Chainsaw Repair

...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