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frans(at)privatepilots.nl
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 Posted: Mon Oct 10, 2011 3:09 am Post subject: Heavy In-flight Vibration Riddle /The Solution /The Winner( |
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On 10/09/2011 09:55 PM, Raimo Toivio wrote:
| Quote: | How could loosing only one plug to make such a hard situation? My only
guess is that it was detonation; below and over certain rpm´s and map´s
the engine was very unhappy.
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Burning of the fuel-air mixture will start at the spark-plug and from
there it will migrate to the other end. This takes some time. This is
why ignition always takes place before the piston is at top dead center.
The Rotax has two spark plugs per cylinder, so the burning starts at two
opposite sides and takes less time. Pre-ignition time is set to a lower
value to deal with this. If one spark plug fails, it takes longer before
all the contents of the cylinder is burned, pre-ignition is of course
not corrected, so probably some burning is still taking place when the
mixture is expelled through the exhaust valve. The result is less
pressure in that cylinder. This causes an imbalance.
Lower RPM gives more time for burning, and a less dense mixture burns
faster.
I wouldn't have guessed that this imbalance would be so noticable. Maybe
the delayed pressure wave in the cylinder prevents fresh mixture to
enter the cylinder so in the next cycle less mixture can be burned,
hence an even lower pressure and more imbalance.
Anyway, it was interesting to learn about this problem. I will take a
mental note that if I ever have to deal with vibration that I will check
if switching off one ignition circuit will improve the situation. Thanks!
Frans
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zwakie
Joined: 03 Aug 2009
Posts: 157
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| Posted: Mon Oct 10, 2011 4:01 am Post subject: Re: Heavy In-flight Vibration Riddle /The Solution /The Win |
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I have another thought on top: what will this have done to the crankshaft and/or piston rod and bearings?
Should one be worried about increased tolerances, excessive wear a.s.o., especially having continued flight for an hour?
I have made the same mental note that Frans has made, I too would not have expected vibrations to become so imminent by one failing spark plug.
Thanks Raimo!
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_________________
Marcel Zwakenberg
Europa XS TG || 912ULS || PH-SBR |
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