Chainsaw Repair

Quote from: 1manband on February 26, 2016, 07:18:26 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).