Soft start unit for Rotax 912ULS

corrado95

Newbie
Joined
Jan 28, 2009
Messages
11
Location
Upplands Väsby
Hi,
I have this engine in my autogyro.
This engine kicks a lot at startup.
I have seen on R.O.A.N. that there now is a soft start unit that Bullyhawk is making. Cost is $220.
Have anyone out there experience of it?
Here in Sweden i have been warned for it. They say it's not good. The idea is stolen from Rotax. Rotax will late this year come with it's own solution.
It will be 2 new units, to replace the original ones. Cost around $800? Not as Bullyhawk do, put a third upon the 2 there is allready.
Should be good to hear what you guys say!
 

Turbo-Tom

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Joined
Feb 22, 2009
Messages
18
Location
Usingen
I have designed something like this for our fixed wing ultralights (Ikarus Comco C-42) that are also utilizing the 912. Actually I reverse engineered a broken CDI to figure out how the system works. The engine hasn't got a true variable ignition timing (like we know from the modern automotive engines). Instead, it's only two distinct timings settings, a delayed one for low engine RPM (and thus starting) and another advanced one for higher RPM.
They distinguish between the two modes by actually generating two trigger pulses each time the notch on the flywheel passes the spark timing pickup coils, one when it closes the magnetic field and another one when it opens again. Since the value of the voltage generated depends from (amongh other things) the speed at which the magnetic field varies, it also depends on engine RPM. Now Rotax just used a zener to limit ignition on the first (leading) impulse to higher voltages.
Yet, with powerful batteries, and especially when it's quite cold (since the zener voltage is somewhat temperature dependent), the ignition box may trigger on the first, leading impulse even during starting, which results in the observed kicking.
We found that starting only on one ignition circuit sometimes helps things a little, but finally I put a relay circuit in one ignition circuit that will be activated with pressing the starter button and that has a capacitor-controlled delayed release. When activated, the system routes the ignition pulses through two diodes so only the late iginition pickup pulses will pass. Since advanced and delayed pulses have got opposite polarity, this is no problem and absolutely bullet-proof. When the starter is released and the relay delay cap dischared, the relay will short the diodes and allows normal operation of the engine. We left the other ignition circuit untouched (for safety reasons) and switch it on after the engine has been started on the modified one.
This system proved to work reliably during the last five years, yet it's only installed on a few UL's that belong to flying school where I got my license.
If anybody's interested, I may publish the circuitry.

Cheers,
Thomas

P.S. Turbines rule... ;-)
 

bartc150

Senior Member
Joined
Apr 10, 2004
Messages
317
Location
Arcadia, FL
Aircraft
Used to own an Ultrawhite, but sold it like a dumb A$$
Total Flight Time
less than 100
So are you saying you will have one starter switch for cold starts and one for warm starts?
 

GyroDoug

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Joined
Sep 23, 2005
Messages
3,664
Location
Layton, UT.
Aircraft
Butterfly Super Sky Cycle
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I would be very interested in that information. I suspect there aren't a large number of 912's being used in Gyros today but the number is starting to grow and in the future there will be many more. I think this would be great information to have on the forum.
 

Turbo-Tom

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Joined
Feb 22, 2009
Messages
18
Location
Usingen
Sorry for the delay, I just returned from Cyprus to do some more programming on my Solar T-62T-32 turbine engine governor (...now we've got data logging capabilities... :D ).

To answer bartc150's question, no, we haven't got additional switches on the system. The "delayed ignition system" is always active on one of the ignition circuits as long as you press the start engine switch. It releases after a short delay of one or two seconds. Since our circuit utilizes a four way relay, we use two of the contacts for delaying ignition while the other contacts are used to disable the second ignition circuit momentarily (by pulling the "brown lead" to ground).

This system always works perfectly well, we even go so far that we press "Start" again while we switch off the engine when it's hot because we found it to be kicking during shutdown as well!

Attached to this posting, you will find the schematic and the placeplan of the PCB for the simple circuitry. Comments on these drawings only in german, sorry...

Cheers,
Thomas
 

Attachments

bones

Gold Member
Joined
Jul 28, 2004
Messages
1,278
Location
Chtrs Twrs Australia
Aircraft
a/c
Total Flight Time
cant remember, didnt have log books before, but since over 3000
I have designed something like this for our fixed wing ultralights (Ikarus Comco C-42) that are also utilizing the 912. Actually I reverse engineered a broken CDI to figure out how the system works. The engine hasn't got a true variable ignition timing (like we know from the modern automotive engines). Instead, it's only two distinct timings settings, a delayed one for low engine RPM (and thus starting) and another advanced one for higher RPM.
They distinguish between the two modes by actually generating two trigger pulses each time the notch on the flywheel passes the spark timing pickup coils, one when it closes the magnetic field and another one when it opens again. Since the value of the voltage generated depends from (amongh other things) the speed at which the magnetic field varies, it also depends on engine RPM. Now Rotax just used a zener to limit ignition on the first (leading) impulse to higher voltages.
Yet, with powerful batteries, and especially when it's quite cold (since the zener voltage is somewhat temperature dependent), the ignition box may trigger on the first, leading impulse even during starting, which results in the observed kicking.
We found that starting only on one ignition circuit sometimes helps things a little, but finally I put a relay circuit in one ignition circuit that will be activated with pressing the starter button and that has a capacitor-controlled delayed release. When activated, the system routes the ignition pulses through two diodes so only the late iginition pickup pulses will pass. Since advanced and delayed pulses have got opposite polarity, this is no problem and absolutely bullet-proof. When the starter is released and the relay delay cap dischared, the relay will short the diodes and allows normal operation of the engine. We left the other ignition circuit untouched (for safety reasons) and switch it on after the engine has been started on the modified one.
This system proved to work reliably during the last five years, yet it's only installed on a few UL's that belong to flying school where I got my license.
If anybody's interested, I may publish the circuitry.

Cheers,
Thomas

P.S. Turbines rule... ;-)
Tom,

I only stumbled onto this tonight but i have been having trouble with my 100hp rotax starting on real cold mornings, it only has the small starter motor, and it is tired, from so many starts, how ever a couple of weeks ago, i decided to try something, as a habit i normally turn ign switches off and turn it over on the starter motor for a while to get the oil pressure to read, then stop turn on both igg and try to start with the common problem of the commpression completely stopping the motor a few times and really straining the starter.
So i decided lately to after i see the oil pressure come up i turn on just one side of the igg, and hey preto, the motor starts first go no probs, as soon as it fires i turn on the other igg, and it runs smooth no probs at all, so i might have just stumbled on the easy fix for my probs.
A lot cheaper than the $1500 option of new starter.
 

carlsnilsson

Newbie
Joined
Apr 23, 2011
Messages
1
Location
Hobart
Soft start for Rotax 912

Soft start for Rotax 912

Sorry for the delay, I just returned from Cyprus to do some more programming on my Solar T-62T-32 turbine engine governor (...now we've got data logging capabilities... :D ).

To answer bartc150's question, no, we haven't got additional switches on the system. The "delayed ignition system" is always active on one of the ignition circuits as long as you press the start engine switch. It releases after a short delay of one or two seconds. Since our circuit utilizes a four way relay, we use two of the contacts for delaying ignition while the other contacts are used to disable the second ignition circuit momentarily (by pulling the "brown lead" to ground).

This system always works perfectly well, we even go so far that we press "Start" again while we switch off the engine when it's hot because we found it to be kicking during shutdown as well!

Attached to this posting, you will find the schematic and the placeplan of the PCB for the simple circuitry. Comments on these drawings only in german, sorry...

Cheers,
Thomas
Thomas:
A friend just pointed out this posting of yours and I wanted to say thanks. I have heaps of trouble getting my old(er) 912 UL to start at all when cold and often get the rough starting chatter. (Well, it's more than chatter - the thing really shouts at me!) Would not be too hard to make a longer delay in one starting circuit so the spark plug fired some degrees AFTER TDC. However, that's a refinement I may or may not get around to trying (yet another on the "gunna do" list!). But your idea of stopping the leading trigger pulse with a diode is excellent. I assume the polarity of the leading edge pulse is negative. I don't understand how Rotax "just used a zener" to limit ignition on the leading edge only. The polarities of leading and trailing edge pulses are opposite. Did (Do) they somehow split the two in the module? In any case, I will make up a circuit according to your design. Do you by any chance still have a PCB as surplus that I could buy from you?
regards
 

dendeze

Newbie
Joined
Apr 29, 2015
Messages
1
Location
wuustwezel Belgium
Hi guy's, has someone build this and know where all the wiring goes to?
I'm not a very good in electronics, so i don't see where K5 and K6 are connected.
I suppose the K3 goes to the startersolenoid and K5 goes to the earth?

I hope someone can help me out.
 
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