Landracing Forum
Tech Information => Technical Discussion => Topic started by: jlmccuan on April 12, 2013, 02:15:51 AM
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How close above or below the bottom of the cylinder wall is the lowest feasible position? Is it possible to have an application where a portion of the wrist pin is out of the cylinder at BDC ? Has anyone attempted to run something similar?
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How close above or below the bottom of the cylinder wall is the lowest feasible position? Is it possible to have an application where a portion of the wrist pin is out of the cylinder at BDC ? Has anyone attempted to run something similar?
"There is nothing new under the sun" Just new combinations of the same thing!
In other words .. .. .. Yes people have tried before. The limiting factor beside's hitting the crankshaft counter weight, is that if the wrist pin gets too close to leaving the cylinder, the piston skirt will gall on the bottom edge of the cylinder when the piston pivots during the change in direction.
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How close above or below the bottom of the cylinder wall is the lowest feasible position? Is it possible to have an application where a portion of the wrist pin is out of the cylinder at BDC ? Has anyone attempted to run something similar?
An engine's build geometry (stroke length, rod length, crank counterweight radius, piston pin height, pin offset, etc, etc, . .) will dictate whether or not this is possible. Prior to designing a build with this type of geometry, do not forget that the portion of the piston skirt below the pin centerline, supported by the cylinder wall, is what stabilizes the piston (and therefore, the ring seal) during "rockover" @ BDC &, to a lesser degree TDC.
Just off the top of my head, I can't think of a "high rpm/speed" engine that uses this type of design, although I confess I'm not trying that hard. Personally, I think it is a poor design choice, because of the BDC instability and the resultant increased wear.
In other words .. .. .. Yes people have tried before. The limiting factor beside's hitting the crankshaft counter weight, is that if the wrist pin gets too close to leaving the cylinder, the piston skirt will gall on the bottom edge of the cylinder when the piston pivots during the change in direction.
If, perchance, the piston "jams" at the base of the cylinder bore, well, . . . . BANG!!
:cheers:
Fordboy
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Just ran across this...I will ad a note from "long ago". The first problem you might have is with the rings. They will roach and you will lose your Dykes effect. If you have to run pretty close to the bottom, but stay in the bore, you may have to run one piece oil rings.....not sure, we never tried three piece in big stroke increase.
We used to pull a lot of skirt out of the bore, back in the days of big stroke increase in small Hondas. We had pistons cut almost to the wrist pins, just to clear the full circle crank. With one piece oil rings, we were ok. Thats how you take a 90cc single and build it to 174cc! The rings are doing a lot of work keeping a piston straight, so skirt length has all but disappeared on some engines. Now they have cage pistons and they work well!
Piston acceleration gets pretty fierce when you add a lot of stroke without longer cylinders and rods. You will subtract rpm if you go down this path. Keep in mind, you can have pistons made with bridged or plug mounted oil rings and the wrist pin moved up. They can move the ring pack up, with thinner rings and closer spacing. I was looking at as much as 4-5 mm shift upward (working with CP) when I was considering taking my 650 twin into 1000 class. That would have added 8-10mm to the stroke and barely got me into a tough class. That usually ends badly. :|
Remember...if you add a lot of stroke, you need to reduce cooling in the bottom of the bores....trim fins if air cooled, or insulate if water cooled. All the new longer stroke production cars and trucks are using some method of keeping heat in the bottom of the bores, due to the longer expansion time. And....Dont pull the pin out of the bore, build the pistons to suit.
Regards, JimL
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Just a thought,
If you increase the stroke a lot, you will probably need to lengthen the connecting rod C/C length by approximately the stroke increase*(.5) just to get some clearance back. . . . . AND, you will want to run a simulation based on the original geometry Vs. the "new" geometry. . . . . . . . you know, just to check. . . . . . .
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Yes, the piston accel goes up significantly as the rod ratio goes down but it's not that violent (as compared to a similar increase in RPM).
Example, a 3.00" stroke engine at 8,000 RPM with a 6.00" rod (n=2) vs. a 4.50" rod (n=1.5), 25% shorter.
Long rod, Z = 109.639
Short rod, Z = 116,948 (+6.7%)
The equivalent RPM change is only about 250.