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

0 Members and 0 Guests are viewing this topic.

Offline Chris-PA

  • 6 cube
  • ****
  • Posts: 203
  • Karma: 28
Re: crankcase vol. (no drilling involved)
« Reply #40 on: March 05, 2016, 09:51:58 pm »
Going back to Figure5, which is also for no tuned intake or exhaust, take a look at the smallest case volume (which is still pretty large at 2.44).  Not only is the rpm of max delivery ratio moved up, but it does not fall off as fast as for larger case volumes.  The peak is wider and flatter.

It is really hard to tell extrapolate this data to the the little engines used in saws at 10,000rpm with no pipes. 

Offline 1manband

  • Nitro Hotsaw
  • *******
  • Posts: 759
  • Karma: 51
Re: crankcase vol. (no drilling involved)
« Reply #41 on: March 06, 2016, 10:19:34 am »
believe that figures #5 and #6 are showing the same thing.

lost some things on my laptop after an update, and lost ability to take screenshots.  will be back, when i get it sorted out.

hiatus

Offline 1manband

  • Nitro Hotsaw
  • *******
  • Posts: 759
  • Karma: 51
Re: crankcase vol. (no drilling involved)
« Reply #42 on: March 06, 2016, 03:33:23 pm »
I still need to wade through all the details of this paper.  However, I think it is important not to lose sight of what the authers were trying to accomplish.  They state this in the opening paragraphs:

By changing the crankcase volume and lengths of the exhaust and inlet pipes, the authors have experimentally investigated the effect of the crankcase volume on the delivery ratio, and the effect of exhaust and inlet systems to compensate for the drop in delivery ratio caused by increasing the crankcase volume.

So once again it is all about the pipes, which makes sense because that effect is so dominant.  But what if there is no pipe?  Here, the only real hint seems to be in the following graphs, if we look at both intake and exhaust as "shortest" (i.e. untuned).  However these graphs don't go any lower than a case to swept volume ratio of 2, so even here it may not be relevant:

Figure 6 shows that there is no rpm relationship to the delivery ratio - the delivery ratio peaked at some rpm, and it didn't matter what the case volume was. 

Figure 9 shows that even by 3000 rpm, with no tuned pipes increasing the case volume reduces delivery ratio. 

I don't know why the no pipe lower rpm plots in Figures 7 & 8 show larger case volume is better - perhaps because there is so much time that other effects dominate, or maybe because the no pipe situation was not really the focus of their efforts and some other effect was going on.   

Still, I think what little data there is in this paper about engines with no tuned intake or exhaust confirms that at reasonable rpms smaller case volume is always better.



sorry, i cannot post the screenshots.  best i can do.  EDIT <<<<<<hahaha fixed it. silly windows.

believe it is best to use the words from the paper in my reply.

imho:
the first one shows the graphs being similar.
the second one shows what i used to come up with the graphs i made.
the third one addresses, part of the intent of the paper.

i'm looking at the 'untuned' pipes as being the 'shortest-shortest' as well.
hiatus

Offline Chris-PA

  • 6 cube
  • ****
  • Posts: 203
  • Karma: 28
Re: crankcase vol. (no drilling involved)
« Reply #43 on: March 06, 2016, 05:55:11 pm »
Looking at Figure 5 again, and thinking about the engines used in this paper, which are 200cc and 150cc with power peaks at 3600rpm. 

What I see is that when the case volume is very large, then you can get a good delivery ratio at low rpm, but it falls off quickly once you go higher in rpm.  That makes sense because there isn't much pressure differential to move the air, but if you go slow enough eventually it fills.  Go beyond that and it falls on its face since there isn't enough time to move the air.   

However, once you get close to the peak power rpm (where presumably the port timing was optimized), and you get the case volume down small enough, then it no longer falls off with rpm.  Now the delivery ratio isn't so much a peak as a threshold. 

This makes sense to me because the relationship between the pressure differential and the case volume isn't linear.  As the case volume goes down the pressure differential goes up fast, and therefore the velocity goes up fast too.  So as the case volume goes down you can move the air much faster and then you don't have this effect where there isn't enough time to achieve the full delivery ratio. 

So if we look at our smaller engines at 10,000rpm it appears to me that smaller case volume is better, and we will not easily make case volumes small enough such that the delivery ratio will fall off at practical rpms for these engines. 

Offline Chris-PA

  • 6 cube
  • ****
  • Posts: 203
  • Karma: 28
Re: crankcase vol. (no drilling involved)
« Reply #44 on: March 06, 2016, 06:05:10 pm »
This is a graph of the pressure volume relationship from https://en.wikipedia.org/wiki/Boyle's_law



If you define the volume as the ratio of Vc/Vs as in the paper, the graph is even more extreme/less linear:




Offline 1manband

  • Nitro Hotsaw
  • *******
  • Posts: 759
  • Karma: 51
Re: crankcase vol. (no drilling involved)
« Reply #45 on: March 06, 2016, 07:06:22 pm »
Looking at Figure 5 again, and thinking about the engines used in this paper, which are 200cc and 150cc with power peaks at 3600rpm. 

What I see is that when the case volume is very large, then you can get a good delivery ratio at low rpm, but it falls off quickly once you go higher in rpm.  That makes sense because there isn't much pressure differential to move the air, but if you go slow enough eventually it fills.  Go beyond that and it falls on its face since there isn't enough time to move the air.   

However, once you get close to the peak power rpm (where presumably the port timing was optimized), and you get the case volume down small enough, then it no longer falls off with rpm.  Now the delivery ratio isn't so much a peak as a threshold. 

This makes sense to me because the relationship between the pressure differential and the case volume isn't linear.  As the case volume goes down the pressure differential goes up fast, and therefore the velocity goes up fast too.  So as the case volume goes down you can move the air much faster and then you don't have this effect where there isn't enough time to achieve the full delivery ratio. 

So if we look at our smaller engines at 10,000rpm it appears to me that smaller case volume is better, and we will not easily make case volumes small enough such that the delivery ratio will fall off at practical rpms for these engines.

yep.

hiatus

Offline 1manband

  • Nitro Hotsaw
  • *******
  • Posts: 759
  • Karma: 51
Re: crankcase vol. (no drilling involved)
« Reply #46 on: March 06, 2016, 07:21:03 pm »
This is a graph of the pressure volume relationship from https://en.wikipedia.org/wiki/Boyle's_law



If you define the volume as the ratio of Vc/Vs as in the paper, the graph is even more extreme/less linear:

^^.......this graph, if volume was on the y-axis, rpm on the x......

so close.  since volume changes with piston movement........

set it up so that it's sqrt vc /rpm and you got it.  (like in the paper)

(you will need a need to pick a known example case volume, and a certain peak value for rpm).

.......from there a simple ratio will get you case volumes and crankcase compression ratios for all other rpm points.

i did not think it beneficial to post this info yet, because it is useless without porting to a certain similar rpm point using time*areas?
hiatus

Offline 1manband

  • Nitro Hotsaw
  • *******
  • Posts: 759
  • Karma: 51
Re: crankcase vol. (no drilling involved)
« Reply #47 on: March 06, 2016, 07:29:55 pm »
......very tough to convince you of anything........hahahahaha.
hiatus

Offline 1manband

  • Nitro Hotsaw
  • *******
  • Posts: 759
  • Karma: 51
Re: crankcase vol. (no drilling involved)
« Reply #48 on: March 06, 2016, 07:37:46 pm »
time*area delivery ratio graph vs. rpm (graph setup as per the paper):

compare this to the crankcase volume/delivery ratio graphs vs. rpm, and draw your own conclusions.

peace

hiatus

Offline Chris-PA

  • 6 cube
  • ****
  • Posts: 203
  • Karma: 28
Re: crankcase vol. (no drilling involved)
« Reply #49 on: March 06, 2016, 08:12:43 pm »
......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. 


 

Related Topics

  Subject / Started by Replies Last post
12 Replies
1217 Views
Last post May 08, 2011, 06:11:27 am
by Spike60
2 Replies
221 Views
Last post November 10, 2012, 12:45:34 pm
by HolmenTree
4 Replies
306 Views
Last post July 01, 2013, 11:42:19 am
by SawTroll
13 Replies
362 Views
Last post November 23, 2013, 06:09:58 pm
by SawTroll
4 Replies
114 Views
Last post June 05, 2014, 01:19:56 pm
by KilliansRedLeo
17 Replies
1629 Views
Last post January 19, 2015, 02:07:53 pm
by Cat-Face Timber
6 Replies
212 Views
Last post September 21, 2014, 10:24:54 pm
by 660magnum
6 Replies
208 Views
Last post December 12, 2014, 09:58:50 pm
by Cut4fun .
23 Replies
192 Views
Last post January 20, 2015, 10:13:22 am
by Cat-Face Timber
066 crankcase

Started by jmester « 1 2 » Stihl

12 Replies
114 Views
Last post August 08, 2015, 10:05:48 pm
by jmester