Author Topic: crankcase vol. (no drilling involved)  (Read 575 times)

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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #60 on: March 09, 2016, 06:02:11 pm »
CCR = (VC+VS)/VC

VC/VS = 1/(CCR-1)

They are both ratiometric (dimensionless) ways of relating case volume to swept volume - all the plots in the paper could be reformatted to use CCR instead of VC/VS, and it would change the shape of the plots but the meaning would be exactly the same.  The relationship between the two measures is not related to rpm, and I'm confused as to how you are introducing this term?

You can actually set the two equal and try to solve it:

VS2+VCVS-2VC=0

https://www.wolframalpha.com/input/?i=a^2+%2Bab-2b%3D0 (had to use c for VC and s for VS)

But it still doesn't mean anything. 

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #61 on: March 10, 2016, 04:05:20 pm »
will post up the estimated crankcase delivery ratio graph in a few minutes, was at a extended union contract update dinner.  working 11-1/2 months without a contract is not cool.  returned with a few brain cells lost last eve to even reply.

since, i am unable to prove the relationship to you, as i have described, if i can get the time in the next few days, by using only values given for motor A......will post up all six graphs, similar to the ones referenced in the paper so you can compare values.  after i post those up, will email you working copies of the graphs, for you to do as you like with.  don't really want to spend the time to rework what has been shown in the paper, but that is what i am willing to do.  those graphs are of no use to me.  my purpose was/is different.

its all cool, always need proof of things myself.

will check out the link later.

hiatus

Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #62 on: March 10, 2016, 06:18:06 pm »
Graphs and charts are fine Joe, but I tend to approach problems from concepts first, and since I do not know what you are trying to show they may not be meaningful to me. 

To that end, VC and VS are fixed for a given engine design.  The authors devised a way to vary VC and look at how that effected delivery ratio at various rpm.  Still, for each value of VC, both VC and VS are fixed and not a function of rpm, so equating different combinations of them (VC/VS vs. CCR) tells you nothing about delivery ratio. 

It is delivery ratio that is a function of rpm and VC and VS.  If you could come up with a way to model Figure 5 for an un-piped engine that might be useful, but it would be very complex - and it appears that if you tune the engine for typical saw rpms, then the simple model of smaller-case-volume-is-better will suffice in most cases anyway.  In my opinion that is what Figure 5 shows, and I think if you look at what the manufacturers are doing it confirms this.  It might technically be possible to make the case volume so tight that the rpm of max delivery ratio moves higher than the port timing can be set up for, but I think in practice this would be very difficult to do. 

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #63 on: March 10, 2016, 09:02:50 pm »
some motor a things.  at this point, i can say that i have seen enough of this motor.  hahaha.
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #64 on: March 10, 2016, 09:25:46 pm »
Graphs and charts are fine Joe, but I tend to approach problems from concepts first, and since I do not know what you are trying to show they may not be meaningful to me. 

To that end, VC and VS are fixed for a given engine design.  The authors devised a way to vary VC and look at how that effected delivery ratio at various rpm.  Still, for each value of VC, both VC and VS are fixed and not a function of rpm, so equating different combinations of them (VC/VS vs. CCR) tells you nothing about delivery ratio. 

It is delivery ratio that is a function of rpm and VC and VS.  If you could come up with a way to model Figure 5 for an un-piped engine that might be useful, but it would be very complex - and it appears that if you tune the engine for typical saw rpms, then the simple model of smaller-case-volume-is-better will suffice in most cases anyway.  In my opinion that is what Figure 5 shows, and I think if you look at what the manufacturers are doing it confirms this.  It might technically be possible to make the case volume so tight that the rpm of max delivery ratio moves higher than the port timing can be set up for, but I think in practice this would be very difficult to do.

the graphs are just an estimate from the relationships the authors gave folks like us to picture what the curves look like.
the values for DR are lower than actual, as you can see.  that is all there is with this.  i understand that.

the only difference with the graphs(mine) i posted before all of this, is that rpm is plotted with a changing case volume.  the graphs are not meant to be compared to any of the graphs in the paper.  my purpose is different.  i for one have never seen a chart of this.

i agree with making case volume as small when raising rpm point within reason.  for me it would be just to know how small, or how big to leave it for matching the case to peak torque point at whatever rpm i am after.  physical limitations would stop you before reaching those goals.

its just numbers.
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #65 on: March 10, 2016, 09:26:21 pm »
........some folks like rc plane guys for example, may not need a small case.  depends on what you are trying to do i guess?
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #66 on: March 11, 2016, 06:33:49 pm »
now here is a head scratcher.

same saw.

first saw graph = estimated t*a DR
2nd graph = estimated crankcase DR

peak rpm points are chosen.  best guess.  disregarding the differing number values........due to these being estimates.

the t*a peak rpm is pretty close to what it actually is, imo, because it matched up with blair recommendation t*a values for that size motor at that rpm.

the crankcase peak rpm, may or may not be at the right rpm point.  my best guess is that it is somewhere between peak tq and peak hp rpm.  saw specs say 8300 hp peak, 11400(max) ? if i remember right.
somewhere it gets air to run up there.

if t*a peaks, at 5500......maybe the crankcase actual rpm of DR peak is higher.....?  and if so, it may tightrope between the two peaks?  or is t*a in control of DR as i was thinking previously?

what say ye?

so this is what i am looking at:

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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #67 on: March 11, 2016, 06:59:44 pm »
think i will heed chris' advice and leave the whole Vc/Vs and CCR separate from the various peak DR's.

there is no way i can be sure the CC DR is at peak tq rpm, and even less sure that it would be located at that peak tq rpm point for all different sizes and porting configurations of motors and cases.  there are definite trends, as  y'all can see, but that is all i have to go on with using these estimates so far.
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #68 on: March 12, 2016, 12:56:09 pm »
here is a working program,

figures out CCR and Vs/VC, based on your crankcase volume.

screenshots to follow.

the working program, is found right below this sentence.
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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #69 on: March 16, 2016, 07:52:29 am »
So here's a question:  If the rpm of peak Delivery Ratio is dependent on case volume, does it also depend on combustion chamber volume?

I've been playing around with two 42cc Poulan engines with different combustion chamber volumes, and thinking about the impact of that.  Obviously larger combustion chamber volumes negatively impact compression ratio, but do they also trap a larger volume of fuel/air mix?

Clearly combustion chamber volume can't be zero or way too big, but what is optimal?

 

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