Author Topic: crankcase vol. (no drilling involved)  (Read 465 times)

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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #30 on: February 20, 2016, 08:15:43 pm »
from what i gather from the nagao & shimamoto paper, reducing the case volume too much can cause the charge to blowback through the carb at lower rpms they were testing, with porting kept the same.

reasoning they had was the pressure builds up too fast in the case.  as the case pressure rise fills the transfers too quickly, a portion of the charge goes back out the intake with a small case, at lower rpms.

------------------

(above must happen because the case pressure gets equal or greater than atmospheric before the intake port closes......my words not theirs here).

the case has harmonic cavity resonance and wave pulses.  changing volume, length of port, port diameter, carb size........ can all change the pulses as well.  at a certain rpm the pulses become tuned to rest the motor.  at others they are out of phase.  intake tuning.

suggest sticking to the paper, not my ramblings.




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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #31 on: February 26, 2016, 07:18:26 pm »
it has been abit, but have made some progress in the crankcase volume thing, along with many setbacks.

long story with not much to say.

first tried to get through some of the multi variable calculus formulas in that paper, to be able to get crankcase pressures. so far a no go.  it's not like riding a bike.  been 25 years.

someone younger needs to take a shot at it while it is still fresh.

after that, tried to find an easier way to get it done.  got distracted and off course with some p-v diagram stuff and heat transfer (thermodynamic stuff) i found from a working program available from a university.  cool stuff, you could plug in some numbers, and it spits out the whole intake/compression/expansion/blowdown cycle on a graph, along with power output.  another graph shows just how much heat the motor tosses around.

of interest was that raising the compression ratio way up, forms two pressure spikes in the chamber.  one before ignition, and a primary pressure spike when it lights off.  there is more to learn there.  but after some time, read some more of blair to figure out why the numbers were off.  long story short, imo would stick to blair's methods, and abandon the regularly used p-v graph altogether for two stroke use.

on another note, figured out some things of interest.  these were derived from page 178 of the paper i quoted earlier in this thread.

found how and succeeded to make a graph of the delivery ratio of a chainsaw motor based on the time area of the motor, how to find optimal crankcase compression ratio for a particular rpm, and lastly how to find the case volume in cc, needed for that particular rpm.

that's all for now.  one mans interest is another mans boredom i guess.   hahaha.  time to crack a beer.  peace out.
 
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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #32 on: February 26, 2016, 09:12:45 pm »
that's all for now.  one mans interest is another mans boredom i guess.   hahaha.  time to crack a beer.  peace out.
It's interesting stuff to me Joe, but sometimes I just don't have the mental energy left at the end of the day to do more analytical stuff with spreadsheets and calculations.  So much of what is written in the texts seems to be mostly empirical stuff. 

I'm more interested in trying to visualize what happens.  So as I said previously, here there is no pipe, and no way to increase the volume of air drawn in beyond what the piston can pull in under (at most) 1 atmosphere of pressure.  Then it pushes that up the transfers with slightly higher pressure. 

The swept volume is always larger than the effective displacement because it is gated by the port timing - the ports are only closed for a fraction of the swept volume.  Combined with the extra volume that will always exist in the case and transfers, it's hard to see how it could ever get "crowded" in the case - there is always more volume than needed.  What would be the negative effect of too little case volume?  Would the pressure during primary (case) compression get too high?  That just seems unlikely.

If you look at a theoretical engine with durations of:

Intake =160, Exhaust = 160, Transfer = 120, Blowdown = 20, Primary (Case) Compression = 40

That primary compression represents only 34% of the swept displacement, and it occurs while the crank is centered around 90deg - so over half the cylinder's swept volume is still exposed. 

The intake has about 41% of the swept volume to pull in air (if you assume it's stops pulling at TDC - I know the air continues to move beyond that, but it's only from inertia), and at 1 atmosphere of pressure. 

The transfers have 25% of the swept volume to push the mix into the cylinder, but that is pushed not pulled, so it's at more pressure. 

It seems to me that the advantages of having a small case volume are to improve the efficiency of pulling in air and pushing it up the transfers, and I'm having a hard time visualizing what the negatives are - at least with no pipe. 

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #33 on: February 27, 2016, 02:21:37 pm »
that's all for now.  one mans interest is another mans boredom i guess.   hahaha.  time to crack a beer.  peace out.
It's interesting stuff to me Joe, but sometimes I just don't have the mental energy left at the end of the day to do more analytical stuff with spreadsheets and calculations.  So much of what is written in the texts seems to be mostly empirical stuff. 

I'm more interested in trying to visualize what happens.  So as I said previously, here there is no pipe, and no way to increase the volume of air drawn in beyond what the piston can pull in under (at most) 1 atmosphere of pressure.  Then it pushes that up the transfers with slightly higher pressure. 

The swept volume is always larger than the effective displacement because it is gated by the port timing - the ports are only closed for a fraction of the swept volume.  Combined with the extra volume that will always exist in the case and transfers, it's hard to see how it could ever get "crowded" in the case - there is always more volume than needed.  What would be the negative effect of too little case volume?  Would the pressure during primary (case) compression get too high?  That just seems unlikely.

If you look at a theoretical engine with durations of:

Intake =160, Exhaust = 160, Transfer = 120, Blowdown = 20, Primary (Case) Compression = 40

That primary compression represents only 34% of the swept displacement, and it occurs while the crank is centered around 90deg - so over half the cylinder's swept volume is still exposed. 

The intake has about 41% of the swept volume to pull in air (if you assume it's stops pulling at TDC - I know the air continues to move beyond that, but it's only from inertia), and at 1 atmosphere of pressure. 

The transfers have 25% of the swept volume to push the mix into the cylinder, but that is pushed not pulled, so it's at more pressure. 

It seems to me that the advantages of having a small case volume are to improve the efficiency of pulling in air and pushing it up the transfers, and I'm having a hard time visualizing what the negatives are - at least with no pipe.

hey chris.
 
what you have shown here is about all the information one could gleam from looking at this using just degree durations and volume displacement.  i don't disagree with the conclusions.

my point of contention, when i posted that a case can be too small is that 'every case is too small.'
the point being, that at any motor rpm, that is less than the peak torque rpm, the case is too small.  at peak torque rpm, it peaks to a value 'as good as its going to get.'  lastly,at any rpm higher than peak torque rpm, it is too large.

looking at this, like you have is a good way to visualize it.

looking at it, adding in both time areas and rpm, lets one look at it a little more clearly.

if somebody works the heavy math involved, to add in pressures and mass flow, we could look at it exactly the way those two guys did who wrote the paper.  in this last way, their calculated numbers matched values that were closely approximated to the results obtained from the actual physical tests performed on the motor.  (would be cool to be able to graph the pressures and mass flow at any rpm).


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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #34 on: February 27, 2016, 02:41:55 pm »
don't know if many folks have read that paper?

the paper's graphs were very confusing to me at first because of the way they are set up.

for example, the term ......  no/n ........   no, is the 'optimum rpm'.  'optimum rpm' is the peak torque rpm.  where the case volume is best suited.  and n, is just the particular rpm is running at.

so, when you look at no/n, it is peak torque rpm divided by motor rpm.

small values of no/n, are high rpm operation.
large values of no/n, are low rpm operation.

the graphs display high rpms, closer to the left side bottom, while low rpm is further to the right side of graph.  backwards to what i am used to.

hope it helps.
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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #35 on: February 27, 2016, 08:08:20 pm »
So thanks to a recent storm I had many hours of shoveling gravel off my yard and dragging the lane with a box blade, which gives lots of time to think about this kind of stuff.....

from what i gather from the nagao & shimamoto paper, reducing the case volume too much can cause the charge to blowback through the carb at lower rpms they were testing, with porting kept the same.

reasoning they had was the pressure builds up too fast in the case.  as the case pressure rise fills the transfers too quickly, a portion of the charge goes back out the intake with a small case, at lower rpms.

------------------

(above must happen because the case pressure gets equal or greater than atmospheric before the intake port closes......my words not theirs here).

the case has harmonic cavity resonance and wave pulses.  changing volume, length of port, port diameter, carb size........ can all change the pulses as well.  at a certain rpm the pulses become tuned to rest the motor.  at others they are out of phase.  intake tuning.

suggest sticking to the paper, not my ramblings.

I am not convinced of this explanation (yet).  On a piston ported engine the intake open event is symmetrical, and no other ports are open at the same time.  So from a low speed/static point of view, whatever volume of air it draws in, the same volume will be pushed back out.  However, if the case volume is larger than the volume it draws in, it is not necessarily the same air that goes back out - some fuel mix stays in the case.  And the case volume is always bigger than what is drawn in.  In my example engine upthread the intake is open for 41% of the swept displacement (and the piston is moving through TDC, exposing the maximum volume cylinder), and that does not count transfers and the con rod slot. 

I'll come back to this...

hey chris.
 
what you have shown here is about all the information one could gleam from looking at this using just degree durations and volume displacement.  i don't disagree with the conclusions.

my point of contention, when i posted that a case can be too small is that 'every case is too small.'
the point being, that at any motor rpm, that is less than the peak torque rpm, the case is too small.  at peak torque rpm, it peaks to a value 'as good as its going to get.'  lastly,at any rpm higher than peak torque rpm, it is too large.

looking at this, like you have is a good way to visualize it.

looking at it, adding in both time areas and rpm, lets one look at it a little more clearly.

if somebody works the heavy math involved, to add in pressures and mass flow, we could look at it exactly the way those two guys did who wrote the paper.  in this last way, their calculated numbers matched values that were closely approximated to the results obtained from the actual physical tests performed on the motor.  (would be cool to be able to graph the pressures and mass flow at any rpm).

If we imagine the case volume at the extremes it is useful to see how things respond to that variable.  So imagine a very large case volume - maybe there is a huge tank under the cylinder.  Then when the piston rises the volume in the case is changed by only a small percentage, and since air is compressible then that small change in volume causes a proportionately small change in pressure - and there is not much pressure difference to drive the movement of air into the case.  So the volume of air intake is very small.

Next imagine if we could reduce the case volume to equal the displacement.  Now the volume of air drawn in is as large as it can be (higher volumetric efficiency), and what is spit back out is too.   As rpm goes up there is simply less time for moving the air, but it is still symmetrical so I don't thing rpm changes the picture. 

Now is the spit back situation improved in any way by drawing less fuel mix into the case (larger case volume)?  I don't see how it could be. 

In a running engine there may well be a continual flow of air into the case that causes a bias towards retaining more of the air drawn in, but a smaller case volume will still make it a better pump.

So I'm still struggling to come up with some mechanism whereby lower case volumes are bad (at least without a pipe).  It might be some secondary effect like the small case volume restricting flow to the transfers, but that could be mitigated by placing the transfer inlets in line with the con rod slot - such as under the exhaust port like in many modern saws......


Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #36 on: February 28, 2016, 08:40:13 pm »
do think you are bringing up some interesting points.  i like the way the paper explains all this.

what i get out of it, so far.

both the time area of the ports and the case volume, are changing the amount of mass flow (delivery ratio) and pressures.
and, both time area and case volume change with rpm in opposite ways.  time area values gets smaller, and case volume becomes to large at higher rpm. opposite of that at lower rpm, so the time area is too large, while the case is too small.

on the discussion page, shweitzer says "with small volume and low speed, the crankcase discharges too fast into the cylinder, and during the rest of the transfer period, there is reverse flow."

thinking, since the time area is too large at lower rpm, it also allows the ease of filling the transfers too quickly with little resistance.  once the trans are filled with fresh mix, it happens during the time where the pressure back-flowing in the trans, from the cyl to the case is quickly stalling flow. for the remainder of the time, the piston, still on the down-stroke is just pushing the remainder of the intake charge out through the carb, and easier way out.

as rpm increases, this all goes away.  and the delivery ratio is just controlled mainly by the time areas.  at least that is what the graph looks like when set it up like they say to.  it follows the shape of the time areas changing with rpm.

that is all i have absorbed so far.  hope it is correct?



 
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #37 on: March 02, 2016, 07:29:59 am »
without working out the heavy math, there is really no way i can to make any further headway on this.   so, my view of how time area is influencing the delivery ratio in opposite ways as compared to crankcase volume, may not be the on the mark.  from what information i can go on at this time, comparing the time area delivery ratio graph i worked out, to the shapes of crankcase delivery ratio graphs as shown in the paper is not a definite.  what looks and quacks like a duck, may not be a duck at all.

leaving this alone for others to explore further. 
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #38 on: March 03, 2016, 03:48:14 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.



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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #39 on: March 05, 2016, 08:48:13 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. 

 

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