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 #10 on: July 04, 2015, 05:09:31 pm »
...have not had much available time lately.  had to reread my previous posts.

anyhow, sometimes a tiny scrap of information can shed some light on motor gizzards.

did not show how I came up with with an estimate of how much a motor ingests into a crankcase on every stroke.  (it also gives me an opportunity to correct myself).

blair did an example of this crude estimate in his 'book,' below is using a j'red in similar fashion.

given:

jonsereds 52
49cc displacement
TDC crankcase volume = 164.6 cc
BDC crankcase volume = 93 cc
crankcase compression ratio = 164.6/93 = 1.77:1

delivery ratio = 0.5
(delivery ratio is amount air-fuel mixture, that is ultimately used for combustion.  In other words... the cylinder volume is 49 cc total.  If this total cylinder volume could be filled completely, the delivery ratio would be 1:1,  or de = 1.  a saw motor which I found data for, had a de = 0.5, so only half of the entire cylinder volume is filled…..if its good enough for blair's saw motor, should be good to use for an estimate).

de = 0.5 was chosen, because that is what blair was using.
 
de of 0.5 = 49 cc/2 = 24.5 cc

(this is very poor because the more air-fuel that can be squeezed in there, the more heat can be made, and more output can be had).

fwiw, the de also = torque curve.

as it relates to this motor's crankcase in particular:

(24.5 cc)/(TDC crankcase volume) = 24.5/164.6 = 0.1488 x 100 = 14.88%

only 14.88% of the total crankcase volume is being filled on every stroke.

case volume ratio (comp ratio), in this particular saw is very loose, at 1.76:1

taking this a step further....

each individual transfer port cc'd out to be 4 cc by measuring.  8 cc total.

so, as the piston descends to BDC it is squeezing down on at least 24.5 cc of new mixture on every stroke, and sending it up through 8 cc of transfer port total volume.

hypothetically...... in order to cram 24.5 cc into the chamber itself, one would need
24.5 cc + 8 cc = 32.5 cc of mix to do so, because the trans are holding 8 cc of the charge.

…...who gives a frog you say!

...hope it may shed some light on effect of transfer port volumes, expectations of realistic modification gains within a motor, etc.

the chamber size w/piston at TDC measured out at a volume of 4 cc.

if some of the 24.5 cc mix did not leak out the exhaust (which it does) .......then 24.5 cc's gets squeezed into 4 cc.


i had no idea that so little A-F charge enters the motor on every spin.  changed the way i think about things motor.
none of which would have happened if i did not get the calculator out.


*there is a good paper on 'crankcase delivery ratio' on the interweb that shows how crankcase volume and port sizing are connected to delivery ratio if anyone does decide to take more interest.


hope it helps
-joe












 
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #11 on: July 05, 2015, 09:55:10 am »
link to the crankcase volume paper from previous post:  http://www.bridgestonemotorcycle.com/documents/crankcase_volume6.pdf
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Offline Cut4fun .

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Re: crankcase vol. (no drilling involved)
« Reply #12 on: July 05, 2015, 11:51:23 am »
I have always wondered the crankcase volume differences in the husky 50 special and the partner 5000 plus. Seems like folks preferred the partner 5000 top on the husky 50 special crankcase.  Always wondered the difference in them or not.

Very neat reading by the way.  8)
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Offline mdavlee .

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Re: crankcase vol. (no drilling involved)
« Reply #13 on: July 05, 2015, 03:11:00 pm »
Good info.

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #14 on: July 11, 2015, 09:15:33 am »
link to the crankcase volume paper from previous post:  http://www.bridgestonemotorcycle.com/documents/crankcase_volume6.pdf

some things from this paper:

the paper uses Vc/Vs (this is slightly different than crankcase volume ratio).

Vc = crankcase volume cc @ BDC
Vs = motor displacement  cc

using the jonsereds 52 as an example:

crankcase volume at BDC = 93 cc
motor displacement = 49 cc

so, Vc/Vs = 93/49 = 1.9

......The larger this number, the more crankcase volume there is.


Figure #18
-Effect of Intake Duration on crankcase pressure.  (Carby sizing also plays a big role).

....if you are interested in whys/hows of intake reversion, and whats going on....possibly worth a read.
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Offline aclarke

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Re: crankcase vol. (no drilling involved)
« Reply #15 on: July 11, 2015, 11:32:39 pm »
Joe,  shows the potential pitfalls of hogging out transfer ducts and loosing more of the already small charge that makes it into the chamber.   Cool stuff. Thks


Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #16 on: July 14, 2015, 06:02:51 pm »
.....i may need to clarify.

the example saw motor in this thread has a box muffler.  pipe motors crankcase requirements can differ, because they are highly influenced by the pipe.
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Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #17 on: July 14, 2015, 06:42:31 pm »
...taking a look at crankcase pressures which were used in a saw motor in gb's writings:

(more of an fyi thing, as i have no way of measuring these in the jon's 52.  so it is not apples to apples comparison).

lowest crankcase pressure = 0.8 atm = 11.7 psi
highest crankcase pressure = 1.5 atm = 22.0 psi

now, here is the interesting part . (hahaha, if these ramblings of mine were indeed interesting at all).

crankcase pressure builds well before intake closes.  goes from 0.8 atm to 1.1 atm

so, 0.8 atm (11.7 psi), is only 3 psi below atmospheric pressure.

...since, atmospheric pressure = 1.0 atm = 14.7 psi at sea level.   so, at anything under 1.0 atm, air-fuel mix (delivery ratio), is entering the motor.

this is the reason where pipe motors put out 3x the power of box muff motors.....they pull it down well below 0.8 atm in the crankcase...pulling in a bigger gulp, ie. more delivery ratio.

after intake is closed, goes from 1.1 atm to 1.5 atm, so, about 22 psi is being forced up the trans ports.  some of this pressure is gained from blowdown, so crankcase pressure has to overcome this.

another interesting thing, (and i say this loosely), how do crankcase comp degree measurements give any inclination as to any of this?  hope someone can explain this to me, because i am not seeing this at all.
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Offline EHP

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Re: crankcase vol. (no drilling involved)
« Reply #18 on: July 28, 2015, 07:08:41 pm »
I was shown abit different way to do crankcase volume mainly so it works with my TSR program , the way I was shown a 3120 ends up at 1.47 to 1 ratio , You want 1.20  to 1 ratio . That much changes will produce alot more power but takes about double the fuel to produce it and to get the 1.20 ratio your pretty much outside the case so alot of extra machine work , If you got the time the best thing is to machine a new crankcase so you can make more volume and 2 carbs on it to get enough fuel to make it run

Offline 1manband

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Re: crankcase vol. (no drilling involved)
« Reply #19 on: July 28, 2015, 09:03:38 pm »
.....i read a good thread on a bike site, that a couple of aprilla race motor engineers chimed in on.  talking about all that stuff.

edit:  just remembered that some of the thread was in italian.  it switched to english the last 30+ pages.  interesting read.



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