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Author Topic: Engine like question...  (Read 1092 times)

Offline jim gilmore

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Engine like question...
« on: May 10, 2008, 02:27:10 PM »
Ok I know my subject is way off topic but engine related math is similar.
Basically if you could change the length of a crankshaft so there was more or less compression the volume of air moved would remain the same ? The actual subject is a bellows drywasher for finding gold. I'm wondering if lowering/lengthening my crankshaft will reduce load on my electric motor but keep moving the same amount of air.
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Offline Ray

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Re: Not really shaft?
« Reply #1 on: May 10, 2008, 03:39:29 PM »
Let's recheck terminology, shall we?  Stroke length changes both compression and displacement.  Shaft length has no relationship to either one. 

Connecting rod length, on the other hand, could potentially change the final compression ratio, while keeping the same displacement. 
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Offline jim gilmore

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Re: Engine like question...
« Reply #2 on: May 10, 2008, 04:17:48 PM »
Sorry You are correct. If I shorten my connecting rod I still move the same volumn of air because my crank/or off set is still 2.5 inchecs each way?

Just not compressing it as much ?
In a drywasher i'm not sure that compression is necessary.
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Offline L0U CRANE

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Re: Engine like question...
« Reply #3 on: May 12, 2008, 09:56:36 PM »
Jim,
I can't quite picture what you are asking...

The only effect a longer or shorter crankshaft (from the crankdisk to the thrust washer behind the prop) could have is the volume of air space in the bore through the shaft from the port to the open space under the piston. I.e., if the choke is far forward, there'd be more volume in that bore space...

However, if by the "...length of the crankshaft..." you refer to the stroke radius, that affects more than the compression ratio. Displacement will change. If nothing else is done with the cylinder ports location, timing will also change.

...and, I don't see how this connects directly with load on an electric motor... Any ideas to clarify??
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Offline jim gilmore

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Re: Engine like question...
« Reply #4 on: May 13, 2008, 02:12:04 PM »
ok, i'll try to explaine simply. A bellows drywasher is basically a box with a door on the bottoms side that pivots from one end. the other end gets attached to a motor with an offset crank. just lIke a crankshaft that drives a piston kind of. Its like the bellows you might use on a fireplace. I can adjust the length of the connecting rod as well as the length of the stroke/oroffset of the crank. Suppose I just change the connecting rod so it does not compress as much? the door/bellows will still travel 5" but will open a little wiser, and not close as far. Will the volume of air it tries to move be the same ? But compression wise it will be less, am I correct ? So with less compression on the overall movement the load to the motor should go down some ?

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Offline Gordon Tarbell

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Re: Engine like question...
« Reply #5 on: May 13, 2008, 06:51:54 PM »
Are you drywashing for gold in them thar hills? Thats the only thing I ever used one for.
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Gordon Tarbell AMA 15019

Offline Gordon Tarbell

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Re: Engine like question...
« Reply #6 on: May 13, 2008, 07:00:24 PM »
Yes you are sorta right, the net effect of a shorter connecting rod length in your case ( point of connection at crank and wrist pin(or gugieon /spelling?/pin) with crank stroke unchanged the compression ratio is lower. which should reduce load to motor.
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Gordon Tarbell AMA 15019

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