Richard,
Anything related to the alternator output would vary with the RPM, but it
wouldn't surprise me if the Ducati regulator was doing something mysterious.
A vibration within the regulator attached to the firewall would allow the
firewall to act as a sounding board as you suggest.
If it can be done safely, you might be able to see if the regulator is
involved by getting someone to press their hand on it while the engine is
running. If this damps the vibration (or the hand can feel the buzz) then
this could confirm it. Alternatively, you might temporarily detach the
regulator from the firewall (but make sure it is still properly grounded)
and see if the resonance is still there. Ducati regulators are not known
for their longevity, especially if they get hot in service (go on, tell us
it's hot in Kenya!). Many regulators had been replaced after 300 hours or
less.
I am not sure how normal is your delay of a minute or two before the voltage
rises from 12 to 14. The regulator ought to provide full charging voltage
immediately the alternator is up to speed, unless there is a discharged
battery drawing a heavy current that drags the voltage down; even then, I
would expect the voltage to rise slowly as the battery charges rather than
jump suddenly, assuming you are not switching any major electrical loads at
the time.
I defer to Gilles Thesee regarding the internal workings of the regulator
and what might cause it to act in this way, if indeed it is the culprit.
Mike Gregory
_____
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Richard Lamprey
Sent: 15 January 2008 00:44
Subject: Europa-List: Rotax 912 electronic noise
Can anyone shed light on an strange noise that emanates from my engine on
startup? The engine is a basic 912UL, with about 520 hours on it. The
noise is an electronic note, almost flutelike, around G in pitch. After
startup, and idle at about 2200 RPM, you hear this noise through the
firewall, loud enough for a passenger to comment. With increasing RPM, it
doesn't increase much in pitch, just slightly, and therefore I think it must
somehow be related to the alternator output, or possible the voltage
regulator (Ducati); and not related to any noisy bearings in anything.
You cannot detect this noise if you stand close to the running engine, with
the cowling off. It seems that the firewall acts as a resonating surface to
amplify it to inside the cockpit.
The noise goes for about 5 minutes during warm-up, then quite suddenly it
completely disappears, usually at about the time of taxying out. All
through this startup, the engine runs smoothly, and temperatures and
pressures behave normally. The big spike-protecting capacitor charges up,
and after a minute or two from startup, the voltmeter shows a switch from 12
volt (not charging) to 14 volt (charging), All quite normal, and seemingly
unrelated to the noise.
This noise didn't happen when the engine was new. It began after about 300
hours of engine time. It is particularly noticeable after sometime without
engine operation, say 2 weeks or longer.
Any ideas on this mystery noise gratefully received!
Richard Lamprey
Europa Classic, 5Y-LRY
Nairobi, Kenya
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