Author Topic: crankcase vol. (no drilling involved)  (Read 448 times)

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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #50 on: March 06, 2016, 08:20:51 pm »
Oh - I'm not sure Crankcase Compression Ratio is the same as the Vc/Vs ratio they used.  It's still not clear what all is included anyway.  If it is supposed to be the same, then it's probably because their setup could only add volume. 

EDIT:  I think CCR is the ratio of the swept volume plus the case volume to the case volume, or:  CCR = (VC +VS)/VC

That is different from VC/VS

If you calculate it out, VC/VS = 2.44 works out to a CCR of 1.41, which is what Engine A is listed as.

So for the red line of Figure 5, the VC/VS = 2.44 is getting pretty close to the practical limit of what you could achieve, and it still is at almost full delivery ratio by 2,500rpm.  That scales to about 7,000rpm for an engine with a 10,000rpm power peak, so practically it would be very hard to reduce the case volume down so far that you push the peak delivery ratio up above your power band.

I'd still like to know if the transfer volume is included - it should be. 

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #51 on: March 07, 2016, 04:30:12 am »
......very tough to convince you of anything........hahahahaha.
LOL, I'm a bonehead for sure!   

But look at the red curve of Figure5 again.  This is at 3000rpm, or if scaled to an engine with a 10,000rpm power peak, it would be at 8,333rpm.  The red curve does not peak and fall off, it plateaus.  This is because once the case volume gets low enough the pressure differential gets very large, and you can no longer get ahead of the flow rate. 



glad you are having fun with it.

if you are on board with graph #5, being similar to #6......?  x-axis scale on #6, has a big 3000 to 6000 jump.

motor A's hp peak is at 3600 in stock form.  thinking it may only rev a few grand or so higher?

the only way to get an better handle on that would be to plug in it's numbers, which i have not done.  will have to look into that.  would be a good check.

i have not been focusing on pressure and mass flow, with this so much.  when the math came to a dead end, lost interest.  will rekindle that angle again eventually.  the fuel's mass compared to the air's is much greater.  kind of like wind trying to blow sand around on the beach.  takes a lot to get it moving because of the mass difference.  as rpm's rise, velocity naturally increases, looking just at pressure differentials may be bit deceiving because it is compressible flow.  lots of things going on.
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #52 on: March 07, 2016, 05:19:59 am »
Oh - I'm not sure Crankcase Compression Ratio is the same as the Vc/Vs ratio they used.  It's still not clear what all is included anyway.  If it is supposed to be the same, then it's probably because their setup could only add volume. 

EDIT:  I think CCR is the ratio of the swept volume plus the case volume to the case volume, or:  CCR = (VC +VS)/VC

That is different from VC/VS

If you calculate it out, VC/VS = 2.44 works out to a CCR of 1.41, which is what Engine A is listed as.

So for the red line of Figure 5, the VC/VS = 2.44 is getting pretty close to the practical limit of what you could achieve, and it still is at almost full delivery ratio by 2,500rpm.  That scales to about 7,000rpm for an engine with a 10,000rpm power peak, so practically it would be very hard to reduce the case volume down so far that you push the peak delivery ratio up above your power band.

I'd still like to know if the transfer volume is included - it should be.

imo, think that the difference in the numbers is not that big a deal.  does sound silly i know.  for the following reason......

both results are interchangeable when using them just to scale a graph.  thinking that you will not buy into this.  hahahaha.

ok, put another way,  the end purpose of figuring any of this out is not to see just how much beer you can drink while juggling numbers in a calculator.
for any of this to be useful for anyone, the end goal is to find the estimated case volume needed for a certain rpm.
both CCR methods, while different, give the same result when a rpm point is equated to it.  the shape of the graph does not change.
so, the CCR is just along for the ride.  think you will find that the author's use this interchangeably, if you stick to the way they set the graph up in the relationship using sqrtVc/rpm, that i quoted.

trans volume....up to you.  if 0.1 or so CCR makes a difference?  wouldn't think it would change anything as far as the graphing is concerned.  10cc looks to only have an impact when you are way way up in the rpm range, possibly well beyond physical limitations anyway.
 
as you have already found, achieving the proper case volume runs into physical limitations very quickly, and it's doubtful that they can be met.

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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #53 on: March 07, 2016, 06:16:06 am »
Another way to look at the reasons behind the peaking of the curves:

Larger case volume means lower pressure difference and lower velocities.  Lower velocity means more time to move the air.  Therefore larger case volumes represent a time delay in moving the air.

However, port timing is always symmetrical about TDC.  So at least at lower rpm there should always be some case volume/time delay that best matches the port time events, trapping the most air before the ports close or some such (probably the transfers would be most important).

At smaller case volumes/higher velocities and higher rpm possibly inertial effects start to come into play, making the shape of the red lined plot different. 

i have not been focusing on pressure and mass flow, with this so much.  when the math came to a dead end, lost interest.  will rekindle that angle again eventually.  the fuel's mass compared to the air's is much greater.  kind of like wind trying to blow sand around on the beach.  takes a lot to get it moving because of the mass difference.  as rpm's rise, velocity naturally increases, looking just at pressure differentials may be bit deceiving because it is compressible flow.  lots of things going on.
I'm not sure there was any fuel in the tests done in this paper?  Also, the fuel is in vapor form in a engine, so its mass should just join that of the air - not quite like blowing sand.

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #54 on: March 07, 2016, 07:53:30 am »
Oh - I'm not sure Crankcase Compression Ratio is the same as the Vc/Vs ratio they used.  It's still not clear what all is included anyway.  If it is supposed to be the same, then it's probably because their setup could only add volume. 

EDIT:  I think CCR is the ratio of the swept volume plus the case volume to the case volume, or:  CCR = (VC +VS)/VC

That is different from VC/VS

If you calculate it out, VC/VS = 2.44 works out to a CCR of 1.41, which is what Engine A is listed as.

So for the red line of Figure 5, the VC/VS = 2.44 is getting pretty close to the practical limit of what you could achieve, and it still is at almost full delivery ratio by 2,500rpm.  That scales to about 7,000rpm for an engine with a 10,000rpm power peak, so practically it would be very hard to reduce the case volume down so far that you push the peak delivery ratio up above your power band.

I'd still like to know if the transfer volume is included - it should be.

imo, think that the difference in the numbers is not that big a deal.  does sound silly i know.  for the following reason......

both results are interchangeable when using them just to scale a graph.  thinking that you will not buy into this.  hahahaha.

ok, put another way,  the end purpose of figuring any of this out is not to see just how much beer you can drink while juggling numbers in a calculator.
for any of this to be useful for anyone, the end goal is to find the estimated case volume needed for a certain rpm.
both CCR methods, while different, give the same result when a rpm point is equated to it.  the shape of the graph does not change.
so, the CCR is just along for the ride.  think you will find that the author's use this interchangeably, if you stick to the way they set the graph up in the relationship using sqrtVc/rpm, that i quoted.

trans volume....up to you.  if 0.1 or so CCR makes a difference?  wouldn't think it would change anything as far as the graphing is concerned.  10cc looks to only have an impact when you are way way up in the rpm range, possibly well beyond physical limitations anyway.
 
as you have already found, achieving the proper case volume runs into physical limitations very quickly, and it's doubtful that they can be met.

i deserve a good kick in my backside.  ignore my ramblings above.  did not see this before i plugged in numbers from motor A.  using a chainsaw motor the difference was slight, with motor A is is quite large.

will post up some motor A stuff today.
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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #55 on: March 07, 2016, 08:43:10 am »
i deserve a good kick in my backside.  ignore my ramblings above.  did not see this before i plugged in numbers from motor A.  using a chainsaw motor the difference was slight, with motor A is is quite large.

will post up some motor A stuff today.
Actually I agree with you, in the sense that CCR or VC/VS are just two different ways of measuring the same effect.  It will change the shape of the graph by changing the X-axis scale, but it does not change the meaning at all. 

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #56 on: March 08, 2016, 05:41:43 am »
graph is motor A from paper

top blue line is crankcase volume.
black is Vc/Vs
red is crankcase comp ratio.

idea is, that you can see what volume/CCR value needed for a particular rpm.

wanted to include the time area delivery ratio and crankcase delivery ratio graphs on this plot.  when i added a 5th range, the graph went to crap.
liking this spreadsheet software, but the graphing functionality needs some work.  going to have to show the delivery ratio stuff on separate graph.

as far as saw motors go, it is a little off.  does not seem to account for straight line, non changing situations, because of the way it is set up.
delivery ratio of a saw is pretty well close to flat.  think it could be tweaked using bsfc's, not sure.  will throw some photos up of the saw graphs later.
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #57 on: March 08, 2016, 07:20:02 pm »
first photo is motor A from paper again, yellow highlight, showing that everything lines up fine.

second photo is saw motor set for 5500 rpm.  the CCR is off.

third photo is the same saw motor now set at 4500 rpm.  there is about a 1000 rpm spread on either side of 5500. but far off from 4500.
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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #58 on: March 08, 2016, 10:43:31 pm »
OK, the relationship between CCR and VC/VS is straightforward, but I'm not sure what you are calculating with the rest of it. 

I have been more focused on trying to understand the big picture effects - what happens as case volume changes.  Near as I can tell from the data they provide, without a pipe it would be near impossible to make the case volume too small on most saw engines, unless you were going for an rpm band well below what is typically used in a modern saw engine.

The recommendation I have seen before is (I believe) for a case volume of 120% of the displacement, but I'm never sure what that includes.  VC would clearly include the volume under the piston and the transfers, but I don't know if the rule-of-thumb does.  It would be tough to achieve this.  Still, these are significantly smaller numbers than the 2.44 minimum they used.  Looking at the 2.44 and 2.99 plots of Figure 5, you can see that things are different at higher rpm compared to the larger case volumes.  What happens if you go smaller still?  There just isn't any data here to show that. 

I tried to make a plot of the rpm of the peak delivery ratio vs. VC/VS from Figure 5, but there are not many data points, and looking at only the peak ignores what is happening at rpms above the peak.

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #59 on: March 09, 2016, 04:59:48 pm »
OK, the relationship between CCR and VC/VS is straightforward, but I'm not sure what you are calculating with the rest of it. 

yes, straightforward.  to get the CCR and Vc/Vs, you need just two things.  case bdc volume, and swept volume.  a plot can easily be made for different volumes.  taking it a step further, put rpm in the mix at peak DR.  peak DR, for any motor has to be below peak hp rpm.  there is a slight rise at peak tq.  check the curves in the paper.  blair shows this much clearer.  for a high rpm motor they are pretty flat.  used that single rpm or close to it in my graph.  a user input value.  best guess.  that rpm, is placed between values of rpm, in 500 increments.  after peak torque rpm, the time*areas of the ports are getting too small. so, the time*area is in control.  the rpms are showing what the CCRs an Vc/Vs would be along their curves if there was air enough.  nothing fancy.

it is a close estimate.  until someone figures out the heavy math, best that can be done, as i had mentioned before.  since the Vc/Vs is always going to be a curve, and CCR is always a straight line.  they will never line up with rpm and the case volume, to the right of their point of intersection.  none of this matters at all, unless you really like to make things go, by balancing the case to the porting using rpm.



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