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Old Dec 06, 2012, 05:27 PM
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Another onboard starter thread

I have a DLE 20 that is not being used with a prop, but as a generator for a silly hobby project (tinkering you know...).

I searched through this forum and found a few threads about on-board starters but did not want to hijack any of them.

I made the starter with a 2 stage gearbox and the flywheel gear is on a sprag. The motor turns over great, maybe 500 - 1k rpm (I can make it faster if I need to).

The crankshaft of the DLE 20 is directly coupled to a big BL motor for the generator; that BL motor weighs about 500 grams so I think it has plenty of mass.

The problem is that the motor keeps backfiring and trashing my starter gears.

What on earth can I do to prevent this? Anybody with experience in this please help.

I thought maybe getting the motor turning really fast and then switching on the ignition?

Maybe some sort of clutch? I will post a pic and/or a vid of the starter soon. Thx,

- Joe

Video:

Starter (0 min 11 sec)
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Old Dec 06, 2012, 08:54 PM
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How large is the gap between the sensor and magnet? Is it the video or does the base plate flex under load?

Greg
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Old Dec 06, 2012, 09:16 PM
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The gap is fixed as far as I know and the screws are tightened all the way down.

Now there IS some flex in that plate. In that version in the video, I added a bent piece of aluminum across the top to stiffen it up, but it still flexed.

I have some 6061 aluminum on the way that is twice as thick and should not flex.
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Old Dec 06, 2012, 11:08 PM
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Did you check the ign. timing?
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Old Dec 06, 2012, 11:54 PM
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I always thought it would be fun to have 2 gassers(auto start) each run a gen and then put them into a V-22 Osprey with brushless motors the prop setup. Make for some autonomous flying with a lot of endurance and recharge capabilities....

your project is pretty cool!
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Old Dec 07, 2012, 05:20 AM
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The spark plug seems not to be hooked-up...

You could trash your engine's ignition system if you do that with it turned on.
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Old Dec 07, 2012, 07:58 AM
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Turk: I have not messed with the timing and I used to have that motor mounted with a prop and it ran great. I suppose if it is possible, I could try retarding the timing a bit.

Hutchinstuff: That would be a really ambitious and cool project. It would have to be massive to make the power to weight worth it. Probably would be better off with a single gasser in the 50cc range, and just run two rectifiers off the motor; one for each BL setup. Just don't try to use two rectifiers in parallel...


DarZeelon: the ignition is not hooked up at all, no sensor, plug, power... its just a motor with a sensor wire dangling down.


I am just wondering if I should be trying to design some sort of clutch in this, like adapt a slipper clutch from an RC truck? that way if it backfires it can just slip and not trash the gears?

Anything of that sort is going to require a lot of planning and effort, that is why I am trying to get an idea about what other people who have designed and successfully implemented starters have done about the issue of backfire.

I also realize that I can buy certain off the shelf solutions but I really wanted to make it myself, just for kicks... like kit building vs buying an ARF.

Thanks again everyone for the input.
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Old Dec 07, 2012, 10:06 AM
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For this kind of engines to start on a boardstarter without any messing with the ignition, need two things: instant acceleration, and a high enough RPM to get the engine passing TDC against ignition on its own inertia.

If you want to start it with a lower RPPM, you need retardation to close to TDC, if you want to start it with a slow accelerating starter, you need to temporary cut ignition until "firing speed" is reached.

Which of the options you choose is up to you.
Judging that video based on my own experience with starting problems on a Zenoah G230, I would say that your current starting RPM is on the low side, and acceleration is way too slow. Fix those problems, and it will most likely start reliably and instantly.

Brgds, Bert
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Old Dec 07, 2012, 10:12 AM
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If you put a clutch in it will slip and the engine still won't start. You might check the actual cranking timing. It sounds as if the ignition may be firing early even tough it should retard to just before TDC. I think some or all of the DLE 20 have a strange timing curve? The rigidity of the starter is important as well. Are the long standoffs rigid enough to prevent them from twisting?

What are you doing with this generator setup? You can buy a 500W generator with a four stroke engine for under 350USD delivered. I love tinkering, but unless the purchased product didn't fit my application I would just buy it.

Greg
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Old Dec 07, 2012, 10:20 AM
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Quote:
Originally Posted by gkamysz View Post
It sounds as if the ignition may be firing early even tough it should retard to just before TDC. I think some or all of the DLE 20 have a strange timing curve?
If I had to estimate that cranking speed I would say somewhere between 400 and 600 RPM.
If the engine is using an "RCexl-like" ignition, the retardation only takes place above 400 RPM. With a slow accelerating starter that means, the engine will start igniting before a sufficient speed is reached, and than timing will be too early, and the backfiring will prevent the starter from reaching sufficient speed.

So IF my suspicion is correct regarding the type of ignition, then first cranking up and only activating ignition if sufficient speed is achieved, would be the most logical solution....

Brgds, Bert
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Old Dec 07, 2012, 10:34 AM
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I start smaller engines with full ~28 advance without issue, but my starters have very high cranking speed and torque available.

The reason I asked about the sensor gap,is because it looks quite large in the video. It may not be retarding as expected. DLE are supplied with RCEXL ignitions, but I understand the curve supplied with the 20 was somehow odd.

One more thing, go to full throttle straight away with the fastest motor acceleration programmed into the ESC. If the motor balks, use a better ESC.

Greg
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Old Dec 07, 2012, 10:46 AM
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Awesome info.

The start sequence will be controlled by arduino eventually so I can program an ignition delay with a relay switch. For now, I will just put my finger on the switch and when the RPM hits what I estimate to be good, i will turn the ignition on.

The generator is just for kicks for now, not sure if it will go "in" anything at this point. I just like playing with robotics, circuits, RC planes and the like. A lot of people have speculated on gas/electric stuff for RC and I would be lying if I said I have not contemplated/attempted this as well - but there are big technical challenges to that...

The standoffs have a tiny amount of twist but not enough to even effect the gear mesh.

In fact, when it backfires, the gears on the starter motor get trashed, not the flywheel gears which is what the flexing would effect the most. I am currently moving from aluminum to hardend steel gears on the starter motor to improve robustness against a backfire.

Maybe that is the wrong direction because trashing the flywheel gear will be expensive - I had to get a machine shop to bore it to an interference fit with the sprag... that is about a $120 gear when its all said and done. The flywheel gear is 416 stainless though, its durable... I really wish I could find a helicopter main gear with an 8mm bore... that would lower the cost of the flywheel gear a lot...

Thanks for the input guys, I have great info now to proceed.
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Old Dec 07, 2012, 02:29 PM
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Maybe I am making a very silly suggestion here.... but if I understand correct, you intend to use a fairly big brushless motor as a genertor, correct?

Wouldn't it be easier, to make use of that motor/generator as a starter? if it can handle the power delivered by the engine to convert to electrical energy, I would figure it would be able to crank that engine into life as well?

Might be a bit complicated electrically, but it would make things a little less complicated mechanically....

Brgds, Bert
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Old Dec 07, 2012, 03:31 PM
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Originally Posted by Brutus1967 View Post
Maybe I am making a very silly suggestion here.... but if I understand correct, you intend to use a fairly big brushless motor as a genertor, correct?

Wouldn't it be easier, to make use of that motor/generator as a starter? if it can handle the power delivered by the engine to convert to electrical energy, I would figure it would be able to crank that engine into life as well?

Might be a bit complicated electrically, but it would make things a little less complicated mechanically....

Brgds, Bert
I really really wish I could do this (maybe I can?).

To an extent I can; in the sense that the generator motor will turn the fuel motor through compression stroke, and being a 270 kV motor, a 2 cell LiFe battery may get it somewhere in the 1.3k rpm range. That should be enough power; however - I am using a bellow shaft coupling which allows for about two degrees of misalignment between the two motors - and I am hoping will soak up some of the power pulses of the IC motor and mitigate harm to the magnets of the BL motor (I really don't know if this is even a concern). But that shaft coupling can only handle so much torque and I don't want to risk stressing it with routine starts/potential backfires.

I have not really thought about electrical implications either, probably just a Y-split where the motor wires go to both a rectifier and an ESC. As long as there is no load on the generator at start-up, it should not make a difference... I would have to think more about it I guess.

Maybe I will go to a rigid shaft coupling and just use the generator motor in the future.
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Old Dec 07, 2012, 05:17 PM
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I have screwed up more on board starters than ANYONE! I can suggest two things...I'd recommend a flywheel of sorts,with some mass,equal to what the engine experiences with the prop-the engine can reverse direction too quickly,without it. It won't affect rpm capability...(just take a little longer to get there)

You do need to check that the ignition timing is being retarded electronically.
I have a Zenoah G-62 with an onboard starter,and I'm fabricating a bracket that will work in conjunction with the throttle to alter ignition timing.

Knowing that your magneto is correctly adjusted will remove one concern.-position the magnets directly in line with the magneto...Fold a piece of paper money (President is inconsequential),and place it on the magnets.Loosen the screws,close the gap,and tighten the screws....Send me the money...lol...Your magneto is correctly adjusted.

There is much to learn here,and a high interest is developing.

There is no solution with gearing-your/our problem is controlling the ignition timing. When the gears get strong enough,you'll start twisting shafts in two.
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