. :lol:
Bore = 4.155"
Stroke = 3.5"
Rod = 6"
IVC = 85 ABDC
VE = 115%

cylinder pressure around 61.5" hg @ IVC
Stan
Wow - 2 atmospheres. That's not your average engine. (115% trapped mass VE @ peak HP rpm.)

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Re: Compression Ratio Vs Cylinder Fill

Post by modok » Thu Oct 04, 2018 5:29 am

Plausible. I was imagining some kind of an average engine, and "when the intake closes" is a little hard to nail down.
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Re: Compression Ratio Vs Cylinder Fill

Post by Stan Weiss » Thu Oct 04, 2018 10:54 am

I would be interested if anyone else did these calculations what they come up with.
Bore = 4.155"
Stroke = 3.5"
Rod = 6"
IVC = 76 ABDC (36/76 - 292 seat-to-seat - 110 ICL)
VE = 100%

cylinder pressure around 47.1" hg @ IVC
Stan
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Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3000 DFW or FLW Files

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Re: Compression Ratio Vs Cylinder Fill

Post by modok » Thu Oct 04, 2018 9:26 pm

You left out compression ratio
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Re: Compression Ratio Vs Cylinder Fill

Post by Stan Weiss » Fri Oct 05, 2018 12:01 am

CR = 13.5:1 for both examples.

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Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3000 DFW or FLW Files

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Re: Compression Ratio Vs Cylinder Fill

Post by modok » Fri Oct 05, 2018 4:46 am

It might be interesting exercise to figure it out freehand by some method, but honestly I never have tried.
There are several engine sims on the market that can simulate the whole PV loop so you can know the pressure at any crank angle you want. No offense intended but I'm getting kind of a weird vibe going on....based on your signature.
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Re: Compression Ratio Vs Cylinder Fill

Post by digger » Fri Oct 05, 2018 5:07 am

1.5-2b depending on rpm send to be what some simulations show

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Re: Compression Ratio Vs Cylinder Fill

Post by modok » Fri Oct 05, 2018 5:18 am

The more I think about it, It's a heck of a trick question. Some kind of riddle. Of course the pressure rises very rapidly with each degree, and when the valve closes COULD be considered negligible since it is at high rpm. If you actually wanted to know when the valve closed you'd have to really define that. It's going to have hit the seat a few degrees before you'd think, but then it's going to bounce, so....when DID it close? But....the low lift flow is negligible at peak power, so, you can functionally ignore it. VE is similarly suspect, as you don't know how much was trapped and how much went through, and EGR %, and is that estimated VE FROM power produced? Or some kind of actual or theoretical...who knows, not to mention temperature.

Just pick a degree that is near when the valve closes, and figure what the pressure was backward from the gross power produced and CR and temp, ( like BMEP estimate).... might work fine, but would that be a suitable answer for..... whatever the question was?
What was the question? That's the question.
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Re: Compression Ratio Vs Cylinder Fill

Post by Stan Weiss » Fri Oct 05, 2018 10:35 am

A test where no software or expensive equipment needed.

Hook a compression tester up to an engine and do a cranking compression test. This will show what you get when cylinder pressure is just about atmospheric @ IVC. Now if you gauge goes high enough start the engine and see what it reads then and as you slowly increase the RPM.

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Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3000 DFW or FLW Files

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Re: Compression Ratio Vs Cylinder Fill

Post by modok » Fri Oct 05, 2018 7:42 pm

Very clever.
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Re: Compression Ratio Vs Cylinder Fill

Post by MadBill » Fri Oct 05, 2018 11:04 pm

Unless the engine is under load, I believe its peak pressure will be lower than the cranking pressure.
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Re: Compression Ratio Vs Cylinder Fill

Post by Mark O'Neal » Sat Oct 06, 2018 12:45 am

MadBill wrote:
Fri Oct 05, 2018 11:04 pm
Unless the engine is under load, I believe its peak pressure will be lower than the cranking pressure.
I learned two things when I was a young feller.

1. A 12.5:1 motor that pumps over 150 lbs makes about 60 lbs of cylinder pressure when it's running.

2. Never start an engine on a engine hoist.

I know that's simplistic....but you take your lessons where you can. I learned these things concurrently.

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Re: Compression Ratio Vs Cylinder Fill

Post by Caprimaniac » Sat Oct 06, 2018 2:53 am

Mark, you don’t have a video of that engine in the hoist, do you?
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Re: Compression Ratio Vs Cylinder Fill

Post by Roundybout » Sat Oct 06, 2018 12:52 pm

Does the start of compression mark the end of the intake process? It doesn't necessarily occur at the same time as IVC. Engine speed determines the point at which the fuel/air mixture stops flowing into the cylinder. At lower RPM, the start of compression is closer to IVC and farther away as RPMs rise. How do you calculate for that?

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Re: Compression Ratio Vs Cylinder Fill

Post by Belgian1979 » Sat Oct 06, 2018 2:05 pm

Stan Weiss wrote:
Thu Oct 04, 2018 10:54 am
I would be interested if anyone else did these calculations what they come up with.
Bore = 4.155"
Stroke = 3.5"
Rod = 6"
IVC = 76 ABDC (36/76 - 292 seat-to-seat - 110 ICL)
VE = 100%

cylinder pressure around 47.1" hg @ IVC
Stan
What is the projected compression pressure ?

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