How can I determine engine cylinder pressure during a compression stroke?

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Discussion Overview

The discussion revolves around determining engine cylinder pressure during a compression stroke, with a focus on the factors influencing pressure calculations and measurement techniques. Participants explore theoretical and practical aspects, including the impact of engine design and operating conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to understand the necessary parameters for calculating peak cylinder pressure, including stroke, bore, cam specs, and compression ratio.
  • Another participant argues that calculating cylinder pressure is complex due to the irregular shape of the cylinder head and the effects of engine wear, suggesting that actual compression tests are more reliable.
  • A different viewpoint suggests that while a rough estimate of cylinder pressure can be made using intake valve closing points and combustion temperatures, it will not yield precise results.
  • Some participants mention that peak cylinder pressure is influenced by factors such as heat release from fuel and gas expansion, but accurate calculations would require advanced computational fluid dynamics (CFD) work.
  • One participant recommends a textbook on internal combustion engines for a more thorough understanding of the Otto cycle, noting that typical peak pressure values are around 10 atmospheres.
  • Several participants emphasize that measuring cylinder pressure directly is the most accurate method, with suggestions for using pressure transducers instead of gauges for capturing peak pressure accurately.
  • There is a discussion about the variability of peak pressure estimates, with one participant recalling a peak pressure of around 15 atmospheres from an exam context.

Areas of Agreement / Disagreement

Participants generally agree that measuring cylinder pressure directly is the most reliable method, but there is no consensus on the best approach for calculating it theoretically. Multiple competing views on the complexity of calculations and the influence of various factors remain unresolved.

Contextual Notes

Participants note the challenges in calculating cylinder pressure due to the irregular geometry of the cylinder head, heat transfer, and leakage, which complicate theoretical estimates. The discussion also highlights the dependence on specific engine characteristics and operating conditions.

5.0stang
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I would like to find out how to find out much pressure occurs during a compression stroke of an engine. I'm using this to help compute sideloading information from different rod ratios.

What exactly is needed?

Stroke? Bore? Cam specs (opening and closing events)? Compression ratio? Etc?

To make it simple, I would like to find peak pressure to keep it simple?

Also, what is needed to find out how hot it gets in the combustion chamber? I thought I had read somewhere that it gets to 2,000 degrees?

Thanks for helping a newb:)
 
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Calculating it would be near impossible because the cylinder head is not an easy geometric shape where you can figure out the volume. Plus as an engine wears compressed air/fuel can blowby the rings and valves, decreasing the actual compression. The only real way to find out is to do an acutal compression test on each cylinder.
 
Well let's say you get 150psi on all 8 cylinders.

This of course doesn't take into account the "spark"...which is heat and creates a lot more pressure.

I figured you could take intake valve closing points (dyanamic compression starts) with IC engine temps at first combustion and this and that...and you could get an "idea"...not exact.

Let's say the cylinder head is 58cc's.
 
Stang, you're not easily going to be able to calculate peak cylinder pressures. You might be able to have a play with looking at how much heat is released by the fuel into the charge, how much this would cause the gas to expand, and how much heat is rejected by the engine, but you're not going to be very accurate, and it certainly won't give you a value which you could use to base any other calculations on. At the very least you'd need to do some pretty intensive CFD work to get anywhere close.

Can I ask exactly what you're trying to accomplish?
 
I was going to use it in a formula to see how much sideloading was present with certain engines and there rod ratios. I have the formula and help from Q Goest, he was very helpful.

I was just seeing if I could get a better idea in what might be a more accurate number to put in for cylinder pressure.
 
It's a difficult one to calculate, because instantaneous cylinder pressure depends not only on cylinder volume change due to the pistons' movement, but in heat transfer to the cylinder block, flow around crevices, and leakage, not forgetting the combustion event itself.

If you have an engine, I'd take some measurements rather than even attempting to calculate PCP, they're easy enough.
 
My advice is to look at "IC engines Fundamentals" of Heywood. He calculates REAL Otto Cycles, taking into account both combustion and compression.

The usual value of peak pressure is about 10 atmospheres.
 
That book would probably be a good idea, I just don't need to shell out money for it.

Yeah, I know aluminum vs. iron heads can change it slightly...or quench area...but I was just seeing if there was a more accurate way of determining cylinder pressure. I wasn't wanting an exact. To many variables.
 
The accurate way to determine cylinder pressure is to measure it. Or, ask someone who's measured it already. :wink:

Ooh, and Clausius' book recommendation is definitely a goer, it'll serve you for life. I think Clausius' estimate for a general peak pressure is low though. (Mate, did you get that from the Rassweiler & Withrow section by any chance?!)
 
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  • #10
So how would I measure it?

The only way I know of is with a compression checker...but that doesn't take into the account of heat expansion (pressure).

Thanks guys:)
 
  • #11
A pressure transducer fitted to a tapped drilling in the head.
 
  • #12
brewnog said:
The accurate way to determine cylinder pressure is to measure it. Or, ask someone who's measured it already. :wink:

Ooh, and Clausius' book recommendation is definitely a goer, it'll serve you for life. I think Clausius' estimate for a general peak pressure is low though. (Mate, did you get that from the Rassweiler & Withrow section by any chance?!)

I don't remember. I am only giving an order of magnitude (you know, I am fluid mechanics people, we only give orders of magnitude :biggrin: ). But I remember to have written in an exam around 15 atmospheres as the peak pressure. :smile:
 
  • #13
They say engineers round pi to 3. Well you fluids people round it to 1!
 
  • #14
Yeah, basically for us \pi\sim O(1) :smile:
 
  • #15
brewnog said:
A pressure transducer fitted to a tapped drilling in the head.

And don't forget to use a gage that locks on peak pressure or the reading will fluctuate so fast, you'll never be able to record the number.

Plan B: Install a check valve in the line.
 
  • #16
Nope, not a gauge, a transducer. The peak will be over so quickly a gauge would never be accurate enough.
 

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