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mcculleyja(at)starpower.n
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 Posted: Sat Jan 13, 2007 3:38 pm Post subject: Lycoming Camshaft Question |
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In Lycoming 4 cylinder engines there are only 6 cam lobes. If I have
deduced it correctly, the four exhaust valves are each driven from their
individual cam lobes, but the intake valves share cam lobes as follows:
Cylinders #1 & #2 share the same lobe and #3 & #4 share another lobe.
This arrangement results in only 6 total cam lobes being necessary.
(BTW, in the 6 cylinder models, there are 9 lobes for the same reasons)
My questions are the following:
1) For those of you with considerable experience in tear-down of these
engines for problem analysis or overhaul have you noted any correlation
in observed cam lobe wear of the lobes that drive the intake valves
versus those lobes driving exhaust valves? Since the intake lobes make
twice the number of follower contacts per crankshaft revolution the rate
of wear would seem to be greater, if all other variables are constant.
2) Assuming that one of the lobes driving a pair of intake valves does
wear significantly, what engine operating symptoms would be a clue to
this event?
3) How would this symptom differ from that of an exhaust valve lobe
wearing to a similar degree?
4) Which situation, if not detected by oil analysis and cutting open oil
filters, would be easiest to recognize via engine performance degradation?
Jim McCulley
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tompkinsl(at)integra.net
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| Posted: Wed Jan 17, 2007 2:20 pm Post subject: Lycoming Camshaft Question |
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You have made a correct observation about which cam lobes are likely to exhibit wear signs first.
A good sign that the cam lobes are worn is good compression but low power. All other things being equal, your engine won't be able to make the WOT static RPM that it could just after the engine was broken in and developing full power and your take-off runs and climb-outs will seem weaker. Top speed would be down slightly.
It will be interesting to see if roller tappets offer a durability improvement for cam lobe wear. The great difficulty is that most of us can't fly our aircraft frequently enough. If the engine operated every day like a car there wouldn't be any internal corrosion issues. Perhaps someone more familiar with with cam lobe surface corrosion, roller face corrosion and internal needle bearing corrosion can comment. Race car engines have used "roller cams" for almost three decades, but it took nearly two decades before that to get all of the reliability problems worked out. If the roller tappet hops and brinnells a needle bearing in the roller, life is drastically reduced. Racing camshaft folks solved this by reprofiling cam lobes and the addition of a "rev kit," which are springs over the tappets themselves to keep the tappet riding on the lobe. I don't see how that device could be incorporated in a Lycoming. I have a hard time convincing myself that there is enought test & development done on new designs, which is understandable since production volumes are so low (compared to the auto industry, for example).
This is probably more information than you wanted, but I hope it helps.
Larry
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archie97(at)earthlink.net
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| Posted: Wed Jan 17, 2007 3:01 pm Post subject: Lycoming Camshaft Question |
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Good answer, Larry
You covered the basics well.
Since the aircraft engine runs at an essentially steady RPM, lifter bounce can be eliminated by adding a few more pounds of spring pressure.
The additional pressure is insignificant when using a roller, and a rev kit would be unnecessary in this case..
Most aircraft reciprocating engines have less than 100 lbs on the seat, whereas our racing engines run anywhere from 270 to 410, and have open pressures around 1000 lbs.
Unfortunately, as I understand it, the factory rollers are not rebuildable, and must be replaced, unlike the racing counterpart.
Archie
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n801bh(at)netzero.com
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| Posted: Wed Jan 17, 2007 3:14 pm Post subject: Lycoming Camshaft Question |
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Roller lifters in an engine that turns 2700 rpms will last longer then your plane will...
Ben Haas
N801BH
www.haaspowerair.com
-- "Larry L. Tompkins, P.E." <tompkinsl(at)integra.net> wrote:
You have made a correct observation about which cam lobes are likely to exhibit wear signs first.
A good sign that the cam lobes are worn is good compression but low power. All other things being equal, your engine won't be able to make the WOT static RPM that it could just after the engine was broken in and developing full power and your take-off runs and climb-outs will seem weaker. Top speed would be down slightly.
It will be interesting to see if roller tappets offer a durability improvement for cam lobe wear. The great difficulty is that most of us can't fly our aircraft frequently enough. If the engine operated every day like a car there wouldn't be any internal corrosion issues. Perhaps someone more familiar with with cam lobe surface corrosion, roller face corrosion and internal needle bearing corrosion can comment. Race car engines have used "roller cams" for almost three decades, but it took nearly two decades before that to get all of the reliability problems worked out. If the roller tappet hops and brinnells a needle bearing in the roller, life is drastically reduced. Racing camshaft folks solved this by reprofiling cam lobes and the addition of a "rev kit," which are springs over the tappets themselves to keep the tappet riding on the lobe. I don't see how that device could be incorporated in a Lycoming. I have a hard time convincing myself that there is enought test & development done on new designs, which is understandable since production volumes are so low (compared to the auto industry, for example).
This is probably more information than you wanted, but I hope it helps.
Larry
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