Nigel,
I remember the kickback thread and also remember being taught (many
many years ago) that the dual ignition on aircraft engines were at
different timing and that some engines disabled the more advanced
ignition to improve starting. So, I dug up my ROTAX 914 Manuals (PDF
format on CD provided with the engine) to see if ignition timing was
documented. The ROTAX 914 Maintenance Manual, Sec. 9.4.1), Page 50
describes ignition timing. I will try to quote the page verbatim
(don't blame wording, punctuation, etc. on me), use of a fixed width
font is recommended:
Quote -----
Ignition
See Pic. 24.
As already stated the engine is equipped with dual ignition of a
breakerless
capacitor discharge design (DCDI). That means that the ignition unit
com-
prises two independently working ignition circuits (separate trigger
coil,
electronic module, charging coil etc.).
*NOTE: The ignition unit is completely free of maintenance and
without any adjustment.
Each ignition circuit consists of two ignition branches. Ignition
occurs on
cylinder 1 and 2 simultaneously every 360d as well as on cylinder 3 and
4 but
180d offset.
*NOTE: Due to engine design ignition occurs also at overlap
T.D.C.,
but this is for engine operation insignificant.
The engine is furnished with an automatic ignition adjustment
controlled by
the edge of trigger cams on the flywheel and the electronic modules.
*NOTE: For easy engine start the ignition timing at start is 4d
B.T.D.C.
As soon as the engine runs the ignition timing will change over
automatically
to operation timing of
26d BTDC on ignition circuit A and
22d BTEC on ignition circuit B
The transition from start ignition timing to the timing for operation
takes place
between 650 to 1000 rpm.
*NOTE The different ignition timing for the top spark plugs and
the
bottom spark plugs take into account the differing ignition
lag resulting in better knock behaviour.
The difference in the ignition timing will be achieved by
different length of the trigger cam.
Trigger cam for ignition circuit A (raised position) is
approx.
4 mm (.16") longer.
Unquote ---
From this information, I don't see that shutting off one ignition
circuit would help. Unless a bit less energetic start of the
combustion is all that may be needed.
Now this if for the 914, the 912 and 912S may be different. You might
check the appropriate maintenance manuals and see what they say about
timing. From there, perhaps, a suitable solution might be determined.
Just my tuppence...
Good building to those still building
Good flying to those fortunate souls,
Bob Borger
Europa Kit #A221 N914XL, XS Mono, 914, Airmaster C/S
(65%) tail kit done, wings closed, cockpit module installed, pitch
system in, landing gear frame in, rudder system in, outrigger mod in,
Fuselage Top on, lift/drag/flap pins in. Working in - 24 Instrument
Panel, 25 Electrical, 29 Main Gear, 30 Fuel System, 32 Tail & 34 Door
Latches.
3705 Lynchburg Dr.
Corinth, TX 76208
Home: 940-497-2123
Cel: 817-992-1117
> There was a thread recently about kickback during start with the
912S. This
> can be quite vicious and I had it happen quite often especially on
cold
> starts. Apparently it can also lead to damage to the starter sprag
clutch.
> The problem is worse with the 912S than the 912 or the 914 due to its
higher
> compression ratio. It was pointed out that it happens because, with
the
> Rotax ignition system there is no provision for retarding the
ignition during
> start. With this in mind it occurred to me that the speed of ignition
may
> be slightly slower if the engine was started with just one ignition
system
> working. I have since found that I get less kickbacks when I start
using
> just one ignition system. It is obviously important to remember to
reselect
> both magnetos after engine start. This technique is only possible if
you
> have separate magneto and starter switches although it would be
possible
> to add extra wiring to short out one ignition system during start
with a
> key starter.
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