## The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

All that has to do with the power train, gearbox, clutch, fuels and lubricants, etc. Generally the mechanical side of Formula One.
johnny comelately
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

The solution is SP400 and SP200.
4 stroke and 2 stroke respectively.

Convert the stroke into Stroke Percentage.

I would appreciate some feedback if this would work.
The only question now is, is there a further modifier required for rod ratio because of its affect on piston speed and acceleration??

Oh and then there is the question of what to do for Rotaries, or shouldnt we bother... the only trouble is they did win Le Mans

gruntguru
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

johnny comelately wrote:
Wed Nov 23, 2022 5:41 am
The status quo seems to be the use of Crank Angle for various particular engine analysis characteristics.
This fails in comaparative applications because it does not take into consideration the Bore Stroke ratios nor the Rod ratios and their effect on the very parameters that are being measured.
Before someone jumps in and says that some of those measurements are independent it would be prudent to see that a normalised factor that reflects all three parameters as a single encompassing new paradigm would be more indicative.
Not being particularly mathematical, any suggestions as to how this could be achieved?
The starting point would be the conversion of the stroke to a percentage.
Methinks this has been touched on before but I could not find it.
Must admit I also am not sure what you are trying to do. I agree with most of what @Hoff says.
One comment - how does B/S affect things? Two engines with identical displacement and L/R but different B/S will have the same volume - CA relationship.
je suis charlie

johnny comelately
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

gruntguru wrote:
Fri Dec 02, 2022 12:02 am
johnny comelately wrote:
Wed Nov 23, 2022 5:41 am
The status quo seems to be the use of Crank Angle for various particular engine analysis characteristics.
This fails in comaparative applications because it does not take into consideration the Bore Stroke ratios nor the Rod ratios and their effect on the very parameters that are being measured.
Before someone jumps in and says that some of those measurements are independent it would be prudent to see that a normalised factor that reflects all three parameters as a single encompassing new paradigm would be more indicative.
Not being particularly mathematical, any suggestions as to how this could be achieved?
The starting point would be the conversion of the stroke to a percentage.
Methinks this has been touched on before but I could not find it.
Must admit I also am not sure what you are trying to do. I agree with most of what @Hoff says.
One comment - how does B/S affect things? Two engines with identical displacement and L/R but different B/S will have the same volume - CA relationship.
From a previous post (with the drawing)
"To illustrate the challenge using a standard 266 point 6 cc cylinder.

The difference between an 80mm bore by 53mm stroke
and a 50mm bore by 135.6 stroke
Both at 20 degrees CA
The rod length was 116 mm for both (rightly or wrongly)

Produced piston heights of 2 point 234 and 6 point 43 respectively.
This translates into the different volumes of 11 point 229 cc and 12 point 625 cc respectively."

The long stoke used is for illustrative purposes only, to exagerate the affect. At both 20 degrees CA there is a substantial (as there should be) difference in volume
For example, If you are comparing different fuels in the one engine for say torque and adjust ignition for MBT it is of no consequence.
But if you are comparing different configs, for example, for a partic application using the one fuel (as a reference) , it matters.
Things like the affect on burn patterns or the ability to compare ignition timings etc are all skewed by the differences in stroke if you use CA. Consequently if you are mapping torque impulses it is necessary to standardise the comparators.

gruntguru
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

That example compares two engines with different L/R.
je suis charlie

johnny comelately
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

gruntguru wrote:
Fri Dec 02, 2022 1:05 am
That example compares two engines with different L/R.
In this particular instance the rod ratio is relevantly immaterial.
I see the danger here of a complicated discussion which may be needless.
I am at fault here for not being able to articulate my point adequately.

Do you agree it is difficult to compare different "square" (or stroke) engines configurations?

These days the capacity is not all that relevant when using the equal comparator of fuel/ energy properties.

So if you were using two different stroke engines and saying one has MBT at 20 degrees but the other has it at 15 degrees, there is a need for a "normaliser", hence my idea of percentage.

Hoffman900
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

I think studying a log scale P-V diagram would be very useful to you.

johnny comelately
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

Hoffman900 wrote:
Fri Dec 02, 2022 3:43 am
I think studying a log scale P-V diagram would be very useful to you.
That is rather condescending Hoff, not appropriate, and not the point.
It shows that you dont understand the proposition, as I said my fault for not being articulate.
We can just agree to disagree.
The PV relationship is not comparable at the same CA in two different stroke engines, adjusted or not for rod ratio.

gruntguru
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

I am sure Hoff's point is that v would a better metric than CA for comparing MBT. In other words a dimensionless "v" is probably what you are looking for.

On your comparison of two engines with different B/S (and different L/R). If L/R were the same for both engines, the "dimensionless v" or "dimensionless piston position" (same thing as it happens) would also be the same for both engines at a given CA.
Last edited by gruntguru on Mon Dec 05, 2022 11:08 pm, edited 1 time in total.
je suis charlie

johnny comelately
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

From the other unsolved aspect of rod ratio effect on piston position, using easy to identify with 70's, 80's ratios. Extrapolate for the effect using 3 or more :1

Hoffman900
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

johnny comelately wrote:
Mon Dec 05, 2022 10:12 am
From the other unsolved aspect of rod ratio effect on piston position, using easy to identify with 70's, 80's ratios. Extrapolate for the effect using 3 or more :1
https://images-wixmp-ed30a86b8c4ca88777 ... 3My2RuRCN4
Thanks, Grunt. Wasn’t meant to come across condescendingly but that graph will show combustion and pumping loss times

Johnny, I’m still not sure what you’re trying to show. Those are all completely different engine packages and I’m not sure how any conclusion for comparison can be made. You can’t look at this without considering piston compression height (which is a function of use / piston design / and deck height). I could make it look like a dramatic difference with a 50.8mm compression height, but that’s saved for 70yo designs, and no one in their right mind would run anything like that in a performance application.

Hoffman900
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

Johnny, you don’t even know what or why you’re asking what you are, and when I suggest other graphs that might, you throw a totally unreasonable fit. And you have forced this metric for mechanical scenarios that never exist in the real world. These aren’t personal attacks on you, despite you thinking they are. I explained to you why most engine designers ignore stroke / rod length to begin with, and now you’re introducing dwell and combustion concepts into it, which a log based p-v diagram will help illustrate as it would also any mechanical differences of different rod-stroke ratios.

I’m not sharing my profession online, especially for a bunch of strangers that act like they do here. It also has nothing to do with it.

Airshifter
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

johnny comelately wrote:
Fri Dec 02, 2022 2:19 am
gruntguru wrote:
Fri Dec 02, 2022 1:05 am
That example compares two engines with different L/R.
In this particular instance the rod ratio is relevantly immaterial.
I see the danger here of a complicated discussion which may be needless.
I am at fault here for not being able to articulate my point adequately.

Do you agree it is difficult to compare different "square" (or stroke) engines configurations?

These days the capacity is not all that relevant when using the equal comparator of fuel/ energy properties.

So if you were using two different stroke engines and saying one has MBT at 20 degrees but the other has it at 15 degrees, there is a need for a "normaliser", hence my idea of percentage.
With absolutely no offense intended, trying to find any "normaliser" is IMHO somewhat meaningless. For any variation of engine build characteristics, differing engines have differing strengths and weaknesses.

I can agree that it's always difficult to compare at the engineering level. But as the end use level, all that matters is the torque curve and it's suitability (or lack of) for the intended purpose, but at the same time fitting within the required weight and packaging requirements. And regardless of those size/weight/packaging/regs requirements, there will be compromise on a number of fronts.

That's what makes it all more interesting really.

johnny comelately
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

That is the problem Airshifter, there already is a "normaliser" and that is Crank Angle.
What I am saying is that in comparative situations it is both inapplicable and misleading.
Therefore what I propose can allow for comparative discussions of valve timing, MBT, combustion etc etc.
If for instance valve timing is being discussed and someone is used to Chev numbers and then the discussion turns to Yamaha M1 engine modifications you simply cannot think along the same lines.
When getting into the minutae of combustion it almost defeats the ability to compare.
I may be wrong but from my experience I am very confident, lucky its not a democracy

gruntguru
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

Musing here. V or V% is a commonly used normaliser in thermodynamic analysis of heat engines and probably does everything you want. As I mentioned earlier V% is the same as Stroke% if you ignore clearance volume.

Unfortunately, none of these (including CA or CA%) allows direct comparison (between two engines) of combustion events like MBT, MFB and location of PCP. One reason is cyclic variation of crankshaft angular velocity. This is different for different engine configurations eg I4 engines suffer extreme speed variation due to inertia torque (all pistons are stopped simultaneously transferring all their momentum to the crankshaft) whereas a 90* V Twin has zero inertia torque. Of course combustion events also contribute to cyclic speed variation - decreasing with cylinder count and relative flywheel mass. Getting back to the point - two engines may have PCP occurring at the same location measured in CA or V% but quite different time period ATDC due to one engine being an I4 and the other a V8.
je suis charlie

Airshifter
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### Re: The Better Alternative to the Folly of CA in the Comparative Analysis of IC engines.

I've always looks at CA or any other metric simply as a comparison point, not a normaliser per se. There are far too many ways to impact the big picture vs the small picture for it to be used for any major engine changes. For certain things it works a bit better but even then has limits. Using CA to compare burn characteristics of pistons of various design but the same chamber volumes might have merit, but if you toss in a longer rod you've just change the entire scope of the point of comparison.