Author Topic: crankcase vol. (no drilling involved)  (Read 464 times)

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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #70 on: March 16, 2016, 06:31:14 pm »
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?

my understanding at this point, is that two factors control what ultimately becomes a motor's delivery ratio.  ie. case volume and porting.

in the first part of the paper on case volume, porting was held constant, while they tested differing case volumes.

i found another paper in which testing for delivery ratio was done by keeping the case volume the same, and changing porting only.
found here (starts on page 17):  http://www.vintagesnow.com/SledU_Folder/delivery_ratio-1.pdf

combustion chamber......so far, i have not found mention of this affecting delivery ratio.  i was not on the lookout for this while reading and may have missed it?

while this may or may not be completely true, my belief is the optimal case sizing has to match the porting sizing for a overlapping range of rpms to work well.  still learning on this.  read 4 papers on this so far, just slogging through it all.  two sources mention that delivery ratio is the most important factor for making power, for motors which lack a tuned exhaust.

thought the estimate i was working on for case size/rpm would have ended better.  the error between optimal and estimated, increases with rpm.  they are two straight line style plots.  could post it.  too bad it was not more accurate.
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Offline Chris-PA

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Re: crankcase vol. (no drilling involved)
« Reply #71 on: March 16, 2016, 08:56:37 pm »
The combustion chamber question was not from a paper, just my own musing.  The more fuel/air you can trap in the combustion chamber, the more energy is stored in there at ignition.  Delivery Ratio is a way of measuring that by comparing the trapped volume to the displacement. 

So if you can increase the volume available to trap that fuel/air by increasing the combustion chamber volume, do you increase deliver ratio?  You still have to turn whatever is trapped into mechanical power effectively, so DR is not the only factor by far - and a larger combustion chamber (lower compression ratio) may really hurt in doing that. 

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #72 on: March 17, 2016, 06:43:36 am »
The combustion chamber question was not from a paper, just my own musing.  The more fuel/air you can trap in the combustion chamber, the more energy is stored in there at ignition.  Delivery Ratio is a way of measuring that by comparing the trapped volume to the displacement. 

So if you can increase the volume available to trap that fuel/air by increasing the combustion chamber volume, do you increase deliver ratio?  You still have to turn whatever is trapped into mechanical power effectively, so DR is not the only factor by far - and a larger combustion chamber (lower compression ratio) may really hurt in doing that.

see where you are going with this.

imo, combustion chamber size sets compression.  since compression is pressure and the higher the pressure the higher the heat, and the more heat, the more power.  so i would side, on that the smaller the chamber - the better.  because of the whole pressure cooker idea.

the heat, got me onto the whole otto cycle p-v diagram/heat transfer thing a few weeks ago.  there is a free program that looks at this.  it is what is used for both 4st and 2st.  it is way off for 2st, when i plugged in heat data numbers for a 2st, from blair's book.  he came up with a better way to look at this.  anyhow, you could plug in different values for compression ratios, and use the program to get your own conclusions.

<edit> added the link here:  http://ronney.usc.edu/spreadsheets/
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #73 on: March 19, 2016, 07:33:06 am »
for those who still have interest in this kind of thing, something to look into, is volumetric efficiency.  this is basically what folks call delivery ratio these days.

there are lots of views on this.  an interesting thing is that once you find the VE, you can find the CFM the motor needs for a certain amount of power/torque at an rpm.  so, a very basic dyno curve like simulation can be had.  this was the initial reason for my interest in delivery ratios.

like i mentioned, unless someone stays at a holiday inn and figures out the heavy math, the best that can be had is an estimate.  using the crankcase DR, therefore is out for me.  the T*A DR is what i will be using, since the values are based on the simple porting program found somewhere here in another thread.

when i find things of interest that may help folks in these papers and such, i post what may be useful.  i'm over the rehashing of jennings to the point that it causes effects similar to the acidic juices of pre-vomiting in my throat after a night of heavy bourbon consumption.  hahahaha.
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